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Old 23 March 2012, 07:11   #1
Country: UK - England
Town: Surrey
Make: Porters Tohatsu
Length: 5m +
Engine: Suzuki DF80
Join Date: Oct 2009
Posts: 18
Porters/Tohatsu Transom mod

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.

V129 is offline   Reply With Quote
Old 23 March 2012, 08:37   #2
Country: UK - Wales
Town: West Wales
Make: Vipermax 5.8, SR4.7
Length: 5m +
Engine: 150 Opti, F50EFi
Join Date: Sep 2005
Posts: 6,298
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.

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Old 23 March 2012, 10:48   #3
Country: USA
Town: Oakland CA
Length: 3m +
Join Date: Feb 2005
Posts: 6,618
Originally Posted by V129 View Post
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.

jyasaki is offline   Reply With Quote
Old 23 March 2012, 11:26   #4
Country: UK - England
Town: Poole
Boat name: Exodus
Make: Tohatsu
Length: 7m +
Engine: Evinrude 150
Join Date: May 2007
Posts: 274
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.
Only two things are infinite, the universe and human stupidity, and I'm not sure about the former --- Albert Einstein
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