Submarine Propellers and Torque

[Dean Markley]

How does a modern nuclear boat cope with torque? I'd assume with such high power outputs and that large propeller that there must be a torque effect?

In a similar vein, why did it take so long for the single propeller to be realized as quieter and more efficient? IIRC, the British R-class of WWI used a single propeller. But those were obsoleted fairly quickly after the war.

[Paul F Austin]

On 11/28/2012 3:52 PM, Dean Markley wrote:
> How does a modern nuclear boat cope with torque? I'd assume with such high power outputs and that large propeller that there must be a torque effect?
>

The center of gravity is much below the center of buoyancy in all
conditions of ballast, providing a righting moment that counteracts the
torque of the screw.

A bigger problem is the lift generated across the sail during a turn
that exerts a rolling force (called snap roll). With the hull rolled in
a turn, there is cross-coupling of the rudder forces into the vertical
plane and dive plane forces into the horizontal plane. There's also a
rolling moment from lift across the hull during a turn, since the center
of pressure acts above the center of gravity, rolling the hull "out" of
the turn.

When first built, Albacore included a movable surface on the back of the
sail to counteract the snap roll forces. After experience in operations,
the dorsal rudder was disabled.

The Navy experimented with contra-rotating screws in an SSN-594 variant,
Jack (SSN-605) as a means of reducing screw noise and eliminating
reduction gear noise. The use of contra-rotating screws also reduced the
torque induced on the hull, but that wasn't a design consideration.

Paul

[Derek Lyons]

Paul F Austin <pfau...@bellsouth.net> wrote:

>On 11/28/2012 3:52 PM, Dean Markley wrote:
>> How does a modern nuclear boat cope with torque? I'd assume with such
>> high power outputs and that large propeller that there must be a torque
>> effect?
>
>The center of gravity is much below the center of buoyancy in all
>conditions of ballast, providing a righting moment that counteracts the
>torque of the screw.

In addition, the sail and aft control surfaces are slightly offset to
provide dynamic forces to counteract the torque.

D.
--
Touch-twice life. Eat. Drink. Laugh.

http://derekl1963.livejournal.com/

-Resolved: To be more temperate in my postings.
Oct 5th, 2004 JDL

[Keith W]

The diesel electric boats of WW2 are quite different beasts
to a modern SSN or SSK. They were basically optimised
for surface cruising and the ability to fire from aft torpedo
tubes along with the redundancy twin shafts, diesels and
motors gave trumped any quieting issues.

The German type XXIII was much more like a modern
SSK with its streamlined welded hull, lack of deck gun and
of course single propellor and no aft torpedo tubes.

Keith

[Paul F Austin]

On 11/28/2012 7:09 PM, Derek Lyons wrote:
> Paul F Austin <pfau...@bellsouth.net> wrote:
>
>> On 11/28/2012 3:52 PM, Dean Markley wrote:
>>> How does a modern nuclear boat cope with torque? I'd assume with such
>>> high power outputs and that large propeller that there must be a torque
>>> effect?
>>
>> The center of gravity is much below the center of buoyancy in all
>> conditions of ballast, providing a righting moment that counteracts the
>> torque of the screw.
>
> In addition, the sail and aft control surfaces are slightly offset to
> provide dynamic forces to counteract the torque.

I had a vague recollection that that was the case (maybe during quals)
but could find no mention of that in Friedman. Can you cite a
description? Offsetting the rudders and stern planes seems
straightforward but offsetting the sail would be a little more complex
since the resulting torque wouldn't act through the same center as the
shaft.

Paul

[bob]

On Nov 28, 9:52 pm, Dean Markley <damark...@gmail.com> wrote:
> How does a modern nuclear boat cope with torque?  I'd assume with such high power outputs and that large propeller that there must be a torque effect?

Also note that some modern submarines use pump jets rather than open
propellors, that have both rotor and stator blades, so will produce
significantly less (if any) net torque on the boat.

> In a similar vein, why did it take so long for the single propeller to be realized as quieter and more efficient?  IIRC, the British R-class of WWI used a single propeller. But those were obsoleted fairly quickly after the war.

I think the resistance came from concerns about redundancy. If you
have a single shaft, you have a single point of failure. Not just
external damage, but also components like gearboxes or final drive
motors, of which you necessarily have one per shaft.

Robin

[Mark Borgerson]

In article <CKCdnQlRr6PlvCrN...@supernews.com>,
pfau...@bellsouth.net says...

When you say "the sail is offset" do you mean that it has a positive
angle of attack to counteract torque, or that it is physically off
the centerline of the boat?

I suppose the worst case would be when going from zero to
full speed in a hurry---at the start you would have max
torgue and no speed for the planes or sail to exert a controlling
force.

Mark Borgerson

[bob]

On Nov 29, 9:48 am, Paul F Austin <pfaus...@bellsouth.net> wrote:
> On 11/28/2012 7:09 PM, Derek Lyons wrote:
>

I read the OP's comment to mean the "sail control surfaces", that is
to say the planes mounted on the sail (as an alternative to bow
planes), which makes a little more sense.

Robin

[Paul F Austin]

On 11/29/2012 10:46 AM, Mark Borgerson wrote:
> In article <CKCdnQlRr6PlvCrN...@supernews.com>,
> pfau...@bellsouth.net says...
>>
>> On 11/28/2012 7:09 PM, Derek Lyons wrote:

>>> In addition, the sail and aft control surfaces are slightly offset to
>>> provide dynamic forces to counteract the torque.
>>
>> I had a vague recollection that that was the case (maybe during quals)
>> but could find no mention of that in Friedman. Can you cite a
>> description? Offsetting the rudders and stern planes seems
>> straightforward but offsetting the sail would be a little more complex
>> since the resulting torque wouldn't act through the same center as the
>> shaft.
>>
> When you say "the sail is offset" do you mean that it has a positive
> angle of attack to counteract torque, or that it is physically off
> the centerline of the boat?
>

The lift generated by the sail generates a couple acting on the hull as
would would the rudders and stern planes. The difference is that each
couple (top and bottom rudder, left and right stern planes) acts
symmetrically through the hull with the torque center aligned with the
shaft. The sail, not having a corresponding force exerted on the bottom
of the hull, exerts torque about a center that is defined by the center
of (hydrodynamic) pressure on the hull and the center of pressure on the
sail, so _that_ couple acts around a center much higher in the hull.
It's been a _long_ time since I had a mechanics class but my
visualization has the sail couple causing an "up-bubble" pitch cross
couple. Keep in mind that as a mechanical engineer, I'm one hell of a
systems guy, which means my notion of mechanics incorporates a lot of
hand-waving.

Paul

[Paul F Austin]

That's still a concern, the reason why there's an emergency propulsion
motor _and_ an outboard.

Paul

[Mark Borgerson]

In article <x5SdndYDEJ0eFCrN...@supernews.com>,
pfau...@bellsouth.net says...

The fact that the sail on most boats is well forward of the horizontal
center of lateral resistance and COG would also seem to generate a
significant pitch effect if there is any upward vector to its lift.

Hmmm----maybe those guys on the planes and rudder controls really
earn their pay during maneuvers that result in large changes
in power, speed, and torgue!

Mark Borgerson