HELP!!! Prop Issues

[Bandchaser]

I have a 86' 50hp tiller that I got in a boat, motor, and trailer combo. The compression, spark, and fuel all tested great. In the barrel, it seems its going to rip your arm off. But under a load, its a real dog. The boat is a aluminum Duracraft 1860.
The motor had a 10 3/8" x 15 pitch prop. Went to a outboard shop, and they let me try a 13 pitch. The boat doubled the speed and almost planed out. When I returned, they said I needed a 9 pitch prop, so they ordered one. Got the prop and installed it friday. Went duck hunting saturday, and the motor is worse now than before.
They ordered a 12.1" x 9 pitch prop. All my research today is telling me that the new prop is incorrect. Please help, as I just don't have the time or the money to be spending on multiple props.

Thanks,
Brian

 

[5150abf]

Re: HELP!!! Prop Issues

I have a 50 on a heavy 16 fishing boat and run a 12 1/4x15 so if your boat is just an empty jon you should be able to run bigger than that 9 pitch is way to low.

 

[Frank Acampora]

Re: HELP!!! Prop Issues

Difficult to say with the sparse info you gave. However, the 12 X 9 is a prop designed for something like a pontoon--a displacement hull. If the stock prop on your engine was 10 inch diameter, then perhaps a 10 X 11 would work. Just be aware that 50 on an 18 foot boat, even a tin boat is not going to move it fast and may just be able to get it on plane and not more.

Just for comparison, I had a 55 on a 15 foot boat and used a 10 X 13 prop and topped out at about 25 mph. The same engine on a 10 foot go-fast hull uses a 10 X 15.

 

[cyclops2]

Re: HELP!!! Prop Issues

My 16' Lowe Aluminum open fish boat almost planes on a Evenrude 9.9 hp.

Will VERY easily plane with 2 ..200# guys with the 30 hp on it. Max USCG rating is 30 hp.

A 50 would cause whiplash & dump my buddy in a quick turn.

 

[cyclops2]

Re: HELP!!! Prop Issues

Is the rear bottom of your hull as flat as the middle of the hull ?
Or does it have a rounded out tunnel where the prop spins ?

 

[steelespike]

Re: HELP!!! Prop Issues

You don't say your brand of motor but:;but boat is about 575 lbs.
I have a 87 50 hp Evinrude on a 77 18ft 375lb Crestliner bare utility that runs a 19" pitch very close to 40 mph. In my opinion your 15" pitch should be close to overreving.
If its a jon it may not be the fastest boat around but should leap up on plane and easily run 30-35. Could be as simple as missing a cylinder.

 

[Bandchaser]

Re: HELP!!! Prop Issues

Im sorry its a 86' mercury. The hull is a mod-v, flat bottom. I was told this evening at the shop that the 9 pitch should have popped it right out of the water. Maybe even have to back off the throttle also. I am getting confused and very upset over this situation. I appreciate everyones reply, and welcome for more ideas.

Also, he told me the diameter would not make any difference in performance.

 

[cyclops2]

Re: HELP!!! Prop Issues

Prop diameter makes a....HUGE ....difference. The greater the diameter the more water you are pushing. Person has very little knowledge of selecting correct props.

I used to call Michigan Wheel Co. for selecting a prop. They usually got it within 1" of pitch. You will spend all your money listening to your locals & this website.

BORROW.............BORROW the dam props!!!!!!!!!!!!!!!!! Stop wasting your money.

 

[steelespike]

Re: HELP!!! Prop Issues

Extremes in diameter can cause different performance Just as props of extreme blade design differences will deliver different results. But typical props intended for your application will work within a couple of hundred rpm of one another.Your boat is a straight forward application.
Not sure of your gear ratio but your 15 should be a good starting point,should plane easily and run strong.You need to confirm the motor is running on all cylinders check condition of plugs.Confirm each cylinder is firing.Is threottle ope ning all the way.Running in a barrel
only confirms it runs.It could be runnug lean a little choke will usually help in that case.
Your prop is not at fault and a 9" pitch is usually seen on a boat like a pontoon that is hard to push.

 

[mommicked]

Re: HELP!!! Prop Issues

While your trying new props/pitch's, try a 4 blade. I have an 1860 G3 jon and a Solas Aluminum 4 blade made a vast improvement in performance. I do have alot more HP though. Also where is your cavitation plate in relation to the bottom of the boat? above, below, even?

 

[cyclops2]

Re: HELP!!! Prop Issues

I sure hope you do NOT have a problem I had. 1 of the 2 Damm spark modules was intermittant. Took 2 years before it was a problem at the test tank & caught crapping out. Plugs would look good or almost good. Another give away to ignition problem, is no hot restart or a rough restart.

Run the correct amount of Sea Foam gas tank cleaner in the next tankfull of gas you buy. That stuff is harmless to engines. But will quickly clean out the fuel system. It also does remove some water in the fuel system.

Get back to the bigger & higher pitches for that motor. What was the original prop size & pitch when it was BRAND NEW ?