0.75 change in pitch?

[dthrckt]

I've got a '57 35hp evinrude on a 16ft arkansas traveler (aluminum semi-v). It does just over 26 mph (gps) when set to the 4th (highest) trim setting. It drops about 1mph w/ each lower notch, I usually run at 3rd position.

I usually have two average size men, gas, fishing gear, battery, 2hp motor, e-motor, anchor, oars and small cooler.

I have three props that all perform very similarly and measured the pitch of each at 13. I think 13.25 was stock, but I'm sure I can't measure the diff between 13 and 13.25.

I don't have a tach...I know, that's bad...but it doesn't take an expert to be able to tell that the motor revs way too high at WOT.

I've been looking for AJC461 and AJC475 for awhile with no luck (one is 10x15.5, the other 10x16, both two blade, one aluminum, one bronze).

I've just found an AJC464 which is 10x14, 2 blade aluminum - is that enough to make a difference? I tried a couple online prop calculators and it doesn't seem like it will...

do i have other options? ie. having a similar prop retrofitted by a prop shop?

Thanks!

 

[hwsiii]

Re: 0.75 change in pitch?

Dt, no it doesn't take an expert to know when a motor is over revving, but it does take a tach to know how much it is over revving so you can make an informed decision as to how much the pitch needs to be changed in order to have the correct pitch prop. And even when you have the correct pitch prop, without a tach you don't know how much prop slip you have so you don't know how efficient that particular prop is with your boat and motor combination and many boats have a speed loss of over 15 to 20% because of having the wrong blade geometry yet the right pitch. Without a tachometer you are in the dark whether you know it or not.
I hope you take this as intended, but you just don't realize how important a tach is for your engines health and your pocketbook.



H

 

[dthrckt]

Re: 0.75 change in pitch?

I do take it as intended, you're right, and thanks, even though you didn't tell me whether or not you think it might be worth acquiring.

I'm going to get a tach so that I can at least cruise below WOT and be comfortable knowing I'm not over 4500 rpm

The problem is, I really don't have many (any!) choices w/ this old motor. This 14 pitch is the first one I've come across that is anything higher than the 3 props I already have, so I'm trying to decide whether or not to jump on it...

I bet there are a whole lot of people in the last 50 years that have made that same switch on a similar setup...so I was just hoping one might see my post and respond...

 

[hwsiii]

Re: 0.75 change in pitch?

Dt, the catalog shows your motor to have a 1.75:1 gear ratio and WOT recommended RPM of 5,000.
By changing from a 13" pitch to a 14" pitch in the same make and model of prop you can expect an RPM change of right at 400 RPM and goung to a 15" pitch brings the RPM down right at 750 RPM.
What you might think about trying is having one of your old props repitched after you know what your RPM are and what pitch you need.
I am trying to be helpful, but I just believe everyone ought to have a tach. Sorry.


H

 

[dthrckt]

Re: 0.75 change in pitch?

excellent info thanks. I'm surprised by that max RPM. I thought I'd read 4500 for this old motor in other threads, but I believe you.

the tach has been ordered. I'll be camping for almost a week so I won't be able to get an rpm reading until after next week. Didn't know a prop could be re-pitched, that's awesome.

Once I get some numbers I'll post again. Then maybe you'll be able to advise whether to have the 3 blade 13 or (if I can still get it) the 2 blade 14 reworked.

thanks again

 

[hwsiii]

Re: 0.75 change in pitch?

You are very welome


H