Cavitation plate hieght???

[tinderbitzin]

I recently bought & rebuilt an 11' Bobcat Mini Tunnel. I've tried twice now to take the boat out & I can't get it to plane. The boat has a Tohatsu 40 with a 3 blade cleaver on it. It's got an adjustable height set back plate on it that I remounted in the same position it was when I bought it. The plate happened to be adjusted all the up the first time I took it out, so I thought simple enough, I'll adjust it all the way down. It improved, but still won't get on plane. I guess the set back plate could have been mounted too high, but I looked at some "before" photos I took, & the plate appears to be mounted in the same location. I re-glassed the transom, but the first thing I did after the glass dried was re-drill the old holes out as they were seen through the glass. Presently, the cavitation plate is sitting 3.25" above the top of the tunnel. The boat is gorgeous, so I don't want "swiss cheese" testing different motor heights, so anyone's help would be wonderful.

Thanks, Tony

[swheeler]

Are you running the same motor now as before the rebuild? The reason I ask is I've had people come into the shop with a used boat they just got and can't plane. I look at it and discover that they have a short shaft engine on a long shaft boat. I'm not familiar with you boat but for a pleasure engine with a standard gear case, you engine seems to be high.

[david bryan]

check the kick out

[WildBill5-A]

I built an custom 11 foot tunnel hull and it uses a stock lower unit on the engine. Seems to do best with the cavitation plate about even with the top of the tunnel. The mini GT tunnels which also use stock lower units, run with the cavitation plate even or slightly above the top of the tunnel. If you are running a stock lower unit 3.5" above the top of the tunnel, it sounds too high. You also have lotsa power and may cavitate too much if you just jam WOT instead of easing it up on plane.

[Fast Jack]

Ventalation --- not cavitation

The plate above the prop is to help prevent ventalation of the prop not cavitation. Ventalation is the introduction of surface air into the water around the propeller. Your set up sounds like it is very high. Do you have power trim? Tucking the motor can help get it up on plane. Cavitation is the lowering of water pressure on the surface of the prop blades, this can cause the surface of the prop to erode -- the water boils off the blade of the prop. Usual causes of cavitation are surface damage to the blade (s) of the prop or something built up on a prop blade. Did you change or repair the botom of the hull? or add weight to the hull? I would lower the motor and trim under . Watch the rpm and try to slowly get it on plane.

[tinderbitzin]

Motor height

Thanks for all the ideas. It's the same motor, lower end, set back plate & hull. The only weight added to the hull was some glass on the deck & at the transom. Can't imagine it was more than 10 lbs. I did try easing into the throttle, with the same results. No power trim, but the motor is positioned in it's lowest setting. The prop looks like new, no burns on it from cavitating. So I must have mounted the plate too high, so that's gonna be my next step.

Thanks again.

Tony

[ram95]

Are you sure the prop isn't spinning the hub? Got another prop? Try it?