Rainer's point about maneuverability also has relevance to the ability of modern submarines to rescue survivors (assuming the tactical situation would allow that kind of exposure and survivors could be transported over the difficult deck surface of a low-noise albacore hull). At low speeds near the surface, the variation in propeller action with depth becomes significant, and the propeller tends to move the stern of the sub sideways as if its blades were walking along a hard surface like a table-top. So a single screw submarine trying to do low speed maneuvering around a stationary lifeboat or group of floating survivors tends to twist clockwise or counterclockwise rather than move in the intended direction; and the power necessary to obtain (or stop) forward motion for steering control is likely to draw survivors through the screw. =AL=