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Prop cavitation 200 Black Max

G

gmflex_78

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"I have a 1985 23 Ft. Sea Ox w

"I have a 1985 23 Ft. Sea Ox walk around cuddy with a 200 Black max (86 or 87? no serial sticker and rebuilt power head) mounted on a Gil bracket. At about 3000 rpm my prop cavitates and I loose my bite on the water, with plenty of throttle left to go. The prop is a 3 blade 14" and I believe it is 17 pitch. The prop was repaired professionally (one bade tip got whacked). The problem is not the hub, that's the first trial and error mistake I paid for. Any suggestions on what to try next. Same size and pitch different style/mfgr? 4 blades? different pitch? No idea what it will tach out at, can't get there."
 
"Hi there,

Is transom heigh

"Hi there,

Is transom height correct? Trim too high?

Brain still frozen, can't think of anything else right now.

Regards,

Benjamin."
 
"Thanks for responding, I don&

"Thanks for responding, I don't believe so. The Gil bracket was installed correctly to their recomended spec's. To me it seemed a little high even with the 1" per foot back rule of thumb. Any way, I removed the bracket and lowered it 3" which gave me the ability to experiment. The engine was all the way down on the bracket and still is. My vent plate is now riding below the surface for sure. I can plane off but shortly after I loose the bite on the water and rev up.

Greg"
 
"Hi,

When the prop loses bi

"Hi,

When the prop loses bite and motor rev's up, what does RPM shoot to? 3k?

Are you running hydro foils?

Remember, the anti-cav plate should be about half inch below the bottom of the hull when motor is trimmed 90 degrees to base of hull. IE: cav plate parallel to spine of the hull.

I'd still check the fuel supply for pressure, pump could be suspect...

How's the compression? Is the timing advancing all the way? Could rev great on the muffs, but would bog down under load.

New prop could solve the problem, but try that as a last resort, loan one from a friend maybe?

How's weight distribution of the boat? Try shifting the weight around when taking off.

Lastly, some hulls have been designed to operate with a particular brand of motor, is your's the correct match? I ask because you mention that you're using a GIL plate... These are not installed as a standard feature in new boats.

Keep an eye on the fuel supply, double check the height of motor.

We'll take it from there when you've checked all these options.

Good luck!

Regards from a chilly Johannesburg in South Africa!

Benjamin."
 
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