Chapter VI
Fuel
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Summary
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Availability of information of fuel
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The use of fuel in operation
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Fuel reports as an aid to radio intelligence
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1. Summary
Fuel 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 fuel
Fuel 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 operation
One 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
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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 intelligence
The 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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