12×15 OJ 3 blade swap?
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12×15 OJ 3 blade swap?

  1. #1
    Junior Member SOMANDELLA75's Avatar
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    Default 12×15 OJ 3 blade swap?

    I was looking for a little more top speed. Currently running an OJ bronze 3 blade 12×15. 9% gears in a 12° casale top loader. Direct drive with a 468 BBC max rpm around 5000-5200. Boats a '75 mandella 20.5' daycruiser. Currently tops out at 63mph on gps. Any input would be greatly appreciated.

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    Senior Member sanger448's Avatar
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    I made a calculator that is slightly easier to use for V-drives. It's on the Hot Boat Database.

    To answer your question more directly, based on the numbers you have provided, you would have a theoretical speed of 80.5 mph at 5200rpm. If you're actually going 63mph that is 22% slip. Based on what I've heard on the forums, for a cruiser/v-bottom type hull 20% is to be expected (flats might make it to 10% slip). So you're not doing too bad there.

    The question then would be is the engine out of power at 5200? If you put more gear or prop in the boat, you will raise the theoretical top speed at that RPM. Depending on how much power you have, it may go the same speed or even slower (unless you are just hitting your rev limiter at 5200 and the engine has more to give, in which case you will go faster). Most likely, your max engine speed will drop. I can't honestly make a prediction about what will happen, but I know that too much prop/gear and not enough power will make your slip % go way up, and that is just bad, your setup is all off and everything will suffer. At 22% slip I doubt there is much you can do in the way of props or gears to go faster without adding more engine to the equation.
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    Senior Member flatwater's Avatar
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    I'll be the guy to say you shouldn't spin a bronze prop over 5k.... lots of info on that if you search it. You are going to want to get a nice SS prop. The bronze one may throw an ear off, resulting in bad things. Bronze is weak.

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    Junior Member SOMANDELLA75's Avatar
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    Quote Originally Posted by flatwater View Post
    I'll be the guy to say you shouldn't spin a bronze prop over 5k.... lots of info on that if you search it. You are going to want to get a nice SS prop. The bronze one may throw an ear off, resulting in bad things. Bronze is weak.


    I was worried about bronze durability at higher rpm. Also what's the pros and cons of a 3 blade vs. 2 blade. Currently finishing the new motor and should have me in the 550+hp range and a redline of 5500. I don't pull skiers just sandbar hop and bash on jets. I'm new to v drives so the more input the better so I don't costly move in the wrong direction. Thanks.

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