- Availability of information of fuel
- The use of fuel in operation
- Fuel reports as an aid to radio intelligence
1. SummaryFuel reports sent by U-boats at sea were of significance in the planning both of U-boat operations by COMSUBs and of anti-submarine operations by the Allies. The amount of fuel which a U-boat had on hand was usually appended to any other report which it made. The mass of data on fuel resulting from such frequent reports provided with ample basis for various assumptions with regard to U-boat operations. Moreover, the same information provided a basis for identification of the type of given U-boat and for determination of fuel capacities and consumption, day's runs, and prescribed speeds. There were occasions when knowledge gained from fuel reports aided in the decipherment of disguised grid positions.
2. Availability of information of fuelFuel reports sent by U-boats at sea had a double significance. For COMSUBs in his conduct of the U-boat war they were essential in disposition of the U-boats and in the planning of operations. For the Allies, the reading of fuel reports was of equal importance in the conduct of the antisubmarine war. The vital nature of these reports is evidenced by the fact that U-boats were generally required to append a statement of the amount of fuel on hand to every transmission which was otherwise necessary, i.e. position, movement, contact, and success reports, as well as specific requests for provisioning. The number of such reports received by the U-boat Command varied, of course, according to the number of U-boats at sea and the extent of their activity. For example, in March, 1943, when U-boat anti-convoy operations reached their peak, 179 U-boats were at sea, and Command received a total of 325 fuel reports, an average of 10.5 per day. The maximum number on any day was 33, the minimum 2.
3. The use of fuel in operationOne of the cardinal principles in U-boat operation was economical use of fuel in the patrol line and during any cruising which was not directly operational; but once contact
was made with a convoy, U-boats operated with a total disregard for fuel, only reserving an amount absolutely necessary for their own safety. When U-boats had sufficient fuel and were not engaged in a convoy operation, they normally sent routine fuel reports along with position reports. When the supply decreased to the amount required for the homeward cruise, they might request provisioning if the prospects of success in the attack area seemed promising, or simply advise COMSUBs that they were returning. A convoy sighting, however, nullified all immediate plans, and the contacting U-boat shadowed to the limit of its fuel or until other U-boats could be brought to the scene. Whenever possible, the movements of U-boat tankers were coordinated with those of the attacking force to lessen the chance of losing the convoy. In some few cases in which no U-boat tanker was available, an outgoing U-boat refueled those at sea, or U-boats engaged in an attack themselves transferred fuel in order to permit continued operation or return.
4. Fuel reports as an aid to radio intelligenceThe abundance of information regarding the fuel status of U-boats was also of inestimable value to radio intelligence activity. In February 1944 COMSUBs confirmed estimates of fuel consumption which had previously been made here on the basis of the daily fuel reports, revealing that the type Vllg boats in anti-convoy groups Command counted on a maximum daily consumption of up to 2 cbm if no operation was going on; for type IXc, up to 3 cbm under the same conditions. From the amounts reported, the Atlantic Section was able to identify or confirm the types of new U-boats as they reported from seas. The daily reports also permitted an evaluation of cruising speeds in terms of a day's run and the identification of these speeds with those ordered by COMSUBs--maximum, economical, cruising, etc. With a knowledge of the type of U-boat operating in a given area and an approximate idea of its fuel capacity, it was possible to estimate the extent of its future operations and to plot a rough course. Furthermore, it was possible to reduce enciphered positions to a specific area with the aid of knowledge of the fuel capacities and consumption of the U-boats concerned.
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