Seawind 1000XL Catamaran Yamaha 9.9 to Tohatsu 20 Outboard Upgrade.

[Sinkers]

Hi,
Currently have 2 Yamaha 9.9 Four Stroke High Thrust Outboards on my boat and am very happy with them ( cruise 6 knots and top speed 7.2 knots)
I would love to have more power however because whenever the boat is punching into a head wind or into choppy head seas, the performance drops off dramatically. Obviously this is to be expected, but an engine with more thrust would have to help significantly.
Nearly all the higher HP outboards are physically too large to fit in the outboard well and are also significantly heavier.
However Tohatsu has recently introduced a brand new 20 hp Four stroke which may be an improvement over the Yamaha 9.9 hp ( Model MFS20)
It’s actually slightly lighter than the Yamaha and will fit in the outboard well.
Also it’s available with an optional 25 inch shaft length and a 4 blade high thrust prop.
The downside is that it’s not available with a different high thrust gearbox like the Yamaha or the reversing exhaust deflection the Yamaha has.
One other advantage is the electrical charging output. Increases from 5 to 12 amps.
So my question is has anybody had experience with these motors ? ( in particular has anybody upgraded from the Yamaha to the Tohatsu) If so, how did the performance change. It would be nice if the cruise speed increased a bit as well.
Should also mention the local Tohatsu Dealer is suggesting that he can extend the shaft length to 30 inches to further reduce any cavitation.
Thanks all
Greg

[44'cruisingcat]

Assuming you have the old model 9.9, which was the 15 de-rated, how about finding which bits are different, and re-rating to 15s ?

[Bean Counter]

Went from Yamaha 9.9 low gear high thrust to Honda 20 standard gear high thrust prop. Made hardly any difference, slight increase in top end speed and more amps and electric tilt was nice. To develop the full 20 horses engine needs to be at high revs which doesn't help at displacement speeds. Tried putting on even finer pitch props - that did not help either. Yamaha makes a 25hp low gear outboard but it may not fit.

[sea-bee]

I have an old Yamaha 9.9 that died. I replaced the powerhead with a new 15 and reconditioned the old leg. Really, it's only the same motor that was originally on the leg. The new models with the souped up 8s are aimed at the trailer-sailer market where weight is even more of an issue. There is no subsitute for the 3:1 gearbox and the big prop that those yammies can spin.

[clockwork orange]

I'm very happy to hear that the 15hp powerhead will fit on the 3:1 leg as I have often thought of the same thing. I have 2 of the older white 9.9s, a 1989 and a 1992. I wish that they would use the low gearing on larger motors. We recently installed a pair of Yamaha 25 hp high thrusts on a 42 ft Simpson cat and they have 2.44:1 reduction vs the 2.92 on the 9 .9hp and they both swing exactly the same dia and pitch.
So, has the 15hp on the 3:1 leg provide a significant improvement over the 9.9 ?

Steve.

[Sinkers]

Thanks for the replies.
Yes the Yamaha 25 hp is too big, so it looks like my 9.9 hp is still the motor of choice ( mine are the newer ones.
Have to wait until someone makes a larger engine that will fit and that has a high thrust gearbox.
Cheers all
Greg

[sea-bee]

Originally Posted by clockwork orange

.. has the 15hp on the 3:1 leg provide a significant improvement over the 9.9 ?

Steve.

I can't comment on the change in performance. I bought the 9.9 with a blown powerhead and never used it. I bought it while building the boat to make sure that I was making the enginebox the right size to fit a motor once the boat was finished. It was that cheap that I could afford to waste a few bucks to make sure the enginebox was good. But once finished, my local outboard mechanic convinced me to fit a new powerhead on the old motor.

In calm conditions, we use this motor alone, and get about 5 knots out of that one motor running at 2200rpm burning about 2 litres/hour.

[sea-bee]

Originally Posted by clockwork orange

.. has the 15hp on the 3:1 leg provide a significant improvement over the 9.9 ?

Steve.

I can't comment on the change in performance. I bought the 9.9 with a blown powerhead and never used it. It was bought while building the boat, just to make sure that I was making the enginebox the right size to fit a motor once the boat was complete. It was cheap enough that I could afford to waste a few bucks to make sure the enginebox was good. But my local outboard mechanic later convinced me to fit a new powerhead on the old motor. It's actually a Chinese clone of the yammie 15, and everything is absolutely identical (except for the quality of metal).

In calm conditions, we use this motor alone, and get about 5 knots out of that one motor running at 2200rpm burning about 2 litres/hour.

Chris

[Dave_S]

Check the OD of the props. The larger the OD the greater the volume of water they push which = low speed response.

If you down size the OD you will loose some of the control you might need for docking in strong winds.

[valhalla360]

As mentioned, historically most 9.9 & 15hp motors are identical with just some simple changes to derate them.

Check the current prop size. If you can get a larger diameter lower pitch prop, that will also likely help. We swapped out a 40hp for a 25hp on our Gemini but also switched to a low pitch work prop and docking it was more responsive. We lost 1/2kt on the top end but the 40hp never spun up to max RPM anyway.

[K38bob]

Hi,
What about the new 25hp? Too big? Any experience? Any other options?
https://yamahaoutboards.com/en-us/ho...st/high-thrust
Thanks!