[typer181]

Hi Guys,
My local reservoir started imposing strict regulations regarding a 9.9hp limit on motors. Being limited on funds, I needed a reasonable solution to the 35hp Evinrude I have been running. Plus the 35 lost a cylinder, and it's a 1979 motor so rebuilding it was a pain according to what I have read.
So i lucked across a 1999 Mercury 9.9 that came off of a pontoon boat, for the nice price of $250. I finally got it hooked up and took it out for a test today. It felt great knowing that I was finally legal and would not get hit with the $2500 fine they soak you if you're caught with a big motor (absolutely ridiculous fine!).
Then I noticed some cavitation, and it was pretty bad. Felt like the prop was losing "traction" when I would get to faster speeds. So I got it back on the trailer and realized that the cavitation plate (I think that's what I'm seeing?) was 1" above the bottom of the transom. Is there any way to reduce the cavitation or improve the performance without hacking a notch out of my transom? Thanks for any help!!

 

[bvomark]

Motor is low enough as it sets. Cavitation plate is above prop and appears to be below boat. I suspect a prop that needs hub replaced. Starts slipping at higher loads.

 

[twaskom]

Eric, I agree with Mark. The cavitation plate is directly above the propeller. And it is well below the bottom of the boat. What size prop do you have on it. The reason I ask is that I have a 9.9hp Merc and you could try my prop on your motor to see of it does better. Just to help you identify the problem. It does sound like the hub is slipping at higher speeds. If so, you can get a new hub pressed into your old prop. Problem fixed.

any thoughts?

 

[twaskom]

Actually, your motor should be able to be raised some. When you are running on plane you should be able to look down on the lower foot and see the cavitation plate right on the surface of the water.

 

[typer181]

Hey thanks Mark and Tom! Glad I don't have to hack my transom.

I'm not sure what size the prop is, it's got 4 blades. Is there a design that works better than another? I'd like to squeeze all of the speed out of this motor possible.

I'd also like to take you up on your loaner prop offer Tom. That will help me save a good chunk of change if a worn hub is not the issue. I'll shoot you a PM. Thanks buddy!

 

[D schuerenberg]

Fewer blades = more speed it may be to small pitch for smaller boat Tom do you know what pitch you are running?

 

[Jbuck]

Bvomark's reference to a prop hub being worn out may be correct. Using Tom's loaner, if the same pitch should confirm. You can try "feathering" the throttle gaining RPM at a slow but steady gradual rate until full throttle if you can get there stop and try getting full throttle again this time like you are in a drag race, if you cannot get there with heavy acceleration it is likely your prop hub. While that will confirm it is your hub it will not confirm it is not, sometimes even trying to accelerate slow they will slip at a given RPM. Good Luck with it.

 

[Torvaldsberg]

I'd like to squeeze all of the speed out of this motor possible.

Not sure about that particular motor, but with a lot of the older motors, the only difference between a 9.9 and a 15 was the carburetor. If this one is the same and you can locate a carb for a 15 that fits, you will see significant improvement. Best part is that unless the officer tears down the motor and measures the carb there is no indication of the modification and since the cowl still says 9.9 most won't bother(at least around here).

 

[typer181]

Not sure about that particular motor, but with a lot of the older motors, the only difference between a 9.9 and a 15 was the carburetor. If this one is the same and you can locate a carb for a 15 that fits, you will see significant improvement. Best part is that unless the officer tears down the motor and measures the carb there is no indication of the modification and since the cowl still says 9.9 most won't bother(at least around here).

That's great advice and I'll definitely look into that. Many thanks!!

 

[Jbuck]

Not sure about that particular motor, but with a lot of the older motors, the only difference between a 9.9 and a 15 was the carburetor.

In many the only difference was the jetting in the carb with some being modified with an orifice jet drill, others just ordered out 15hp jets. You are correct though, I had a 9.9 Evinrude, comparing the spec. sheet the bore, stroke, compression, and valve body was identical to a 15hp. just different carburation.

 

[twaskom]

Fewer blades = more speed it may be to small pitch for smaller boat Tom do you know what pitch you are running?

D, I would have to go out to the barn and look. If I remember right it is a 2002 9.9hp 4 stroke, rope start.

The picture won’t tell the pitch but this is the 9.9 before the tree fell on it. It knocked a big hole in the engine cover and broke the shift lever off. You can see the manual shift lever in the pic with the three dots on it. It is a plastic I think.

I was trying to remember the attributes of a 4 blade prop and if I remember right it gives more lift but the three blade provides more speed. A blade can also be adjusted by cupping which will also add lift. I can see where these differences might make a difference in a big motor but don’t think you would see a lot of change in a small motor.

 

[River_monster91]

4 blade will result in better fuel economy amd will get a heavier load on plane easier. 3 blade will run a little faster top end but lose a little fuel economy.

 

[typer181]

I'm going to give Tom's 3 blade a try and see if my hub is indeed at fault. Unfortunately, I haven't had a chance to see how the boat operates under a heavy load. Stay tuned!

 

[Shootrj2003]

Please forgive me gents but the extent of my powerboating is a Minkota on my Jon boat,what is cavitation?I’ve heard the word before but as an effect in powerboating,I’m ignorant but willing to learn.sorry!

 

[twaskom]

Shoot, cavitation is the propeller on a boat sucking in air. Like when you pull the plug on the kitchen sink and you get that spiral vortex making noise as air is sucked into the drain. It usually happens when the propeller is lifted to high. The cavitation plate is that flat horizontal fin right above the propeller. If it stays at or just under the surface of the water when on plane, you very close to having the motor adjusted right.

When cavitation occurs the boat is usually above half throttle and thus the transom is being lifted to the top of the water. Two things happen when cavitation occurs. The RPMs jump up fast and the boat loses the pushing effect the propeller should supply.

Usually just takes some adjusting to get it corrected.

No forgiving needed.

 

[D schuerenberg]

Just thinking if it came off too probably pitched low to push it

 

[twaskom]

Fewer blades = more speed it may be to small pitch for smaller boat Tom do you know what pitch you are running?

D, the prop that s on my 9.9 Merc is a 9 inch x 9 pitch.

Here it is on eBay.

Mercury Propellers Aluminum 48-828156 A12 9 X 9p Black Max Propeller Prop Merc for sale online | eBay

Find many great new & used options and get the best deals for Mercury Propellers Aluminum 48-828156 A12 9 X 9p Black Max Propeller Prop Merc at the best online prices at eBay! Free shipping for many products!

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tight lines

 

[cspot]

Prop Selector | Mercury Marine This is a prop selector from Mercury that may be of some help.

 

[twaskom]

Prop Selector | Mercury Marine This is a prop selector from Mercury that may be of some help.

Cspot, I tried to get the prop selector to work but it kept giving me only one choice for a motor. A Legacy, 150 hp.

Give it a try and see if you get anything different.

small aluminum boat,
tiller,
either mid range or speed
year 19xx

Here is the first information needed.

 

[cspot]

Cspot, I tried to get the prop selector to work but it kept giving me only one choice for a motor. A Legacy, 150 hp.

Give it a try and see if you get anything different.

small aluminum boat,
tiller,
either mid range or speed
year 19xx

Here is the first information needed.

If you select the year 19xx then that is what you get. Seems like if you select 2000 or after then it gives you more choices.