The English worked out some average figures when conducting their offensives against the U-boats in the Bay of Biscay. Michael Gannon recites them in "Black May," pp. 263-264. Paraphrased:

A typical U-boat could proceed underwater at a rate of about 2 knots for a distance of about 50 miles. This was an optimum speed to provide the greatest underwater range. Ater that time (about 25 hours at economy speed) it would be forced to surface to recharge. Running flat-out at about 9 knots, the batteries would be exhausted in about an hour. The exponential rate of battery drain associated with increased speed mandated the use of measured quantities of "full ahead" while submerged. (Consider the improved Type XXI's -- which could do about 17 knots max underwater, but then again for only about an hour.)

To fully recharge the batteries required a run of 7 to 8 hours at an optimum surface speed of 12 knots. Clutching one diesel to the propellers and one diesel to the dynamotors was a typical arrangement when recharging, and gave the U-boat about 12 knots on the surface. A U-boat could clutch both diesels to the dynamotors and simply sit and recharge but this was somewhat hazardous -- i.e, "a sitting duck."

U-boats could, and did, of course reduce their submerged speed still futher to extend underwater battery life as much as possible, although trim became adversely affected when there was no forward motion -- a U-boat could not hover underwater indefinitely and maintain static bouyancy. However, after about 40 hours submerged, the U-boats air supply typically dictated a rise to the surface, at a minimum to ventilate the boat (although there were longer submergences recorded by determined commanders). Assuming a surfacing to ventilate and then an immediate dive to evade pursuers, the ventilation cycle consumed the equivalent amperage of 7 minutes of recharging time.

The advent of the schnorkel late in the war changed this, of course. A U-boat need not rise above schnorkeling depth at all during a patrol; however, the schnorkel limited the U-boats speed to about 5 knots and provided the most basic of ventilation for human needs.

A typical underwater running pattern would be to proceed at something just above economy speed if not in danger of attack (maybe 4-5 knots) for 7 to 12 hours. This coincided with a "day/night" type pattern of operations (submerge by day, surface by night) since only U-boats seeking to transit the Bay of Biscay or simply stay out of sight would run their e-motors for 25 hours at 2 knots. U-boats did most of their work on the surface.

During D/C attack periods U-boats could apply "full ahead" to try and get away during the explosions - a period when the cavitations of high-speed underwater propellers were unlikely to be picked up by the hydrophone operators on destroyers and corvettes. Again, because of the much faster rate of battery depletion during full ahead maneuvers, such bursts of speed were saved for when really needed.

Although I am no expert (and there are some out there!) I hope this information is helpful.

Chris M.
Miami, Florida USA