The Tohatsu Jackplate N Prop Diaries.

[seanehauck]

Good day All,

Long time lurker, first time poster.

I have a 1998 Tohatsu 25hp tiller operated short shaft motor I have been tweaking over the past few weeks. The boat it sits on is a 15" Troller fiberglass trihull. The boat itself is quite heavy. There are front and rear casting decks built with 3/4" marine plywood. I'm not sure of the weight, but I would say it added 400 pounds to the boat. With one-two people in the boat, it would plane, but would take a long time.

I started doing my reearch and found out that my cavitation plate was sitting about a inch below the bottom of the transom. Thus heading into the jackplate. I borrowed a old jackplate from my dad (total overkill for this boat, but it was free) and installed it. I have the motor sitting a 1 1/2 about the bottom of the hull. With the addition of the jack plate. I also added a Solas stainless steel 10x9 prop, removing the old 9.9x13 OEM prop. The boat was night and day difference. Holeshot and planning were instant. My concern was, WOT rpm on the motor. I clocked with two men and a livewell full of water (25 gallons) I was reaching 28mph.

I bought a tachometer to run this weekend with the factory prop and the Solas unit to see what I am getting. My concern is two things currently. I am still getting some spray with the motor 1 1/2 above the transom (picture included). I understand that raising the motor also increases RPM as well. How much more should I go before?

Thanks,
Sean Hauck

 

[steelespike]

Re: The Tohatsu Jackplate N Prop Diaries.

It appears the motor is really high most pictures I've seen the plate is barely visible.
I usually say raise the motor to a point just before venting becomes a problem.
Just be sure it pumps water.
A quick calculation guessing 10% slip indicates an rpm of 6780 .
Motor is rated for 6000.
I think your amazing improvement in hole shot is more pitch change than ss change.
I think you'll end up with an 11 or 12" prop.
The tach will tell the story.

 

[pootnic]

Re: The Tohatsu Jackplate N Prop Diaries.

Have you tried the 13 pitch you took off?
With the jackplate,I'd be curious to see how it preforms.
Incase you didn't notice what SS mentioned,you could be around 800 rpms over the recommended WOT range.
As far as the spray...I get the same but I'm using a jet tunnel hull with a prop outboard and I want the outboard as high as possible,alot of rocks here.You could try lowering it abit,see if that helps...nice jackplate by the way.