Prop sizes for 1976 70HP Evinrude

[cp4000]

Hey guys,
I just bought a project boat with a 1976 70HP Evinrude on it. The prop is in really bad shape and will need to be replaced at some point. What size do you guys recommend for top end speed? The boat is a 1976 16' Eldorado Eldocraft II. With a prop setup for top end speed and assuming engine / boat is performing well....would I be crazy to think I could get 35-40 MPH out of it?
Thanks for any feedback

 

[steelespike]

Re: Prop sizes for 1976 70HP Evinrude

Start with the same size that is on it. The size should be on the barrel or under the prop nut. probably 13 1/4 X 17.Let us know what you find.After you get her all done do some test runs and get us the numbers wot rpm and speed.
Yes you could expect 35-40 with a proper setup.We can try to help fine tune the setup.

 

[cp4000]

Re: Prop sizes for 1976 70HP Evinrude

Thanks for the response, it has a 14x17 on it now. My other question is motor height poistion...Not exactly sure which one is the cavitation plate that people are referring too? Is it the plate that is just below the water outlet holes (where you would mount a whale tail) or the one above it? The plate above the water outlet holes is about 3" above the bottom of the hull, the plate below the water holes is about an inch above the bottom of the boat? The motor is mounted on the second mounting holes to the bottom...Does that sound like a good position to start? Testing it in a tank tonight with the newly installed pee-hole mod. If tests well, open water tests soon after

 

[steelespike]

Re: Prop sizes for 1976 70HP Evinrude

anti vent plate is just above the prop.Yes where you would attach a foil.
Yours sounds about right at about an inch above the bottom.
Should be visable on plane.See how the performance is. Most likely it will work as is but every boat is different. Prop size sounds good.Good luck.

 

[cp4000]

Re: Prop sizes for 1976 70HP Evinrude

Hey guys,
Finally have some numbers...

The prop it came with was a 14x17, I am currently using a 13x19 just to test the condition of the motor. The cav plate is an inch above the bottom of the boat. (The lowest mounting hole.) Under moderate waves with or against the wind, hit 35.4 MPH today at max RPM of 4900. There is no trim on the motor yet so I may have been able to squeek out a few more. The max RPM I belive is 5500-5800 RPM so which way should I go? 13 1/2x17 or 14x17? Think it will be possible to tick 40 with trim and the right prop?

Thanks!!!!

 

[Dhadley]

Re: Prop sizes for 1976 70HP Evinrude

If you are going to make a major change like adding power trim then we need to do that first. After adding trim the prop choice may change.

 

[steelespike]

Re: Prop sizes for 1976 70HP Evinrude

You should fine tune the trim so we are sure you are reaching max possible rpm.
It will be good to know how the boat responds to up trim.

 

[cp4000]

Re: Prop sizes for 1976 70HP Evinrude

Ok,
Looks like I will have to go with a CMC unit, the OEM ones are impossible to find. The weather up here in WI is cooling off quickly so I may not have that on and tested until Spring. (Not cheap! haha) Thanks guys!!