i need help choosing prop pitch

[mullet89]

Well my current prop has seen better days, its now missing a few chunks after my last trip in the river. So i'm in the market for a new one. It looks like I have a choice of pitch's 7" 9" 10" or 11". Is it really that important to get the correct pitch? I know with the right pitch you get better fuel economy and more speed etc. but I'm not really dealing with a whole lot of boat. I have 1960 lone star capri 14' fiberglass boat its a little heavy and I'm running a 1956 evinrude 15 hp. i'm looking for speed but i don't want to strain the motor either. with the current prop I'm getting 15mph on my gps. Which i think is pretty good for what it is but i'm not sure of the pitch on my current prop.

 

[steelespike]

Re: i need help choosing prop pitch

It is critical that you get propped right.In 56 your motor was rated to make about 25 mph on a light reasonably fast boat.
While not light most Fiberglass boats do well for speed because the bottom is clear of rivets.seams and various extra strakes or keels.
I think if your motor is healthy 20 mph would be a reasonable figure.
It is important that your motor be able to rev freely within its rpm range.4000-5000.Too low will lug the motor and lead to premature repair problems and of course poor performance,fuel economy and you can see at 15 mph your wasting your time as well.Too much rpm will lead to premature motor failure.
To do this right you need a tach a induction tach like a Tinytach will work, be sure to tell them its going on a outboard.
Your present prop size may be near the base of the blades.
Your boat dry weight is 310 to 370 depending on options.Its rated for 18 to 40 hp so you can see your badly under powered but still lightly loaded you should see 20 mph with the right prop.

 

[mullet89]

Re: i need help choosing prop pitch

thanks for help, so I took the prop off and the only markings I can find say Michigan smc 43. I will say the boat is rarely lightly loaded. heavy items usually in the boat: trolling motor, battery, normal 4 gallon tank, spare 5 gallons, I'm a heavy guy plus I usually have someone with me and all the fishing gear. Is it worth the time money to go through the steps you said or should I just go for like the 7" or 9" because the motor is small for the boat?

 

[steelespike]

Re: i need help choosing prop pitch

If you email michigan wheel they can tell you what size and probably what it fits and what it cost new.
Guessing what size is like russian roulette. Is 15 mph with your typical load ?Does it appear to struggle to get on plane?
If your going to reach into the dark with a blindfold on you don't want an 11" pitch
and I don't think a 7" is enough.If I was going to guess I think a 9" is a good start.
Assuming the motor is healthy enough to reach 5000 with the 9" you may be able to reach that 20 mph I mentioned. With that load your slip will probably be high so 20 may be in question.
If it turns out you all ready have a 9" you'll just end up with a new prop that still may be wrong.
Looking forward to your results.

 

[mullet89]

Re: i need help choosing prop pitch

Well going straight it doesn't seem to have a lot of trouble. its only a 15 so doesn't jump out of the water but it does get on plane with the right trim setting. If I crank the wheel all the way to one side you can hear the motor straining and it slows down a bit. so I guess i'm going to see if I can't figure out my current pitch and go from there. thanks for the help

 

[jestor68]

Re: i need help choosing prop pitch

Running a similar size/weight boat through the prop selector comes up with a 9 X 10 prop in Mercury standards.

That was based on a 500 lb boat/motor, 500 lb of people/gear, and 12 gal gas with a 2.42 gear ratio and 15 HP.

I believe the rpm range on that older motor is like 4000-5000 rpm.

Since Evinrudes don't seem to be as powerful as Mercurys, you would probably need a 9 pitch prop.