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Old 19-12-2005, 15:22   #1
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Adjusting a feathering prop

My newly acquired boat has a 2-blade feathering prop, the operation of which is still something of a mystery to me, but that sems to function ok.

I know that the pitch of the prop blade can be adjusted, but I have no idea how one goes about determining the "optimum" blade pitch. Presumably, the ideal pitch would depend on the vessel displacement, the engine hp and the "normal" operating rpm of the engine?

Is there a formula that one can apply, or a reference book that details this information, or is it a matter of tweaking it until it seems to be optimal?
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Old 20-12-2005, 20:01   #2
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I recently bought a boat with a Max-prop that was badly overpitched. Fortunately, the owner's manual specified the pitch. If you do not have that information, the definitive text on the subject is "Propeller Handbook" by Dave Gerr. The formulas can be intimidating, though. After that, you have to determine your current pitch and how to change it.

Short of all that, you can probably estimate if you are pitched correctly. If you motor close to hull speed at 80-85% of your engine's maximum RPM, you are probably about right. If you start belching black smoke at or before you reach 85% of max RPM, you may be overpitched (provided your prop and bottom are clean). On the other hand, if you cannot get close to hull speed in flat water at close to your engine's max RPM, you may be underpitched. Or you may have insufficient horsepower or too small a prop (in surface area/# of blades).

In my case, I had smoke at 50-60% of the max RPM with a clean hull and prop. I also was moving at 3.5 knots at idle. After taking about 4" of pitch off, I'm still a touch overpropped but I can run at about 80-85% of max RPM without smoke.
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Old 21-12-2005, 03:45   #3
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Here's a nice calculator, props are at the bottom.

http://www.image-ination.com/sailcalc.html

I thought the Volvo site had info on that but I was wrong.
What size is your current prop? how many RPM will it pull before heavy smoking?
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Old 21-12-2005, 13:44   #4
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Thanks for the info

Because I am relatively new to the boat owning thing and did not, until recently, have sufficient information about the maximum revs that I could safely put the engine to, I have been running the motor at quite low revs. The next time I take the boat out (for a 4 day cruise, leaving boxing day), I shall be a little less cautious and see how it goes with higher revs.
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