Dumb newbie question

[jtl_90]

I was riding with a friend in his 16' flat bottom alum. john boat with a 40 horse merc outboard. The boat and motor run great up until 3/4 throttle. From 3/4 to wot the rpms go up but no increase in speed. Is this normal for this set up? I have no experience with outboards so I am no help to my friend.
The motor has a 15" 4 blade s.s. prop.
Any help or a direction to go would be great because it's bugging my friend to death!

 

[BIGcarpy50]

Re: Dumb newbie question

could possibly be a slipping prop, carb issues, coils, or stator. check the HUB on the prop, and check the electronics.

 

[reelfishin]

Re: Dumb newbie question

Is it truly increasing in RPM's or just getting louder and bogging down?
I'd say maybe it's dropping a cylinder or has too big or too steep a prop pitch.
40 hp on a jon boat should all but fly. I tried a 50 hp on a 17' once and could never think about wide open throttle and needed to keep some ballast weight in the bow.

Is the motor the right length for the transom?

 

[jtl_90]

Re: Dumb newbie question

The motor seems to run very well. It does not have a miss to it or seem to be doing anything else to make me think there would be a problem with it running. So maybe the 15" 4 blade is to much? There are always atleast 2 peaple onboard with fishing gear and/or hunting gear so the motor needs to be able to work.

 

[tashasdaddy]

Re: Dumb newbie question

also could be trimmed too high, and prop starting to blowing out. (loose bite)

 

[JB]

Re: Dumb newbie question

Sounds like prop ventilation to me, but could also be cavitation.

I suspect that fancy, 4 blade, prop is entirely unsuitable for the application and/or the outboard is not ideally rigged.

Let's put this in the Prop Forum and see what the prop gurus think.

 

[jtl_90]

Re: Dumb newbie question

Sorry if I posted in wrong forum. I am trying to learn as much about boats and everything about boats as I can. I am getting ready for my 2nd season as a boat owner,so I figure if I can help my buddy out,it will just give me more knowledge for the future.

Thanks for the help so far!

 

[reelfishin]

Re: Dumb newbie question

The prop on my 40 hp, which was last on a small glass boat, about 600lbs in weight, is only about 10" in diameter and maybe a 13 pitch or so.
I don't think I could even get a 15" on that motor?
Try your specs on one of the online prop selectors and see what it gives you.