Long shaft /short shaft? & Tohatsu 9.9 question

[Peter Lawrence]

It seems the standard for most small outboards is 15'' Short and 20" for long shaft. my question is the measurement is taken from the underside of the engine mounting bracket from the top of the transom down to ...? the water level? Also, the Tohatsu 9.9 2 stroke conversion to 15hp I see discussed in earlier threads.... Is long term reliability affected? Is there a prop upgrade needed? one assumes it is just higher revs to achieve the hp gain? I have seen different weights mentioned for this motor? one @ 27kg, another @ 41kg?

 

[Sea Rider]

Re: Long shaft /short shaft? & Tohatsu 9.9 question

Long term reability is not affected as both engines has same CC, one is carb restricted, the other no, weight is about 41 Kg for both. According to owner's manual both share same 8.5 prop which is factory standard on 9.9 & 15 D2 models.

Happy Boating

 

[pvanv]

Re: Long shaft /short shaft? & Tohatsu 9.9 question

Luis is correct... Depending on model (you didn't mention that -- very important), the 2-strokes can be converted from 9.9 to 15 with a timing cam swap. No durability or reliability change as a result, since the 9.9 is a de-tuned 15.

4-strokes are a different story. Again depending on model, a carb swap could possibly convert you from 9.9 to 15.

Shaft length is nominal, 15-20-25", and the boat is measured vertically from the top of the transom (where the motor rests) to the bottom of the transom (that's below the waterline). Modern Tohatsu and Nissan motors are physically a little longer than the nominal length, so a short shaft is OK for up to about 17" transom height; Long shaft for 17-22", and UL for 22-27" transoms. Rule of thumb (for "normal" boat designs) is that the ventilation plate on the LU should be within an inch of the bottom of the transom. Too long and the boat will be slow. Too short, and the prop will ventilate.

 

[Sea Rider]

Re: Long shaft /short shaft? & Tohatsu 9.9 question

Paul, now that you mentioned, have had some splashes issues at back transoms with Tohatsu's and no problem with Yamaha's, definitely must be the little longer issue you stated. That's why are using 40 cm transom heights to compensate. So no need to change carbs, just a carb timming cam swap, right ?

Happy Boating

 

[pvanv]

Re: Long shaft /short shaft? & Tohatsu 9.9 question

Right Luis... The 9.9/15 conversion is just the timing cam.