I am hoping someone may have intimate knowledge of my prop problem. Little history: I bought a 16.5ft deep V boat in 1997 with a 40hp Johnson (for Erie) then added a 9.9 hp for the local state park lakes. Dry weight of boat only is approx 750 lbs.

I got tired of maintaining two motors and did not like the performance of the 9.9hp (5mph ugh!). This summer I sold both motors and bought a Yamaha 20hp.

Here is my dilemma. With a 9 1/4 x 8 prop, the boat will get on plane and will get to 19 mph (calm lake). At 19mph the motor exceeds maximum rpm and the motor starts knocking due to the internal rev limiter. Motor is rated 5000rpm-6000rpm and rev limiter hits at 6200rpm.
I went to the next larger pitch prop, 9 1/4 x 9. Normally 1 inch of pitch will only change the max rpm by approx 200. In my case the 9" pitch prop could not get the boat on plane so the rpms stayed below the power zone and my top speed was about 14 mph.

I am searching for a middle zone (16-17 mph) but not sure how to get there. I have two ideas to run past you:

There are 9 1/4 x 8 four blade after-market props. Since four blades would push more water, I thought this might lower the rpms slightly versus the 8" pitch 3 blade prop.

My other idea was to stay with the 8" pitch 3 blade prop and add weight (50-100 lbs?) to the bow of the boat.

Any thoughts? I have all winter to think about.

Jerry

Try same size blade with more surface area/cupping or go to a 9 1/4 x9 stainless
Buy used props on ebay then sell them til you get it right-should only loose postage

Marty,

The Yamaha 8" prop already had a high degree of cup.

I considered your stainless 9" recommedation but I didn't think it would gain enough rpms over the aluminum prop. I will research this option more.

Thanks.

I may of made a mistake above.
Stainless may lower rpm because it won't flatten out like an aluminum.
This makes a bigger difference with higher hp but a thinner prop could have the same results.
9 1/4 x 8 SS could work
There are so many variables that this can be a nightmare/expensive especially with a high HP performance boat prop.

9 1/2" x 8 or 9"x9 might be other options
Honda and Mercury props might also work-most have same spline

Does your 8" pitch prop have issues with getting your boat to plane out?

It seems wierd to me that just one inch of pitch would make that profound difference.

Ok check the 9 pitch prop, make sure you can read on the prop that it is infact a 9 pitch (lookin is free, new props are not free)

Do you have your motor set up correctly? with tilt trim, motor height tuned in? Do you have a good balance of the weight in your boat? All of those help/hurt your preformance.

On smaller outboards I have found that props tend not to follow the rule of thumbs when it comes to selectiong a prop. I have played around extensivily with my 25hp motor trying to drain every last drop of preformance out of it. And realised why the mfger puts a huge WOT range on the motor, thats because with smaller power things change very fast in RPM. But going from rev limiter to stuggleing to plane is very extreme, thats why i question the 9pitch, while your at it verify that you 8pitch is truely a 8 pitch.

I made a guess that your outboard is newer, so I have a link i posted below. Props generally are ot returnable (it sucks) but you may need to buy several to figure this out. With a smaller motor personally i would stay away from SS, From what i read motors will not start to see the benefits of SS until the 50-70hp range (more torque, heavier loads) I think SS props are super cool, they look awesome and make you feel good inside... But i feel with your case it isnt worth the extra $.

So with assuming your props are the pitch you think they are and the motor is set up correctly, If i were you the next prop i buy is a Solas Amita 4 in 8" pitch (on the page i put the link to). Changing brand of props can yield different results, but i think it is a safe starting point. the forth blade should help lower rpm, and provide better holeshot with more grip and hold better in corners. Solas aluminum props have the best quality and finish i have seen in a AL prop.

If you still bump rpm limiter i would then go to a Amita 4 with 9" of pitch. One of those two props should be a good fit for you, and the prop that you dont use keep as a spare.
http://boatpropellers.iboats.com/Yamaha-Outboard-Propellers/?pitch=8.00&chart=68&engine_id=810&cart_id=081266797

go to a bigger pitch the 9 is ok but the pitch will decrease or increase a example is a work prop on merq is 16x16 and factory was 15 x 17 1/2 this lowered the rpm but put the boat on plane fast and at lower rpm a ss prop will lower rpm but if you run the river i would be aware of that the blade will eat rocks and can damage the lower unit

Go back to your original prop and adjust your throttle stop to keep it in the rpm range.

Anzomcik,
I was amazed that one inch of pitch made that much difference also. Both props were labeled correctly. The 8" four blade Solas is the prop I was considering in my original post. This may be my best bet.

Since it is a manual tilt motor, the trim adjustment is a pin with optional hole placement. It is currently placed in the second from the bottom hole which seems to be ideal for plane angle. Maybe I could try placing the pin in the bottom hole. This would push the bow down more which should lower the rpms on the 8" pitch prop.

Another option that Marty got me thinking about is blade cup. Both the 8" and 9" pitch Yamaha props I used are marketed as "cupped blades". Yamaha also lists same size props that do not have the "cupped" designation (actually marketed as "weedless"). Maybe the 9 1/4 x 9 non-cupped prop would get me better rpm range.

Bulldog,
Since I currently have the original prop installed, I had considered that option. Although I worry that I need the full throttle range to get me on plane.

Thanks to all for the ideas. You got me thinking about various experiments to perform this spring.

Jerry

My brother and nephews go to a site called Scream and Fly - you might want to check their forums!

http://www.screamandfly.com/content.php?35

When adjusting the tilt trim you want the antivent plate thats on the motor to be parrallel to the bottom of the boat, or the setting that is closest to that. Also once you found that tilttrim setting idealy you would want the antivent plate then to be adjusted vertically. The rule of thumb for that is to have the antivent plate level with the bottom of your hull, but every boat is different, my boat runs the best with my antivent plate sitting 1 below the bottom of the hull.

The height is easliy adjusted with some scraps of wood the width of the motor bracket cut to the height you want.

Also you have the boat loaded correctly? (distrubiting the weight of the contents)