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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?

Thanks

Last edited by fundysailor; 09-12-2014 at 09:04 AM.
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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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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.

(P.S sorry for harboring thoughts of the Dark Side)
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Last edited by BunnyPlanet169; 09-12-2014 at 11:11 AM. Reason: ps
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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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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.

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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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Old 09-12-2014, 04:42 PM
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get a few recommendations. michigan wheel can do it also.
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Old 09-12-2014, 07:25 PM
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Much appreciate the comments/suggestions! Its looking like a 3 blade will be the answer. Prop is in an aperture between keel and rudder, so not much room for a larger diameter, and as I suspected, re-pitching would be iffy for the existing prop. Just wanted to hear some confirmation of my suspicions from those who know! I'll consult with my local prop wizard over the winter and evaluate the options. Meanwhile, its sail fast, motor slow! By the way, I'm already missing my A4's performance - am reminded of a cruise a couple years back - several of us were approaching a narrow passage in thick fog (no wind) so motoring slowly. I had to keep shifting into neutral to stay back with the rest of the fleet! But I guess driving an 'oil burner' has its compensations, too. Hope so, anyway!

Gord
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Old 09-12-2014, 07:30 PM
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Yeh, talk to a prop wizard... I concur. Typically a 30 hp diesel will be swinging a 13 or 14 inch prop. All the best.

RE: you comment on speed. Was going through Sambro Ledges a few weeks back and two Diesel powered boats couldn't keep way on in the chop and headed in. One of them has a folding 2 blade and 30 hp....he couldn't hold 3 kts and was getting knocked by waves. Didn't really know the other guy and don't know what he had except that he was moored next to my friend once they got shelter in Sambro Hbr. I held 4 to 4.4 with 25kts on the nose ... a bit of chop with that...was slowed a few times but she kept her heading and kept way on.
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Last edited by Mo; 09-12-2014 at 07:35 PM.
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Old 09-12-2014, 08:56 PM
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I flew an airplane once with a similar issue. It was a surplus Air Force trainer configured for Colorado Springs, i.e hot and thin air. At sea level here in Maryland the big engine and fine pitch prop were like having a Corvette stuck in 2nd gear. It climbed at 100 knots and then sped up to 100 knots in level flight.
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Old 09-13-2014, 08:26 AM
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Why are you only running the Perkins at 2000 rpm? Is that your personal choice or is it a manufacturers recommendation?
Martec in 7/8'' shaft size makes 13X13, 12X12, and 12x10 that have been used by Atomics 4s. Probably with a reduction gear.
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Old 09-14-2014, 09:42 AM
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Quote:
Originally Posted by Sony2000 View Post
Why are you only running the Perkins at 2000 rpm? Is that your personal choice or is it a manufacturers recommendation?
No reason - that's just the speed/rpm's I noted while testing out the new installation. I used to cruise at 4-5 kts at fast idle with the A4, so was just contrasting the difference.
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