Propeller Sizing Question - Help! - SailNet Community

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Go Back   SailNet Community > On Board > Gear & Maintenance > Engines > Diesel
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Diesel This is a new forum dedicated to diesel engines and their applicable accessories.


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Old 10-13-2011
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Propeller Sizing Question - Help!

Ok...I am a newbe so please don't beat me up too badly here, but I have a sincere question I hope someone can help point me in the right direction.

I recently bought a Morgan Out Island 33. The previous owner "swapped" or replaced the engine with a Yanmar 20hp deisel out of ? I suspect he used the prop that was on the boat the engine came out of (SMALL!) and I get very little thrust and don't make hull speed at full throttle.

I know the engine seems small to me for the Morgan 33, but can anyone give me an idea what the diameter and pitch should be for the original prop on the Out Island 33? I will at least be starting off closer to what should push the boat acceptably and can adjust from there...

Thanks...Paul
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Old 10-13-2011
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It would be best to contact a propeller shop and let them determine which prop is right for the boat/motor/trans that you have. You want to size the prop so that the engine will be able to reach the recommended rpm in gear. If you put a prop on that is too large or has too much pitch it will be extremely hard on the engine. With the engine model, transmission ratio and the length/displacement of the boat any competent prop shop should be able to get very close on sizing a prop for you. It would also help them if you know the dia. and pitch of the prop that is on now and what your top rpm is at WOT in forward.
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Old 10-14-2011
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Absolutely agree but what I am hoping & asking for are any of the stock prop sizes anyone knows of as what is currently on the boat is of no use in referencing the correct size .

I am asking prop shops as well, but when I provide existing prop info they get confused & want to recommend something for a 19' daysailer...
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Old 10-14-2011
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You can get advice from these guys by completing the form.

West by North Enterprises - Propeller Recommendations
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Old 10-14-2011
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Decide if you want a fixed of folding style, and contact the manuafacture with you boat type, motor, trans ratio among things you will need, and they can tell you the prop diam and pitch needed. SWAG would be a 13-15" prop. Two vs three is another debate per say. The size etc is the same, but the 3 blade has more catch for lack of terminology on my part. So if you go against currents etc, a 3 will not slow down as much as a two blade. BUT< the 2 blade has less drag sailing. Hence a folder if you go with a 3 blade would be the best option for your boat. Good motoring, along with low drag sailing.

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Old 10-14-2011
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Thank you all for the responses...Please keep them coming.

FYI I have sent as much info as I can to West by North and Max-Prop company for thier recommendations.

Again they and I are at a disadvantage because the "existing prop" was never sized for this boat and is of no use as a starting point of reference. Also the labeling for the engine and transmission are missing so I am hard pressed to provide the most accuate information. What is on the boat now is a VERY SMALL 2 bladed prop that has NO reverse power at all and can't make hull speed at max RPM's now....

Which is why I am posting the questioon hoping someone out there has some prop info from thier Morgan or comparable boat.....

Oh...and FYI...I am going to be going with a fixed 3 blade prop....

Thank you all for your help and support!
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Old 10-17-2011
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No one has a Morgan Out Island 33?
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Old 10-17-2011
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and can't make hull speed at max RPM's now....

What RPM are you able to attain at wide open throttle? Are you hitting Yanmar's max RPM?
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Old 10-18-2011
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Originally Posted by Maine Sail View Post
What RPM are you able to attain at wide open throttle? Are you hitting Yanmar's max RPM?
Should reach max yanmar r4pm,IFIRC 1 in diameter or 2 in pitch = 100 rpm.,diameter increases power ,pitch for speed.marc
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Old 10-18-2011
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You must advise exactly which engine you have. The data plate will show this. Also need the transmission ratio. Again the data plate will show this.

The information on the Morgan is readily available (27'6" waterline, 14,500 Lbs) but we need the HP @ RPM and reduction ratio. With this info I can work out a prop calc for you. The prop should be sized for the particular engine/ transmission/ boat combination.
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