Top speed out of a 1969 Evinrude 115hp on a 18ft Larson Shark Tri-Hull???

[1968pontiacgto]

So I just redid about every tune up item on my engine and I am wondering what type of speed should I be getting out of this motor. Right now the speedo is showing about 45mph, I think the RPM's are about 4100 at top end. This is fitted on a 1969 Larson Tri-Hull Shark (bowrider). I also am wondering is it correct to blow through 6gal. of gas in about an hour?

Your thoughts?
Thanks,
Damian

 

[windsors03cobra]

Re: Top speed out of a 1969 Evinrude 115hp on a 18ft Larson Shark Tri-Hull???

Expect to blow thru 6 gallons of gas in an hour at WOT doing 45 in a 18 foot boat, that's cookin.

 

[ezmobee]

Re: Top speed out of a 1969 Evinrude 115hp on a 18ft Larson Shark Tri-Hull???

Your speed sounds to be just about perfect and your gas usage is normal (actually good). The rough formula would be 11.5 gallons in an hour at WOT. (HP/10) However your RPM is way low. You should be able to hit around 5500. Something might not be right with your tach though because I wouldn't think you'd be able to hit 45 at 4100 RPM.

 

[1968pontiacgto]

Re: Top speed out of a 1969 Evinrude 115hp on a 18ft Larson Shark Tri-Hull???

I am also thinking about adding a Hydrofoil to it, what are your thoughts on those? Think it will save me some gas, maybe get me some extra MPG?

 

[ezmobee]

Re: Top speed out of a 1969 Evinrude 115hp on a 18ft Larson Shark Tri-Hull???

I'm doubting your motor has power trim...correct? I believe the foils can cause problems if you can't adjust your trim to an optimal position once on plane. I would skip them. If you have low-speed planing issues I'd consider smart tabs.

 

[1968pontiacgto]

Re: Top speed out of a 1969 Evinrude 115hp on a 18ft Larson Shark Tri-Hull???

Your right no power trim, the smart tabs would just level it out? I looked into them but cant see how they would know how to adjust without hydrolic or electric assist. I think I would prefer to have control of them if that makes sense?

 

[tboltmike]

Re: Top speed out of a 1969 Evinrude 115hp on a 18ft Larson Shark Tri-Hull???

My 17.3' trihull powered by a 1975 115 Johnnie runs about 39 by gps and speedo. However, it had been de-tuned to run on unleaded and lost about 3 mph. It gets about 20 miles per 6 gals bay running at mixed speeds mostly not WOT, but more like the RPM's you mentioned. WOT is 5100 rpm's on mine.
The rig is about 2000 lbs in the conditions mentioned above.

 

[1968pontiacgto]

Re: Top speed out of a 1969 Evinrude 115hp on a 18ft Larson Shark Tri-Hull???

Any ideas about changing props to maybe pick up a few MPH more?

 

[NoKlu]

Re: Top speed out of a 1969 Evinrude 115hp on a 18ft Larson Shark Tri-Hull???

You can't trust the speedo on your boat as they are not very accurate and give a rough estimate only. Get a gps of some sort to confirm your speed. You will also need to know the true rpm as well so try and get your tach confirmed. Look on the prop forum for the information you need so you have a starting point for the prop tuning. Thats all that might be wrong with the set up.

 

[26aftcab454]

Re: Top speed out of a 1969 Evinrude 115hp on a 18ft Larson Shark Tri-Hull???

back off the throttle a little to save more gas-like the other replies- you should hit 5,000-5,500WOT.- I also think 45mph is as fast as possible with that boat/motor combination.