Re: surface speed vs. submerged speed
A number of factors affect surfaced and submerged speeds. In most WWII era boats, surfaced speed was considerably higher than submerged. In a Gato, about 20~21 knots surfaced, 9 submerged. A lot of this had to do with hull design. The old boats were really surface warships with a limited ability to dive, so they were designed for surfaced efficiency. Once submerged, deck guns, the conning tower fairwater, radio antennas, and any number of projections interfered with the smooth flow of water, slowing the boat.
Late in the war, Germany reversed that concept, designing boats with streamlined hulls for greater efficiency under water. The type XXI u-boats are generally thought of as the first production submarines designed to be faster submerged than on the surface. A second factor in this was adding a lot of cells to the batteries, as battery capacity was a limiting factor in both speed and range. Run at full speed, a submerged fleet sub would run her batteries flat in about an hour.
The design of the screws is also important, but in that case depth is even more important. Cavitation--the swirling stream of bubbles you see in movies--robs efficiency, but as you reach a critical depth the water pressure becomes great enough that cavitation ceases and the screw gets about as close as possible to 100% efficiency. This happens rather abruptly, with the motor/turbine suddenly speeding up without any increase in power setting as cavitation-caused drag ceases.
The same pressure factor also makes the hull more efficient at depth, with the energy that was formerly wasted in creating a wake--which you get, even if you can't see it, to a considerable depth--now available for speed as the water pressure suppresses the wake and forces the water to flow smoothly around the hull. (By now tear-drop or cylindrical in shape.)
The periscope has little to do with speed, as no submarine intentionally travels very fast at periscope depth in order to avoid throwing up a lot of visible spray and possibly giving away her position. In the old boats, the exposed periscope shears certainly created turbulence and robbed speed, of course. In modern boats, everything that sticks up above the fairwater can be fully retracted.
J.T. McDaniel
Webmaster, FleetSubmarine.com
Author of: With Honour in Battle and Bacalao
General Editor: American Submarine War Patrol Reports series
