Logo

Repower with a Tohatsu 20hp?

jmomc345

Member
I have a 14" aluminum Starcraft Seafarer that currently has a 9.9hp 2 stroke. I am thinking of putting a 20hp Tohatsu short shaft with power tilt on it. Anyone have experience with a similar combination? What is your top speed? Will it troll at low speeds ok? Does it vibrate an aluminum boat too much at trolling speeds? Thanks
 
The 4-stroke (especially with power tilt) will be a lot more weight. That aside, the MFS15/20 idles well. You may want more prop pitch for WOT runs than you will want for trolling. Those 2-cylinder 4-strokes run a lot better/smoother at 1,500 RPM than they do under 1,000. Any 4-stroke will probably require a pad between the transom and clamps to dampen vibration.

Another option, if you don't need a ton of WOT speed, would be to go with the 9.8, which is available in power tilt as well.
 
Paul, thanks for the info. Yes, the weight is considerably more. But I draped a 50 lb sack of corn on top of my 9.9 and the boat seemed to handle the extra weight fine. I bet people were wondering what kind of motor I had :). I maybe had one less inch of freeboard at the transom. The boat planed and handle fine.

I was considering the 9.8 but I have heard top speed would be slower than my 9.9 2 stroke. The 9.9 is 15.6 cu in and the 9.8 is 12.8 cu in. So I am concerned I would lose top end speed with the 9.8 and especially if I add another person. Plus my boat has a 15" transom and the Tohatsu 9.8hp only comes with power tilt on the 20" model. Ironically, the Tohatsu 20hp has a 15" model with power tilt (and the 15hp doesn't).

Any other thoughts you have would be appreciated.
 
Your present 9.9 is a de-tuned 15 (smaller carb, depending on manufacturer). That's why the bigger displacement vs. the new 9.8. The 9.8 would be comparable top speed, but likely slower hole shot, and slower to plane. IIRC, your boat is rated for something like 25 or 30 hp, so a 20 would be a good choice. BTW, the Tohatsu 15 is a de-tuned 20 (smaller carburetor). You are currently a little under-powered with your 9.9.

Yes, the 9.8 and 15 only come with PTT in L and UL (intended for difficult-to access sailboat locations, etc.) The 20 is used a lot for duck hunting, etc, and so comes with available PTT in S.
 
The boat is a 1967 and is rated for 40hp, believe it or not. Of course the HP ratings then aren't the same as today. The boat should handle the Tohatsu 20 but I am concerned about whether it will troll slow enough. I guess I should be thinking about the right prop. I don't need more top speed than I have now when I am alone, but I would like more speed with 1 or 2 more people aboard. And I would like it to troll slow and without a lot of vibration. So maybe I should err on the side of a smaller pitch prop (size?) and just not open it up all the way when I am by myself. Would I need something different than the stock prop?

The power tilt is not really a trim function. But according to the manual you can use it at slower speeds. I would like to navigate under power thru rocky shallows. Obviously I will be going slow, but at what speeds does 20hp tilt/trim cease to function?

Thanks again for your help!
 
The 20 will come with a 10 pitch prop. You will need to determine whether that's low enough pitch for trolling by trial. You will want a transom pad to dampen vibration.
The idea of not trimming with the motor at speeds is so that you don't overwhelm the hydraulics. Works fine at moderate thrust. You will know if you are pushing too hard, as it won't be able to trim up against the thrust.
 
I have a 14' Valco with a 2014 4strk 20hp Tohatsu with PTT. WOT speed with me and my fishing gear is 25. with two persons it's about 21-23 depending on the wind conditions.
My trolling speed is 1.2-1.3. Also, I get very little vibration at idle and this motor just sips the fuel.
Couldn't be happier with this motor.
 
Last edited:
Back
Top