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I was thinking the key was not installed right as others have mentioned, but that doesn't track with it working fine when rev'd and in neutral.

The damaged/melted impellor leads me to think it is sucking air.

I would try to run it on a conventional hull, i.e. aluminum riveted V-hull and see what happens :twocents:

What type of floor is in the Zodiac? Nice and rigid?

Also, what do the leading edges of the prop blades look like? Any signs of pitting?
 

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Discussion Starter #23
That last picture looks like it is telling you the problem. Look at the bubbles coming off the strakes on your hull. I believe your hull is pushing air right into your intake at speed - just plain bad luck that your hull design and motor depth work out that way. You need to get that intake into 'clean' water without any air cavitation off the hull.

Buy a Bay Kit to extend your motor down another 5 inches. Install (another) new water pump to clean up any previous damage. Problem will be solved.
Here is the video that the photos came from. I think my plaining fins are mounted to the cavitation plate and that appears to be well underwater. It is logical that the hull design somehow injects air just perfectly as to cause the intakes to be covered with air. That would be most unfortunate. That would make extending with a bay kit a proper solution although I always enjoyed having a shallow motor draft when beaching. I will work on it and let everyone know what I come up with. I will also continue to take suggestions.

https://youtu.be/E67lcV7DAbY
 

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You could try a jack plate instead of a bay kit. That would let you try various heights until you got it to cool without moving it down an entire 5 inches of a bay kit. Bit more complicated than a Bay Kit since you'd need to find a jack plate that would allow you to adjust downward (most are meant to lift the motor, not lower it) while still making sure you had clearance to make everything fit - but would be just the ticket to find that exact depth where cooling worked without going the full extra depth of a bay kit. Might be able to make your own out of aluminum angle bracket too.
 

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It sounds like you know your way around a water pump.

That plate people are seeing in the last picture is not the cavitation plate. Your cavitation plate is under water in all three pictures, and the video. The plate people are referring to is the one I have circled in red. It can, and should be be well above the water line on plane. The plate circled in yellow is the cavitation plate. It should be right at the waters surface on a skiff like this on plane. The black spot with the green arrow is the water intake obviously.



Your outboard looks like it's mounted correctly from the information provided. Your cavitation plate is definitely at the surface or under water while on plane. In the video you can see the forward edge of the cavitaiton plate spraying water up over itself, the water trailing off the plate circled in red is being thrown up to it from the cavitation plate. Dropping the lower unit 5" with a bay kit would be the same as running a long shaft outboard on a short shaft boat. The extra drag from the wrong size shaft kills performance and affects handling, it can be so bad that certain hulls will not plane with the wrong shaft length installed.

It also looks like you have some sort of Doel Fin or other hydrofoil mounted to the cavitation plate. Those also create a lot of drag and can suck performance out, although that's not what you are trying to solve here. What I'm wondering is if the hydrofoil or it's mounting bolts are causing some sort of situation where it's creating turbulence and allowing air to be drawn into your water pump intake. Do you know if the pump runs dry without that fin? If not, take it off and see what happens.

If you can take a picture of the boat out of the water, everyone will be able to see how the outboard is mounted more clearly. A picture of a straight edge from the bottom of the hull to the cavitation plate would be good. Maybe a picture of the bottom of the hull as well. Something is obviously off here allowing air into your water pump, but I don't think it's how the outboard is mounted. You also need a new gasket on your outboard flush port :)
 

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I would take the Doel fin off and look at the wake. Looks like much more cavitation/air in the wake than normal. Just before you shut down at 1.09 is what to me is normal off the back end.
 

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Maybe try offsetting the motor to port or starboard (like a kicker) to see if there is cleaner water available away from the keel? It doesn’t look like you have much room before the transom steps up, so if that doesn’t help, try making a hard turn (if you can do it safely at speed) to to the same side the motor is offset to... if the keel is causing cavitation, turning hard to the inside should send the bubbles to the outside and it might give you some cleaner water. Turning the other way should cause extra cavitation.

Good luck!
 

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That last picture looks like it is telling you the problem. Look at the bubbles coming off the strakes on your hull. I believe your hull is pushing air right into your intake at speed - just plain bad luck that your hull design and motor depth work out that way. You need to get that intake into 'clean' water without any air cavitation off the hull.

Buy a Bay Kit to extend your motor down another 5 inches. Install (another) new water pump to clean up any previous damage. Problem will be solved.



The picture I saw showed water above the cavitation plate (the lower larger longer plate just above the propeller is the cavitation plate). It's hard to see the position of the cavitation plate in relationship to the bottom of the transom in that picture but it is very logical that "something" is introducing air off your hull at high speed/on plane. Is there a transducer or perhaps a pitot tube mounted too close to the engine? Also check the bottom of the boat, just before/forward of the transom. Some boats have a "spoiler" lip that might be too severe. ???
 
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