Outboard Motor Mounting Height

What is the correct height for the anti cavition plate on a small RIB? With the plate in-line with the bottom of the V-hull the prop comes partially out of the water during tight turns!

[Tom Lathrop]

First we need to know just what the problem is. I suspect that you mean that the prop ventilates in turns not that it actually comes out of the water. This can be caused by having the plate too high but can also be caused by the hull bottom configuration. If there is a centerline keel or a deep V ahead of the prop, it can also cause ventilation in turns. The stern of the boat goes sideways in turns and the keel draws in air from aft. the prop is operating in this slipstream and gets the air.

The soution is to remove part of the keel to about 14" or so, forward of the transom or to lower the engine.

What is the correct mounting height for an outboard motor? I have an old 14' Crestliner fibreglas boat (circa 1960?) with a new 60HP Yamaha 4-stroke, and when I 'raise' the motor using the electric trim to improve the high-speed performance and smooth the ride on choppy water, the prop sometimes ventilates on turns.

When I took the boat out of the water this month and winterized it, I also lowered the motor one more notch (3/4") to the last bolt-hole. That is as far as I can go without modifying the transom, but the anti-ventilation/anti-cavitation plate now appears to be even with the bottom of the transom (hull).

I won't be able to try it until next May, but I wonder if I have it right, and whether or not the 3/4" lowering is likely to make a difference.

Any assistance would be appreciated. Thank you.

[gonzo]

The cavitation plate should be even with the keel. However, hard turns will ventilate your prop. Cupping the blades helps to keep some "bite" .

[sukibe]

I found that adding a "whale's tale" to the cavitation plate on my Tohatsu 55 that the boat got out of the hole much quicker, stayed on plane at lower speed and did not ventilate on tight turns. Seemed like an easy inespensive solution.