"Proper Pitch" - SailNet Community
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post #1 of 11 Old 04-16-2008 Thread Starter
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"Proper Pitch"

I have a 25' Coronado that I am putting a 9.9hp Merc outboard on. The motor was purchased used and the former owner stripped the rubber drive from the center of the prop. Price of repair vs. new prop pretty close, so my question is what would be the best pitch for my boat. Stock pitch is 9 degrees (motor). And I send this thread at risk of offending SD, I swear I searched old posts.
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post #2 of 11 Old 04-16-2008
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Jewelled-

First, prop pitch is measured in INCHES PER REVOLUTION, not degrees generally. What diameter is the prop? How many blades does it have?

What I'd recommend is that you get a Pirahna Propellors composite prop. They have a blade exchange policy, so you can experiment with different pitches for the prop until you find the one that works best for your boat.

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post #3 of 11 Old 04-17-2008
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Usually the boat engine and prop manufacturers have programs to calculate prop diameters and pitch. They need data on waterline length, beam, weight, gearbox ratio, engine type and HP and such. Even then, its a bit of a dark art. Having an adjustable prop, like the Maxprop, helps experiment with pitch, but the diameter has to be right to start with.

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post #4 of 11 Old 04-17-2008
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Or, you could just go with the factory spec prop knowing that the 9.9 hp will be pleanty of power to push your boat.

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post #5 of 11 Old 04-17-2008
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Get one of the elephant ears type.
Most owners seem to pefer these for the great stoping abillity to stop and work best on through the prop exhaust.
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Factory props for outboards often don't work well for sailboats, since sailboats are usually far heavier than most of the boats small outboards are normally used for.

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post #7 of 11 Old 04-17-2008
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Quote:
Originally Posted by sailingdog View Post
Factory props for outboards often don't work well for sailboats, since sailboats are usually far heavier than most of the boats small outboards are normally used for.
..... although that may depend on the motor as the (edit-Yamaha) 9.9 I had on a Nimble Arctic 25 a few boats ago Was designed with a lower gear ratio and 'high thrust prop' with pushing heavy boats in mind. It was basically comparable to thier own 15hp for torque. Of course part of that was marketing to the 'under 10hp' thing that was an issue in some places for a few reasons that I don't recall.

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Wickford/Narragansett Bay RI

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post #8 of 11 Old 04-17-2008
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Quote:
Originally Posted by sailingdog View Post
Factory props for outboards often don't work well for sailboats, since sailboats are usually far heavier than most of the boats small outboards are normally used for.
That's a bit of a generalization since saildrive and long shaft outboards are made specifically for sailboats.

Have you looked at the hp of the outboard and the specs of the factory prop, as they relate to the size and weight of the boat, before deciding that the factory prop is inadiquate?

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post #9 of 11 Old 04-17-2008
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If there is an optional Merc propeller with a lower pitch than the stock 9 inches, get that. The stock size is based on powering a small powerboat at much higher speeds than a sailboat. You'll get better performance and fuel economy with the lower pitch prop.
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post #10 of 11 Old 04-17-2008 Thread Starter
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Thanks everyone, I didn't realize how much there was to prop sizing. I figured length and approx weight would be enough. Lots of stuff to look at thanks.
Jeff
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