Changing rpm on new outboard

[san dimas]

Hello everyone. I have a new 9.8 Tohatsu outboard that is mounted on a 14' Valco tinny. I'm at the point in the break-in procedure where I can run full throttle for 1 minute or so. I noticed on plane going about 22 mph gps full throttle, the rpm is only 3,000 according to my Tinytach. I should be 5,000 -6,000 according to the owners manual. The current prop is a 8.9x10. If I go to a 8.9x7 will I come up enough in rpm. I've tried adjusting the trim and the antiventilation plate is about even with the bottom of the boat. Doesn't seem to change rpm much. I weigh about 295 pounds plus the fuel (3 gallons), so most of the weight is in the back. The boat probably weighs about 170 pounds. The motor is 81 pounds.

 

[san dimas]

Re: Changing rpm on new outboard

I also wonder about my Tinytach. On plane at full throttle it seams to lock on about 2,960 rpm and only jumps about 10 rpm every 5 seconds or so. I can't believe my rpm would be that stable.

 

[pootnic]

Re: Changing rpm on new outboard

I'd wonder about the tach also...22mph seems to good,at 3000rpm with a 9.8hp.
I could probably go to a prop calculator to find out for sure...

 

[IamCanadian]

Re: Changing rpm on new outboard

Tinytach is set for 2 stroke, I am sure you have a 4 stroke as you say it is new, 2960x2=5920rpm

 

[san dimas]

Re: Changing rpm on new outboard

The Tohatsu has a wasted spark ignition so it fires both cylinders once each revolution of the crank shaft just like a single cylinder engine.. My idle rpm reads 900-950 which is what it's supposed to be.

 

[san dimas]

Re: Changing rpm on new outboard

I have a second Tinytach that I'm going to put on it so that I can compare the two. I also have another tach that is hand held. You put a piece of reflective tape on the flywheel and point the tack at it which is not so fun when your're going wide open across the lake. But I know this tach is accurate.

 

[IamCanadian]

Re: Changing rpm on new outboard

ok well, with that prop and your motor only going 2960rpm you would only be going 12.1mph

 

[jestor68]

Re: Changing rpm on new outboard

A 10" pitch prop with a 2.0:1 gear ratio at 22 mph equals 4646 rpm.

What is your gear ratio?

 

[san dimas]

Re: Changing rpm on new outboard

Thanks Canadian. That leads me to believe it's the tach.

 

[san dimas]

Re: Changing rpm on new outboard

My gear ratio is 2.08 (13:27)

 

[jestor68]

Re: Changing rpm on new outboard

A 10" prop at a 2.08 ratio equals 4832 rpm at 22 mph(theoretical).

 

[steelespike]

Re: Changing rpm on new outboard

The tach is probably set wrong. I get about 5300 rpm at 22 mph.A very respectable speed. 2.08 gear ratio,10" pitch, guess 10% slip = 5300. rpm range is 5000-6000.
By the way a quick check here at iboats seems to indicate a 10" pitch is off the chart on the high end.

 

[san dimas]

Re: Changing rpm on new outboard

Sorry guys. I gave you wrong info on the prop. It's 8.5 x7.5. My mistake.

 

[san dimas]

Re: Changing rpm on new outboard

I'll have to recheck my speed because according to the prop calculator I should only be able to go 18 or 19 mph.

 

[steelespike]

Re: Changing rpm on new outboard

ya,6000 rpm 10% slip = about 18 mph.
At 295 you could probably have better luck with a tiller extension to get your weight more forward.
certainly would plane quicker and stay on plane better.

 

[san dimas]

Re: Changing rpm on new outboard

Thanks guys for all your help. I called Tinytach and explained my problem. Steve said to rap the red wire one turn around EACH of the plug wires. Took the boat out yesterday and the tach works perfectly. I'm a little concerned that I may have broken in the motor a little hard. Nothing I can do about it now. Still seems to run fine.

 

[san dimas]

Re: Changing rpm on new outboard

And by the way, the motor was taching 5,980 rpm at 18-19 mph.

 

[IamCanadian]

Re: Changing rpm on new outboard

Glad you got it sorted out.