PART III. SEAMANSHIP AND NAVIGATION
Page Chapter 1. Steering and boat handling 273-283 Section A. General 273-274 Section B. Characteristics of the PT boat at sea 274-275 Section C. Characteristics of the PT alongside dock 275-278 Section D. Maneuvering table (idling speed) 278 Section E. Mooring a PT boat 279 Section F. Anchoring a PT boat 280-282 Section G. Addenda to notes on anchoring a PT boat 282-283 Section H. Conclusion 283 Chapter 2. Dead reckoning 284-287 Section A. Dead reckoning 284 Section B. Current 284-287 Chapter 3. Charts and compasses 288-295 Section A. Charts 288-291 Section B. The pioneer airplane compass 291 Section C. Observer compass 291-293 Section D. The spherical compass 293 Section E. Variation 293 Section F. Deviation 294-295 Chapter 4. Publications and records 296-299 Section A. Publications 296-298 Section B. The log 298-299 Chapter 5. American buoyage system 300-301 Chapter 6. Piloting 302-318 Section A. Introduction 302-303 Section B. Instruments used in piloting 303-305 Section C. Plotting a course 305-306 Section D. Methods of determining position 306-313 Section E. Practical hints 314 Section F. Storm warnings and weather symbols 314-316 Section G. Sea anchor 316 Section H. The lead 317-318
Chapter 1. STEERING AND BOAT HANDLING
SECTION A. GENERAL
This important phase of PT operation cannot be stressed too highly. Basically, all naval actions depend upon placing the ship in an area where she may attack the enemy, and maneuvering
her in that area after arrival. It is the function of good seamanship and navigation practices to place the ship so that she can do the most damage to the enemy with the least damage to herself. In PT boats, the importance of good boat handling is magnified, because of the boat's small size, high speed, and, particularly, its vulnerability to damage by the action of the seas.
Good boat handling largely depends on three factors: (1) Common sense, (2) proper instruction, and (3) experience.
Common sense is something which no amount of teaching will develop, unless the student himself is trying his best. Proper instruction is added to this, and the student goes out to an operating squadron with these two factors as tools. Experience alone will tell whether he uses these tools to become a good boat-handler or just another dock-smasher.
SECTION B. CHARACTERISTICS OF THE PT BOAT AT SEA
PT boats in their present stage of development are a specialized weapon. The ones in current use are the best answer to the question: "Is it possible to build a small ship, extremely fast and still seaworthy, which can deliver a real knockout punch to a capital ship?" It is manifestly impossible to combine all the seaworthiness of a round-bottom sailing ship with the speed of a Miss America. Some designers chose to emphasize speed. Their boats performed brilliantly as long as the sea was calm, but simply could not take rough weather. Others built boats that could take the worst weather in their stride, but could not make speed enough to close the range on an aircraft carrier. The Navy's PT boat is one that (a) can outrun anything that floats and carries weapons big enough to sink her, and (b) can make reasonably high speed in bad weather.
Naturally, such a compromise type of boat requires special handling in a seaway. Up to about a 4-foot sea, the PT boat can make her maximum speed on any course.
When the seas increase to about 8 feet, she can still make her maximum speed down wind or across it, in fact on every course except dead into the seas. If the normal course is directly into the wind and sea, in heavy weather, the boat may be injured by
trying to make maximum speed. In addition, the crew will be unable to man their stations properly and may even be injured, thus placing them out of action at a time when all hands are needed. Perhaps most serious of all is the fact that, forcing the vessel at high speed into the seas causes so much spray to come-over the bow that visibility is reduced to a very great degree. This is particularly undesirable, because the very success of a mission, to say nothing of the lives of the crew, usually depend, upon spotting the enemy before being spotted, and hitting him where it hurts, before he can bring his guns to bear effectively. All these considerations point to the necessity for tacking the boat. This means to steer a zigzag course, taking the wind and seas quartering over the bows instead of from dead ahead. Then the boat can resume much of its speed, and visibility will improve notably. Just how much the base course must be altered to do this depends upon the seas and the condition of loading. 30° to 45° is ample under the worst conditions, and usually 20° is sufficient. Another controlling factor is the question: "Which is more important, that I get there fast, or that I go farther?" Naturally, at increased speed on zigzag courses, the boat uses more gas to cover the same distance made good than she does plugging along at slow speed dead into the wind. The course and speed must be chosen with this fact in mind.
If the seas continue to increase, the boat will be able to maintain speed and come closer and closer to steering right into the seas and wind. When the sea gets to about 20 feet and greater, she can still steer down wind or across it, and she can even steer into it very well, using only a turn or two of the wheel at the crest of each sea, to momentarily slant her bow into the next sea at a favorable angle. Doing this first to the right, then to the left, and so on, will permit the boat to make good practically a straight course, dead aweather, at good speed.
SECTION C. CHARACTERISTICS OF THE PT BOAT ALONGSIDE THE DOCK
The PT boat has three right-hand propellers, and two or three rudders, each rudder set directly aft of each propeller and in the slip-stream. Even in a single-screw vessel with a right-hand propeller there is a slight torque, or tendency for the vessel to go to-
the left with her engines ahead and rudder amidships. This torque is very noticeable in a PT boat, since all three propellers turn right-handed. This is caused by the fact that the top blade of the propeller is working in water of less pressure than that in which the bottom blade is working at any given moment. This gives the bottom blade a stronger thrust against the water in a sideways direction, and tends to throw the stern to starboard and the bow to port at slow speeds--approximately 1,000 r. p. m. on all three engines. To compensate for this torque it is necessary to give the helm about 3° to 4° of right rudder. As the speed of the boat increases, she begins to rise up and plane on top of the water. This results in "packing" a good deal of water under the boat on which a good deal of pressure exists. The term for this is "impaction," and it results in pressure on the water directly under the after part of the hull, but not extending below the hub of the propeller. At about 1,500 r. p. m. this impaction about equals the water pressure below the hub of the propeller, and the result is that both top and bottom blades of the propeller are turning in water of equal pressure, canceling out the torque completely. Therefore at 1,500 r. p. m. or thereabouts, with all engines engaged, and the rudder amidships, the boat will go straight ahead, with no tendency to turn right or left. When the engine speed reaches approximately 2,000 r. p. m., the impaction increases so much that the effect is to have the top blade of the propeller turning in water of greater pressure than that in which the bottom blade is turning, and thus the torque is now applied so as to throw the stern to port. The result is a tendency of the boat's head to go to starboard, which must be corrected by 2° to 3° of left rudder. The torque effect is less in the newer PT boats.
The torque effects should be borne in mind whenever handling alongside the dock as well as when under way outside.
Another characteristic of the PT boat is her ability to turn in her own length, at slow speed, with one engine ahead and one astern. The rudder, during the maneuver, should be hard over in the direction the handler desires to have the boat turn. Thus, to turn her in her own length to port: starboard engine ahead, port engine astern, wheel hard over left. To turn to starboard everything is put opposite: port engine ahead, starboard engine astern, and wheel hard over right.
Since the wing propellers are offset from the centerline, a comparatively quick turn right or left can be made by using just one engine ahead, and the rudder hard over in the direction you want the bow to turn. Thus, to turn left, put your wheel hard left and go ahead on your starboard engine. Customarily, only the wing engines are used when maneuvering alongside the dock, while the center engine is left idling, in neutral, as a stand-by.
Going ahead on the port engine, with rudder hard over left, makes the boat go practically straight ahead for a length or two. With the starboard engine ahead, and rudder hard over right, she will have a slight tendency to go to the left.
Backing on the port engine, rudder amidships, the stern tends to go strongly to port. Backing on the starboard engine alone, rudder amidships, it will tend to go to starboard. The rudder, at slow speeds, has very little effect except as a baffle plate for the propeller slipstream to work against and to give the stern an initial thrust to right or left when the engine is put in ahead position. To steer the boat while backing on one engine it is often necessary to use the propeller thrust of the other engine in ahead to slew the stern in the desired direction. In backing, if the wind is strong, the boat will tend to back into the wind regardless of which engines are engaged.
Whenever possible, approach to a dock should be made at a very flat angle. This means approaching the dock so as to be nearly parallel to it when you touch. Many times this is impossible, and the approach will have to be more nearly at right angles to the dock. Backing the outboard engine, and going ahead on the inboard engine if necessary, will swing the boat parallel to the dock just before she touches.
When backing away from the dock, put the rudder hard over toward the dock and go ahead briefly on the outboard engine. This will throw the stern off the dock, without appreciably moving the boat ahead, and then you can go astern on the inboard engine and back clear.
A good landing is always made at comparatively slow speed.
Before making a landing, the engines should all be disengaged to test engine telegraphs, at a safe distance from the dock. This also serves the purpose of insuring that the clutches are not "frozen" in ahead. Reversing the engines should also be
accomplished as a test on their availability, if needed, to stop the headway of the boat.
In this connection every effort must be made to conserve machinery. Excessive use of clutches and engines in jockeying back and forth to make a four-point landing at the expense of engines, clutches, and reverse gears is definitely a damaging operation. A good landing is one that causes a minimum damage to the boat. MTB's are very light and can readily be hauled in by hand once the dock is approached close enough to get a line across.
SECTION D. MANEUVERING TABLE (IDLING SPEED)
Arrows indicate how sharply and in which direction ships head will turn.
ON A WINDY DAY, THE BOAT WILL TEND TO BACK INTO THE WIND.
For practical maneuvering, the boat when backing on the port engine alone, will usually back straight for a length or two.
SECTION E. MOORING A PT BOAT
Figure 28.--2-line mooring.
Figure 29.--Breast and spring mooring.
In areas where tide range is over 4 feet this type of mooring will have to be tended. Lengthening the bow and stern leads will provide a mooring which will need little or no attention during use and fall of tide with less than 12 feet range.
Figure 30--Mooring alongside another PT. This mooring can also be used for towing alongside.
SECTION F. ANCHORING A PT BOAT
Three factors enter into the art of anchoring a PT in a snug and seamanlike manner. They are: (a) Proper tackle, (b) selection of good holding ground and lee, and (c) intelligent use of gear. These factors will be considered singly.
1. Proper tackle.
PT's are equipped with two anchors of the patent type the Northhill, weighing approximately 25 pounds, and the Danforth weighing approximately 50 pounds. Both anchors, although of different patterns and weights, depend more on design than weight for their holding power. Of the two, the 25-pound North-hill is the better anchor for general duty, although either one is good. The anchor line, or rode, is of 31/2-inch manila, approximately 35 fathoms long, with a thimble spliced into the end. The Samson post, or fore bitt, is used to take the strain of the anchor line which leads out through the bullnose. A 2-fathom shot of chain is always used between the anchor and the manila line, as a precaution against cutting the line, and also to help the light anchor hold down against the surge of the boat.
2. Selection of good holding ground and lee.--The boat will ride best to ground tackle if she is in a protected spot, preferably
in the lee of a bit of high ground, and away from heavy seas or swells. If it becomes necessary to anchor in an open roadstead, particular care must be taken that the best holding available is utilized.. Mud holds to PT anchors best, with sand, gravel, and rock coming next in that order. (Coral can be made to hold the anchor absolutely fast by jamming it, but injury to the gear is almost certain to occur when getting under way again.)
3. Intelligent use of gear.--First, pick your anchorage from the chart or by eye. Note any available bearings on which to approach clear of all dangers. Use your lead all the way in. Assure yourself that you will have room to swing 360° in case the wind changes, and have all your gear laid out on the foredeck clear for running. Approach your selected anchorage from the leeward, if possible, at dead slow speed. Drift slowly up to your bearings or ranges, and when they come on, let go your anchor. Back down on one engine briefly as soon as you have let go, so that your boat will be making slight sternway, enabling the anchor to get a good bite. Pay out enough line to insure that the anchor will not be pulled out by a pull too nearly vertical. (A safe rule is, 5 fathoms of line to every fathom of water at the anchor. You cannot pay out too much line, unless it puts you into dangerous proximity to rocks or shoals.)
When you are sure your anchor is firmly set, secure your engines and recheck your bearings or ranges on the beach, so that you can tell if you drag. Frap the manila line with old rags, canvas, or other chafing gear where it passes through the bull-nose. Every watch or at least every six hours freshen the nip at the bull nose. That is, slack out or take in a foot or two of your anchor line, and re-frap it with chafing gear, so that no one spot has to take all the strain and chafe over a period of time.
If you feel there is danger from a sudden blow or if you do not trust your gear overmuch, let go another anchor alongside your bow, on a slack line about two-thirds as long as the anchor line to which you are riding. Secure the inboard end of this new and slack line to a different cleat or bitt, and coil down the slack on deck, stopping it off with cut yarns, if your boat is pitching or rolling badly. In case the anchor line to which you are riding drags or carries away, the second anchor will automatically take hold, and save you from dragging or drifting into danger.
In general, once your Northill--called the right bower--is set, and if you have plenty of scope in your anchor line, you should be able to ride out anything short of a hurricane. Above all, care for and inspect your anchor gear frequently and well. Your boat's safety and the success of a mission may depend on it.
SECTION G. ADDENDA TO ANCHORING A PT BOAT
1. Boats operating in tropical areas, where coral bottoms are to be found in almost every anchorage, have reported that neither of the present anchors on PT's can be expected to stand up in continual use under such conditions. The relatively light flukes catch in coral, and when the time to get underway arrives, the anchor is so hard and fast that it can only be pulled clear by main force--"steamed out." This results in a bent or broken anchor.
Also, these boats report that for anchoring in the above coral conditions, in deep water especially, the 2-fathom shot of chain mentioned in the text is insufficient to prevent cutting and chafing of the manila anchor line. Five or six fathoms are recommended.
2. Steps are being taken to procure 75-pound Navy standard anchors for boats operating in coral seas. The length of the chain shot will be increased to 5 fathoms.
Towing another PT boat.
Standard towing equipment includes a fitting on each side of the clime close to the water line with a bridle and quick-releasing hook. In towing it is important that adequate line be payed out to obtain the most comfortable riding compatible with the particular circumstances.
If possible the towing vessel should tow at a speed that will permit the PT to capitalize on as much of her planing as possible.
A secondary method of towing is to use one end of the 30-fathom anchor line of 31/2-inch manila, two or three round turns are taken around the 20 mm. gun mount and the end made fast with a bowline to its own standing part. The other end of the line is passed or drifted to the boat to be towed where it will be made fast at the bow either to a towing pennant or directly to the Samson post. The strain must be taken up gradually. For a protracted tow in heavy weather, it may be best to shackle
two anchor lines together, thus doubling the length of the tow, and allowing both boats to ride easier.
SECTION H. CONCLUSION
Always bear in mind that the sea is bigger and more powerful than anything that floats. Remember that the good seaman is one who yields to the sea when necessary, and opposes her with headwork rather than hull construction. The PT boat has been described luridly with respect to its riding qualities as a cross between a bucking horse and a streamlined pile driver. It is true that careless people can get hurt aboard one, but you will soon discover that she is a real boat, capable of performing well in any situation, and one that can get right out there and deliver the goods.
Chapter 2. DEAD RECKONING
A. Dead reckoning.--If the navigator had no means of observing celestial bodies to determine his position, he would still be able to calculate his approximate position at a given time by keeping track of all his courses and distances sailed, and noting the points at which they placed him. This process is called dead reckoning, and may be solved either by use of computation and table or by plotting. If computation is used, middle latitude sailing is ordinarly employed. However, the navigator generally uses plotting sheets to solve his dead reckoning problems, and this is the only method we will consider here.
Plotting sheets for any latitude may be obtained from the Hydrographic Office. Using the plotting sheets for the approximate latitude in which the ship is located, it is only necessary to lay off, from the last well-determined position, the courses and distances run successively. The resultant point is called the dead reckoning position, and is labelled D. R., followed by the time and date.
Since any good navigator takes frequent celestial observations for position, the use of dead reckoning today has been reduced to plotting a short run between observations, or to keep track of the ship's movements during periods of poor visibility when observations are impossible.
Because the dead reckoning problem is based primarily on courses steered and distances run, the dead reckoning position may be in considerable error due to poor steering, failure to compute distance correctly, or currents in the ocean. Steering and computation errors admit of no set rule to compensate them, but currents, when known, can be offset quite feasibly. An explanation follows.
B. Currents.--The difference between current effect and wind effect, insofar as the navigator is concerned, is so slight that when both strength and direction of either leeway or drift
due to current are known, the same diagram will solve the problem involved. Two cases are possible:
1. Given course and speed: To find the course and distance made good through a current of known set and drift.
2. To find the course, at a given speed, that will allow the ship to make good a given course, through a current of known set and drift.
(In the above cases, set and drift of current might be replaced by set and drift resulting from wind effect. In such instances, the problem would be one of leeway, but would be solved precisely the same.)
Case 1--To find course and speed made good.--A ship steams on course 211°, speed 12 knots, through a current whose set is 075°, and whose drift is 3 knots.
See figure 32.
Lay off on your chart or plotting sheet the true course steered from a point A on a Meridian NS, to a point B which is 12 miles distant, along the course line, from A. Point B is the location
at which your ship would have arrived in 1 hour, had there been no current. Now, from point B lay off a line in the direction of the set of the current (075°) for a distance equal to the drift of the current (3 miles), call the end of this line point D. Now
connect point A and point D. The length of the line AD is the distance made good (10 miles), while the direction of line AD is the course made good (199°).
Case 2.--To find course to be steered to make good a given course over the ground.
As in case 1, the ship's speed is 12 knots, and the set of the -current is 075°, drift 3 knots. To find course to steer to make good a course of 195° see fig. 33.
From a point A, on the meridian, NS, lay off a course of 195° for an indeterminate length. From A lay off another line, AD in the direction of the set of the current and for a length equal to the drift of the current. From point D swing an arc of 12 miles (your speed), so that it intersects your course line of 195°. Call the intersection B, connect the points D and B. Then the line DB represents the course to steer to make good a course of 195°.
(In cases 1 and 2, all courses are considered to be true, and measured from the meridian NS clockwise through 360°.
Chapter 3. CHARTS AND COMPASSES
SECTION A. CHARTS
Charts are extremely accurate maps of portions of sea and coast areas. Each, chart contains particular information as to the peculiarities of the bottom, depth of water, lighthouses, lightships, location of buoys, wrecks, Coast Guard stations, and other information that will be of aid to the mariner. It indicates the obstacles to be avoided, and the best courses to follow when making passages. To understand the chart thoroughly, it. is necessary to understand something about its construction.
The earth being globular in shape, the representation of its surface on a flat piece of paper is necessarily of artificial construction. The construction of a chart, on a fiat piece of paper, to represent the earth, whose surface is round, requires the use of a scheme known as a projection, in which curved surfaces appear to be flat. All projections have some errors, but the most common, or Mercator projection, is sufficiently accurate in lower latitudes to be used for navigation. The polyconic projection is used to a lesser extent, particularly by the United States Coast and Geodetic Survey.
An explanation of the Mercator is briefly this: The earth is imagined as a ball which has been placed inside a large cylinder and caused to swell until its surface coincides with the surface of the cylinder which encloses it. By cutting the cylinder along a meridian and opening it out into a flat surface, we have a representation of Mercator's chart. All the meridians on a Mercator chart are parallel, straight lines, and the degrees of longitude are all equidistant. The latitude parallels are everywhere at right angles to the meridians, and therefore the rhumb line or the course of a ship can be shown on this chart as a straight line. For navigational purposes, the Mercator chart has a great advantage over the polyconic chart. However, in the Mercator chart, the degrees of latitude are all unequal, being increased in length from the equator toward the poles.
Tables of meridional parts go with this chart, which give the length of run as minutes of longitude at the equator. With the aid of these tables, it is possible to construct a Mercator chart.
Charts are divided into two classes; general charts and plans.
Plans are used primarily in survey work, while the navigator uses general charts. General charts are those that take in a large part of the ocean, the entire ocean, or a considerable part of the coast line and its rivers and waterways. They are divided into sailing charts, for use in offshore navigation, and charts of the coast. Charts of the coast are for use in coastwise navigation when the vessel, during most of the time, will be in sight of land, and can fix her position by buoys, landmarks, lights, and soundings. The numbers in water areas on a chart indicate depths of water.
The charts have markings which stand for such different things as lighthouses, lifesaving or Coast Guard stations, anchorages, buoys, and light ships, etc. The various kinds of bottom are indicated. Every chart has a key to the markings and symbols. A few of the chart symbols are represented below.
These markings are found on the charts of the Atlantic and Pacific seaboards, and of adjoining coastlines, such as Nova Scotia, Newfoundland, Mexico, the West Indies, and Central America, which are published by the United States Navy Hydro-graphic Office and the United States Coast and Geodetic Survey. The markings of these two publishers of charts are almost the same, but they differ in a few instances, and they have a large variety.
On general charts great curves are laid off. These curves represent the 10-, 20-, 30-, and 100-fathom curves. They are of great value when navigating along the coast.
SECTION B. THE PIONEER AIRPLANE COMPASS
This is a small, compact, liquid compass secured to the top of forward bulkhead in the cockpit directly over the wheel where best seen by the helmsman. The dials are slightly luminous so as to be seen clearly for night steering. The liquid in the compass is composed of highly refined kerosene oil. The lubber's line in this type of compass is on a small glass plate directly facing the helmsman, and is vertical rather than horizontal, Unlike other compasses it indicates the swing of the stern, rather than the bow. To lower reading of compass (from 250°,to 230°), turn wheel to left; to increase reading (250° to 270°), turn wheel to right. With a little practice this will be readily understood.
Suggestions.--To prevent bubble when compass is exposed to direct rays of sun, build small wooden screen to go over the compass, leaving one end open so that the compass may be seen to steer by, and in this way protecting the compass liquid from overheating and leaking.
Care.--Do not place any iron or magnetic metal in vicinity of this instrument while under way. Cover when not in use. Keep face and case clean.
SECTION C. OBSERVER COMPASS
Instructions for Use and Care
The observer compass is a precision-built instrument designed especially for obtaining distant shore bearings. The magnetic element and compass card are especially "dead beat" so that any
unsteady motion of the ship transmitted to the hand or compass will not disturb or interfere with its accuracy. Construction of the compass and flashlight is entirely of brass or bronze so that there is no local magnetic disturbance. Do not attempt to use the observer compass close to iron or steel fittings.
The hinged-prism mounting is made rigid so that the prism cannot be put out of alignment and change its focal length. Should the image in the prism become indistinct it is probably due to dirt or water lodged between the prism and its bracket. To clean the prism, simply remove the screw from the chromium V-piece and the bracket, clean the prism, and replace it. Notice the alignment before setting the screw down hard to make sure that the alignment is correct.
The compass bowl is equipped with a corrugated expansion chamber to take care of contraction and expansion of the liquid, and there is also a very large and capable bubble trap. As the compass is filled with a high-grade aircraft compass oil, it sometimes develops a bubble due to cold weather. Should this occur, it is only necessary to tip the compass upside down and then level it slowly. This will put the bubble in behind the bubble trap where it will not be visible nor disturb the magnetic action of the compass card.
Care should be taken when removing and replacing the instrument in the carrying case. This case is designed to protect the prism from being knocked out of place, as well as removing the possibility of vibration reaching the jewel and pivot. It is recommended that the carrying case and compass be stowed in a flat or horizontal position. This prevents wear and tear on the jewel and pivot.
Should it be necessary to insert a new bulb in the flashlight, first remove the batteries, then unscrew the flashlight tube. The bulb may be replaced in its reflector. If the occasion arises where a spare complete flashlight is needed, simply unscrew the three small screws on either side and the forward side of the compass.
To use the observer compass, it is necessary to hold it at eye level (at convenient distance from the eye), and get the object on the prism at the same time, holding the lubber line in line with the chromium V-sight. The compass should be held approximately level when observing these bearings. The intensity
of the light which shines through the compass is sufficient to illuminate the section of the chart on which you may be plotting the bearings.
The compass itself may be used as a spare or telltale by fixing its carrying case to a vertical bulkhead. With the compass in the case, it is usually necessary to turn the flashlight on to read it.
The compass is over 91/2" all, 31/2" outside diameter. The carrying case is of mahogany and nicely fitted. Its dimensions are 11" long, 41/2" deep and 4" wide.
Care.--Place in case on board built for it. See that case is securely fastened. Wipe carefully to remove any water and see that glass and prisms are always clean. Secure in its locker, made especially for this instrument.
SECTION D. THE SPHERICAL COMPASS
It is located in the pilot house. It is used as a checking compass with the Pioneer on the bridge or as a steering compass when vessel is being conned from the pilot house.
Care.--Same as for Pioneer compass.
SECTION E. VARIATION
The influence on the compass needle due to magnetic attraction of earth or influence outside of the ship in called Variation. It is the angular difference between true North and Magnetic North. Its value for any locality is found on compass nose on chart of that area, and varies for different localities.
SECTION F. DEVIATION
Error caused by magnetic substances such as iron and steel on board the boat itself is called Deviation. It exists to a greater or lesser degree on every boat. Moreover, deviation on any boat is not constant; that is, it is different in amount for every different heading of a boat or ship. Cards are made up for each compass on board, showing the deviation on different courses.
To correct a course for deviation and variation, the following diagram should be memorized.
In the preceding diagram, the arrows indicate the direction in which the corrections are to be applied, i.e., whether the course is being corrected to obtain true or compass course. The letters stand for the following quantities:
To find deviation of compass.
There are several methods commonly used, for example, by azimuth of the sun, and by reciprocal bearings of a stationary object on the shore, such as a church steeple, mountain peak, etc. The compass error can be checked on any heading by known bearings such as ranges on coming into harbors, or by known bearings of any object or objects on shore.
In calibrating the compass, it is best to previously lay off on the chart, from prominent objects, ranges which will give magnetic headings not more than 15°-20° apart, obtaining sufficient ranges to cover the circle of 360°. Then the boat is taken out, and her head put on the various ranges, noting on a card (Napier's diagram) any differences between the charted range and the actual heading by compass when on the range. Such differences, if they exist, constitute the deviation of the compass on that heading. These deviations are arranged in a column,
opposite their respective headings, and using any two adjacent headings it will be possible to obtain the deviation for any intermediate heading by interpolation. A card of such interpolations should be made up, giving deviations for headings every 15° from 0° to 360°.
This deviation card must be checked at regular intervals, and always when any alteration involving sizable quantities of metal on the boat is involved. It is necessary to calibrate the compass with all metal objects in their standard stowage spaces, and with the boat in her normal sea condition.
Chapter 4. PUBLICATIONS AND RECORDS
SECTION A. PUBLICATIONS
The tide tables contain the time of high and low water, the range, etc., of all principal ports of the world, also the tides of all navigable rivers in the United States. They are published by the Coast and Geodetic Survey.
Light lists contain information regarding the characteristics of all light and lighted buoys, also characteristics of fog signals and stations equipped to sound them, as well as stations fitted to sound submarine bells and the characteristics given. Radio beacons also are listed. Each publication shows on its cover the area covered. They are published by the United States Lighthouse Service.
The current tables contain the time of slack water, minimum ebb and maximum flood, also velocity of the current for all the principal ports of the world. These tables are of particular assistance to a ship's captain or navigator planning to go alongside a pier or dock in any port in the world. They are published by the Coast and Geodetic Survey.
The Coast Pilot
The Coast Pilot is published in several volumes and contains information regarding the entire coast line and navigable sounds, rivers, and harbors of the United States. There are five volumes for the Atlantic coast, one for the Pacific coast, two for the coast of Alaska, one for the Hawaiian group, and two for the Philippines. These volumes are published by the Coast and Geodetic Survey.
There are three chart catalogues: for the Atlantic, Pacific, and Asiatic stations. The catalogues contain index charts from
which can be found the portfolio numbers covering all the areas of the world. By referring to the correct portfolio the desired chart of any locality in these areas can be found. They are published by the Hydrographic Office.
The Nautical Almanac contains information regarding the sun, moon, stars, and planets, of value to the navigator for fixing the ship's position when out of sight of land. Published by Naval Observatory, Washington, D. C.
1. Chart corrections are made from a publication called "Notice to Mariners," which is published weekly by United States Coast Guard and United States Hydrographic Office, jointly. This contains all important corrections to charts, and notes changes in aids to navigation which have occurred since the publication of the charts in general use at the time of issue.
Two copies are sent weekly to ships and stations, or to anyone requesting them. Notices to mariners requiring immediate dissemination are sent out by radio, under three classes of security: (1) Plain, (2) Restricted, and (3) Confidential or Secret.
2. Each notice should be read carefully. Charts to be corrected first are listed in bold-face type at bottom of notice. These charts are largest scale (detailed) of the area containing correction. Upon completion of corrections on charts designated, a note should be made in the lower left-hand corner of each chart, stating date of notice.
3. Next, corrections should be made in the United States Light List, and if any new lights or buoys have been established, the notice should be cut out of the issue, and placed in the proper place in the Light List, and U. S. Coast Pilot.
The bottom of each notice shows the charts and publications to which the corrections pertain, as in the following example:
H. O. Charts, 1061, 611, 1013, 981.
U. S. Coast Survey Charts, 290, 294, 1215, 1218.
U. S. Light List, Atlantic Coast (North) 1942, page 70 or No. 345. 1-348.
U. S. Coast Pilot, Section B, 1940, page 105.
These references should be checked off the list as soon as the corrections have been made to all the publications aboard.
SECTION B. THE LOG
The log is the only complete official record of the ship during her commission. It is imperative that the log not only be complete, but also accurate and clear. Because of many routine inspections required to be entered in the log, it also serves as an important check on whether or not they have been made. The boat captain is responsible for the accuracy and completeness of all entries, by whomsoever made.
PT boats use the district craft log which is simpler and less detailed than the standard deck log. It is better to enter events as they occur so that nothing remains to be done but to verify the columns and sign the remarks. It is required that the ship's log shall be a careful, detailed, and accurate record of current events, since it is frequently used as evidence before courts and boards, and is consulted in many cases which come up years later. The smooth log is the ship's deck log. Therefore, officers should take pains to collect all the data required and to enter them into the log, using the proper phraseology.
There must be no erasures in the rough log. In civil courts, the rough log and the quartermaster's notebook are considered better evidence than the smooth log. Any erasure in them opens to question their competency as evidence.
The quartermaster has charge of the preparation of the log. By the regulations he is required to carefully examine the log book to see that it is prepared in accordance with instructions, and to call the attention of the watch officer to any inaccuracies or omissions in the entries. The quartermaster is responsible to the boat captain for seeing that the entries in the log are in proper form, but the officer of the deck is responsible for the entries during his watch and, unless directed by orders of the boat captain, he is not compelled to make changes in any entry he may have made.
The regulations require that the smooth log shall be signed by the boat captain and submitted to the squadron navigation
officer for his approval monthly, or when requested by the navigator, or called for by executive officer or commanding officer.
Every boat captain and quartermaster should frequently read "Directions for Keeping the Ship's Log," and "Extracts from the United States Navy Regulations, 1920, Relative to the Log," both of which are in the front of the rough Deck Log Book (N. Nav. 330). Extracts from "Instructions for Writing the Log," Watch Officer's Guide, will be found in Appendix II of this book.
Chapter 5. AMERICAN BUOYAGE SYSTEM
Buoys--Entering Harbor (Channel) From Seaward
On RIGHT-HAND side of channel (starboard) are Conical or Nun-shaped buoys, painted Red with Even numbers (2, 4, 6, 8, etc.). Red or White lights are placed on starboard side of channel.
AND side of channel (port) are Cylindrical, or Can buoys, painted Black with Odd numbers (1, 3, 5, 7, etc.). White or Green lights are placed on port side of channel.
Obstruction buoys--Painted Red and Black--horizontal bands. May be passed on either side.
Bell buoys.--Flat-topped float with skeleton framework supporting a Bell.
Figure 37.--Conical or nun shaped buoys: even numbered and red. Cylindrical or can buoys: odd numbered and black.
Gong buoys.--Have flat top with skeleton framework supporting a series of four gongs of varied tone.
Whistle buoys.--Are conical with whistle on top.
Fairway buoys.--Are painted Black and White--Vertical stripes, and may be passed on either side, close to, as they indicate deep water.
White buoys.--Are anchorage buoys, designating anchorage grounds.
Spar buoys are long slender buoys of wood. They may be used with the other types of channel buoys in which case they are painted red or black, depending on the side of the channel.
White lights may be either side of channel for entering vessels-- but colored lights are only on sides indicated above.
Character of lights.--To avoid confusion, lights are given distinct characteristics. These are indicated by abbreviations as follows:
Lights which do not change color Characteristic phases Lights which show colors in various combinations F.=Fixed A continuous steady light Alt.=Alternating. Fl.=Flashing A fixed light varied at regular intervals, the duration of light being always less than that of darkness. Alt. Fl.=Alternating flashing. F. Fl.=Fixed and flashing. A fixed light varied at regular intervals by 1 or more flashes of greater brilliance or different color, or both. A flash is preceded and followed by a diminution of light or an eclipse. Alt. F. Fl.=Alternating fixed and flashing. Gp. Fl.=Group flashing. Showing at regular intervals groups of flashes. Alt. Gp. F1. -A1-ternating group flashing. Qk. Fl.=Quick flashing. Shows not fewer than 60 short flashes per minute. I. Qk. Fl.=Interrupted quick flashing. Shows quick flashes for about 4 seconds followed by a dark period of about 4 seconds. S-L. Fl.=Short-long, flashing. Shows a short flash of about 0.4 second, and a long flash of 4 times that duration, this combination recurring about 6 to 8 times a minute. Occ.=Occulting A steady light totally eclipsed at intervals. The period of darkness exceeds the period of light. Alt. Occ.=Alternating occulting. Gp. Occ.=Group occulting. A steady light totally eclipsed by a group of 2 or more eclipses.
Chapter 6. PILOTINGSECTION A. INTRODUCTION
As long as man has been going to sea, one of his major concerns while afloat has been the determining of his position. He has always been trying for the best possible answer to the question: "Where am I now, and where do I go from here"? The answer to this question we call navigation or the art of guiding a ship from place to place on the earth's surface. Navigation is a general term, covering all branches of the art. It can be broken down into the following branches:
(a) Celo-navigation, or nautical astronomy. This is the art of finding the ship's position on the earth's surface by the use of observation of celestial bodies, such as the moon, sun, stars, and planets. This art is at its best offshore and is not in general use along the coast.
(b) Geo-navigation, or piloting. This is the art of guiding a ship from place to place, using terrestrial objects for help and guidance. These terrestrial objects may be natural landmarks, such as a mountain or headland, or they may be man-made aids to navigation, such as lighthouses, lightships, or buoys. Depths of water, configuration of the land, tide rips, and other natural features of geography are also employed.
PT boats, with their high speed, shallow draft, short range, and limited navigational facilities perforce are more adapted to piloting than to nautical astronomy. While it is possible to take a sun sight or star sight and work it out aboard a PT, it is usually much more satisfactory and practical to use local aids to navigation. In other words, if a battleship navigator could be called a "star gazer", then a PT navigator should be called a "buoy bumper." This is not an uncomplimentary term, since piloting alongshore calls for just as accurate and skillful work as does nautical astronomy. In fact, an error of 5 miles in a noon observation of the sun while the ship is a thousand miles offshore is not in itself a very serious thing, while an error of even 1 mile
when sailing alongshore in some regions may result in the loss of the ship through grounding on a weather ledge.
This chapter is concerned with means of finding your position in a short time with an acceptable degree of accuracy, and with the methods of checking the position of the ship while moving from place to place.
SECTION B. INSTRUMENTS USED IN PILOTING AND THEIR OPERATION
(1) The Chart
Charts are simply very accurate maps of a region. They contain data on depths of water, currents and tides, location of aids to navigation, shape and configuration of the land, compass data, and local conditions which may be of value or interest. The best ones in the world are published by the Hydrographic Office, United States Navy, and are available free to naval vessels.
(2) The Parallel Rulers
This instrument is composed of two identical wooden rulers usually made of ebony, so joined by a pair of hinges as to be capable of being separated from each other to the limit of the hinges while still remaining parallel at all times to each other. They are used to transfer a selected course to the compass rose on the chart, and thereby enable the navigator to obtain his true or magnetic course in terms of degrees.
(3) Dividers and Draftsman's Compass
There are two similar instruments. The dividers consist of two legs, hinged at the upper ends, and having points at the lower ends. They are used mainly to "step off" distances on the chart, being set by hand from the latitude scale at the side of the chart to the desired distance in miles. The draftsman's compass is identical with the dividers, except that one of its points is a pencil lead, allowing the navigator to employ it for drawing circles of any prescribed diameter, such as, "a 12-mile circle from a lighthouse, indicating its limit of visibility as represented on the chart."
(4) Protractors and Universal Drafting Machine
These instruments are devices which can be used for the same purpose as the parallel rulers, and may be described briefly as improvements on them. The method of operation is similar, and results are the same.
The protractor consists of a transparent plastic disk, marked off as a compass rose. (Usually it has a smaller, movable disk mounted concentrically on the center pivot with it, used to offset courses for variation.) There is a long plastic arm also pivoted in the center of the compass disk.
To transfer courses, the card is set over the first point of the course, and turned so that its north points in the same direction as the north on the chart's own compass rose. The movable arm is then turned to coincide with the desired course line, and the magnetic or true course read off directly from the compass rose of the protractor.
The Universal drafting machine is a device much like the protractor, except that it is attached to the chart table, and its rose set to coincide with that of the chart. Then the machine can be moved around from course to course on the chart with no further setting necessary.
(5) The Observer Compass
This is the trade name given by its manufacturer to the portable hand bearing compass. Its function is to determine the magnetic bearings of objects in sight. The compass card is reversed, and has a reflecting prism attached to its case. A flashlight is included in the handle for use at night.
In operation, the observer compass is held up at eye level, and turned so that the prism is toward the object sighted, such as a lighthouse or tower. When the sight mark on the prism and the lighthouse are in line, then the magnetic bearing of the lighthouse will be seen reflected in the prism. This magnetic bearing is considered to be free from deviation, since the hand compass is always held well above the metal parts of the boat.
(6) The Sextant
Sextants are used mainly for celestial navigation. However, they are useful in piloting for the measurement of horizontal angles between objects, after which the angles are laid off on the
chart to fix the ship's position. Occasionally, vertical sextant angles are taken to determine the ship's distance off a chartered object.
(7) WSN Speed, Time, and Distance Calculator
This device is as a means of quickly obtaining time, speed, or distance run, when any two of the three are known. It is a transparent plastic rectangle with three scales on it, marked respectively "Time," "Distance," and "Speed." In use, a straightedge is laid across the scales, intersecting the proper scales at the two known points. Then the third factor is read off at the point where the straightedge intersects the third scale. (See fig. 38.)
Speed: 20 knots.
Distance: 4 miles.
Required: Time 010.
Answer: 12 min.
SECTION C. PLOTTING A COURSE
The job of plotting a course is one on which the safety of the vessel may depend. It should be done properly, and above all accurately.
The steps in plotting a course are:
(1) Select the two points involved on your chart; for example, Lighted Bell Buoy No. 2, from which you intend to sail and Fairway Buoy No. 2A, at which you intend to arrive.
(2) Lay your parallel rulers (or protractor) on the chart, so that they connect the two points (buoys in this case).
(3) Draw a light line connecting the two buos7s, and remove the parallel rules. Notice carefully that your course line as drawn does not cross any dangerous spots, such as shoals or ledges, and that it does not pass too close to any other dangers, such as wrecks, points of land, etc.
(4) Replace your parallel rulers exactly on the line you have drawn. Now, holding one of the rulers firmly and moving the other, "Walk" them to the nearest compass rose, being sure that they are not joggled away from their original angle. Stop your parallel rulers with one edge directly on the cross in the middle of the compass rose.
(5) Now note the reading of the outer compass rose where the parallel rulers cut it. This is your true course. To obtain the magnetic course, add or subtract the variation (found on the compass rose) to the true course, adding if westerly and subtracting if easterly variation. Or, you may note the reading of the inner compass rose in points where your parallel rules cross it, and convert the points to degrees to obtain magnetic course. If any deviation exists in your compass for this course, apply it to your magnetic course to obtain compass course.
(b) Label your course line plainly but lightly, as: "C. 179° true," "C. 015 Mag." etc.
(7) Now take your dividers to the side of your chart. Spread the points to a convenient number of minutes of latitude, say 2 minutes on the scale. This distance equals 2 miles on the chart. Step off, along your course line, the number of units between the two buoys, and if you have a space left over which is less than one 2-mile unit, take this distance between the points of your dividers and carry it to the scale at the side, to see exactly how far it is. When you have determined the length of your course line, label it: "Distance 7.5 miles," "16. 0 miles," etc.
SECTION D. METHODS OF DETERMINING POSITION
(1) Bearings in General
(a) Bearings are imaginary lines drawn between objects, an expressed in degrees or points. The bearings we are most con-
cerned with in PT boats are bearings taken of an object from the PT. Such bearings may be expressed as true bearings, magnetic bearings, or relative bearings.
(b) True bearing of an object from a PT boat is the angle made at the PT between the Geographic north pole and the object sighted, expressed in degrees, clockwise through 360°.
(c) Magnetic bearing of an object from a PT boat is the angle at the PT boat between the magnetic north pole and the object sighted, expressed in degrees, clockwise through 360°.
(d) Relative bearing of an object from a PT is the angle at the PT boat between the PT boat's bow and the object, expressed in terms of points (1 point equals Examples of these three bearings are given in the diagram:
The bearings taken from the observer compass are considered to be Magnetic Bearings. They can be converted to true bearings by adding or subtracting the variation in the correct direction. Cross bearings from a PT are best taken in this manner. Bow and beam bearings are of course relative bearings, and can best be taken in terms of points on the bow, rather than by compass.
(2) Cross Bearings
1. Suppose you are sailing up a coast line, a few miles offshore. You have in sight two prominent objects, say a lighthouse and a water tower, whose positions are indicated on the chart. To obtain your position: Sight the lighthouse over your Observer Compass, and note the reading in degrees. This is a magnetic bearing, of the lighthouse from you. Lay out this bearing from the lighthouse on the chart. You now know you are somewhere on the line you just drew.
Now, sight the water tower, and get a similar magnetic bearing of it. Lay this bearing out on the chart, from the water tower. You now know you are on this line somewhere, also: You will notice that the two lines cross on the chart. Since you are on bothlines (or bearings) somewhere, the only place where you can be is the spot where the bearings intersect. To eliminate the possibility of error, it is always wise, if possible, to obtain and lay out bearings from three objects. This method gives you a good cross, or fix, which must be your position.
Cross bearings may be taken of any visible objects which appear on the chart.
Figure 40.--Cross bearings.
(3) Bow-and-Beam Bearings
Suppose that now you are sailing up a coast line on a steady course, and you have one lighthouse or other prominent charted
object in sight. To get a fix or position, you proceed as follows: Take a bearing when the object bears 4 points on your bow, that is, broad off the bow. Take the time at this instant, and note your speed. Keeping speed and course constant, take another bearing when the object is right abeam, or 8 points, and note the time. Figure out the distance you travelled between the time you took the first bearing and the time you took the second bearing. This distance is equal to your distance off the object when it was passed abeam. (The distance can be figured this way: 60 divided by speed in knots--number of minutes required to go one mile. Then, elapsed time between bearings divided by number of minutes required to go one mile equals distance between bearings.
Example: Elapsed time--8 min.; speed--30 knots. Then: 60 divided by 30 equals 2 minutes required to go 1 mile, and 8 divided by 2 equals 4 miles, which is the distance run.
Figure 41. --Bow-and-beam bearings.
The bow-and-beam bearing has several variations. Perhaps the most useful is the 2 point-4 point bearing. This one is worked similarly to the bow-and-beam, except the bearings are taken when the object bears 2 points, or 22 1/2 degrees, relative; and 4 points, or 45 degrees relative respectively. The distance run will be the distance fromthe lighthouse at the time of the second bearing. Notice in the example below, that the distance runis notthe distance from the lighthouse when abeam.
Figure 42. --221/2°-45° bearings.
There are two advantages to the 2 point-4 point bearing.
First: It gives you a fix beforeyou get to the position abeam of the object. Since many lighthouses are placed near shoals or ledges, it is a good idea to know your position before you get to the danger area.
Second: With the data already obtained and assuming your course and speed do not change you can easily predictthe distance you will be from the lighthouse when you do pass it abeam. This is found by multiplying the distance off the lighthouse at
B by .7. In the example above 10 miles X .7=7 miles=distance you will pass off the lighthouse when it is abeam. This is called the .7 rule.
(4) The Running Fix
This is a means of determining your position by two bearings of one object with a run between. The running fix is not absolutely accurate, and should be considered only as an approximation. It should never be used as a departure fix. With only one prominent charted object in sight and your course and speed constant, first take a bearing of the object and note the time and speed. After you have run far enough to change the bearing of the object at least 30°, take another bearing of the same object, and again note the time.
Now figure the distance in miles which you made good between the time of the first and the time of the second bearings. Set your dividers at that distance. Lay out your two bearings on the chart, from the object. Now by the use of your parallel rulers, move the dividers along the two bearings, parallel to your course line, until the points just touch both bearings. The point where the dividers touch the second bearing is your position at the time of the second bearing.
An alternate method of plotting the running fix is to: Lay off both bearings as before. Then, from any point on your first bearing lay off your course line, for a distance equal to the distance run. From the end of this course line, lay off a line parallel to your first bearing. Where this line intersects your second bearing is your position at the time of the second bearing.
Figure 43. --Running fix.
(5) Danger Angles and Bearings
When running along a coastline where a hidden danger such as a shoal or reef exists, it is sometimes advantageous to make use of the danger bearings or danger angles. If one prominent charted object is visible, lay off a danger bearingas follows: From the prominent, charted, object in sight, lay off on your chart a line running clear of the shoal on the side on which you wish to pass. Find the magnetic or true bearing of this line, and from your boat, see that you do not get over on the other side of the danger bearing.
Figure 44. --Danger bearings.
If two prominent charted objects are visible in the vicinity of the dangerous spot, lay off a danger angleas follows: Pick a point outside of the shoal or danger well clear of it. From this point on your chart, lay off the angle between the two charted visible objects, and measure this angle. Then, using your sextant to observe the actual angle between the objects, see to it that the angle remains always smaller than the one you have laid off on the chart. (The same procedure can be followed by passing inside the danger, keeping the observed angle always larger than an angle laid off on the chart well inside the danger.)
Figure 45. --Danger bearings.
(6) R. D. F.
The Radio Direction Finder is used similarly to the observer compass. The bearings are plotted in the same manner, although the distances are usually much beyond visible limits. Cross bearings are the most frequent R. D. F. bearings used. Single, dead-ahead, bearings are also favorites, since they give a course from the P T to the R. D. F. station as a "beam" to run on. Bow-and-beam bearings are seldom taken from an R. D. F. station, but are perfectly possible, as is the running fix.
(7) Chain Soundings
Charts are the source of much valuable data for the navigator. One method of determining position relies upon the soundings printed on the chart. The method is: Take a series of soundings, evenly spaced, perhaps 1/8 of a mile apart, and in any constant direction. Lay out on a piece of transparent paper the depths of water obtained, spaced at the proper intervals, on the same scale as the chart you are using. Now lay your strip of transparent paper over the chart in the vicinity of your supposed position. Move the paper around until the soundings on them coincide with the soundings on your transparent strip of paper in the direction of the course line followed while taking the soundings. The ship's position will be the location of the last sounding taken, at the time of the last sounding.
SECTION E. PRACTICAL HINTS
When running in clear weather, keep a constant rough check of your position, such as "I am now approximately 4 miles from Vineyard Lightship bearing about SW," or "Point Judith is one point on my port bow, distance 5 miles." These rough checks will be of great help to you, should fog close in or should you suddenly have to do a series of maneuvers which may make accurate dead reckoning impractical.
Charts of the largest scale available should be used. An error of a fraction of an inch on such a chart may be only a few yards, while on a small scale chart the same error might mean being miles off.
Remember that sound signals in fog are subject to various errors. Regard them always as approximate in estimating distances and directions.
Practice taking bearings, even when you know your position. Besides increasing the accuracy of your navigation, it will inspire confidence in your own work.
Say to yourself at frequent intervals: "Suppose I were booming along as I am now, and suddenly I saw an uncharted red-and-black buoy to starboard, what would I do?" or, "If that tow boat ahead turned sharply to port, what should I do?" and the like. Try always to keep "one jump ahead" of the present situation.
Bear in mind the phrase "Red right returning" as a guide in determining how to pass buoys.
There are two things you must always keep aboard your boat: Keep your reckoning, and keep your head.
SECTION F. STORM WARNINGS AND WEATHER SYMBOLS
Because of their small size, high speed, and relative lightness of construction, PT boats are vulnerable to violent weather at all times, and dependent upon the weather much of the time. While he can, if necessary, take his boat to sea under any weather
conditions, the good PT boat captain is at all times properly informed as to the state of the weather and its possible hampering effects on his proposed operations. He knows that ice can reduce his boat bottom to splinters, that bad weather will decrease his visibility greatly, and that seas can slow him down materially. Therefore he takes an unusual interest in weather conditions in his area.
Weather Symbols(Letters to be used in recording the weather)
b--Blue sky, cloudless.
be--Blue sky with detached clouds.
c--Sky mainly cloudy.
d--Drizzling, or light rain.
e---Wet air, without rain.
f--Fog, or foggy weather.
g--Gloomy, or dark, stormy-looking weather.
p--Passing showers of rain.
r--Rainy weather, or continuous rain.
s--Snow, snowy weather, or snow falling.
u--Ugly appearance, or threatening weather.
w--Wet, or heavy dew.
Great intensity of any weather feature may be indicated by an underline thus: r., heavy rain.
SECTION G. SEA ANCHOR
The function of a sea anchor is to hold the head of a ship to the sea and enable her to ride out a gale. With small ships it has been used with good results. PT boats do not head into the wind well when riding to a sea anchor, but it will materially lessen the amount of drifting.
Figure 47.--Sea anchor.
SECTION H. THE LEAD
The instrument used in determining depth of water is called the lead. There are several kinds of leads in use. The band lead used on PT boats weighs from five to eight pounds, and is used in coming into harbor, to find the depth of water the boat is in. The lead line is fastened to the lead with a piece of leather or line well tied on; the line is marked thus:
2 fathoms with 2 strips of leather.
3 fathoms with 3 strips of leather.
5 fathoms with a white rag.
7 fathoms with a red rag.
10 fathoms with a piece of leather with hole in it.
13 fathoms with the same as 3 fathoms.
15 fathoms with the same as 5 fathoms.
17 fathoms with the same as 7 fathoms.
20 fathoms with 2 knots.
25 fathoms with 1 knot.
30 fathoms with 3 knots.
35 fathoms with 1 knot.
40 fathoms with 4 knots.
The object of taking a sounding is to find the depth of water, and get a specimen of the bottom. The lead may be armed with tallow and made to pick up pieces of the bottom by which the character of the bottom is determined. Comparison of the depth of water and specimens of the bottom with the markings on the chart gives a fairly accurate check on positions found by dead reckoning.
It is used along the coast when any doubt is in mind as to the boat's position or the exact position of lights or landmarks.
So What!! Buck Rogers Did That Last Week
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