Speed of submerged submarine

[Bushranger]

WWII era subs used diesels on the surface and attained faster speeds than when switched to electric motors while submerged.  However, modern atomic powered subs attain higher speeds while travelling submerged than on the surface.  This seems counterintuitive in that more of the hull is in contact with the water and therefore more friction with the water than when on the surface.  So my question is, how/by what mechanism do submerged submarines travel at higher speeds than while on the surface?

[Mordred]

I would think that the design of the hull itself is a more relevant factor than the type of engine that propels the sub. The overall designs of each type may significantly differ. In particular the shape of the Bow is important in this case as one main factor. If the bow design is more aerodynamic in so far as better suited to slice through water while submerged as opposed to the surface. Diesel subs spend more time on the surface so the bow is designed to the surface for average travel speed. While a nuclear sub can stay submerged for far longer periods and the bow and hull design takes this into consideration.

[Bushranger]

Your answer does not make sense to me.  If a big portion of the hull is out of the water, its design is moot as to slowing it down in air.  

[zapatos]

22 minutes ago, Bushranger said:

Your answer does not make sense to me.  If a big portion of the hull is out of the water, its design is moot as to slowing it down in air.  

If a big portion of the hull is out of water, its design at the surface is critical wrt speed. It also has to deal with waves.

I imagine the thrust generated by the propeller is also impacted by its depth.

[Mordred]

A hull designed to run on the surface is also a design that incorporates lifting the craft as high upon the surface to decrease drag. A design that spends most of its time under water will design the bow to slice through the water on all surface areas. Where as the first design would focus on the lower portion of the craft. Secondly in both cases one can use the backflow reaction of the water from the turbulence curling back upon its rear to which creates a region of high pressure much like how a sail allows certain angles towards the wind direction.

 Also placement of the propeller would vary in location the surface design can be closer to the keel.

edit: In all cases for all types of water craft the hull design is a critical factor for speed.

31 minutes ago, zapatos said:

If a big portion of the hull is out of water, its design at the surface is critical wrt speed. It also has to deal with waves.

I imagine the thrust generated by the propeller is also impacted by its depth.

x-posted while in edit

Edited January 26, 2019 by Mordred

[Endy0816]

Yeah I would say hull shape mainly. They can also roll like a tin can depending on surface conditions.

Interesting question.

 

[Sensei]

4 hours ago, Bushranger said:

So my question is, how/by what mechanism do submerged submarines travel at higher speeds than while on the surface?

Submerged submarine does not have to fight with unfavorable weather conditions which can be on the surface of ocean..

 

[Mordred]

3 hours ago, Sensei said:

Submerged submarine does not have to fight with unfavorable weather conditions which can be on the surface of ocean..

 

During my 5 year term as a shipwright, I had a year working on diesel subs. Its amazing how cramped they truly are. The structural engineering designs taught me a lesson I took to heart. Every structural member that has a sharp corner is a stress point. In submarines this is strongly applied. For each joint is designed as rounded as possible in radius as that shape distributes mechanical stresses more readily. No welding or drilling beyond engineered design was permitted. 

 Anyways that being said, you raised an extremely important strong point in engineering design as to hull deign to active employment design. In particular combat readiness but also stealth. You'd be amazed at the science that goes into silencing the noise a sub makes is involved in the design. A strong aspect being turbulence with the hull. 

 However cylindrical isn't the greatest for surface stability and cutting through waves nor turn radius. 

Edited January 26, 2019 by Mordred

[swansont]

9 hours ago, Bushranger said:

Your answer does not make sense to me.  If a big portion of the hull is out of the water, its design is moot as to slowing it down in air.  

Do the hulls have the same surface area? A nuke’s hull is a cylinder with rounded ends (a cigar) while a diesel is a “V” with a wedged front.  A circular cross-section minimizes the surface area for a given volume. So it’s not obvious that there is more hull exposed to water, since the diesel will have more surface overall, if all else is equal.

And the front of the boat matters - whether it’s designed to move water around it in 1 dimension or 2. A nuke on the surface generates a pretty big wake, because of the the rounded nose. It doesn’t “cut” through the water, like a surface ship design. It’s more efficient when submerged.

 

 

 

[Klaynos]

We've had a discussion on this before. 

 

I suspect this want true for traditional diesel Vs electric due to the maximum output for the two engine types. 

[swansont]

5 hours ago, Klaynos said:

I suspect this want true for traditional diesel Vs electric due to the maximum output for the two engine types. 

One could compare diesel on surface vs submerged while snorkeling. Unless the power is limited by snorkeling, which is a possibility.

[J.C.MacSwell]

The drag is much lower as you get significantly below the surface, as wave making drag disappears, leaving essentially the form drag and skin friction.

On the surface you have that extra energy loss from the pressure differential creating gravity waves, a surface effect that disappears with depth, without which much of that energy would be recovered. In fact all of it would be recovered if not for surface effects (wave making), and viscous effects which is drag in itself but also triggers separation.

Furthermore depending on the length of the hull, the wave making drag starts increasing substantially as "hull speed" is approached unless enough lift is created to allow the hull to plane and skim across the water. This of course does not happen with submarines due to there weight and hull shape.