76 Evinrude 85hp prop?

[jafitzwater]

I have a 1976 85hp Evinrude on a 17ft stinger bass boat fiberglass. Carrying 3 people tackle, 2 batteries, troll motor, and 10 gal of gas I do about 38Mph.

My current prop is a 13 1/4 x 17 stainless steel
It hits 4500rpm at wot.

This motor says max rpm is 5500.

What is the recommendation for the size I should use to get more top speed and what's the estimated speed I should be getting?

Thx let me know if anything else is needed.

Not sure of total weight of boat unable to separate boat and trailer for weighing

 

[fab74]

Re: 76 Evinrude 85hp prop?

Hi, 4500 rpm seems to be a little low for that boat. Your right in that the upper end rpm should be around 5500. Evinrude gives a range of 4500 to 5500 rpm, and they like the high end of that. Just for comparison, my 74 Fabuglass 18 ft trihull with 74 Evinrude 85 with an OMC 13 1/4 x 17 SST II prop will run 44 on the GPS at 5600 rpm with just me and 16 gals of gas. With 3 adults, 4 kids, and loaded with big cooler, fishing gear, etc, it will run 40 GPS at 5400 rpm. How high is your motor mounted on transom? Are you trimming it out any once up to speed? Has the boat been weighed recently? Sorry for the long reply, just trying to help out.
D.

 

[fab74]

Re: 76 Evinrude 85hp prop?

Hey, just reread your post about unable to weigh boat. You could weigh the whole rig at a truck stop scale and then subtract the estimated weight of the trailer. At least that would get you in the ball park... hull not water logged, etc. I would think a trailer for a 17 footer would not weigh more than 6-700 lbs. Also, what brand of prop?
D.

 

[jestor68]

Re: 76 Evinrude 85hp prop?

According to NADA guide, the dry weight of your boat/motor is about 1200 lbs.

A quick look at a prop selector calls for a 17 pitch and says it should do 41-42 mph at 5300-5500 rpm.

Also; a 2.0 gear ratio turning 17 pitch at 4500 rpm equals only a theoretical 36 mph. If you add an inch of pitch for cupping(18) it equals 38 theoretical mph with no slip; which is impossible. A prop cannot function with no slip.

Try this on for size.

A 2.0 gear ratio turning 17 pitch at 38 mph, assuming an 8% slip equals a theoretical 41 mph at 5094 rpm.

Could it be that your tachometer is wrong and you're actually turning a little over 5000 rpm?

Only way to find out is get your tach checked for accuracy and speed by GPS. Any dealer can check your tach against their shop tach.

 

[steelespike]

Re: 76 Evinrude 85hp prop?

I think basically your tach isn't accurate.It is reading low .Is your speed by gps?
Assuming about 12% slip the prop calculator indicates your rpm at 38 mph should be about 5300.
You need to confirm your speed by gps and varify your tach.Perhaps borrow a shop tach.
It appears your setup is pretty good but for your tach.