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Old 05-12-2011, 12:55   #1
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Plastic Prop on 36' Sailboat ?

Hi friends. I have recently got a Catalina 36. The previous owner replaced the standard 2-blade 15X9 bronze prop with a plastic one that appears to be quite a bit smaller. His only explanation was "it's better for the boat." I doubt that since I can only reach ~ 2100 rpm (according to the tach) and WOT is supposed to be 3300.

Has anyone ever encountered a plastic prop on such a boat, or can anyone shed any light on why on earth this may have been installed?

Thanks,

Nick
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Old 05-12-2011, 12:59   #2
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Re: Plastic prop on 36ft sailboat ?

I've seen plastic props on that sized boat before but the brand escapes me. If the prop is smaller your revs should be way up there. Something strange here, have you had the engine checked?
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Old 05-12-2011, 13:07   #3
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Re: Plastic prop on 36ft sailboat ?

Found it, think they were called flexprop.
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Old 05-12-2011, 14:15   #4
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Re: Plastic prop on 36ft sailboat ?

Quote:
Originally Posted by ChannelIslander View Post
Has anyone ever encountered a plastic prop on such a boat, or can anyone shed any light on why on earth this may have been installed?
The urethane prop will not corrode, they are popular with the Atomic-4 crowd and yes, I have seen them on boats of all shapes and sizes.
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Old 05-12-2011, 16:09   #5
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Re: Plastic Prop on 36' Sailboat ?

The Kiwi Prop is a plastic feathering 3-blade prop.

I doubt you could make a plastic prop blade as rigid and thin as a traditional metal blade, but these days you can also easily use CAD/CAM software to design a blade that is "wrong" when at rest, but distorts into the correct shape when running at speed. So if the design was done well, plastic shouldn't hurt at all.

Plastic or metal--it is still easy to put the wrong size prop on a boat.
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Old 05-12-2011, 18:10   #6
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Re: Plastic Prop on 36' Sailboat ?

Thanks guys.

Thanks Rick I; those guys sure make it seem like the bee's knees, don't they? I am not sure what make of prop I have; will ask the PO and see if he remembers.

Although the engine doesn't get up to rated max RPM, it does drive the boat at 6.5kts+, so it seems to be quite effective.

I will investigate more and report back.

- nick
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Old 05-12-2011, 18:21   #7
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Re: Plastic Prop on 36' Sailboat ?

Something seems amiss with the engine only getting up to 1/3 the expected rpm's. I am not a mechanic, but is it possible (if the rpm's are being sensed from the alternator) to have a bad diode in the alternator's rectifier?
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Old 05-12-2011, 18:34   #8
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Re: Plastic Prop on 36' Sailboat ?

I would agree with Duke 48 - get an optical tachometer from a mechanic and check and see if the boat's tachometer is or is not displaying the actual rpm's.
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Old 06-12-2011, 10:45   #9
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Re: Plastic Prop on 36' Sailboat ?

Kiwiprops are plastic too.

I do not get the part about the prop being smaller and the revs (?) lower? Perhaps it is seriously fouled.

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Old 06-12-2011, 10:53   #10
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Re: Plastic Prop on 36' Sailboat ?

If you can't find or afford an optical tach you can mark the flywheel on your boat and use a florescent light running on shore power, the light is not actually continuous, it flashes at 60hz. At 3600rpm you should see the mark at one consistent location, 2 locations at 1800, and 3 locations at 1200 and 2400.
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