German U-Boat

Almost invariably, convoy interceptions are a result of contact reports provided by BdU. The convoy’s location, estimated speed and bearing are broadcasted together in a contact report. U-boat commanders would use this information to determine whether the convoy was within its reach, and if it was, to plot the most probable point of interception.

A convoy’s speed was given as slow, medium or fast; (slow = 4 to 6 knots, medium = 7 to 9 knots, fast = 10 to 12 knots). Based on this, and together with the current convoy position, it would be possible to calculate how far the convoy would have traveled in a given space of time.

For example, since a fast convoy travels at between 10 to 12 knots, therefore it could travel anywhere within a 120 to 140 nm (nautical mile) radius in a 12 hour timeframe. This is a simple calculation of 10 knots per hour multiplied by 12 hours = 120nm. (see Fig A). Assuming that the convoy does not change course, then it would be further possible to narrow down the most probable location of the convoy after 12 hours. Hence, a fast convoy traveling south at between 10 to 12 knots, should probably be between 120 to 140nm south of its current location in 12 hours time (see Fig B).

Since U-boats travel up to speeds of 17 knots, then it could reach up to a distance of 204nm in 12 hours. If the convoy’s most probable location was within this range, then the chances were good that it would end in a successful interception (Fig C). If not, then it would be possible to recalculate the positional bearings using a 24 hour timeframe. The maximum was usually 48 hours, as the chances grow progressively dim with every extension of the timeframe.

From here, the commander can determine exactly if he had any chance of intercepting the convoy at all. If the chances were good, then he would proceed to plot his course of interception. Otherwise, he could always bang the table with his fist and curse, as he would have missed his chance. Finally, luck is also a factor, because if the convoy changes course, the U-boat would be left far out of position.