Transom/Stern.

[Lc99g]

Can someone kindly tell me what height my outboard motor cavitation plate should be in relation to the stern of this boat ? It’s a 15ft long planing hull , I’m just not sure if it should be slightly below the V in the hull or in between the flat hull and the V. Any input very welcome thanks. Stan.

 

[jfm]

That's a very difficult question because that hull shape is so unusual. Anyone answering is guessing, including me.

Trial/error is the only way to find out.

I would say never below the V. Always between the flat and the bottom of the V, but exactly where depends on speed.

If boat is slow (say 40hp or less engine) then I would put the cav plate 2 inches below the flat of the hull.

If the boat is fast (say 70hp upwards) then I would put the cav plate 1 inch above the deepest part of that V.

Hopefully you can adjust it and therefore apply trial/error. Good luck!

 

[Lc99g]

Thank you Jfm, that’s exactly the way I am thinking, I’ve never come across a step like this at the stern it’s almost as if someone had added on length at the stern but that doesn’t show on the hull ! Engine wise I’m going to fit a 50hp.

 

[Hot Property]

From experience I put my outboards around 40 mm above the lowest point of the hull.

As jfm suggested, trial and error on this one....

If you do have ventilation problems because it's too high, a 4 blade prop should help no end and it's a lot easier than lifting or lowering an engine.

 

[simonfraser]

don't all outboards come with height adjustment holes ?

 

[Hot Property]

don't all outboards come with height adjustment holes ?

View attachment 192576

don't all outboards come with height adjustment holes ?

View attachment 192576

Anything above 30hp will have adjustment holes but it's a fiddle to adjust and the holes need sealing with sikaflex to prevent water ingress, thus rotting the transom over time if not done correctly....

 

[Rappey]

I have a just under 16ft planning hull and the manufacturer states 28knts with a 50. If you get any less then that could suggest the engine would need raising.
I had a15ft powered cat and had a similar problem of what height to mount a 90hp.
My chosen height have me 30knts but each time i went through a wave i could feel drag. Raised engine 5" and hit 45knts. I was amazed at how much extra drag an outboard causes when too low.
Are there not any previous mounting holes ?

 

[Lc99g]

Many thanks to all for the input.