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Old 09-08-2018, 02:00 PM   #1
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George&Jill

Hello!
Iím wondering if someone out there can help us with a propeller pitch question.
We have a 34í CHB here in Puget Sound that Iím pretty sure is propped wrong.
Our hull speed should be 7 knots to maybe a bit better but to get it, we have to push our poor old 120 Lehman to 2200 rpm which is way past its ideal operating RPM of 17-1800. Hence, we poke along at around 6.5 knots at 1900 RPM, wasting fuel as we go. We have a Paragon transmission with a 2/1 reduction.
Right now, our prop is a four blade 25L x 13.
Iíve spoken with our local prop shops and the concensus seems to be that we need to re-pitch it. Itís such a hassle to do that Iíd like to get it right the first time. I guess each inch of pitch has the general effect of shifting RPMs by about 200 RPM.
Could anyone with a similar hull (34í Grand Banks or CHB) let me know what theyíve found to be the ideal prop for this hull.
Thank you,
George&Jill
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Old 09-10-2018, 10:24 AM   #2
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We have used Tacoma Prop in the past with great results. Their service is fantastic even for a small job.
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Old 09-10-2018, 10:28 AM   #3
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There is a great prop calculator here.
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Old 09-10-2018, 10:48 AM   #4
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I went through this when I repowered with greater hp several years ago.
My original configuration had me getting to a comfortable cruising speed of 7.5k at 2700 RPM on props that were 19x13 with a 2:1 reduction gear.
I changed nothing but the hp and found that the speed at 2700 RPM increased. My fuel economy (mpg) improved about 10% due to the more modern engine.
Then I looked at prop curves, Boatdiesel.com has a good prop calculator, which I used.
I took my props to Osborne in Vancouver and followed their advice, so I now have 19x17s, 8k is achieved at 2000 RPM and my fuel economy (mpg) has improved another 5%.
All this was done in 2000 and I am still very happy with the outcome.
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Old 09-10-2018, 11:44 AM   #5
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The first point in this conversation is "what is the accurate, ie phototach calibrated, rpm of your engine at wot". Your Lehman is rated to produce its 120 hp at 2,500 rpm so that is the second point. That is the second point and in most cases you want to prop the boat to reach rated rpm at wot.


Tell us the answer to the first point then we can continue the conversation. Also all good prop shops will ask the same question before discussing any changes that may be recommended.


An from what I have read, changing the pitch by one inch changes the rpm by 100.



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