trying to figure it out

[raevin51]

Hi all I have a 1969 Seabird runabout with a 1983 Johnson 115 It currently has a 17p x 13 3/4 alum prop. I can not go to WOT at 5500 rpm I am at roughly half throttle and don't want to push my luck with opening all the way. I had this same engine on my 65 Cobia until the boat decided to flop over on its side after crossing a small wake at very low speed, (subject of an other posting) I have tried using a prop cal calculator to find the right pitch to use but that only gives approx speed for any given rpm and pitch, it does not take into account prop diameter and boat weight. I have also tried the Evenrude site and the parets list for this engine shows everything from an 11p to a 23p prop. The question is does anyone have a Johnson 115 and what pitch props are you running with what results.

 

[JoLin]

Re: trying to figure it out

You need to get more specific.

I think you're saying that your motor reaches 5500 rpm before you've advanced the throttle all the way. Is that correct? If that's the case you definitely need more pitch, the trick is figuring out how much.

Is this happening when the boat is carrying an average load of passengers, fuel and gear?

What is the manufacturer-specified WOT rpm range for your motor?

What is the size of your boat? What is the hull design? Semi-V, deep-V, tri-hull, other? I doubt many people know much about a 1969 Seabird runabout. Simply asking how others are propping a 115 won't get you anywhere, as they're mounted on all manner of different boats.

I can understand your reluctance to push the motor past the redline, so you may need others with similar type boats to provide you with a starting point. If that doesn't work the prop calculator should at least give you a clue, but you might need to experiment.

 

[steelespike]

Re: trying to figure it out

If you are reaching max rpm at 1/2 throttle I would suspect a tach error perhaps the setting is wrong.Without the size and weight of your boat it is al;so difficult to tell where you are at.It would seem a 17 is in the ball park unless the boat is small/light.Give us some real details.

 

[72SideWinderSS]

Re: trying to figure it out

Where's Dhadley Or WH when you need them?
Personally, I wouldn't be afraid to spin it up to 62-6400 RPM for a short blast.
A stock 115 in good shape, normally will not spin up (with a load) much more than that. They run out of breathing room.

EDIT:
I do agree with the previous posts. Not discounting anyone here.

Maybe D or WH will chime in.

 

[raevin51]

Re: trying to figure it out

Sorry for not giving more info the boat is a closed bow runabout 1969 Seabird. The weight unloaded is around 1600 lbs. with all of my gear and my grandson and I about 2000 lbs. The engine is a 1983 Johnson 115 in near new condition and properly tuned. The boat is used in salt water in the south bay of Long Island and occasionally out into the ocean along Fire Island to Montauk Point.
I will check the tach setting in my handy dandy but mostly useless Sealoc book. I have been considering trying a 23p prop and working down from there since the 17p seems to be revving so high. Hope this info helps in finding an answer.

 

[Dhadley]

Re: trying to figure it out

We're still looking for the answer Jolin & Steelspike asked - are you saying it reaches 5500 before you get to full throttle position?

I agree with the others - a 17 should be in the ball park. Make sure your tach is set on position 6. If it is, it's not going to hurt to run the throttle up some more to see what you get. You're looking for 5800 with an average load.

 

[raevin51]

Re: trying to figure it out

Sorry I missed that one. It crosses 5500rpm just past 1/2 throttle. I also checked the prop hub and it appears secure.

 

[Dhadley]

Re: trying to figure it out

And the tach setting?

 

[raevin51]

Re: trying to figure it out

Thanks everyone I replaced the tach this morning and solved the problem

 

[walleyehed]

Re: trying to figure it out

Dhadley has a knack for sniffin' out those tach issues...