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Old 16-12-2009, 09:02   #16
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I've only seen them on high performance power boats - some even have two props contra-rotating on each shaft. And they're mostly on the big offshore V-hulls. Not that my observations are definitive!!!

I've never seen them on a catamaran sailboat and I can't imagine it would have any practical use. I don't get any significant prop walk on my boat.
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Old 16-12-2009, 12:51   #17
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No need to have counter rotating props on a cat with relatively small engines and therefore small propwalk. This propwash is considerable when high power engines are in motion.

The twin prop set up is used on high speed boats to utilise the powerloss from the fist prop in the row of the two. Surface props are used on even higher speed vessels to minimise drag from the prop blades and should be considered extreme technology.

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Old 16-12-2009, 13:46   #18
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Counter rotating vs. similar on a sailing cat should make essentially no difference. The main benefit of counter rotating props is realized the closer they are together, as on a monohull (as mentioned earlier several times.) Everything from 7.5knot trawlers to 40 knot sportfish to 80knot go-fast boats utilize and take advantage of the "cleaner" water that counter rotating props travel through (by not sending their propwash towards each other.) That's the deal. SO on a sailing cat, with a large beam, the wash from one prop would never see the wash from the other...so the need is negated.

The issue of propwalk on a cat is also somewhat of a moot point. Mainly because you actually DO have twin screws. So use 'em. You'll find yourself making way and with steerage long before you're small screws are significantly pulling two hulls/keels one way or the other laterally through the water.

I wouldn't sweat it as long as the engines/transmissions are set up right and are doing what they are designed to do. If you have a counter rotating engine, and they switched the linkage on you without sayiing anything...that might raise a question about the yard in my mind though...

Cheers,

Rob
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Old 16-12-2009, 13:54   #19
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Many years ago (about 40), I watched a yard launch a brand new power cruiser. The yard guy hopped onboard started the massive engines, threw both engine shifters into forward and backed hard into the ways, crunching the stern platform. Either the props were on the wrong engine or the engines were on the wrong side of the boat - never found out which.
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Old 16-12-2009, 14:55   #20
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Ayyup....I seen that done.
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Old 16-12-2009, 15:07   #21
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Ouch. Stings just to read that little ditty.
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Old 16-12-2009, 15:33   #22
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take advantage of the "cleaner" water that counter rotating props travel through (by not sending their propwash towards each other.)
Propwash is in the "wake" of the screws - it doesn't get thrown sideways. Counter-rotating props either negate undesired propwalk or enable it to be used effectively for manoeuvring.
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Old 16-12-2009, 16:40   #23
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Why?

Having driven vessels that are set up the way you stated (fixed pitch), vessels where the screws turn the opposite direction when going forward (controllable-pitch), and vessels where both props turn in the same direction, I'm curious.

Having both props turn in opposite directions can benefit manoeuvrability, and I can see with high-performance speedboats with lightweight and large props, this would be essential for control and efficiency, but in a cat it shouldn't matter greatly. IMO.
I said twin screw monohulls benefit by outboard turning, counter rotating props. I also said it does not matter much for catamarans.
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Old 16-12-2009, 20:31   #24
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Actually David you said twin-screw monohulls need to have outboard-turning screws. The vessels I described were twin-screw monohulls that didn't have outboard-turning screws, so I was curious about your statement. My opinion about counter-rotating screws not being necessary in catamarans was directed at the OP - sorry for the confusion.
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Old 16-12-2009, 20:40   #25
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Both of mine turn the same direction, I have no propwalk and can pivot in place.
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Old 17-12-2009, 05:46   #26
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There may be some catamarans that have counter rotating props, but none that I have seen. ....
Brian
I guess the Manta Sail & Power Cats are the exceptions to the rule then.

Port side is LH rotation, Stbd side RH rotation. I have pics if anyone disbelieves.

I agree that it may not be necessary on a cat due to prop separation.

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Old 17-12-2009, 17:14   #27
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Hi

It has been my experience that you do not need counter rotating props unless the wash of one interferes with the other,Yes they will cause problems in a narrower power cat generally in a sailing cat the beam is so wide that they have no effect on each other.
It is easily seen if the wash is crossing from prop to prop.

best wishes john
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Old 17-12-2009, 17:21   #28
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BBW

sorry I did not notice before but the comment about prop wash not going side ways,
I found this quite interessting the majority of thrust from a prop will find the point of least resistance hence it does go side ways as well,If this were not the case why do tugs and comerial boats use nozzles to control their thrust and increase the same..

All thr best john
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Old 17-12-2009, 18:23   #29
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Are you talking about Kort nozzles? They are used to vector the thrust in a specific direction - not really that common on commercial vessels except for tugs and small ferries. The majority of thrust is straight back from the screw - there is lost thrust in circular turbulence around the blade tips from the high-pressure area to the low pressure area in front of the screw. Good grief, I can't believe I'm arguing this - go turn on a fan and feel where it blows.
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Old 17-12-2009, 20:19   #30
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Kort nozzles do supply a small fraction of an increase in effeciency.......to realize the benefits you'll need to run the boat 24/7/365. They also increase available thrust as the prop is designed a little differently, the downside is that when the wear ring around the prop is out of tolerance, all benefits of the nozzle are lost.
but back to he counter rotating.......deep water vessels with twin screws use the counterclock/clock rotation setup for reasons of control, shallow water boats such as Louisiana mud boats use a clock/counterclock to keep from sucking debris up between the props. Both setups can have controllable pitch probs, variable pich props, or fixed pitch props.
All of this in not needed on a recreational vessel with low horsepower and wide prop spacing.
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