Re-prop advice

[Nitro2014]

I have a 1985 2 stroke Johnson 115 hp outboard. It runs 4500 rpms at WOT all the way trimmed up with just me. When I add an extra 300 lbs it will run 5000 rpms at WOT trimmed all the way up. I have been told that the max rpm should be between 4500 rpms and 5500 rpms and I am over-propped and lugging the engine. Right now I have a a 13 1/4 x 20 turbo-o 4 blade stainless prop. I would like to keep the next prop between $100 and $200. I prefer another stainless 4 blade but am not opposed to trying out an aluminum or 3 blade. (Just need to choose the right one without shooting in the dark). Any suggestions on a lower pitch change and/or different prop styles in general?

 

[Sea Rider]

Why are you running trimmed all the way up. When boat is on plane running parallel to water level you need to trim it to have a perpendicular engine provided that deck weight is evenly distributed. Wariables such as correct engine/transom height, trim, deck weight ballance will make a difference.

Been reading different engine owner's manuals brands that states you should trim engine bit less (-) when boat is on plane trim + to have boat running parallel to water surface, test that and check if works better.

Happy Boating

 

[Nitro2014]

Thanks for your reply. What I usually do is trim as far as I can up to prevent as little drag as possible. If the boat starts porpoising, I trim it back down a little. I was able to trim the prop all the way up because the water was extremely smooth. Most of the time I am not able to trim as much; resulting in even lower WOT rpms as previously stated.

 

[jestor68]

How is it that you run higher rpm with more weight in the boat?

Describe the boat ( yr make and model ) so we can look it up and see a recommendation for a prop.

The only thing in your price range is aluminum, unless you can locate a used SS.

 

[Nitro2014]

I sort of wondered about the rpms being higher with more people on board too! I did the test on two separate days with about a week in between. Perhaps maybe my motor was running a little better with more people because I just changed the plugs prior to going out? The boat is a 1985 Nitro MX17 Ski/Fish. Fiberglass hull. 17 feet long. 12 gallon fuel tank, 3 batteries, and 20 gallon capacity live well all at stern. Usually run the boat with around 450 lbs of additionally weight and gear on top of weight of previous sentence. We tube with it as well. Motor current set up right now will push the boat 53 mph with one person. Gets out of hole fine without extra people and/or livewell filled, but is otherwise slower than I like. 5-6 seconds

 

[steelespike]

53 isn't possible at 5000 rpm. I suspect the speed isn't accurate.A gps will deliver best speed indication.
At 5000 speed would be about 42.If you can get the gps speed it will help tell if the tach is accurate.
With your stern weight bias I think 5 or 6 seconds hole shot is pretty good.If 5000 is accurate it appears a 17" prop may be in order.
Changing to an al prop makes predictions difficult. But dropping 2" in pitch should improve hole shot.Stick with as 4 blade.
Dropping 2" will reduce top end.

 

[Nitro2014]

I checked the speed via gps and the speedometer on my boat is correct. However, I believe you are on to something by saying that 53 mph is not possible with 5000 rpms. Perhaps my tach is off? The boat seems to sound fine when running. The idle rpms seem to be correct. Is it possible the tach is out of adjustment at only higher speeds? (May be getting into another thread here but any help is appreciated)

 

[500dollar744ti]

It would make sense that your tach is off because you shouldn't be getting more RPM with a heavier load. There's probably a bad connection or some resistance in the tach causing the bad reading.

If you have verified 53mph via GPS I would bet you are propped correctly and your tach is just off.

 

[jestor68]

Let's try this. Using a 9% slip value and a 21 pitch(SS 20" cupped prop) with a 2.0 gear ratio at 53 actual mph equals a theoretical 58 mph, which works out to about 5833 rpm.

And you're showing what rpm?

 

[Nitro2014]

The boat will run a max of 53 mph with just me and will go 50-51mph when I had a couple of other adults on board. I can tell you that the last speed reading I got recently with the two other adults on board was 50-51 mph and the rpms read exactly 5000. This was smooth water with as high as I could trim the motor. Should I calibrate the tach, adjust the engine height to raise or lower the anti-ventilation plate, or re-prop?

 

[steelespike]

If the tach has a plug, unplug it a few times to clean the contacts.If there is a selector note the position and operate it few times to clean the contacts.
If your tach reads 5000 obviously your almost 900 rpm off ,you need to lower the rpm.
It develops it hp at 5000, 5500 is a reasonable max rpm A 2" increase in pitch should lower the actual rpm about 300 to 400