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 Question about prop on Yanmar engine		  New Topic  Topic Locked
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lrjetav8r
Deckhand

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USA
14 Posts
Initially Posted - 08/15/2013 :  18:56:43  Show Profile
The service manual on this model of Yanmar diesel engine (Model # YSM12, 10 H.P, 3,000 rpm and the propeller shaft rpm is 1,518) calls for a three bladed 13 inch prop. Mine is more like 9 inches. Does anyone have one handy enough that they can put a tape measure on it and tell me what their's is...this seems like a big discrepancy. Thanks for any advice!
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dmpilc
Master Marine Consultant

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USA
4593 Posts
Response Posted - 08/16/2013 :  13:24:01  Show Profile
Do you have enough clearance for a 13" prop? Odds are a PO had to replace the prop and the 9" one was available. Do you have performance problems? Will the currant prop push the boat to hull speed without maxing the RPM's?
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awetmore
Master Marine Consultant

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USA
1144 Posts
Response Posted - 08/16/2013 :  15:04:51  Show Profile
Prop pitch and size is a bit of an art, there isn't just a single recommendation that works. There are various calculators online that will help you get close in sizing.

If you go at hull speed in calm conditions at about 70-80% of max engine RPMs then you are in good shape. If it is significantly fewer RPMs then you are over propped (the prop is trying to push too much water per revolution and taxing the engine). If you can't get to hull speed then it is under propped (you aren't pushing enough water to hit full speed) and you are in danger of running the engine over it's max speed.

The prop pitch matters too, and you didn't mention that.
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lrjetav8r
Deckhand

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USA
14 Posts
Response Posted - 08/17/2013 :  20:22:05  Show Profile
Thanks guys, I will try the performance stuff once I get her in the water. And David, you are absolutely right, I don't know why I didn't catch it. There is only a little over an inch clearance with that 9 in prop. Thanks for the advice
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Notice: The advice given on this site is based upon individual or quoted experience, yours may differ.
The Officers, Staff and members of this site only provide information based upon the concept that anyone utilizing this information does so at their own risk and holds harmless all contributors to this site.