9.9 hp Johnson Beat up Prop? Cavitation? New Prop Size?

[grrr_evans]

I have a 14 ft Harborcraft tin boat with a 9.9 Johnson and I usually carry about 3 guys with just fishing gear, I want it to go pretty fast. Right the prop has issues it looks pretty beat up. I start going and I putt along just fine when in Idle when everything is running, but when I give it gas it seams like the prop air locks and starts to cavitate because there is no more power or no more thrust! should I just buy a new prop? or file it down? I am clueless? any suggestions?!?

and if I buy a new one what size should I buy too go as fast as possible! and I am usually on mountain lakes! so fresh water and not very rough!

Thanks Greg!

 

[DJ]

Re: 9.9 hp Johnson Beat up Prop? Cavitation? New Prop Size?

The prop hub is spun. Time to have it rebuilt or buy a new one.

Plenty of props here on ibaots.

 

[grrr_evans]

Re: 9.9 hp Johnson Beat up Prop? Cavitation? New Prop Size?

Ok that was my plan what kind of prop should I get? same one? I wouldn't mind it to go faster, so if it has more pitch will it go faster?

 

[Tacklewasher]

Re: 9.9 hp Johnson Beat up Prop? Cavitation? New Prop Size?

Kinda hard to say without knowing how the boat is running now, especially what RPM the motor is running at. In general, a higher pitch will move through the water faster, but will put more load on the motor slowing down the rpm's, making the boat slower. No one will really be able to guess what results you may have without more info.

Lot's of info here on what happens to motors that are run too slow (rpm) by going to a higher pitch prop, and it is not something you want to do. But if your rpms are too high now, a higher pitch prop is the way to go.

You really need some kind of tach to work this our properly.

 

[Silvertip]

Re: 9.9 hp Johnson Beat up Prop? Cavitation? New Prop Size?

You have 9.9 HP and are carrying three people. How fast do you expect this boat to go? Install the standard prop and be done with it. Messing with other props will not make it a speedster no matter what you do. We know you have a 9.9 Johnson but we don't know what year so we can't help you with after market equivalents. To fine tune the setup (prop and engine height) you would need to install a tach. You make several runs at wide open throttle with an average load while noting WOT rpm. If the rpm is higher than the manufacturer recommends you need more pitch. If the engine cannot reach the high end of the recommended rpm band then you need less pitch. On those small motors you don't have much to choose from because of the lack of actual horsepower.

 

[fishrdan]

Re: 9.9 hp Johnson Beat up Prop? Cavitation? New Prop Size?

What is the altitude of the lake(s) you are going to? How much does your rig weigh; hull, gear, people? How does it run at near sea level or maybe a few thousand feet elevation? Have you done anything to the outboard for high altitude operation (if you are running at high altitude) carb jetting, prop with less pitch? I'm not saying you don't need a new prop, but think there could be other factors involved and that you should possibly drop pitch to pick up performance.

I've seen the exact same thing you are describing when there was nothing wrong with the outboard. The outboard would run great at 1000', but take it up to 9000' and open the throttle, it would bog and give about as much power as idling,,, too much fuel. Then another problem comes into play, the thin air will make the outboard less powerful. So your 9.9 might only be making 5-7HP.

My 7.5HP outboard will run the boat at about 25MPH at sea level, but take it up to 9000' and it will only run 12MPH with the same load, not enough power to get it up on plane. That's with the carb leaned down.

Another boat I had,,, Ran perfect as 1000', but take it up to 9000' without any modifications (carb, prop) and it would do the exact same thing you are describing. It was actually faster with the 45# trolling motor on high speed

 

[grrr_evans]

Re: 9.9 hp Johnson Beat up Prop? Cavitation? New Prop Size?

I am thinking of buying a 4 blade prop over a three blade what is the difference?

 

[grrr_evans]

Re: 9.9 hp Johnson Beat up Prop? Cavitation? New Prop Size?

The engine is a 1980 Johnson 9.9 hp, and it powers a 14ft harbourcraft tin boat, I usually fish with it and just go crusin on small lakes to good fishing spots, so I don't have a tach or gps on it I was just wondering what would be the proper prop pitch! I cannot find any indications on the pervious propany suggestions where too look?

 

[steelespike]

Re: 9.9 hp Johnson Beat up Prop? Cavitation? New Prop Size?

Generally a 4 bld prop is to improve low end performance or push extreme loads.Usually producing less top end.If you select a prop that allows the motor to run beyond its rpm range you could end up with a blown motor
if the prop makes it run below its rpm range it could cause premature maintenance problems and possible failure.The wrong prop could result in poor performance and economy.
A tinytach for your motor would cost about $40 very easy install and it will fit any 2cyl. 2 stroke motor.Just be sure to tell them the type of installation as the wires can't be spliced or cut.
Without a tach and a borrowed gps there is no way to evaluate what you have.All that said if I was going to gamble on a prop for your application it would be a 9" pitch.With any small motor weight placement and motor trim is critical.Also raising the motor vertically to a point just before venting becomes a problem could gain a little speed and perhaps improve hole shot slightly.If you have no venting problems now the motor is probably too low.
Be sure it continues to pump water.New prop then test.Then raise the motor
a little at a time.Just add 1/4" sticks on the top of the transom.

 

[Silvertip]

Re: 9.9 hp Johnson Beat up Prop? Cavitation? New Prop Size?

Here are your choices:

http://boatpropellers.iboats.com/Jo...r=1980&**********=078557103&*******=992638626

You would likely need an 8 or 9 pitch for the load you carry. Understand that you can't just slap a high pitch prop on the engine and expect it to become a speedster. With a heavy load the engine does not have enough power to twist that much pitch. Without a tach y0u (and us) have no idea what to suggest other than middle of the road.

 

[grrr_evans]

Re: 9.9 hp Johnson Beat up Prop? Cavitation? New Prop Size?

Hey Everyone thanks for all the help! I just wanted to show you my results from the work I did on my little fishing boat and say thank you again to everyone who help out!

http://www.facebook.com/home.php?#/photo.php?pid=5701724&id=546905611&ref=mf

 

[champ21]

Re: 9.9 hp Johnson Beat up Prop? Cavitation? New Prop Size?

I have a 9.9 mercury on a 14 ft. alumacraft.
The engine overrevs with the standard prop,
After reading everything I coud find on the net the manufacturer recommends moving up to a 10 1/2 inch prop on this motor with a 14 or 16 ft aluminium boat.the standard prop is a 9 inch pitch.
basically after you hit max speed and you still have more throttle and rpm's your proboblly under pitched.

champ

 

[Sandhammaren05]

Re: 9.9 hp Johnson Beat up Prop? Cavitation? New Prop Size?

4 blade unnecessary. Buy std. prop or 1" pitch less than standard.

Also, you can boost the motor to 15 hp just by buying a carburetor for a 15 on Ebay.

I am thinking of buying a 4 blade prop over a three blade what is the difference?

 

[bxrxt79]

Re: 9.9 hp Johnson Beat up Prop? Cavitation? New Prop Size?

yes you need a new prop put it in gear and try to turn the prop better yet pull the prop look at the back side if you see chunks of rubber replace it i just had to on my 1984 9.9 you spun the hub man if it is as old as mine or even close that prop dosent owe any body anything mine is stock 9 1/2 x 10 pitch so hope it helps

 

[12guns]

Re: 9.9 hp Johnson Beat up Prop? Cavitation? New Prop Size?

Well, If you're interested in more speed w/ a 9.9, you won't gain much. Prop will help some, but I found that the carb change mentioned before, and a few other minor things will give you the most out of a 9.9. Here's what I did. my 77' 9.9 will top out at 23 w/ just me and a fuel tank on my 14' Duracraft "fisherman".
Here's what I did:
15 hp carb - easy swap.
removed exhaust baffle
left top half of air silencer off
Some will argue the carb change doesn't work, other say it does. I'll say the motor runs AWESOME for a 9.9, but it's still a 9.9-15 no matter how you look at it. Hope that helps.
I think I'm running a 10" pitch, but have a worn out 9" that runs at higher RPM's. I don't have a tach, so I use the 10" more to avoid over rev'ing.