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| Name: |
John boy
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| Subject: |
Prop pitch question
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Date:
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3/18/2008 10:25:44 PM
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I have a 60hp outboard Mariner on a 24 foot pontoon boat and need to buy another prop for it. I would like a little more on the top end but don't want to give up what little fuel economy I have now. I have a 3 blade 10.5 pitch 12 inch prop now. Any help would be greatly appreciated. thanks John
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I'm not and expert, but from my experience, you will gain very little top end with that small of a motor on a 24' boat. A little more pitch could possibly get you better fuel economy by producing the same speed at a lower RPM in the middle band, but that will also depend on weight, loading, and drag issues for your particular boat and too much pitch will cause acceleration problems. Sorry about the lack of an answer; these are just things to think about.
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| Name: |
Kizma Anuice
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| Subject: |
Prop pitch question
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Date:
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3/21/2008 4:48:12 PM
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in order to fathom a guess one would need to know what your wot rpms are and the speed at wot.
it appears that your boat may be seriously underpowered. another thing might be the conditions of the pontoons. if you have any scum on them then it could decrease top speed by as much as 50%.
that would also increase you fule consumption 50%.
when gauging speed you should use a gps not the speedo on the boat.
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| Name: |
Kizma Anuice
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| Subject: |
Prop pitch question
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Date:
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3/21/2008 4:57:26 PM
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copy and past these urls they will give you some idea of the speed you should expect and the prop that you should use
http://www.yamaha-motor.com/assets/products/otb/bulletins/bulletin_otb_2StrokePerf_HPMidPort_50hp_02-13-GFY-A.pdf
http://www.yamaha-motor.com/assets/products/otb/bulletins/bulletin_otb_2StrokePerf_HPMidPort_50hp_02-20-SMK-A.pdf
they are both examples of 50 hp motors which should not be much different than yours
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| Name: |
CAT BOAT
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| Subject: |
Or, use this....
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Date:
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3/21/2008 6:55:10 PM
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This method/calculation will pretty much give you a good indication within 3 mph or so. You need the following information... Drive unit gear ratio. Propeller pitch. RPM's at wot.
Example Prop.pitch x RPM divided by gear ratio x 1056 minus slip.
Use the info like this... 26" of pitch x 5,000 rpm = 130,000
drive ratio of 1.5 x 1056 = 1584 So, 130,000 divided by 1056 = 82.07. Now, most effecient V-bottoms have a slip around 12-13 percent, and Cat hulls in the 9-10 percent range. (Notice, I said most. And a pontoon boat don't even count because none of them are effecient). Just for kicks, lets say the above boat is V-bottom -- 82.07 mph minus 13 percent slip would come out to about 71-72mph. Sorry this post got so long, but most will find it interesting. All this being said, when your buddy tells you his boat runs some unbelievable speed, walk around to the business end of the boat and gather some information....
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| Name: |
Psycho
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| Subject: |
Prop pitch question
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Date:
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4/2/2008 11:15:38 PM
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I ran into this several years back...had a pontoon with a 115 on it...I wanted more speed, so I went up a number or two on the prop.....I actually lost top speed because the motor wouldnt turn as many RPM.
...then I went down a number or two from my original prop...my engine turned more RPM's and I gained a couple miles of top speed.
if you can talk with someone that does boat repair, they may have several used props around....If you are lucky, they will let you try a few different pitch props to find the one you are happy with...good luck !
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