Honda 9.9 ?

[HAL]

What to do?

We have a Honda 9.9, 2005 long shaft ,with Power Thruster that will reach only 4500 rpm wide open.
Honda says 5000-6000rpm is were its supposed to run. We didn't have a tach before the Power Thruster, so I don't know if the Power Thruster might some how be responsible.

More details are: H260,lightlly loaded maybe 6000 lbs, centerboard down, speed 6.2 mph with reverse course for compensation, motor mount lowered 2” for more rough weather and backing bite, hull clean, 2 props, 9 1/4 x 8 and 6 1/2 x 10 with no rpm change ,tach reads 950 at idle which seems right.

Many have said a 9.9 will easily push this boat to hull speed ,which I guess is about 6.2 mph at 3/4 throttle. Normally you would go to a prop with less pitch to raise rpm, but Honda only makes one lower at 9 1/4 x 7 which wouldn't raise it 1000rpm.

I worry about carbon build up at low rpm and low hull speed unless throttle is wide open. What are we missing ?

 

[Charisma]

Tach

Hal, We just purchased a new Honda 9.9 High thrust/auto raise, and I to thought it did not rev very high. I have not checked my boat speed yet. Where did you get the Tachometer?
Chris

 

[jimq26]

I'll bet it simply involves prop pitch & diameter.

We just put a Diameter 5 x Pitch 7 x 6 Blade propeller on our Tohatsu 9.8. Finally we can get the engine performing the way it's supposed to when pushing 8000 lbs.

 

[Granfalloon]

Tiny Tach

A couple of questions.
What is the condition of your bottom?
Is the prop free of dings or small barnacles (should not have any assuming you keep it up when not running the engine)?

A good product for Tachometers is "Tiny Tach" Google Tiny Tach for Diesel they have one gas engines as well. Easy install and you can power it with a 9volt battery or connect tour 12v batteries. Very accurate and under $100. And some bonus features - it has Countdown and Count Up time modes, max rpm, and tracks total engine hours.

 

[Ross]

Honda spec's indicate a 10x6.5 prop for that engine

and that would give about 65 per cent slip at 6 knots. That is about the area you want. The gear ratio for that engine is 2.33 which yields a prop speed of about 2500 rpm. The 10 inch prop will give more low speed thrust than a 5x7.5 mentioned on the Tohatsu. The 9.25x8 will yield nearly 75 per cent slip and that is excessive. Your prop and boat may be absorbing all the power the engine can deliver and that would indicate a lower pitch. I don't believe that you run any risk or fouling the engine if you run at less the WTO. See what it takes to get 5.5 knots.

 

[HAL]

Honda 9.9 Thanks for ideas

The tach is a Teleflex and is set on the 6p position as recommended.

The Tohatsu 9.8 with 5 d x 8 p x4 b has significantly less bite than what we've tried and pushes 8000lbs ,which is significantly more than us. A prop of those dimensions would raise rpm, but Honda doesn't go below 91/4 d x 7 p x 4 b on a 4 b. They do offer 3 b which would raise rpm, but this is far off of their original recommendation.

The bottom is clean and the props are undinged and unfouled. I forgot to mention we are towing a 8 foot Zodiac which is pretty light weight but may be creating more drag than we think.

We started with the 91/4 d x 7 p x 4 b that came with the motor, then we switched to the 10 d x 61/2 p x4 b for more low speed thrust ,at the same time the Power Thruster was added and then the tach. More low speed thrust was definitely gained ,but now we saw low top end rpm, well below the max power band of the motor ,so we switched back to the smaller dia higher pitch and saw no rpm change. Do we not want more slip for higher rpm and more power .But then that slip will mean less thrust delivered to the boat.

When the throttle is advanced ,the 4500 rpm is reached at the 3/4 open amount ,advancing it further to wide open yields no further increase. This makes it seem like the motor is lugging because it won’t accept more throttle.

It takes 3850 rpm to get a hull speed of 5 mph .We had kinda settled on this because the motor seemed to run allot easier than at the 4500 and the higher hull speed ,but still ,this seems slower than it should be, with the motor below its recommended rpm range.

 

[Ross]

Hal, I run a 10 hp inboard with a 3:1 reduction

gear. When I start from a dead stop and go to a wide open throttle it takes about a hundred yards before the engine reaches full rpm because the rate of acceration is limited by the hp. My boat weighs about 9500 pounds and I can't expect the little engine to perform like a dragster. I did reduce the propeller pitch this year and it is much better. If you have a prop shop in your area they can probably take an inch out of that 10x6.5 and also advise you for more or less than that. Mine is a heavy bronze 3 blade and it cost 120 dollars to take 2 inches out and true it up.

 

[48dodge]

Is this a concern?

"I worry about carbon build up at low rpm and low hull speed unless throttle is wide open."

Is this a concern with 4-strokes?

 

[Ross]

Not in my cars .

Four cycle engines run pretty clean if they aren't overloaded.

 

[Benny]

You need to get a prop that will allow it to rev

to its maximum rated RPM. If Honda does not manufacture one look for an aftermarket one. Let the engine serve the boat, not the other way around.

 

[Manny]

I'm missing something

If the main concern is carbon buildup, wouldn't the majority of gasoline powered car and motorcycle owners have this concern? I don't know anyone who drives their car in a lower gear just to keep the revs to just below the redline. Unless your driving something like a Formula One car you couldn't even reach the maximum rpm in top gear.
The rpm range supplied by Honda is the full throttle range, it doesn't mean you have to stay at full throttle all the time, or even close to it. Why would you want to? The important thing is to not stress the engine. If you reach hull speed at a certain throttle setting, no more gas is needed or desirable. Your wasting gas and putting unnecessary pressure on the engine components. The boat can only go so fast.
Prop choices for small outboards are limited. Try Michigan Wheel for a decent selection of aftermarket props. I was able to source a lower pitch prop from them for my 1998 Honda BF8. I can't get to the engine's max rpm but it gets close, and I'm pushing a lot more weight plus I have big 3 blade prop for the inboard slowing the boat down. After about 3/4 throttle the engine doesn't rev any higher but I hardly ever go past half throttle. If you need to rev it up to blow out the carbon, just rev it neutral for a second or two every once and a while.
Best thing to do is keep the outboard in good tune, follow Honda's maintenance schedule, and use quality oil and fuel. Also run it dry if not going to be used for a while. Might not hurt to send Honda an email to get their thoughts or talk to your dealer's service department.

Manny