Long shaft or extra long shaft

[MortenH]

Hello

I have a 115 Merc, long shaft, mounted on a 20 fot, deep V hull, boat. The boat was originally designed for I/O engine, but have been rebuild for outboard engine, and the engine is mounted at a steel bracket. The distance from the transom to the end of the bracket where the engine sits is about 20 - 25 inches. The cavitation plate of the engine is about 2 to 3 inches above the bottom of the hull. Is this the correct position? In the water, I have a cavitation problem when I turn over, but straight forward, she is running like a rabbit. When I am cruising in about 40 mph, I can see the whole top of the cavitation plate.

-M-

 

[deejaycee_2000]

Re: Long shaft or extra long shaft

the cavitation plate should be in line with the bottom of the boat ... not 2 to 3 inches above ... you will have to lower your motor to solve your cavitation problem .....

 

[Silvertip]

Re: Long shaft or extra long shaft

Two things to consider. First, I would install a water pressure gauge to make sure the engine is not so high that it's not getting enough cooling water. Second, I would not rush into lowering the engine as it currently appears to perform well in its current position. If I read correctly, its sitting on a jack plate of sorts with a fair amount of set back. That means the engine can be mounted higher than it would if sitting directly on the transom. Yes, the ventilation in turns is due in part to the engine height, but it might also be due to a prop that can't maintain its bite. If you don't want to experiment with props, then lowering the engine would be the solution. It appears you have a setup that is optimum. You might just try throttling back slightly when making sharp turns.

 

[tashasdaddy]

Re: Long shaft or extra long shaft

with my boat i can make wide sweeping turn, but anything sharp i have to throttle back. how far back is the engine setting from the hull. the rule of thumb is, for every 12 inches of set back you raise the engine 1/2 inch. i like to start at 1 inch about the bottom of the hull, mounting on the transom and go from there.

 

[Scaaty]

Re: Long shaft or extra long shaft

Long as your get water pressure, go have fun. I had a Merc XS150 short shaft that ran with the prop half out the water, but those motors were made to run like that...had a low water pickup in the gearbox.

 

[MortenH]

Re: Long shaft or extra long shaft

Thanks for your replies. I will do some measurements and see what I find. I have access to a kit to make the engine an extra long shaft, so it is a possibility. Only drawback is that I will have to open the LU again.. Maybe it ia better to just run as is. She is running a little "nose down", but this boat is known to do so.

-M-