Why do submarines have higher top speed when fully submerged?

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Two principal factors causing the reduction in surface speed of submarine are cavitation and drag.

Drag 

The modern submarines are designed to operate fully submerged, so they have less drag when submerged than on the surface. There is more than one type of drag:

  1. The resistance from water is skin drag, and this skin drag does go up as the wetted surface increases when the sub submerges. Modern submarines approximate a teardrop shape, which minimizes this type of drag.
  2. Another type of drag is wave drag. This type of drag occurs on the surface where the water is free to create waves. All vessels traveling on the surface of the water must contend with this. The shape of the bow has an impact. A sharp angle bow or a bulbous bow can reduce wave drag. But the blunt bow of a submarine, which helps reduce skin drag submerged, increases wave drag.
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Visual presentation of the result of hull resistance calculations. Water pressure distribution over hull.

In an ideal fluid (one with no viscosity) there would be no drag on a streamlined shape underwater, but it would still produce wave drag on the surface. Of course, water is not an ideal fluid, but it is close. And even well-streamlined submarines have holes and ridges and antenna.

When submerged, the displaced water is pushed in all directions, and gravity has a much smaller impact. This effectively reduces the hull-speed induced drag to a point where it can be ignored.

The viscous drag goes up underwater because the wetted surface increases, but this is more than compensated for by the loss of wave drag.

Cavitation 

One of the most important limits on the performance of your propeller is “Cavitation” – this occurs when the water is moved too fast and that causes a pressure drop that is enough to cause a bubble – a void. These “bubbles” or “voids”, collapse and can generate shock waves that may damage machinery.

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Cavitating propeller model in a water tunnel experiment. (Image: Wikipedia)

For a submarine this is a triple problem – it reduces “thrust” – wears out the prop and makes a noise – not what a submarine wants.

Submerged the water pressure will make the propeller to have less tendency to cavitate. The deeper it goes it will be less cavitation and with no cavitation the propeller will be fully efficient creating forward propulsion. Hence all the force from the engines will be in use.

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Cavitation damage evident on the propeller of a personal watercraft.

As you go deeper the initial pressure goes up – so if your prop cavitates at 5 meters depth it will not cavitate at 15 meters depth

At 100 meters you can put a lot more power into the prop without it cavitating – so the result is that if you have enough power then you can go FASTER at depth

History

Before and during WW2, submarines spent most of their underway time on the surface. This was due to the submarines’ limited endurance while submerged. It was more important to optimize their speed on the surface so the hull form of submarines looks like that of any other surface ship.

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USS Drum (SS-228) on display as a museum ship

You can see in the above photo that there are lots of stuff sticking out above the waterline. While this stuff does not add to water drag when the ship is surfaced it does add a lot of drag when submerged. This drastically affected the submerged speed of the submarine. As WW2 progressed anti-submarine tactics became more effective and the need for greater submerged speed and endurance became apparent. The Germans were the first to start streamlining all aspects of their submarines to improve submerged speed and endurance.

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German Type Xxi Submarine

The above photo is of a Type XXI submarine, you can see that all of the protuberances visible in the first photo is now enclosed. This allowed the submarine substantially higher submerged speed compared to the Type VII submarines.

The first nuclear powered submarine, the USS Nautilus, used a very similar hull form as the German Type XXI submarines.

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USS Nautilus

The US Navy built the USS Albacore, a research submarine for the specific purpose of determining the optimal hull form, propulsion configuration, and control surface geometry for submerged operation.

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USS Albacore

With this hull form, it was found that the USS Albacore could operate at the same submerged speed as the older submarines (even those that are streamlined like the German Type XXI) for about half the required shaft power. This significantly improved the submerged endurance of the submarine.

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USS Los Angeles

The above is a photo of the USS Los Angeles transiting on the surface at speed. You can tell by that wake that it is not cutting through the water very cleanly.

This is why modern submarines, with their hull form optimized for submerged operation, are slower on the surface.


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