4.3L stern drive + hydrofoil with heavy cavitation?

Question

I have a 4.3L yamaha stern drive. The Cav plate has a stingray brand hydrofoil on it. The boat has plenty of power and when you slam the throttle it gets out of the hole very fast, but as soon as it planes the prop begins to cavitate and I lose traction in the water. I can play with the trim, but cannot get it to accept more than 3500 rpm without cavitating. Cruising rpm is around 4000 and I know I am losing fuel economy as well as speed.

The outdrive and steering has a considerable amount of play in it (its 16 years old)....could this be part of the problem? The aluminum prop is not in the best shape, but its just old, has had no major damage or repair.

I also believe the hydrofoil may have something to do with it. I have read alot about them online, and believe it is helping me get to a plane more quickly, but believe it may be the culprit.

Answer 1

hydrofoils are for underpowered boats.

is your prop cavitating, or is it ventilating?

Cavitation is what happens when something interrupts the flow of water over the blades.  You know how water under pressure boils at lower temperatures?  A cavitating prop is actually boiling the water, sort of pulling the air out of it, so to speak.  Cavitation leaves black marks (kind of like it's been burned), or even pitting on a prop.  It can be caused by a foreign object caught on the lower unit, or dents or dings in the prop.  

Ventilation is when air gets introduced into the blades, so they're pushing air instead of water.  Two sources of ventilation:  surface air and exhaust.  Prop too close to the surface will draw air.  Defective or missing diffuser ring, or foreign object caught in the ring, allows exhaust air from the prop hub to bleed back on to the blades.

Not all props have diffuser rings, those that don't, are flared and the aft end of the hub, accomplishes the same purpose.

No matter how old it is, it shouldn't have play in the steering.  Like everything else on a boat, the cables need to be tightened every once in awhile - boats operate in a harsh environment with lots of shock and vibration.

If the prop isn't in the best shape, take it to a prop shop, it's not expensive to have it worked over.  They can add cup to the blades if you want 'em to, but you should really resolve the issue before looking at a prop change.

Take the hydrofoil off, get your cavitation/ventilation problem solved, then put it back on (or decide not to).

Answer 2

Get a new prop with more cup> That's your problem>

Answer 3

Are you sure that your prop hub isn't spun?  Take those wings off,  I don't recomend them on sterndrives, except in the case of a super deep V hull, and only to increase stability.