cav plate height /effect on trim

[aussiemik]

Hi I just changed leg & box on motor now cav plate sits just above water level when on plane but trim does not have any effect on ride or very little the old leg was a little longer and trim had more effect is the cav plate supposed to be under the water?? if so how much ?? thanks Mick

 

[MikDee]

Re: cav plate height /effect on trim

Theoretically the cavitation plate should be flush, & paralell, with the bottom of the boat as a starting point. If you find it works for you a bit higher, that is even better, less lower unit (drag) in the water, unless you have it so high, there's a problem with picking up enough water for your water pump, then lower it down just enough to remedy that. Tilting the motor up some while under way, raises the bow, and takes more boat out of the water, makes it faster, just before it cavitates (because this will cause prop slippage, and it's not good) lowering the tilt to much while underway, will cause the bow to drop, excess will causing plowing, (also not good).

 

[aussiemik]

Re: cav plate height /effect on trim

thanks mate from reading other posts I thought that I had the right height set for the motor but the trim does not get the boat to ride on the back as much as when the longer leg was on, when I trim up the motor only goes to a point where the pressure must stop the motor from going up any more because it wont trim up to a point where the motor cavitates or ventilates?not sure what the difference is but it wont raise the prop out of the water while the boat is moving fast,could prop size make any difference?? tilt & trim seem to be working fine when not moving. certainly wont trim up enough to porpoise. and I cant lower motor any more without cutting transom down that would be a very last resort .any thoughts would be greatly appreciated thanks Mick

 

[MikDee]

Re: cav plate height /effect on trim

Not knowing what motor or year it is, I am guessing that with the motor in forward there is a limit to how much you can go up on trim, usually not enough to start cavitating, but you might get some cavitation when turning at this setting, maybe it's a safety issue? At least that's what I've noticed on my son's boat an 89'Bayliner with a 125hp Force on it. By the way, you don't want your prop out of the water, unless it's designed for surface drive (but that's whole other issue)

 

[aussiemik]

Re: cav plate height /effect on trim

your right I dont want my prop out of the water i was just using as example probably not a good one the motor is a 55hp but it wont trim up and ride as far back as when the longer leg was on which was not to far that it would ventilate or be a saftey issue just enough to ride better Thanks Mick