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Old 10 January 2012, 14:20   #1
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Prop Sizing?

Hi, My rib with a Yamaha 70hp 2 stroke will do about 35.5 knots with just me onboard or 30.5 knots at 5400 rpm with two adults. I always carry about 50L of fuel, anchor, fire extinguisher etc.

I was thinking about changing the prop and increasing the pitch by about 2" to bring the rpm down to 5000 and hopefully improve the top end and cruising economy.

Does this sound about right to you?
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Old 10 January 2012, 14:27   #2
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Sounds good to me where it is. If you go for more pitch then you'll put more load on the engine, probably resulting in more fuel burn, slower to get up on the plane and possibly even lower top end.
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Old 10 January 2012, 15:05   #3
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Are you getting 5400 rpm with just you onboard as well?
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Old 10 January 2012, 15:15   #4
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Don't know it was getting late and I was heading into a very strong head wind. I haven't been out on my own since.

The Avon website said it should do 37 knots with a 70?
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Old 10 January 2012, 15:20   #5
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My prop is a 13 x 19 if that helps.
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Old 10 January 2012, 16:15   #6
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K series?
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Old 10 January 2012, 16:16   #7
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Yep, that's the one.
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Old 10 January 2012, 16:18   #8
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Originally Posted by whisper View Post
Yep, that's the one.
Got a 21" here if you want to try it out.
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Old 10 January 2012, 16:20   #9
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Quote:
Originally Posted by martini View Post
Sounds good to me where it is. If you go for more pitch then you'll put more load on the engine, probably resulting in more fuel burn, slower to get up on the plane and possibly even lower top end.
What Martini said...
Top end might get a little faster.
Hole shot is going to suck.

I am running a 60hp but empty I am aiming for the near or just over the top rpm so that fully laden I am no lower than 5000rpm. but I put 1500lbs of people and scuba gear in the boat.

If you change to 5000 rpm with 2 adults you'll be overpropped with 4.
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