Proping question for a Evinrude 120

[biglurr54]

I have a 1989 Evinrude 120 VRO that I need a new prop for. I am currently running a 14X17P prop. I also recieved a prop with the boat that is too beat up to use and it is a 13 1/4X19P. Currently the boat hits 5100RPM's at WOT. Speed is not an issue at all. The boat tops at around 50MPH. Just this past weekend i had the boat loaded to the max. I had 5 people and camping gear for 8 poeple. The boat popped up on plan like there was no one in it. I would like to Prop the motor so that it has the load taken off of it. I would liek the motor to not have to work as hard so that it will last longer. And as we all know due to gas prices it would be nice to conserve some gas as well. I feel that the prop on the boat is not the correct prop. I cant tell if i need to go up or down in size to get the RPMS up and unload the engine.

 

[steelespike]

Re: Proping question for a Evinrude 120

While I think a 17" prop is about right I don't think it would produce 50 at 5100 rpm.on a boat that I think has a dry weight of about 1200 lbs. A long winded way of saying your numbers don't add up.
A few things to check; Is the antivent plate just above the prop about even with the bottom of the boat.Is your speed by GPS? You should varify your tach.You may be able to borrow a shop tach if you have a regular marina.
To answer your question if your tach is accurate you need to reduce your pitch 2" to gain 300 to 400 rpm. Your motor needs to rev freely within its rpm range.

 

[biglurr54]

Re: Proping question for a Evinrude 120

my speed is not by GPS but i can get a GPS speed next time im out. The dry weight of just the boat is around 1000lbs. Its all aluminum. The Tach has been funky at times. when i got the 5100 rpm it was acting like it should. Other times it sticks at certian RPMs. It was being responsive to the throttle when I got the 5100 reading. It has never reached above that mark in the three years ive owned the boat. I dont have a regular marina that i bring the boat to. Im the regular mechanic! Ive been trying to avoid the local marina because they charge way too much for stuff. I may go to them to test props and pay idiot tax on it and order it through them. I was hoping someone here might have an idea how to avoid that.

 

[steelespike]

Re: Proping question for a Evinrude 120

my speed is not by GPS but i can get a GPS speed next time im out. The dry weight of just the boat is around 1000lbs. Its all aluminum. The Tach has been funky at times. when i got the 5100 rpm it was acting like it should. Other times it sticks at certian RPMs. It was being responsive to the throttle when I got the 5100 reading. It has never reached above that mark in the three years ive owned the boat. I dont have a regular marina that i bring the boat to. Im the regular mechanic! Ive been trying to avoid the local marina because they charge way too much for stuff. I may go to them to test props and pay idiot tax on it and order it through them. I was hoping someone here might have an idea how to avoid that.

((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((
I won't quibble over 100 or 200 lbs.My info indicated all the Phantoms were 1100 to 1200 lbs.
I did suggest if you feel the tach is accurate to go down 2 inches in pitch to gain 300 to 400 rpm.
I still think the numbers don't add up I checked a series of tests your same basic motor
but 115 hp same gear ratio 2.00 1100 to 1300 lb boats all running a 17" prop.
Speeds are low 40s at 5,500 to 6,000 rpm. your motor same gear ratio same weight 17" prop
5100 rpm 50 mph one or both of those last 2 numbers has to be wrong. Without knowing which one we can't tell even which way to go with a prop selection.I guess a gps speed might help.
If it turns out to be say 43-45 mph ;with your 17" prop, hole shot and effortless weight pushing
we can assume your motor is well within its max rpm area and no change is needed.