1993 evinrude 40hp prop and performance combination

[Maddmunkey]

Hello everyone, I have been a lurker on these forums for a while and got some great info. I'm having an issue with the correct propping on the boat and motor combo I'm running. First off the boat is an early 90's tidecraft 1300 series 15ft with a shallow v hull. The boat weighs in dry at 700lbs + 550lbs of people and gear. The motor is a 1993 evinrude 40hp 2cly 2stroke with a 13x11 prop that was originally on it.
The motor had issues with the timing advance and was fixed along with rebuilt carbs and a new power pack. Motor is synced well and runs apparently good. I have played with pitch, prop height and hydro fin non has made a noticeable difference in speed and performance.
At wot motor runs at 4000rpm moving 16mph with13x11 and my partner has to counter balance front of boat and does not plane.
Switched to 12x15 per forums I've read. Motor runs at 3800 rpm at 16 mph same issues as previously noted.
Switched to a 13x10 motor would cavatate at roughly 5mph
Then over revved till I reduced throttle.
What am I missing? Motor issue still? Motor underpowered for boat? Recommended prop? Any info advice would be greatly appreciated. Feel free to ask for any more info needed. I am determined to figure this thing out. Thanks guys and gals.

 

[Sea Rider]

Welcome to Iboats,

What's the max HP transom rated for ? What's the Min-Max Wot RPM range factory stated ? If OB is badly height seated and trimmed prop will lose best thrust. Firstly will need to "determine" if OB sits at the sweet trasnom height, if not correct OB height to go from there testing props pitches till one of the many you have tested dials middle to max wot rpm range. Max range is preferred as currently loaded.

Happy Boating

 

[Maddmunkey]

Thanks for the reply. Transom I believe is rated up to 100hp
Data I've gathered for 40hp recommended rpm is 5500-6000.
I currently have ob mounted one notch up from the full bottom position. It has 4 height adjustments I tried all four and lowest and second lowest are the best with out cavatating.
Transom is a fixed trim with 3 bolt adjustments. I currently have it in center "neutral" position.

 

[Sea Rider]

OB should be height seated at a hole that achieves water flow at speed to skim right under upper water deflector plate, check which lower holes dials that condition. OK to have trim set at neutral, boat & anticav plate will ride parallel to water level when on plane provided that deck weight is evenly distributed.

Lower leg should be riding like this : https://www.youtube.com/watch?v=oBeZjaA76VM

When have dialed the best engine height position, try your props and stay with the one that dials middle to max wot range (5800-6000 RPM).

Happy Boating

 

[Maddmunkey]

Thank you for the info and video. I believe I have the ob height and and in right tilt position. I can't seem to find the optimal prop. How much of an rpm gain will placing a shorter diameter prop? For ex. I'm running a 13x11 at 4000rpm vs 11 1/5 x 11.
I previously had 12x15at 3800rpm and a 13x10 that would cavatate at any rpm over idle.
Thanks for the help.

 

[Sea Rider]

According to boaters diam varies very little RPM. What's the diam/pitch of the factory delivered OEM prop ?

Your achieved wot tests are extremely low, way under the lugging side of the wot range which is bad for OB, too much stress on pistons, piston rods.. You're definitely testing wrong props...

Better check an Evinrude props selector finder as a prop starter and go from there maximizing pitch if in need.

Happy Boating

 

[b.gagnon]

If the mounting height is good and the hull is not filled with water (adding extra weight) something is not right with the motor... Does it have power trim? If it does are you trimming it out?
You could also have a bad Tach.... I would think that the motor would redline with an 11 pitch prop.

 

[anzomcik]

What stands out to me is that the rig has a ceiling at 16mph and 3800-4k RPM (probably right to where the boat wants to jump on plane) That is where it is likely needing the most power.

So IMO it is either the motor is having issues while under that amount of load, or whats more likely is the boat is underpowered and reguardless of the prop you will always be teetering on that plane/noplane.

I have a boat that is just under 17' glass and running wiehgt maybe 1800 on a light day. It had a 70hp on it. It would plane out but took 10 seconds or so with alot of trim work. With my boat I do not think a lesser motor could have planed me out, so i bought a 115 maxed out the HP and not I got a nice matched rig that I am very happy with. My experience