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Old 01-31-2018, 06:10 PM   #1
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The fun of info searching.

I have been having tons of fun, very expensive fun, trying hard to make the "right" boat out of what was close to start with. Least fun, and most expensive, was by me trying to make the boat a little nicer for a novice boating spouse. More on this later after I have completed my therapy and recovered from the lobotomy. The one thing that I just can not seem to get a consensus on is still something I would like to solve. So here we go with the simplest question I have had about the boat. What is the single best prop option for your basis CHB 34 style trawler with a single Lehman 120. I have been told 3 blade, 4 blade and even one propeller store that said a new high tech 5 blade. I just looked at him, thought high tech for 40 year old tractor motor, shook my head and left. But if any and/or all of you out there have a good answer as to blades, diam. and pitch it would be greatly appreciated.
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Old 01-31-2018, 06:28 PM   #2
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Greetings,
Mr. CO. Welcome aboard, if I missed you. Have you seen this? https://www.vicprop.com/calculator.htm
What prop do you have now and what is your WOT engine RPM at full throttle under load (in gear and under way)?
As a general rule, as I understand it, the "best" prop should allow WOT to be in the 2400 RPM range for the Lehman 120. Any less and you're over propped, any more and you're under propped (I think....)
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Old 01-31-2018, 11:20 PM   #3
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There is theory and then there is practice. I wouldn’t spend any time worrying about 3 blade vs 4 or 5 blade on a displacement boat. I would worry about wheel size and pitch.
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Old 02-01-2018, 03:01 AM   #4
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Go with the largest diameter prop that still gives you the required gap between prop and hull. You should have a minimum space of 15% of prop diameter above the prop.

The pitch will then be dependent on maximum engine rpm and gearbox ratio. Plug the numbers in the vicprop calculator as suggested by RT.

3 blades is fine; 4 blades may possibly give slightly better efficiency. It will definitely cost more.
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Old 02-01-2018, 05:20 AM   #5
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Go with the largest diameter prop that still gives you the required gap between prop and hull. You should have a minimum space of 15% of prop diameter above the prop.

The pitch will then be dependent on maximum engine rpm and gearbox ratio. Plug the numbers in the vicprop calculator as suggested by RT.

3 blades is fine; 4 blades may possibly give slightly better efficiency. It will definitely cost more.
More blades are always less efficient. 4 blade may be smoother and quieter. The less blades, more efficient.
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Old 02-01-2018, 06:39 AM   #6
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What is the single best prop option for your basis CHB 34 style trawler with a single Lehman 120. I have been told 3 blade, 4 blade and even one propeller store that said a new high tech 5 blade.

Give us your concept of "best".

Best speed , least fuel burn , quietest , tow a big dink behind, cheapest to buy ???
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Old 02-01-2018, 08:46 AM   #7
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Are you trying to solve a problem? Is this a new boat to you? If so, you should give it time to see if there is even a need. You should proceed with caution. Changing props is expensive and you can find yourself chasing your tail; pouring good money after bad for minimal return.
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Old 02-01-2018, 09:33 AM   #8
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Originally Posted by C.O.S.T. View Post
I have been having tons of fun, very expensive fun, trying hard to make the "right" boat out of what was close to start with. Least fun, and most expensive, was by me trying to make the boat a little nicer for a novice boating spouse. More on this later after I have completed my therapy and recovered from the lobotomy. The one thing that I just can not seem to get a consensus on is still something I would like to solve. So here we go with the simplest question I have had about the boat. What is the single best prop option for your basis CHB 34 style trawler with a single Lehman 120. I have been told 3 blade, 4 blade and even one propeller store that said a new high tech 5 blade. I just looked at him, thought high tech for 40 year old tractor motor, shook my head and left. But if any and/or all of you out there have a good answer as to blades, diam. and pitch it would be greatly appreciated.
3 blade cupped most efficient 4 blade smoothest
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Old 02-01-2018, 10:40 PM   #9
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Itís a boat. Unless thereís an actual problem, the best propeller for it is the one thatís on it.

Actual best will be some sort of compromise based on the specifics of your vessel. No one can tell from here.

But Iím sure many forum-goers may enjoy discussing theories anyway. Generalizations can be fun.
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