F9.9 prop size help?

[quadracer392]

I have 20' sweetwater pontoon with a 07 Yamaha 9.9 4-stroke. I can get about 7-8 MPH on a good clear calm day. I know I'm not going to get much speed out of it, but I was wondering if anyone thought this seemed low or not. At WOT the motor is only reving right at 2900-3000 RPM. I know it should be around 5500 or so. Would changing the prop get me anymore speed or just get it in the right WOT operating range. The prop that's on it now is a factory dual thrust 3-blade 11-3/4 by 9-1/4. What would be a good size prop to start with?

[TownsendsFJR1300]

Here's a link to get you in the ball park:

Performance Bulletins | Yamaha Outboards

Fill in your info and it'll give you an idea.

[boscoe99]

Please clarify what model motor you have.

An F9.9 gear case will not accept a propeller with a diameter of 11 3/4. I suspect that you have a T9.9.

If you are only getting 3000 RPM at WOT you are way over propped. The motor won't make rated HP until it gets to approximately 5500 RPM. HP is what is needed to make the boat go as fast as is possible.

If you have a T9.9 then the Yamaha propeller choices available are a 5 3/4 pitch or an 7 pitch. I would start with the 5 3/4.

You may or may not get much more boat speed. You will get improved engine life by dropping to a lower pitched propeller.

[quadracer392]

Sorry I did put the wrong model. It is a T9.9ELR2. I was cleaning the boat up some tonight and hook the water hose up and ran it for a couple of minutes. I put it in forward and ran it to WOT just to see if it would make it up to around 5K or so. The rev limiter kicked in right at 3600 RPM, so now it's got me wondering if the tach. is right. It's idling right at 950 like it's supposed to. I have a digital timing light that has RPM readout I'll hook up tomorrow to see if I can verify what the tach is telling me. If the tach actually is wrong, could it very well have the right prop on it?

[quadracer392]

Well, I've been doing looking up different and researching things all evening and came to the conclusion that the factory had the wrong pole setting on the tach. The motor is a 6 pole alternator and the tach was set on 12 pole hence the 2900 RPM at WOT. I have verified that the tach is reading much more closely now than it was so it's fixed now. Thanks for the help!

[TownsendsFJR1300]

Don't ya love boat riggers???

I had the wrong prop on mine(not even close), the trim tab set on the LU in the wrong direction (its in black and white) which side to set it at (it was at the opposite).. Ahhhh..

[boscoe99]

I hope that you are only running the motor at WOT with the boat in the water. Sounded like you were running it on the hose. Maybe not.

Also, that model appears to have a wasted spark ignition system whereby the plug fires every time the piston is approaching top dead center. Make sure the timing light/tachometer is set appropriately.

If the tachometer is indicating the motor is hitting the revolution limiter at 7200 RPM this leads me to believe something is not right. Most rev limiters will engage beginning at 6200 RPM.

[quadracer392]

Yes, I did run it at WOT on the hose but just momentarily. Enough for it to hit the rev limiter to see where it cut out at. I know from all the years racing quad and building small engines, it's not good for any motor to free rev that long without a load on it. And yes the tach did show it hitting the rev limiter right at 6200

[svsunbeam]

I have 20' sweetwater pontoon with a 07 Yamaha 9.9 4-stroke. I can get about 7-8 MPH on a good clear calm day. I know I'm not going to get much speed out of it, but I was wondering if anyone thought this seemed low or not. At WOT the motor is only reving right at 2900-3000 RPM. I know it should be around 5500 or so. Would changing the prop get me anymore speed or just get it in the right WOT operating range. The prop that's on it now is a factory dual thrust 3-blade 11-3/4 by 9-1/4. What would be a good size prop to start with?

quadracer,

I had the exact same problem with the prop on my 96 F9.9 MSHU. I hit a rock in the Bahamas and broke my prop. My flea market spare was a 9 1/4 x 10 1/2 and I though I had screwed up something in the motor hitting the rock as all of a sudden I couldn't get any rpms and my 10' Apex RB wouldn't get up on a plane. Ultimately found out the problem was the prop. Didn't have the old prop and had no idea what size it was or that it was such a problem to find out what was OEM. Couldn't find what the OEM prop was, but local mechanic suggested a 9 1/4" x 8". Bought and installed one. RPMs back up and instantly back up on a plane with full power.