Possible Cavitation Issue?

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possom813

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Apr 25, 2011
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Corsicana Tx
I just picked up my new to me motor.

1970 Johnson 40hp :mrgreen:

I'm not sure, but I believe the little fins on the lower unit are supposed to be almost dead even with the bottom of the boat.

My transom is 21 inches, and the measurement from the hanging bracket to the fins is 18 inches.

Will 3 inches above the bottom of the boat have a great cavitation effect? Do I need to find a jackplate to lower the motor 3 inches?

Is it really going to matter during any times, other than full speed?

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cant you just move the pins in the mount to move the lower end back some to level correctly? Am i correct on this because it just seems like you have it all the way down and since your transom is angled you have to compensate for that.
 
Agree with the first comment, move your trim pin out. If you run like this your going to be pushing your bow down. At higher speeds, the prop may not get any water to it, or it will be dirty water killing your perfomance.

You may have to cut your transom down, or get a jack plate...sawzall blades are cheap, jack plates are not.

cutting will also give you a chance to inspect the transom material itself...may be worth the time to check.
 
I would set your trim properly then try it, watch the pee hole and see if it looses flow at full Th. If you are like most of us you do not spend much time at FT anyway, cant catch any fish then :mrgreen:
 
I moved the trim pin, it didn't dawn on me to do that until after the post.

As for the transom, it's brand new, I just finished up all the little details today and hung the motor on to see what it looked like.

I still have my new braces to put on and then to the welder to fix the two welds I had to break to get the transom in.

I moved it back a little bit and it started to look right, I think it's going to be alright.

Ya'll are right, at full throttle, I don't think the catfish can keep up :mrgreen:
 

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