Atomic to Universal repower - SailNet Community
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post #1 of 7 Old 11-15-2009 Thread Starter
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Atomic to Universal repower

I have a 1970 Cal 29 which recently had the Atomic swapped for a universal m-18. The prop remained, and there is my question. Does anyone know if I should re-prop, and if I should what should I change to? Where can I learn more about the necessary pitch/diameter?
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post #2 of 7 Old 11-15-2009
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You should be able to run at the maximum rpm for the engine you have., but not over that rpm with a clean hull and prop. And you should be able to attain hull speed or close to it. With your waterline of just over 24', your hull speed is about 6.5 knots. If you go to West by North Enterprises, they have a lot of good information on choosing a prop and if you fill out the form on their site they will email back their recommendation.
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post #3 of 7 Old 11-15-2009
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The A4 was direct drive and the diesel most likely has a gear reduction of ??? 2:1 ??? so the prop will move the boat BUT be to small

Its time to talk to some prop makers as its a bit of a guessing game and may take a few times to get right

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post #4 of 7 Old 11-15-2009
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What is the ratio of your transmission on the diesel? As long as the propeller speed at governor of both setups is the same, it will be close assuming it was setup right in the first place. The test that mitiempo proposed is also a good way to get a rough idea.

There is no such thing as perfect prop pitch without a controllable pitch propeller because you would ideally change pitch based on sea and wind conditions. As long as you can hit governor and you are doing hull speed, you are in the right ball park although it would be nice to do better than that.
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post #5 of 7 Old 11-15-2009
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Years ago we swapped my brother's Atomic 2 for a Volvo diesel - propshaft speed was 1/2 so a very different prop was required. I'd expect the same in your case.


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post #6 of 7 Old 11-15-2009
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You have to re-prop or you'll do in your new engine because it's running at half the rpm and won't be able to get up to nearly full load with the A-4 prop which went at twice the rpm. Diesels need to be able to run at power or they have problems. You A-4 prop was probably the stock 12" dia. by 6" pitch.

Since the Universal diesel's a common replacement ask other Cal 29 owners and see what they're using and how it works. If you're looking at a fixed prop go to the Michigan Wheel website and send in a request for a prop calculation.

Michigan Wheel asks for rated horse power (not shaft horsepower which is about 90-95% rated BHP). So I think that their calcs will be on the conservative side. If you're reeeely into it you can do the calcs using Dave Spurr's Propeller Handbook as a check on Michigan Wheel

I'm repowering my Cal 29 with a Beta 16 the data I used for prop calcs:
Displacement 9500# (1500" greater that the base boat to account for gear)
LOA 29'
LWL 24'
Speed at power 6 kts
Shaft Horse Power: 15
Beam at the waterline: 8'
Prop depth 2'
Max diameter 13”
I've asked around and other Cal 29s have put in 13" props successfully. The issue is that the slower moving shaft wants more blade area (bigger diameter)to keep from cavitating at power.

The decision you'll have to make is whether you want two blades or three.
Michigan Wheel came up with a 13R9 for a 2 blade and a 13R8 for a 3 blade using thier Sailer model props

My check with Spurr's methods lean towards the 3 blade at a 9 in pitch and that the 2 blade would be close to cavitation. But I don't know how it will work yet as I'm still putting in the engine. Once you get the prop in if it's off you can take it to a prop shop and that can correct the pitch to be right on.

All this applies to a fixed prop. For feathering props check with the manufacturer. You can adjust the pitch on these props yourself.
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Thanks to everyone for the time and brain power!
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