WOT rpm?

[ozarkt165]

Searched thru some threads.... What would wot rpm be on 1976 70 hp johnson? What prop would I need?

 

[walleyehed]

Re: WOT rpm?

Listen to your question....You have asked the same thing in 2 ways..

I'd shoot for 5700 with a Normal Load, which "should" answer both questions.
#2 being "the prop that gives you 5700 with a normal load". A lot depends on the size and style of boat you have. If you'll list that we can likely figure a good starting point.

 

[ozarkt165]

Re: WOT rpm?

1976 ozark t165 tri hull 16 ft. The main beam is around 78 inches. Most old tri hulls i see are narrow. Think it weighs around 1800 lbs

 

[steelespike]

Re: WOT rpm?

If you read the stickies at the top of the page you will better understand prop selection.
Are you guessing weight with the motor and gear? or dry weight.
An 84 is listed at 1,0oo dry.
Your sorely under powered if it is indeed 1,800.
There should be a number on the barrel or under the prop but. It came with a 13 1/4 X 17 but the final selection would depend on the application.

 

[ozarkt165]

Re: WOT rpm?

K found a site says a 1976 t165 bass fisherman has a dry weight of 800lbs. Hull on that critter is same as mine, interior totally different. If i am solo and full on fuel prob about 1250 lbs assuming fuel around 8lbs a gallon. It will carry me 34mph running around 4k rpm wot. if the tach is right.... Speed figured gps, pitot gauge agrees... full loaded weighs around 1650 would run out 26mph. I added a hydrofoil and the loaded speed went up to 32mph. so if max rpm is 5700 that would make for a small prop. Has stainless prop.... assuming oem prop... definetly not a high dollar stainless by the looks. Need to pull prop again and see if dimensions are on hub.

 

[steelespike]

Re: WOT rpm?

I think speed should be closer to 39 or low 40s.
Obviously rpm is low.It will be interesting to see what size prop.

 

[1kruzer1]

Re: WOT rpm?

Original RPM range spec for a 1976 Johnson 70 HP is 4500 to 5500 RPM. You definitely want to prop towards 5500. Some say to run a little faster, some say not. One thing is for certain: If you are getting 32 MPH out of you current 17 inch prop, you are running more than 4000 RPM, so your tach is questionable. With your motor's gear ratio, 32MPH on a 17 pitch prop at 4000 RPM is mathmatically impossible. Also, I have the identical motor ('76 Evinrude 70HP) and with my 16' boat running an OEM aluminum 17 pitch prop I run 33 MPH at 5400 RPM. No disrespect to Steelespike, and many boat speedometers read fast, but I don't think 40 MPH with your boat on a 70 HP is realistic. If you can prop for an accurate 5500 RPM and be close to 35 MPH you will be doing well.

 

[walleyehed]

Re: WOT rpm?

Gonna touch on a sore spot here that keeps showing it's ugly face and many of us that have experimented, some for many, many years have found that the "RECOMMENDED RPM RANGE FOR A 1976 60, 65 AND 70HP TRIPPLE IS 4500-5500" was just fine....in 1976 for a few years. Since our fuel is 1/3 the quality of the 1976 fuel, several manufacturers and several of us tearing powerheads down through the years have PROVEN that with current day fuels, this engine SHOULD NOT be set-up for anything lower than 5500-"this figure" is what we now recommend as a minimum rpm. This engine will last much longer if you target about 5700, even 5800.

 

[1kruzer1]

Re: WOT rpm?

Gonna touch on a sore spot here that keeps showing it's ugly face and many of us that have experimented, some for many, many years have found that the "RECOMMENDED RPM RANGE FOR A 1976 60, 65 AND 70HP TRIPPLE IS 4500-5500" was just fine....in 1976 for a few years. Since our fuel is 1/3 the quality of the 1976 fuel, several manufacturers and several of us tearing powerheads down through the years have PROVEN that with current day fuels, this engine SHOULD NOT be set-up for anything lower than 5500-"this figure" is what we now recommend as a minimum rpm. This engine will last much longer if you target about 5700, even 5800.

Didn't know it was a sore spot. As stated above: "Original RPM range spec for a 1976 Johnson 70 HP is 4500 to 5500 RPM. Some say to run a little faster, some say not." Walleyehed says to run a little faster.

 

[ozarkt165]

Re: WOT rpm?

Okay after a week on the water i have discovered that my memory stinks..... This critter is actually turning around 5000, so i need another 800 rpm. Still need to pull prop and see if it is labeled and go from there.