Submarine submerged vs surfaced speed

[pavelcherepan]

I was just wondering if anyone knows why submarines normally have lower surfaced speed compared to speed when submerged?

 

It's understandable for diesel-electric submarines, where diesel engine is there more to charge batteries than to provide propulsion and as a result surfaced speed is lower, but what about nuclear subs? They have exactly the same propulsion in both cases, but for example, US Ohio-class SSBN has a top speed 12 knots surfaced and 20 knots submerged. Same story with Russian/Soviet Akula-class 10 knots vs 28-35 knots.

 

Is it due to lower performance of the propeller when it's close to the surface or is it something else?

Edited January 22, 2016 by pavelcherepan

[Klaynos]

Have a look at hull speed and wave drag.

 

To put it simply it is harder to move when you're between two media than when completely within one or the other.

[studiot]

Agree totally with Klaynos, but here is some extra info.

 

The surface (in this case the water - air interface) has a surface energy / surface tension that is disturbed by a vessel cutting through it.

This is the source of the increased resistance at the surface (klaynos : wave drag) not normal friction between the hull surface and the water, which is obviously still present when submerged.

 

Submarines submerge to lower resistance to motion.

 

Hydrofoils raise up to take as much hull area as possible out of the interface.

Edited January 22, 2016 by studiot

[pavelcherepan]

Thanks guys! Makes much more sense now!

[studiot]

Edit I realise I said 'normal' friction.

 

Of course I should have said 'ordinary friction' to avoid any misunderstanding that I meant in a normal (perpendicular) direction.

 

The friction acts in the usual direction, at right angles to any 'normal' forces.

Edited January 22, 2016 by studiot

[Endy0816]

Rough on the surface too. Rolls like a tin can.

[MountainGuardian]

On the surface you are dealing with drag caused by the "surface tension" of water. The surface tension of water is quite high and adds a great deal to drag. You also have wind and waves to deal with at the surface of the water, although those could help your speed in any instance the same as they could hurt your speed.

.

My official answer would have to be the surface tension of water, although that is simply an educated guess.

.

In the Navy they warned about surface tension when having to jump off of a ship at any height, in fact at the 70 feet from an aircraft carrier flight deck they described it as being similar to hitting the ground at that height. If you do not jump correctly the surface tension can break bones when you hit.

[studiot]

Hello Mountain Guardian, and welcome.

 

When did the Navy start guarding mountains?

 

 

When discussing waves it is important to distinguish between pre-existing waves due to ordinary air/sea interaction which act as you say and the wave drag as Klaynos described, which will still occur in a flat calm sea.

 

The interaction between a hull at the surface and the surface causes waves to spread out from bow and stern, sapping energy from the boat's progress.

These are caused by the moving boat and not a characteristic of the sea state.

Edited January 24, 2016 by studiot

[Enthalpy]

It is known experimentally and explained that the drag of a submarine increases at the surface. Even at identical engine power, it gives a higher speed underwater.

 

The extra drag at the surface results from the skull making waves, which doesn't relate with surface tension.