Ultra long Tohatsu outboard on sailboat: ideal length of shaft under water?

[Hugo B]

Hi

I have been googling this far and wide but I do not seem to get a definite answer. Perhaps someone here can enlighten me.

I am mounting a 6HP Tohatsu 25 inch ultra long shaft outboard on my 27 ft yacht. Yesterday I spent a lot of time on the boat trying to figure out where the bracket should go, particularly the bracket height.

It is difficult to measure, but where the bracket ideally sits, in the downward position, the shaft (all the way to the bottom of the prop) will be about 350 mm or almost 14 inches under water. To my best calculation, if I lift the bracket and tilt the engine, at that position the outboard can still be lifted clear out of the water (but only just).

The length of the shaft below the cavitation plate to the prop is 210 mm (about 8 inches), so the cavitation plate itself will sit about 140 mm or 5.5 inches below the waterline.

Is that depth sufficient to stop cavitation in open water when there is quite a bit of wave action and swell?

Thanks

Hugo

[Munoz]

Generally you want to get it as low as possible without drowning the power head when there is a bit of swell.cIt depends a bit on the individual boat (transom vs counter stern etc.) 5 inches might be adequate, but I wouldn't want anything less.

I would give it ago and see how it goes. In the end it becomes a trade-off between theprop sucking air vs drowning the power head. Having water momentarily reaching say 1/2 way up the power head every now and then is OK, but if it spends too much time under water that's going to end badly......

Having some sail up to reduce the rolling also helps a lot...