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Go Back   SailNet Community > On Board > Gear & Maintenance
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  #1  
Old 05-28-2007
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Prop size?

Assuming I can get this *&%^ boat running again, what size prop is appropriate?

The engine is a Yanmar 1GM. Max RPM is 3600. Horse power is about 6.5. The gear ratio is 2.62. The boat is a Cal 27. 23'3" at the waterline. Displacement is about 5200#.

Thanks for your help.
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Old 05-29-2007
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Not a simple question! I am assuming you want a fixed two-blade prop, not a folding or three-blade. The key criteria are diameter, pitch, shaft diameter, and right-hand or left-hand in selecting one. Measure the shaft - it's likely 3/4 inch, 7/8 inch or 1 inch. Determine the direction the prop will turn when going ahead: clockwise = RH, counter clockwise = LH. Diameter and pitch are trickier. I would try to get a recommendation on these from someone who knows the boat. For reference my CAL 9.2 (30 foot) has a 14 inch diameter, 10 inch pitch folding prop which seems to work well.
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Old 05-29-2007
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Thanks for replying.

You're right - I want a 2-blade prop. I'm trying to do this as inexpensively as I can.

It's a RH prop. I'll measure the shaft tonight.

She's currently got a 14-10 as well. However, wide open under load I can only get 2500 RPM. My mechanic says that's due in part to having the wrong prop on there. He suggests a 12-12 or a 12-10. I've run the math, but my results are dubious at best, so I was casting about for a more experienced opinion.
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Old 05-29-2007
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Check out:http://www.westbynorth.com/Choosing.aspx
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Old 05-29-2007
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Just my two cents, I think the 12x10 should be fine and your cruise RPM @ 5.5 kts should be around 2800, with the engine capable of reaching 3600 if it needs to, but the speed of the boat will not inprove much. She will push a bow wave, raise her nose and double fuel consumption.

At 2800 RPM and 5.5 kts, she should hum along sweet as a single can and burn less than a quart an hour.

I could be wrong, but if I am, it's not by much.

Hawk
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Old 05-29-2007
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Quote:
Originally Posted by Hawkeye25
Just my two cents, I think the 12x10 should be fine and your cruise RPM @ 5.5 kts should be around 2800 . . .
Thanks for the information. I think it is consistent with what my mechanic is saying.

A couple of points, just for clarity, though. She's got a 14-10 right now and can't make 2,800 rpm. I can only get to about 2,500 rpm.
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Old 05-29-2007
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Select a prop manufacturer and give them these details and ask them. They will also need the distance from centre of shaft to bottom of hull.
They are usually very quick with a reply and certainly are familiar with the Cat 27s.

Gary
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I agree with getting recommendations from a prop manufacturer. Keep ijn mind that going to a smaller diameter prop but leaving the pitch the same will basically increase slip. The RPMs may go up, but you are not getting more thust to move the boat - inefficient!. Reducing the pitch will increase the engine RPM at a particular boat speed which sounds like what you want. I am however surprised a 10 inch pitch doesn't work for you as the prop RPM is pretty low with the 2.62 gear ratio you report.
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