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Old 03-11-2011, 06:04   #1
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Calculating Prop Pitch on Repower

SV Kush is going through a repower, and I'm adding a Max Prop. I am interested in suggestions for the starting point when setting up the new Max Prop. I've researched the net, and I can't find what I'm looking for. The engine is a 4JH2 54hp, and the weight of the boat is 27 Tonns. It's a Max Prop 2 blade classic.

If you have a similar configuration, or an opinion on where a good starting point for the pitch setting, I would like to hear it!

Fair winds, Dave
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Old 03-11-2011, 07:37   #2
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Re: CALCULATING PROP PITCH ON REPOWER

I’ve seen calculators embedded in manufacturer’s web sites... when I get to a real computer I’ll look..... or PAGING MR. MAY TO THE RED PHONE......
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Old 03-11-2011, 09:00   #3
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Re: CALCULATING PROP PITCH ON REPOWER

There's numerous Propeller Calculators online
ie ➥ Propeller Calculator / Prop Calculator
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Old 03-11-2011, 09:55   #4
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Re: CALCULATING PROP PITCH ON REPOWER

Dave:

If you will post your boat's lwl, your engine's max wot rpm, your transmission ratio and the diameter of your MaxProp, I will run the numbers through boatdiesel's prop calculator. BTW if you join for $25, you can do it yourself. The info on marine diesels on this site is priceless.

David
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Old 03-11-2011, 10:33   #5
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Re: Calculating Prop Pitch on Repower

This book nailed the exact size for me when I repowered. There are a LOT of variables and I don't know if an internet formula would cover all the variables.

http://www.amazon.com/Propeller-Hand...0337847&sr=1-1
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Old 03-11-2011, 20:41   #6
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Re: Calculating Prop Pitch on Repower

Quote:
Originally Posted by David M View Post
This book nailed the exact size for me when I repowered. There are a LOT of variables and I don't know if an internet formula would cover all the variables.

http://www.amazon.com/Propeller-Hand...0337847&sr=1-1
This is my concern. I am 48' lwl, and 27 tonns. It would seem to me that there are factors other than the 5 used in the on line calculators. Given the max rpm of 3300, the 20" 2-blade prop, unknown slip, unknown pitch... I'm just looking for a good starting point. I can guess and go from there, but I thought that there might be others with a thought that would get me closer for the initial run. Let me know if you have an idea of a starting point. Thanks for taking the time, Dave
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Old 03-11-2011, 21:18   #7
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Re: Calculating Prop Pitch on Repower

The guys at PYI are a good resource--give them a call--but you will also need to tell them the gear ratio of your transmission.
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Old 04-11-2011, 09:44   #8
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Re: Calculating Prop Pitch on Repower

Dave:

First of all, 54 hp is low for a 27 tonne (60,000) lb boat. You will unlikely be able to get up to hull speed even at wot. Sailboats and efficient trawlers need 1-1.5 hp per 1000 lbs to reach hull speed.

But given that issue and assuming a 3.06:1 transmission ratio (The highest ratio availabe from Yanmar. The ratio is an absolutely critical parameter for figuring prop pitch) I plugged your data into boatdiesel's prop calculator. It gave a 20x15 two blade prop. It said that you would be able to cruise at 2,800 rpm at 6 kts and maximum speed would be 7 kts.

Boatdiesel is pretty conservative for sailboats which have relatively efficient hulls so you may be able to do better, but it is in the ballpark.

David
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Old 04-11-2011, 10:05   #9
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Re: Calculating Prop Pitch on Repower

the specs I can find say its a 14 ton boat and the installed engines are in the 80hp range--a 55hp would be definitely on the low side.
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