![]() ![]() So this explains why nuclear submarines have higher speeds underwater. This is why for example competitive swimmers try to stay underwater as long as the rules allow after a turn in racing - gliding in a streamlined position completely underwater is faster than being on the surface and actively propelling themselves. Surface effect (surface tension) and the ability to produce and interact with waves on the surface more than offset the fact that the submerged surface is less. True, there is more drag from the effect of fluid flowing over the surface of the submarine, but on the surface the effect of drag from the surface itself is more important. Your intuition that there is more drag underwater is incorrect. All things being equal, submarines should move faster underwater. Some of this is a history question, and some isn't.įirstly the non-history part. The round bow of a teardrop hull is the ideal shape for moving underwater where you can push water out of the way in all directions, but when operating on the surface, lifts the water into a bow wave that produces a great deal of drag. Modern teardrop-hull submarines take this to the extreme: they're fully optimized for underwater travel, with no consideration given for surface performance. This resulted in a submarine that was slightly faster submerged than on the surface, at the cost of poor seakeeping abilities when surfaced. The bow was lowered and rounded, the deck was stripped bare, the conning tower was streamlined and fully enclosed, and similar improvements were made. ![]() The German Type XXI was the first submarine to break this pattern: the massive battery banks permitted extended underwater travel, and the hull was shaped appropriately. A submarine captain was expected to figure out where his prey would be heading, get there first, and submerge to lie in wait. When submerged, the turbulence produced by this superstructure imposes severe drag penalties, and so submarines were expected to operate as ambush predators, not pursuit predators. When you're traveling on the surface, a deck gun imposes no speed penalty, and the pointed tip of a flared bow improves seaworthiness in heavy waves. Consider the American Tench-class, the British S-class, or the German Type VII: all of these have very ship-like superstructures. A submarine might spend 90% of its patrol time on the surface, including all its high-speed travel, and so the ship's hull was given features that reflected this. Although these submarines had electric motors, the batteries couldn't power them for very long, and so the air-breathing engines were the primary source of propulsion.Įarly submarines were essentially surface ships that could dive to attack, and were optimized for this role. Early submarines were faster on the surface because they were designed around the needs of their diesel or gasoline engines. PhilS has the right general idea, but approaches it backwards. What changed to make submerged submarines actually faster? Typhoon-class submarine, but it seems the trend also holds for modern nuclear subs too: 22.22 knots surfaced, 27 knots submerged.Type XXI submarine, the reversal is already present in later WW2 models: 15.6 knots surfaced,.However, in later models, submarines are actually faster when submerged, which seems counter-intuitive to me (again, more submerged hull should lead to higher drag, while modern propulsion methods should be equally usable while on surface or submerged). Type VII submarine from WW2 Germany: 17.7 knots surfaced, 7.6 knots submerged.This seems intuitive, as a surfaced submarine will only have part of its hull in the water, leading to lower drag, in addition to better propulsion options (diesel vs. In early submarine designs we see that a surfaced submarines are faster, while submerged submarines are slower, but gain the obvious benefits of stealth and being harder to target (requiring specialised weapon systems). ![]()
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