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Old 02-12-2010, 10:54   #1
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Higher Prop Pitch/Lower RPM

I've had the prop pitch changed from 15 to 18' on a 17" prop, this on a 42' Choey Lee cutter with a Perkins M60 diesal engine. Now as soon as I slip into gear, at about 1000rpm, I am really at/near hull speed of 6.2 knots...while this arrangement now gives me much greater torque and available power am I creating a potential engine problem with the low rpm? I will admit here that yes I do a lot of motoring. Thanks in advance.
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Old 02-12-2010, 11:54   #2
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If I'm reading your post properly, it sounds like you've grossly over prop'd your boat. Increasing the pitch decreases the available RPM and greatly lessens the torque available. You will be running your engine way below the optimimum torque/hp range. Worse, you won't have the highest hp/torque available because the engine won't have the oomph to spin up to higher rpm. Next time you take the boat out, run the throttle to max and see how much RPM you get. Would expect it will be way less than the maximum rpm reccomended by the manufacturer. In adverse conditions, this will result in a way underperforming enging without the power to punch through headseas and headwinds. Expect getting into the slip will be very interesting as you won't be able to slow the boat sufficiently in gear and will have to shift to neutral to keep the speed down.

Wonder about increasing the prop pitch by 3". It was my understanding that a change of an 1" or so was all that was reccomended.
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Old 02-12-2010, 12:02   #3
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It is equivalent to driving a manual transmission up hill in high gear. There is a general rule that states a 1 inch increase in pitch changes rpm by x amount. The info should be available online
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Old 02-12-2010, 12:12   #4
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I think 1-inch equals about 200rpms if I recall correctly.
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Old 02-12-2010, 12:13   #5
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It will also cause overheating in the long run if it doesn't already. You might want to do a search in the search engine after my signature using the term proper prop.
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Old 02-12-2010, 12:28   #6
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I think you could be running the risk of lugging or bogging down the engine, and this can increase wear on the engine components.
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Old 02-12-2010, 13:15   #7
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I thought it was all about the smoke....I met a guy in the PI with an adjustable pitch prop\trans (on the fly).....he said he would continue to steepen the angle until the engine started to smoke...then back it off (the pitch) a little and that was the optimum pitch for a given RPM.
I have no first hand experience with this.
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Old 02-12-2010, 16:51   #8
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Quote:
Originally Posted by James S View Post
I thought it was all about the smoke....I met a guy in the PI with an adjustable pitch prop\trans (on the fly).....he said he would continue to steepen the angle until the engine started to smoke...then back it off (the pitch) a little and that was the optimum pitch for a given RPM.
I have no first hand experience with this.
that would be correct if you had a constant speed propeller. Ie; an on the fly variable pitch.
With a fixed pitch. You want to prop so the engine can reach the maximum rated RPM of the engine.
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Old 02-12-2010, 18:25   #9
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Please do not take this personally.....I have seen more boaters ^%&$ up their boats by "trying to get something for nothing".

On large vessels, we have pyrometers which measure exhaust gas temperature. This allows us to run the mains without overloading them.

On a sailing yacht that I was Chief Engineer, we had Hundestadt Variable Pich props.....by observing the EGT I could dial in the pitch for optimum performance............I had to disable the system, because when I was off watch...the owner or his assholian son-in-law would futz with it to try to make the boat go faster. From my berth in the crews quarters, I could tell when they were fiddling with it.

You will have more power for manoevering with a less pitched prop....

You won't be lugging the engine.

How much does it smoke?
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