Porters/Tohatsu Transom mod

V129 · 23 March 2012, 12:11

I have a tohatsu "one design" rib. The transom height at the boat centre line is 28 inches and it was originally fitted with tohatsu extra long shaft (26 inches) outboard. These dimensions place the anti-cavitation plate about 2 inches above the keel of the boat. I always thought that the cavition plate should be level or just below the bottom of the boat. Can anyone advise as I need to modify the transom for a standard shaft (20 inches) outboard.

Downhilldai · 23 March 2012, 13:37

Was a Twatsoo 50 fitted originally?

You will need to modify the transom for a longshaft motor. Transom needs to be cut down to 21" at the centreline, or you could fit a transom bracket with enough setback to allow a longshaft (20") motor to tilt up, but chopping the transom would be a better option, if you can't find a suitable XL motor.

jyasaki · 23 March 2012, 15:48

Quote:

Originally Posted by V129

I always thought that the cavition plate should be level or just below the bottom of the boat.

Depends on the boat. Generally a good starting place is anti-ventilation plate even with the lowest part of the hull, but quite often, the motor can be raised without causing problems (which puts less leg in the water, which decreases drag, which increases efficiency.)

A lot of it is trial and error; no "rule of thumb" that I know of to get a definitive placement. Well, other than start at even, that is.

jky

Into The Blue · 23 March 2012, 16:26

I've got the Tohatsu 7.1mtr and it has a very similar set up. The transom is around 28" and the ventilation plate on the 150 evinrude is around 2" above where the rule of thumb suggests.
It'll do 40 knts on a good day, and only starts to ventilate when I'm running at max revs with the leg trimmed out and even then it's very rare. Usually it ventilates a wee bit and speed drops off very slightly.

Can't say that unless I'm turning rather more aggressively than needed, just for fun, with the leg still trimmed out that I've ever noticed any issues.

Have you been out in the boat yet? I'd suggest giving it a thorough sea trial before changing anything.