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  #1   IP: 142.167.231.186
Old 09-12-2014, 08:55 AM
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Question New prop time

OK, this isn't directly A4 related, but I hope you'll bear with me. My A4 died this spring, so I re-powered with a Perkins M30. Problem is, the A4 was a direct drive while the Perkins has a 2:1 reduction drive. My prop is a 2 blade 12x8 (definitely 12" diameter, pretty sure its 8" pitch). So now I'm motoring at 2 1/2 kts at 2000 rpm. Question is: Can the old prop be re-pitched, or do I need a new prop, and if so, what specs? I'm guessing its new prop, and I'm leaning to 3 blades for a little more push in heavy weather ('new' transmission specifies sailing in neutral, so no big drag penalty with 3 blades). Any suggestions, advice?

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Last edited by fundysailor; 09-12-2014 at 09:04 AM.
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  #2   IP: 72.45.14.161
Old 09-12-2014, 10:47 AM
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I can't imagine you could repitch enough. You may also find you will need 3 blades unless you have room for a 14 or 15 inch prop. Long ago I looked into getting a diesel and did not because of this issue.

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Originally Posted by fundysailor View Post
OK, this isn't directly A4 related, but I hope you'll bear with me. My A4 died this spring, so I re-powered with a Perkins M30. Problem is, the A4 was a direct drive while the Perkins has a 2:1 reduction drive. My prop is a 2 blade 12x8 (definitely 12" diameter, pretty sure its 8" pitch). So now I'm motoring at 2 1/2 kts at 2000 rpm. Question is: Can the old prop be re-pitched, or do I need a new prop, and if so, what specs? I'm guessing its new prop, and I'm leaning to 3 blades for a little more push in heavy weather ('new' transmission specifies sailing in neutral, so no big drag penalty with 3 blades). Any suggestions, advice?

Thanks
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  #3   IP: 67.244.62.45
Old 09-12-2014, 10:56 AM
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Quote:
Originally Posted by joe_db View Post
I can't imagine you could repitch enough. You may also find you will need 3 blades unless you have room for a 14 or 15 inch prop. Long ago I looked into getting a diesel and did not because of this issue.
Prop selection is a little art, a little science - I know neither.

That said, for reference, I quoted a Beta 16 for my 29 foot 8000 pound boat - it also has reduction gearing - and it was recommended to go with a 12" x 9" pitch 3 blade, 13" if I had room in my keel/rudder opening.

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Last edited by BunnyPlanet169; 09-12-2014 at 11:11 AM. Reason: ps
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  #4   IP: 206.125.176.3
Old 09-12-2014, 12:51 PM
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fundy, welcome back, even if you have an oil burner in your boat now.

I'll second what joe says. My understanding is you can re-pitch a prop a couple inches, but probably not enough to go from direct drive to 2:1.

I'd talk to a prop shop..they know these things.
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  #5   IP: 71.118.13.238
Old 09-12-2014, 12:57 PM
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Get that diesel up to 4,000 RPM's and the performance will be about the same as the A-4.
Seriously no way to tweak the prop enough. A good prop shop or the nice folks at Perkins should get you started in about the right place.
Remember with the diesel getting to the full throttle spot is far more important than with a gas engine, so I'd recommend looking to sources with a similar application RE HP~weight~waterline.

Dave Neptune
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  #6   IP: 76.11.110.224
Old 09-12-2014, 03:23 PM
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Oh yeah, you need a prop. Problem is that clearance and angle of strut will change as well. So you are going to be looking at a strut and prop plus....plus likely a heavier shaft and having to line it all up. Generally, changing out from gas to diesel is a matter of a day...it's the " accessory " changes that really put a bite in things.
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