Proper Shaft length for 10' Tunnel Hull

[toyz4roy]

http://forums.screamandfly.com/Gallery/showphoto.php/photo/9426/cat/500/ppuser/22287/sl/t
Would like to know how to measure for the proper motor shaft length on a 10' Cougar Cub type boat (tunnel hull). Should the cavitation plate be level with the bottom of the transom or the bottom of the sponsons? The motor seems to cavitate as its going up on plane, a hydrofoil helped get it on plane quicker but it still seems to cavitate abit. There are some pics of the boat in gallery.

[David]

Better get a water pressure gage. I'd try the propshaft just below the bottom of the sponsons. But that assumes enough water pressure, a surfacing prop, and a light enough load that the prop doesn't need to be low to carry the bow.

Maybe someone who has run one of these can offer better advice.

[evilntwisted]

I have a 13 funcat and my prop shaft is a half inch above the bottom of my sponsons and i am running a 24p chopper.