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  #11  
Old 04-08-2012
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Re: Fixed vs folding prop

Slightly off topic.
When sailing, I been told to always keep the stopped engine in gear (forward or reverese) so the prop folds back quickly.

This doesn't seem right to me.
Has anyone else heard this?
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  #12  
Old 04-09-2012
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Re: Fixed vs folding prop

My 1981 C&C 30 MK1 manual says 6.7 kt at 3400 rpms.
Well here is the thing the last owner put a folding prop on because he raced. I here a thud when I engage the prop. It donít back up very good. And I can only get around 2,000 rpms? If I push past that very long it starts wanting to smoke black smoke? Like ,I may be pushing too hard on the engine. any advice?


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  #13  
Old 04-09-2012
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Re: Fixed vs folding prop

Quote:
Originally Posted by chrism33 View Post
Slightly off topic.
When sailing, I been told to always keep the stopped engine in gear (forward or reverese) so the prop folds back quickly.

This doesn't seem right to me.
Has anyone else heard this?
Check your transmission manual.. most do say to put in reverse while sailing, some do not, incl Yanmar, and suggest you leave it in neutral. Hurth says not to let the prop freewheel while in forward gear..

Either way, stopping the shaft will promote faster folding.. sometimes an unfolded prop will continue to spin otherwise... same with feathering. If your transmission advises against leaving the gear in reverse, you can always just do it long enough to promote folding and then leave it in neutral afterwards.

Quote:
Originally Posted by ltgoshen View Post
My 1981 C&C 30 MK1 manual says 6.7 kt at 3400 rpms.
Well here is the thing the last owner put a folding prop on because he raced. I here a thud when I engage the prop. It donít back up very good. And I can only get around 2,000 rpms? If I push past that very long it starts wanting to smoke black smoke? Like ,I may be pushing too hard on the engine. any advice?

Sounds like you're overpitched.. a prop shop may be able to fix that for you and repitch your existing prop. btw that style of prop can sometimes only spin one blade open, causing a lot of vibration. If you find that happening get into the habit of giving things a quick spin in reverse prior to shifting to fwd.. seems to help. This mostly occurs when the prop comes to rest with the blades on a vertical axis, and one blade drops open due to gravity and lack of boat speed. The other blade, sitting in line with the shaft sometimes won't flip open leaving you with effectively a one blade prop - a very unbalanced circumstance!

The poor reverse performance is one of the costs of the reduced drag.. if you're not racing and want to avoid all this the Campbell Sailor prop is a good, relatively inexpensive option.
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Last edited by Faster; 04-09-2012 at 03:57 PM.
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Old 04-09-2012
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Re: Fixed vs folding prop

The opening thud is a characteristic of the folding prop. So is the poor reverse performance.

Another thing to check in terms of cruising RPMs is whether the tach is accurate. In-accurate tachs are pretty common, being off by a lot is not unusual. See

diesel rpm
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  #15  
Old 05-23-2012
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Re: Fixed vs folding prop

Quote:
Originally Posted by ltgoshen View Post
My 1981 C&C 30 MK1 manual says 6.7 kt at 3400 rpms.
Well here is the thing the last owner put a folding prop on because he raced. I here a thud when I engage the prop. It don’t back up very good. And I can only get around 2,000 rpms? If I push past that very long it starts wanting to smoke black smoke? Like ,I may be pushing too hard on the engine. any advice?

It looks like you have a Martec prop, which is the worst of the folders. I had one of those on my boat, until I found a good deal on a used Gori prop, which is the same idea except there are gears so the blades open and close together. It is definitely smoother, and reverse is much improved; still not great, but better than the Martec. It definitely sounds like the prop isn't sized correctly for your engine.

Last edited by Faster; 05-23-2012 at 12:01 PM. Reason: fixed quote
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Old 05-23-2012
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Re: Fixed vs folding prop

SchockT,

Low RPM in gear and black smoke at speed? This description is the classic one for a prop that has too much bite for the motor driving it.
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  #17  
Old 05-23-2012
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Re: Fixed vs folding prop

Quote:
Originally Posted by dacap06 View Post
SchockT,

Low RPM in gear and black smoke at speed? This description is the classic one for a prop that has too much bite for the motor driving it.
ergo the prop is not sized correctly (I guess I should have said "matched"?)

Whether the diameter is to big or the pitch is too great, the symptoms will be the same will they not?

Sheesh! A guy sure has to choose his words carefully on this site! :-)

Last edited by SchockT; 05-23-2012 at 09:38 PM.
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  #18  
Old 05-24-2012
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Re: Fixed vs folding prop

Even for cruising there is no question that a folding or feathering prop is a massive increase in speed while sailing. Probably the single best thing you can do to increase the performance of a boat.

Yachting Monthly did a really nice right up of a number of different folding/feathering props, and what they found was pretty surprising.
1) the drag from a fixed prop is roughly the same as HALF the drag from the entire rest of the boat.
2) backing characteristics of the good folding/feathering props was actually better than that for a fixed three bladed prop
3) feathering/folding props has less prop walk than a fixed generally
4) forward thrust from the feathering/folding props was within 5% of a fixed prop.

Based on this study I actually just replaced my fixed with an auto stream, and couldn't be happier. I wanted the flex-o-fold but couldn't find a US distributor. The results have been exacally what I expected. Similar forward speed and control, much better control in revers with less prop walk, and greatly increased speed while sailing.

A reprint of the study http://www.flexofold.com/upload_dir/...onthly_low.pdf

Last edited by Stumble; 05-24-2012 at 05:13 PM.
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