Motor height ?

[Saboat]

I have a Johnson 1972 40hp fitted on a 14 foot double hull RIB.Have read in other posts that the cavitation plate should sit level or just above the bottom of the hull.In my case does this mean I should take a line across the bottom of the 2 hulls as the reference ?

 

[MarkAB]

Re: Motor height ?

I have also read that sometimes a little bit of experimentation is required to get the maximum effeciency. I would start at the point you suggest, and lift a little higher if necessary. Please check that the water inlets are submerged sufficiently otherwise you may have overheating problems. If you can, link up a water pressure gauge to monitor pressure when experimenting

 

[Saboat]

Re: Motor height ?

Thanks.Now if it has to be raised (which I think it does cause it's a long shaft),how do I extend the height of the transom ?

 

[WillyBWright]

Re: Motor height ?

There are two transom heights. 15" and 20". I believe your motor is an older style that has to run with the antiventilation plate an inch below the hull line. If the propshaft is round with the prop retained by a pin, then it has to be below the hull line. If it had the newer style setup with a splined propshaft and exhaust out of the prop, then you'd want it about even. The older 40s had extensions to make them longshafts. With the right parts, that extension can be removed to make it a shortshaft. That would be your best option rather than raising the transom.