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cherod

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for those with twin screw arrangement ,, is there any advantage to haveing a RH and a LH prop , at present ( from standard i presume ) i have both RH but time has come to replace ( on a cat ) and i am wondering if there is any advantages / draw backs in having RH and a LH . and is it possible to fit a LH on to what is a RH shaft ?,, as is it drives straight so no current issues ,,any ideas / opinions will be appreciated , ty R
 

pvb

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Catamarans typically have handed props, but the physical distance between them means it's not as important as in a twin-screwed monohull. As HissyFit said, you'd need to check whether the gearbox is appropriate.
 

cherod

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Catamarans typically have handed props, but the physical distance between them means it's not as important as in a twin-screwed monohull. As HissyFit said, you'd need to check whether the gearbox is appropriate.
when you say " handed " , do you mean both the same or one of each ,,, yes i need to know about the box,, that is why i am asking .;) , not that i have any preference , just wondering about any advantages / disadvantages.
 

pvb

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when you say " handed " , do you mean both the same or one of each ,,, yes i need to know about the box,, that is why i am asking .;) , not that i have any preference , just wondering about any advantages / disadvantages.

Contra-rotating. On a monohull, the advantage is more accurate steering. On a cat, with much more distance between the props, it's less of an issue. And, as you said in your OP, your cat drives in a straight line OK.
 

cherod

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you mean one of each , a right an a left ??,,, yes it tracks true , handles like a dream , no issues at all as it is now ..
 

HissyFit

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What would you need to change for no appreciable gain? First thought would be the need to reverse the forward/reverse on one control lever, or else it could become confusing.
Leave well alone, maybe?
Wot he said.
 

Bluemu

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On a cat, if you run on one engine there are some benefits - fuel consumption, maintenance, better load on the Single engine at lower speed. But those benefits can be offset by crabbing or the turning force by the single engine. Having each prop running to slightly turn the bows inward from the outer prop being used could be beneficial. Only of use when running trawler speed on one engine.
 

Ian_Edwards

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Just to add a bit, out of interest, perhaps.:unsure:
A prop' adds axial trust, but also spins the water, the spinning water doesn't drive the boat forward.
Not a big problem on boats, it's just inefficient,
Some big stern drives have counter rotating props' on one shaft, which improves the efficiency, but at the cost of additional complexity.
I twin screw powerboat with the props close together can gain some efficiency and improved handling by having counter rotating props'.
In the extreme case of a torpedo, the prop' could spin the torpedo, so the are sometimes fitted with counter rotating props on one shaft, or stators, propeller shaped vanes which are designed to take the spin out of the prop ' wake.
I've often wondered why pleasure craft designers don't use one of the available techniques to make props more efficient, on big powerboats, it could make a significant difference to the fuel consumption.
On a cat, as said above, there is no benefit and may well be difficult and expensive to implement.
 

cherod

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Just to add a bit, out of interest, perhaps.:unsure:
A prop' adds axial trust, but also spins the water, the spinning water doesn't drive the boat forward.
Not a big problem on boats, it's just inefficient,
Some big stern drives have counter rotating props' on one shaft, which improves the efficiency, but at the cost of additional complexity.
I twin screw powerboat with the props close together can gain some efficiency and improved handling by having counter rotating props'.
In the extreme case of a torpedo, the prop' could spin the torpedo, so the are sometimes fitted with counter rotating props on one shaft, or stators, propeller shaped vanes which are designed to take the spin out of the prop ' wake.
I've often wondered why pleasure craft designers don't use one of the available techniques to make props more efficient, on big powerboats, it could make a significant difference to the fuel consumption.
On a cat, as said above, there is no benefit and may well be difficult and expensive to implement.
Multi prop shafts have never been my consideration
 

Ian_Edwards

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Yes, I understand that, I was just trying to add some context as to why counter rotating props can be an advantage in some circumstances.
If you understood the context, you wouldn't have asked the question, just trying to be helpful.
If it's not useful, just ignore it.
 

cherod

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Yes, I understand that, I was just trying to add some context as to why counter rotating props can be an advantage in some circumstances.
If you understood the context, you wouldn't have asked the question, just trying to be helpful.
If it's not useful, just ignore it.
thank you for your reply, yes i understand and that is why i asked , i also understand multi propped shafts , and that is why wouldnt want them , i dont doubt that they are good on a power boat but not on a sail boat
 

vas

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since noone mentioned it (I think!) keep in mind that the ratio of the box in reverse may be different to ahead. so turning one side the other way round, wont only affect the morse controls but you may need to have a different pitch to that prop. Seems too complicated for nothing...
 

Neeves

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Our cat has left and right props. We do all our own servicing. I do the heavier stuff and Josephine does the fiddly bits - which means she is in charge of the props. I take them off she prepares and paints them. I replace and add the anodes.

We had done this a number of times without thinking about it and without problem.

On one occasion (and it only happens once) we replaced the props the wrong way round (and I don't recall which is which) with fairly disastrous and, on reflection, comical results, as we went forward in reverse and reverse in forward. This was more complex than I describe as on floating off we had to turn the cat round so it was not simply going backward. It took some time with an addled brain to work out what was wrong.

We had to reverse or go forward back onto the slipway and swap the props round (before we missed the tide).


You don't need to change the morse controls if you get it wrong, or not with Volvos. The lever to which the morse cable attaches on the sail drive (assuming you have a sail drive) can be swung through 180 degrees and this will 'reverse' the gear change. If you need to do this you also need to move the support 'structure' for the morse cable - but that is obvious when you do it. The gearing may be different (I don't know) but if you get it wrong there is a quick fix until you return the props to the correct hull. I don't recall why I know this - but it may have been the time we had a morse cable fail and I had need to manage the gear change with a long piece of string and some small bullet blocks (we carry a spare cable now).

Jonathan
 

scottie

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Our cat has left and right props. We do all our own servicing. I do the heavier stuff and Josephine does the fiddly bits - which means she is in charge of the props. I take them off she prepares and paints them. I replace and add the anodes.

We had done this a number of times without thinking about it and without problem.

On one occasion (and it only happens once) we replaced the props the wrong way round (and I don't recall which is which) with fairly disastrous and, on reflection, comical results, as we went forward in reverse and reverse in forward. This was more complex than I describe as on floating off we had to turn the cat round so it was not simply going backward. It took some time with an addled brain to work out what was wrong.

We had to reverse or go forward back onto the slipway and swap the props round (before we missed the tide).


You don't need to change the morse controls if you get it wrong, or not with Volvos. The lever to which the morse cable attaches on the sail drive (assuming you have a sail drive) can be swung through 180 degrees and this will 'reverse' the gear change. If you need to do this you also need to move the support 'structure' for the morse cable - but that is obvious when you do it. The gearing may be different (I don't know) but if you get it wrong there is a quick fix until you return the props to the correct hull. I don't recall why I know this - but it may have been the time we had a morse cable fail and I had need to manage the gear change with a long piece of string and some small bullet blocks (we carry a spare cable now).

Jonathan
 

cherod

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Thanks again all , no it is not my intention to be using reverse for forward or anything like that ?, would only consider the handed props if it proved an advantage and could be used on as is , b tw , prop shaft , not sail drive and also Still to check if prop shafts both rotate in same direction ??
 

rogerthebodger

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Still to check if prop shafts both rotate in same direction ??

I would say that if the prop shafts rotate in the same direction the propellers must be the same hand. If the prop shafts rotate in opposite directions then you must have handed propellers.

Other wise you will be going round in ever decreasing circles.
 

cherod

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I would say that if the prop shafts rotate in the same direction the propellers must be the same hand. If the prop shafts rotate in opposite directions then you must have handed propellers.

Other wise you will be going round in ever decreasing circles.
Well whatever way they are they are correct as ( at least on the boat ) i do not go round in circles !!
 
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