adjusting trim and stopping cavitation

[Andy in NY]

16' starcraft aluminum hull 1974 evinrude 50 hp 13 x 19 3 blade aluminum prop


last season someone had suggested trimming my motor out to the 3rd hole for better performance. currently on the 2nd hole i hit 26mph gps and plane in about 5 seconds, but doesnt seem like the hull is coming out of the water as much as it should. so yesterday i tried the 3rd hole. noticed a HUGE difference in how fast it accellerated and got on plane. one problem though, by the time i got to about 20mph it was cavitating bad. so back to the 2nd hole i went.

is there anything i can do to combat the cavitating?

 

[Andy in NY]

Re: adjusting trim and stopping cavitation

30 views and 24 hours and no one has any opinions???

 

[bhammer]

Re: adjusting trim and stopping cavitation

Andy, the only thing I can think to try is a higher rake prop?

 

[QC]

Re: adjusting trim and stopping cavitation

Or a four blade, or some cup. There can be big changes with any prop change. Switching to a stainless of the same design could even be right. You need to find a prop shop that will allow you try and swap. I see some sort of jack plate in your restore thread. Is that still there? If so, maybe go slightly down and try that. Any combination change can make a difference.

Oh, and FWIW, what you describe is actually Ventilation. Cavitation is actually fairly rare, and somewhere along the line it became common to call Ventilation, Cavitation, but they are significantly different issues. What you describe is the prop grabbing some air off the hull, or lower unit, or surface.

Edit: Oh and I moved this to the Prop section to see if we can get you some more action. Really is a prop issue although I can see the trim confusion.

 

[steelespike]

Re: adjusting trim and stopping cavitation

First what you have is ventilation.Cavitation is a differenrt animal.
Is your prop in excellent shape?Your speed should be closer to 30 unless you are heavily loaded.
Tell us where the antivent plate(just above the prop) is in relation to trhe bottom.
Most should be about an inch above the bottom.You can use a straight edge along side the keel to check. A prop like the Solas Amita 4 blade should solve your problem and may even allow more height.Thew higher the motor the less drag.A 19 may be too much prop.Do you have the max rpm and speed?

 

[Andy in NY]

Re: adjusting trim and stopping cavitation

my top speed is 26 loaded. dont know wot rpm, no tach. actually just got one today, installing it tomorrow.

i had asked the question about thr right size prop a while back and a 17p was suggested. i dont know if going that route is right, because i have a 85 hp motor that im working on getting going so i dont want to dump much in this 50 hp. i was just wondering if there was anything else (low cost) that i could do to combat the problem.

 

[steelespike]

Re: adjusting trim and stopping cavitation

Get your tach reading.Have you checked the motor height? Could net you 2 to 3 mph under ideal conditions.If you change the height its free. Is the bottom clean and nothing that might disturb the water ahead of the prop?Is the prop in excellent conditiopn?If you can't reach speed with proper trim its difficilt to be sure of a prop change.It does seem to respond well with the 19 but for the venting.
Motor could be too high or the prop unable to resist venting as it should.
Perhaps a person added at the stern would allow it to reach top speed for a test run.

 

[Andy in NY]

Re: adjusting trim and stopping cavitation

gonna get the tach hooked up today.

lowering the motor is out of the question... its as low as it goes.