Propeller Selection Assistance

[RCDuke]

Hello All!

This is my first post here...

I have a 14' Nova Ultra Jon with a 6' beam and a 2005 Yamaha F9.9. Google has taught me how to make a homemade jack plate and how to set the motor at the proper height, but I still have one issue. My engine reads about 4500 RPMs @ WOT traveling at 16 MPH according to my GPS (with my daughter and I on board [315 lbs combined], a battery, and a trolling motor.

I recently purchased a Solas stainless prop 9 x 9 (stock size and pitch) because the stock one was spun.

My questions are:

1. Will a 7 pitch prop correct the RPMs and give me more speed or will it just correct the RPMs?

2. If I don't correct the RPM problem, will it damage my motor?

Here is a picture of my setup.



Thanks for your assistance!

 

[Bamaman1]

Re: Propeller Selection Assistance

Do you actually have a tachometer on a boat with a 9.9 hp engine? Is the tachometer used properly set for one cylinder? Is the tachometer accurate?

Does the engine run smooth, and do you feel you're getting your full 10 hp?

Sounds as if the engine is just too small for the hull.

 

[jestor68]

Re: Propeller Selection Assistance

Check your owner manual for the proper WOT rpm range. I believe it's 4500-5500rpm.

Each inch of pitch will change your rpm by 150-200 rpm.

The smallest one I see here in the prop section is a 7" pitch, which will gain you 400 rpm.

 

[Texasmark]

Re: Propeller Selection Assistance

I can't believe you are only running 16 mph. That's just on plane for a lot of boats. For your john with that much width, you probably plane out around 13. With that wide of a boat your load isn't very important as the boat has plenty of surface area and being flat bottomed, at the speeds you run, should just scoot along. I used to run a 7.5 on a similar boat and I know I was running at least 20.

Something is wrong.

Mark

 

[RCDuke]

Re: Propeller Selection Assistance

Thanks for all of the responses!

I will try to give all of you more information than I did the first time around...

I fish in a place that has a 10hp rule otherwise I would have something bigger

Yes, I have a tachometer. This is the one I have: http://www.motosport.com/HARDLINE-TACHHOUR-METER

The motor idles at 1000 or so and the numbers seem to be right.

Motor history:

I bought the boat and motor used. It had been sitting for a year and a half. It started fine, wouldn't idle, and would only push the boat 7mph. I rebuilt the carburetor (soaked it for 24+ hours), replaced the fuel line, fuel filter, cleaned the tank pickup, replaced the spark plugs, and drilled out the carburetor idle mixture port so I could adjust it (present setting 3 turns, the factory was 1 3/4). After all of that, I started using Ring Free Plus and (blue) Stabil. The propeller was spun so I replaced it as well. Then I installed the homemade jack plate and tested the boat out at all of the different trim settings. It ran a max of 18 mph, but the motor was too high on my first test run (the cavitation plate was slightly out of the water and the prop was coming out of the water in a turn - so I lowered the jack plate to where it is in the picture). The motor seems to run good, but I don't know what "good" is. I am a vehicle motor-head DIY type, but outboards are new to me. There are similarities, but some things are still a mystery.

I will gladly take any advice you have.

I contacted my local Yamaha dealer, they told me that they will diagnose it for $125.00 - how does that sound?

Thanks again!

 

[jestor68]

Re: Propeller Selection Assistance

Sounds fine, as I'm not paying. But with these 4 stroke motors, that's the best way to diagnose questionable performance or confirm that everything is all right.

 

[RCDuke]

Re: Propeller Selection Assistance

If the motor turns out to be ok, will be new 7 pitch prop make the boat faster or will it just allow the engine to get up to a higher RPM (which puts it @ its optimum for max performance)?

And...if I leave things as they are with the wrong prop can it damage the engine?

Thanks again to all who have taken time to respond...

 

[Texasmark]

Re: Propeller Selection Assistance

If the motor turns out to be ok, will be new 7 pitch prop make the boat faster or will it just allow the engine to get up to a higher RPM (which puts it @ its optimum for max performance)?

And...if I leave things as they are with the wrong prop can it damage the engine?

Thanks again to all who have taken time to respond...

Less pitch means more rpm, usually less speed, but better ability to handle larger loads.

Damaging your engine usually results when you lug it, operate it at the low end of the recommended rpm range or less at WOT.

Mark

 

[jestor68]

Re: Propeller Selection Assistance

If the motor turns out to be ok, will be new 7 pitch prop make the boat faster or will it just allow the engine to get up to a higher RPM (which puts it @ its optimum for max performance)?

And...if I leave things as they are with the wrong prop can it damage the engine?

Simple answer: at 4500 rpm, you are not even developing full power, which probably is around 5000 rpm. Right now as the motor is operating, you'll gain about 400 rpm with a 7", putting you barely at 5000 rpm.

Raising the engine rpm by 400 will most likely result in a small gain in speed in your case.

Operating the motor outside of the recommended rpm range (low or high)will shorten it's life .

 

[steelespike]

Re: Propeller Selection Assistance

On smaller motors a 1" pitch change usually results in larger rpm change than on a larger motor. say 40 hp or more.
Is the bottom of the boat clean and smooth. No bulges or hollows?
Are you able to get the boat up so it runs on about 1/2 or less of the bottom.
Jons sometimes have 4 or 5 keels and lots of rivets.Not good for high speed.
My info indicates your rpm range is 4500-5500. So your at the bottom of the rating.Usually suggested to prop
a 4 stroke near the top of the rating. I think a 1" pitch reduction would help your rpm Might improve speed.
Would surely improve hole shot and responce to added weight.
A 9" prop is right in the middle of the selection chart Would seem reasonable for your setup.
The 8" also would be treasonable.
It would be interesting to see the results with you alone and no other gear with the 9.
Some times with manual; trim one hole sometimes doesn't seem like enough and one more
hole seems like too much. You could buy or make wedges to create a half a hole change either up or down. Have you tried having your daughter move back or forward for best balance at speed.