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Old 09-12-2009, 19:53   #1
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Should the Mechanic Know Cats Need Counter-Rotating Props?

After switching out props and shaft recently, I am not confident my marine mechanics know what they are doing. Chapman reads 'counter totating' for full maneuvering on a multihulll.

Now I am steering similar rotating props, but I have been told eventually this will compromise my transmission.

Anyone with experience in either of these situations - similar rotating or counter-rotating, that is the question.
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Old 09-12-2009, 19:58   #2
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IF you have one counter rotating engine you need a left hand prop to go with it..you will be basically thrusting in reverse if not!
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Old 09-12-2009, 20:03   #3
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Damn, I wont have counter rotating props on mine

The guys who are doing my props (and hundreds of powered cats) and the designers who said they aren't necessary clearly have no idea what they are talking about.
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Old 09-12-2009, 22:25   #4
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Both props on my FP rotate in the same direction when going forward.

The only two differences between the port and stbd engines I have been able to discern is a slight offset in mounting angle on the port engine and a 90 amp alternator add-on to the stbd engine for rapidly charging the house battery bank.
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Old 09-12-2009, 22:46   #5
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What?

If the engines were counter rotating before

They should still be counter rotating

If they changed props and shafts and didn't put the proper props on....with both engines going ahead or astern...you would be going around in circles

Quote:
Originally Posted by Brewster View Post
After switching out props and shaft recently, I am not confident my marine mechanics know what they are doing. Chapman reads 'counter totating' for full maneuvering on a multihulll.

Now I am steering similar rotating props, but I have been told eventually this will compromise my transmission.

Anyone with experience in either of these situations - similar rotating or counter-rotating, that is the question.
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Old 09-12-2009, 23:18   #6
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I think that I've read in this forum that there are trannys that have the same ratio in forward and in reverse, and that they work equally well in either direction, so you can have counter rotating props by reversing the the shift mechanism on one engine to run one engine in reverse gear to go forward. If you originally had counter rotating props and now don't, then they would have had to change the connection on one of the shifters as well or you would be motoring in circles as CE pointed out.

I would think that counter rotating would be desirable because it might slightly increase efficiency, and would make motoring with one engine easier, the guys in the real world as posted above don't seem to miss it. Those with sail drives (horizontal thrust) I don't believe care at all.

This shouldn't have had anything to do with the mechanic though, unless he deliberately modified your boat.

John
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Old 10-12-2009, 05:08   #7
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There may be some catamarans that have counter rotating props, but none that I have seen. I don't think it matters much when the engines are so far apart. The interaction of the counter rotating props is only important on monohulls, espescially when at planing speeds.
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Old 10-12-2009, 11:16   #8
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I would think that counter rotating would be desirable because it might slightly increase efficiency, and would make motoring with one engine easier,.....

John
How would having your props turning in opposite directions make a difference when you're only using one of them?
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Old 10-12-2009, 11:23   #9
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There may be some catamarans that have counter rotating props, but none that I have seen. I don't think it matters much when the engines are so far apart. The interaction of the counter rotating props is only important on monohulls, especially when at planing speeds.
Brian
I think Brian nailed it. Additionally, with two right hand props, all you need to carry is one right hand spare. Twin screw monohulls clearly need outboard turning (as seen looking forward, starboard clockwise, port counterclockwise), which by definition are also counter-rotating props.

BTW, many transmissions are set up to travel in either direction. You change the direction by how you connect the shift linkage.
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Old 10-12-2009, 12:05   #10
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Cats are probably less prone to "walking" than power boats.
I used to have a powerboat with twin outboards and it was set up originally with both props rotating the same way. when backing up the boat wants to walk sideways due to the prop rotation. Sometimes that really helped backing into a slip.

Later the lower unit was changed on one engine for a counter rotating setup. The boat then backed up perfectly straight. Took a while to get used to that!

Cats would grab the water a bit better and resist sideways motion.

In forward motion, fast or slow, I don't think it matters.
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Old 10-12-2009, 14:10   #11
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How would having your props turning in opposite directions make a difference when you're only using one of them?
If you're motoring with only the port engine, the boat will have some tendency to turn right. If the unbalanced thrust (p-factor, walking thrust, whatever you call it) also tries to turn the boat right, they are adding together. If the unbalanced thrust is trying to turn the boat left, it is attempting to negate the tendency to turn right due to the offset thrust.

I would guess that in most boats this is probably subtle or not noticable.

I've only chartered cats twice, but did enjoy being able to rotate the boat easily in place for maneuvering around docks. We also practiced making the boat move sideways, but that trick on the boats we chartered would not have worked against any sort of wind. Did not notice or care if the props were counter-rotating and did not motor on one engine as they were underpowered to begin with.

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Old 10-12-2009, 14:22   #12
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Twin screw monohulls clearly need outboard turning (as seen looking forward, starboard clockwise, port counterclockwise), which by definition are also counter-rotating props.
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.
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Old 10-12-2009, 14:42   #13
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What chapmans says and what applied in the real world are two different things. Sure, if you had contra rotating props you'd negate the minimal prop walk you've got. The added expense isn't worth the trouble for a pleasure craft. Large commercial vessels, in their simpilest form, use generally a twin disc transmission that is identical in output in either direction. this allows them to use the same engines, same transmissions, same shafts. The difference is a LH and a RH prop. This translates into a very small percentage of fuel savings by keeping the wash from the props and the hull shape from robbing power from eachother.
you can dive even deeper into the world of counter rotating, cp, azimuthing, whatever, but for a sailing cat it's no big deal.
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Old 11-12-2009, 16:57   #14
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On my cat with 24ft beam with around 18ft between the props, I have right the hand hull's prop rotating clockwise going forward, and anti-clockwise for the left hand. Can turn 360 on its own length.
NB. the props are driven by Cessa aircraft hydraulic motors driven by a single pump on a single diesel engine.
Agree that on the modern day cat with more than 12 ft between the props, counter rotation not an absolute requirement.
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Old 11-12-2009, 17:07   #15
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Counter-rotating props are very important in a heavy displacement boat where it takes so much force to move the boat forward from a stopped position that too much of the engines power is translated to prop walk until the boat begins its movement forward. This does not seem to occur on catamarans. I have owned a couple and chartered several large cruising cats. I have never felt significant prop walk despite the fact that they have never had counter-rotating props.
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