40hp '67 super seahorse top speed.

[briandistler]

I have a 67 40hp rkl-29e on a 68 starcraft explorer 15ftish. It's a trihull of sorts. I haven't had the motor long. It starts and runs everytime no issues. It currently pushes the boat to about 23mph on the gps. Is that the top speed I should expect? If not, what should I be looking for? Thanks

 

[racerone]

Used to get about 28 MPH on a 14' runabout with 1965 Evinrude 40.-----Post the compression test actual values.

 

[jimmbo]

Welcome to iboats

Many factors come into play when it comes to top speed. Engine Condition, is only one. Is the Engine propped to run in upper 1/2 of the WOT Rpm Range. On that Engine it is 4000-5000rpm. I would guess that 12 inch Pitch would be a good starting
Altitude
The condition of the Boats Hull, is the Bottom Straight, is it smooth, do the Bottom and the Transom meet with a nice Sharp Corner, ensuring a Clean separation of water from the Hull?
3 15 ft boats, can give 3 different Top Speeds, just because of Differences in Hull Design, including Beam, Deadrise, Weight.
Even things like Air Temp, Elevation above Sea Level, Fresh or Salt Water will affect the speed.
I don't put much faith in GPS Speed Readings if there is any kind of Current, as that will measure the Boats Speed across the Bottom of the Lake/River, not what it's Speed across the Water. To get an accurate GPS speed, 2 runs in opposite directions will reveal any effects of a Current.
We had a Wide Heavy Sow 14ft Skagit, it got about 25mph according to the Water Pressure Gauge. We were at about 2000ft above Sea Level. A narrow 15 ft would likely have beaten us.

 

[airshot]

Check compression on motor...if motor is running at its peak, what rpm are you getting at wot ?? Anything else is a guess, you need rpm at wot on a proper running motor !! Trihulls have drag from there design, no way around it. It won't be as fast as a tinnie with the same motor. Check your engine and give us the wot rpm, then we can guide you.

 

[briandistler]

Check compression on motor...if motor is running at its peak, what rpm are you getting at wot ?? Anything else is a guess, you need rpm at wot on a proper running motor !! Trihulls have drag from there design, no way around it. It won't be as fast as a tinnie with the same motor. Check your engine and give us the wot rpm, then we can guide you.

The weather today is no good. I bought a tach and will get the wot rpm as soon as I can. Thanks for the help

 

[Texasmark]

I pulled up a picture of it and that is an easily planed hull. Assuming that the boat isn't waterlogged, that is a pretty light boat. Remembering the inlaws 15', windshield, front deck and associated steering fiberglass with cross simple wooden seat, about a '60's 40 HP Rude, stock prop, with a couple of us in the boat 28 rings a bell.....a good clip for a reasonably soft ride; pulling one pair of skis and a driver was a good enough match. Good suggestions herein for a path of action.

 

[flyingscott]

Your speed is where I would expect it to be.