Bringing the RPMs up.

[Mattedhead]

I have a 14' McKee Tri-Hull that I recently restored. I added a tach for diagnostic purposes and I have found the RPMs to be well below reccomended at WOT. The engine is an '83 Evinrude 70hp. Curently at WOT I am getting around 4900rpms at 29kts with a 13.25x17 MWC prop. THe prop has seen better days, but more importantly it is lugging the engine. I would like to get my RPMs up to 57-5800WOT. I understand that this would bring the pitch down ~ 4-5". I would like to get the Hustler aluminum prop, but I am unsure as to which one to go with. As the pitch drops into the 11-13" range the diameter jumps up to 14". Any ideas on what prop might fit the bill, I don't want to continue to lug the engine.

Thanks-
Matt

 

[WillyBWright]

Re: Bringing the RPMs up.

A 70 on a 14'er should twist at least a 19 with no problem. Your motor isn't running on all three.

 

[QuadManiac]

Re: Bringing the RPMs up.

Sorry, don't mean to insult your ability, but should probably ask the obvious: Are you sure that you have your tach set for the proper number of stator poles? (I'd rather see this be the problem instead of a lost cylinder, you know?)

 

[Silvertip]

Re: Bringing the RPMs up.

My last boat was a 16 foot walleye boat powered by a 50 Evinrude and it had no problem reving to 5800 with a 17P. The prop you have is not the issue.

 

[WillyBWright]

Re: Bringing the RPMs up.

I kinda hate to ask, but is that 14'er rated for 70?

 

[Mattedhead]

Re: Bringing the RPMs up.

I hate to bring this thread back from the dead...

The tach is at the right setting, and the compression is good across all three cylinders. And yes the boat was rated for a 75 in '83...though I'm not sure how.

 

[Dhadley]

Re: Bringing the RPMs up.

You say the tach is on the right setting - what's it set on?

 

[Mattedhead]

Re: Bringing the RPMs up.

It's a teleflex set to 6P. From what I understand this motor as a 6pulse/12pole alternator. I hope that's correct.

 

[Dhadley]

Re: Bringing the RPMs up.

That is correct. Bummer in a way because a 17 is what we use on those (and Whalers) most all the time even for (light) skiing. My next step would be to look at the motor and verify everything is working as it should. Timing, carb going wide open, timer base advancing all the way etc. Obviously after a visual check it'd be nice to put a test wheel on and see where we're at. That would tell us if the problem lies with the motor or the hull.

We're assuming the motor is all the way down and transom mounted (no jackplate). If the visual checks out and you can't get it to a dealer with the correct test wheel, I'd raise the motor and see how the r's respond. Again, assuming it's all the way down now.

 

[Mattedhead]

Re: Bringing the RPMs up.

I really appreciate it. I will check all of this and take pics and post later this evening.

 

[Dhadley]

Re: Bringing the RPMs up.

Great. BTW - I like the avatar.......

 

[Silvertip]

Re: Bringing the RPMs up.

While you may have good compression, this thing is down on power. A 12 or 13 P prop would be a real waste of money. Fix the engine first. The boat may also be full of water soaked foam so make sure you are not lugging part of the lake with you.

 

[Mattedhead]

Re: Bringing the RPMs up.

Any ideas on where to find a set of scales? Or would it be obvious by the waterline?

 

[Dhadley]

Re: Bringing the RPMs up.

That's why I suggested a test wheel. That seperates motor issues from hull issues. If the motor comes up to (test wheel) rpm, then the issue is with the hull. It may be weight, it may be shape. Once we verify the motor is OK - or not - we'll know which part to look at.