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Old 14 April 2005, 18:38   #1
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counter rotating twin outboards

Just out of curiosity....

I once did a photography job from a Boston whaler with twin yam 115hp (or 150, can't remember) on the back. Each engine was counter rotating, i.e. it had twin props.

Now my question is:

Is this completely pointless? My understanding is that if you have twin engines, you set them up to go spin in opposite directions to counteract prop torque, so why the need for twin counter rotating outboards? seems unnecessary.

Is there an additional advantage to counter rotation? More grip perhaps? better acceleration? I would imagine top speed is NOT increased.

over to the technical wizards...
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Old 15 April 2005, 12:02   #2
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"My understanding is that if you have twin engines, you set them up to go spin in opposite directions to counteract prop torque, so why the need for twin counter rotating outboards?"

You've lost me - if you set your pair of motors up to spin in opposite directions, that is counter-rotating
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Old 15 April 2005, 13:05   #3
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Quote:
Originally Posted by MadMat
"My understanding is that if you have twin engines, you set them up to go spin in opposite directions to counteract prop torque, so why the need for twin counter rotating outboards?"

You've lost me - if you set your pair of motors up to spin in opposite directions, that is counter-rotating

Lost me a bit as well but maybe he is on about duoprop???

If so the props themselves spin in opposite directions so there is no torque effect - putting 2 of them on would also mean there is no torque effect - or three or four.......

apparently they get a bit expensive if you pick up a bit of rope!!!
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Old 15 April 2005, 13:48   #4
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Sorry, it seems I didn't explain it very well. Codprawn is right, I mean duoprops or whatever they would like to be called. So in simple terms you have two props, one in front of the other sticking out of the same gearbox and spinning in opposite directions. Yamaha are the only company I am aware of that make (made?) them for outboards, probably wrong though.

So, as Mr Cod says, since these units counteract prop torque themselves, whats the point in rigging a boat with two of these outboards? I assume the counter rotating gearboxes are more expensive, so it seems a waste of money to me, unless there is some other advantage.

Apologies if that is still reads like a load of crap, but I'm tired and completely incapable of stringing a sentence together

Tim
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Old 15 April 2005, 22:37   #5
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Quote:
Originally Posted by slimtim
Sorry, it seems I didn't explain it very well. Codprawn is right, I mean duoprops or whatever they would like to be called. So in simple terms you have two props, one in front of the other sticking out of the same gearbox and spinning in opposite directions. Yamaha are the only company I am aware of that make (made?) them for outboards, probably wrong though.

So, as Mr Cod says, since these units counteract prop torque themselves, whats the point in rigging a boat with two of these outboards? I assume the counter rotating gearboxes are more expensive, so it seems a waste of money to me, unless there is some other advantage.

Apologies if that is still reads like a load of crap, but I'm tired and completely incapable of stringing a sentence together

Tim

I think the point is the owner wanted 2 engines - either to go faster - use more fuel or prove he has a bigger penis than anyone else!!!

You couldn't really have one duo prop and one normal engine or you would have torque effect again - however if you wanted 3 engines you could have 1 duoprop in the middle and normal outboards left and right with contra rotating normal props!!! Or even 3 duoprops but that would be a waste!!!

Duoprops ARE very rare on outboards but very common on volvo sterndrives!!!
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Old 16 April 2005, 08:35   #6
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Why duoprops?

Tim,

When a propeller rotates some of the energy is converted into thrust - which you want.
However, some of the energy is converted into rotating the water that the propeller has just passed through - this serves no purpose.
If you put a second propeller, rotating the other way to the first one, into this rotating water it is possible to convert some of this rotating water into thrust - which you want.

So you get more thrust for the same power used - increase in speed, acelleration and a reduction in fuel consumption. A secondary benefit on a single installation is no torque reaction.

This principle was first used i believe on torpedoes.

Volvo Penta picked up on it in the 1980s and now almost all of their sterndrives utilise it.

Duncan
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Old 16 April 2005, 15:51   #7
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oh right, that'll be the one! Cheers Duncan.
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Old 17 April 2005, 18:07   #8
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AFAIK, if these were counter-rotating duoprop type engines, then they must have been Yamaha 150 V-max.
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