Proper relationship of IO cavitation plate to bottom of hull?

[Boomyal]

I have a 20' Gulfstream runabout with a 1978 OMC outdrive driven by a OMC 302. I am running a 15 x 17 cupped Michigan prop and I am experiencing a signifigant loss of hook-up of the prop to the water. With no load in the boat, at full throttle acceleration, the prop breaks loose at 2400 rpm, before the boat has achieved plane. I have to pull the throttle way back to hook back up again. Also once up on plane at a constant 2200 rpm, I nail the throttle and the prop breaks loose at 2700 rpm and I have to back way off again to re-ingage.<br /><br />I have no obstructions in front of the prop but the cavitation plate on the outdrive is about 1 1/4 inches above the bottom of the hull. I suspect this to be a problem.

 

[Bondo]

Re: Proper relationship of IO cavitation plate to bottom of hull?

I think you Might have a problem with a Spun Prop Hub, or Engine Coupler.....<br />Your X-Diamention is Fine....

 

[Boomyal]

Re: Proper relationship of IO cavitation plate to bottom of hull?

Thanks Bondo! I've sent the prop to the shop to be inspected. I couldn't find anything in the manual regarding the "coupler" and how it could slip.<br /><br />Would still like an answer RE: the IDEAL relationship of I/O cavitation plate to keel. My drive has no trim capability so I recently added an additional shim under my rear motor mounts to raise the bow in an attempt to deal with a porpoising problem in CALM WATER, with full passenger load, at 30 ish plus MPH. (gets kinda scary). I only noted the cavitation plate/keel dimension when trying to solve the prop issue. I'm sure that the plate was higher than the keel b4 adding the shim but I undoubtedly increased the dimension. <br /><br />I could pull equal shims out of front and back mounts if necessary to lower the whole drive assy. but I would have to buy a gear reduction starter as lowering the motor would set the stock starter down on the stringer.

 

[Bondo]

Re: Proper relationship of IO cavitation plate to bottom of hull?

RE: the IDEAL relationship of I/O cavitation plate to keel. <br /><br />Every hull is different.... A Good Base line is Even with the hull....<br /><br /> manual regarding the "coupler" and how it could slip. <br /><br />Coupler bolts to the back side of the flywheel... Outdrive splines slide into it.... It's bonded in rubber.....

 

[Floyd]

Re: Proper relationship of IO cavitation plate to bottom of hull?

Having the cav plate 1-1/4 inches above the keel might cause some ventilation. I got lost reading about your shimming.<br />If you have porposing, then you want to trim the outdrive in towards the transom to lower the bow.<br /><br />FWIW, after I put a Cobra hydrofoil plate on my I/O, I couldn't get it to porpoise at all, even by trimming up, and ventilation was reduced somewhat, though it wasn't a problem to begin with.