Speed difference

[redboat219]

How come subs are slower when running surfaced as compared to running fully submerged? Take the case of the LA class, their surface speed is about 20 knot's while submerged it's 30 knots ++.

[Guest]

Re: Speed difference

How come subs are slower when running surfaced as compared to running fully submerged? Take the case of the LA class, their surface speed is about 20 knot's while submerged it's 30 knots ++.

Well the way I understand it there are a couple of problems, first the prop needs to be away from the surface so it will not cavitate ( I hope I spelled that right) the pressure at deeper depths keeps this from happening., also they need the sail to counteract the torque of the prop shaft.
Or at least that is what my buddy's who are bubble heads said.
Andre

[petn7]

Is cavitation really a propeller

Is cavitation really a propeller churning up the water? I thought it had to do with high speed movement and vapor pressure.

[robse]

I'd imagine that surface tension

I'd imagine that surface tension is in the equation as well.. ?

[Guest]

Hasn't it got something to

Hasn't it got something to do with wave making resitance and Mr froude?

Fluid mechanics- good bedtime reading.

Andy

[hakkikt]

As I understand it, on

As I understand it, on the surface part of the energy put into propulsion is converted into making waves.
Under water it is not (or much less), so propulsion is more efficient.

Try it: you can swim faster under water than on the surface. Thats why it is prohibited in competitive swimming.

[petn7]

As I understand it, on

[quote]As I understand it, on the surface part of the energy put into propulsion is converted into making waves.
Under water it is not (or much less), so propulsion is more efficient.

Try it]

But most swimmers often spend a significant portion of their race underwater doing the dolphin kick. Is there some rule that forces the swimmer to surface by a certain point?

[chips]

Speed is determined by hull

Speed is determined by hull design, horsepower, prop design and several other related factors. Todays subs are designed to be more efficient running submerged than surfaced.

Take a look at the GATO Class: surfaced speed 21 knots form 5400 hp; submerged speed 9 knots from 2740 hp. Early subs were designed primarily as surface ships that could operate under water.

The LA class produces about 35,000 hp to go 25 knots surfaced or 30+ submerged. Modern (late 1950's) subs are designed as true subs, that sometimes operate on the surface.