If my old Tohatsu 6HP achieved 5 knots max per hour on my yacht.
What will my new Tohatsu 9.8 achieve ?

Swarbrick S80 (26 foot) 1600 kg

my calculation is 5.2 knots, having Yamaha 9.9 4 stroke,
full bore pushing boat about 6.5
regardless my comfortable , economical speed
is just under 5 kn, about 2 litres / h

Harb said..

Bundeenaboy said..
If my old Tohatsu 6HP achieved 5 knots max per hour on my yacht.
What will my new Tohatsu 9.8 achieve ?

if we're having a poll put me down for 6.2 kts going downwind

So someone knows how to calculate square root x1.3 . Ha

A trailersailer?

Love the input
Thanks all

Two Dogs the Castle 650 is no more a planing hull than an S80 is. Much to my displeasure.

Out of interest I did a test between a 9.9hp merc and 15hp merc same motors just different jetting and intake size. At the same speed the 15hp was slightly better fuel economy. Max speed didn't change it just pushed the stern deeper into the water. Boats a Seaway 25.

I sailed on an S80 that had 10 Evinrude on the back, I seem to remember it doing 8knts or could have been more.

Bananabender said..


So someone knows how to calculate square root x1.3 . Ha

That's like the pirate code, more like what you call a guideline then a precise formula. When it comes to pushing a 1.6T boat using a small outboard the prop, shape of hull , weight and a clean bottom can affect the calculations.

Yara, can I generalise?
Finer Pitch (lower number) will give me more low end power but less high end speed
Courser pitch (higher number) will give me more high end speed but less low end power

since we are all drifting further, what if your lucky enough to have 2 hulls?

With the cat, weighing about 2T, Yamaha 9.9HT gets about 6.5-7 knots max, if the hulls are clean, and maybe 2l per hour also.
The Yamaha 9.9 high thrust probably like Charriot couldn't get a prop for, is geared to push, so add gearing into your thoughts also.
Models from about 5 years ago onwards have a different gearbox ratio,the head unit is slightly narrower, but they aren't the same anymore.

Charriot the prop should be is still available, both original and knock offs.
I bought one admittedly a few years ago, but it is common to have to replace them, as the rubber bonding 'doughnut' that it drives through perishes in the end, meaning you lose 99% of the drive all of a sudden, in my case it was a long slow motor back to the mooring, relying on friction to keep driving the prop.

I've looked at different props on the same 9.9 motor, as well as imported copies of the Yamaha engine.
Unless it can spin the original high thrust prop, which looks like a dinnerplate size in comparison , you can get a little higher hull speed, but no grunt, especially in reverse.

Has it got to do with the wavelength the hull creates?

i.e, the wave rises at the bow, drops down amidships, and rises again at the stern.

This wave length can't change due to the length of the displacement hull? Once this wave is achieved, you are at hull speed.

Is this right?

Yara said..

Bundeenaboy said..
Yara, can I generalise?
Finer Pitch (lower number) will give me more low end power but less high end speed
Courser pitch (higher number) will give me more high end speed but less low end power

No. Basic fact of life is that for a boat that goes though the water (dispacement hull) you hit a brick wall at a certain speed where more power does not give you any more speed. You want a prop that wiil give you max power at a speed just below that.

Hi Yara,
I was generalising that we could achieve higher speed, not specifically on my boat.
Thanks again , you have helped me alot.