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Old 23-12-2010, 06:49   #1
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Cal 29 Prop Size

Hello all,

I am desperately trying to find someone that can tell me what diameter prop I can install with out getting too close to the hull and vibrating. I need to replace mine and i really don't think it was big enough. i could only get 4.5 knot out of it at 2300 rpm. I could have run the rpms up more but i dont like running diesels up too much higher than that. The diver that cleaned my boat did say that my prop was shot(pitted badly and pink) but didnt say what size and or pitch it was. What do you guys think. do you think it is just my ratty prop thats the issue?

Troy
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Old 23-12-2010, 08:20   #2
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Simply buying a larger prop is not likely to solve any issues you may be having. Your engine/transmission/displacement will require a specific-sized prop to perform correctly. And this means correct diameter and pitch. It's possible that another Cal 29 owner will be able to give you the size you need, but only if your boat has the same engine and tranmsission gear ration. Your best best is to find out what these numbers are and talk to a prop shop about the correct prop for your boat. Or you can use one of the many online prop size calculators. Do a Google search, you'll find a bunch.
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Old 23-12-2010, 12:22   #3
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Aloha Troy,
fstbttms has given you some great advice. Your prop diameter and pitch depend on the "real" engine power (not rated) and the ratio of your transmission. Running at 2300 rpm with a rated 30hp engine at 3000 rpm means you are getting maybe 23 hp not 30 so calculate your prop to that.
If your bottom was clean and your prop was clean and the water was flat at the time of 4.5 knots at 2300 rpm then what you want to do is determine what your hull speed should be as determined by your waterline length at shoot for that. I would imagine 5 knots motoring at 2300 should be about right but I don't know your boat.
Try using the search engine after my signature line and type in "Proper Prop"
Kind regards,
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Old 23-12-2010, 12:27   #4
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Aloha again Troy,
Yes, it just might be your ratty prop.
If your library has a copy of Skene's Elements of Yacth Design there is a good simple to understand diagram to help you understand proper prop size in there.
kind regards,
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Old 26-12-2010, 23:48   #5
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i dont have that book but i might be able to find it. thanks for all the help guys.

Troy
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Old 27-12-2010, 11:46   #6
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I did scan the diagram into one of the threads that you'll find under "proper prop" in the search engine after my signature.
Good luck in your research. The long calculations are tricky but the shortened version requires your waterline length, your propeller speed calculated by the speed you want to run your engine and your transmission gear ratio. So those items are essential.
kind regards,
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Old 27-12-2010, 15:33   #7
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Another good rule of thumb is your prop should spin at the rated maximum rpms of your engine when the throttle is wide open. If you are unable to see max engine rpm's on the tach then the prop is over pitched / too large. If the max rpms is less than wide open it could have a high pitch or diameter. Diameter has to clear the opening for obvious reasons. These are simple tests you can do to see if the prop is close to proper. In any case a good prop shop will tell you the results for free. If the diameter of the prop is fine you can adjust the pitch by 3 inches up or down and save some money.

Propellers are a science and thinking it's too small is a common misconception. Best to prove it by the numbers as noted above. I had a prop that was too big. When I went smaller the boat got faster.
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Old 27-12-2010, 15:35   #8
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Quote:
Originally Posted by Pblais View Post
If the diameter of the prop is fine you can adjust the pitch by 3 inches up or down and save some money.
Well, you can if the prop is in good condition or hasn't been previously repitched. The OP indicated his prop is badly corroded, probably negating any possiblity of repitching it.
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Old 27-12-2010, 15:54   #9
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Probably so but just looking isn't always the best test. You can usually see a mark to tell if it has been re pitched before and I would agree it's a very bad idea to re pitch a re pitch.
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Old 18-02-2011, 21:59   #10
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Hello everyone. we got the prop off and it is a 3 blade 13X12. we were able to clean it up and although I do need another one, we put it back on. I did run it up to full RPM with was about 2800. I did get 6 kts out of it also. I just dont like running it up that high. it seems as though I could get away with a 13 X 13 and a little less RPMS. I hate running an engine full throttle for extended periods of time
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