Propeller size for more speed and less RPM on 1975 50hp evinrude.

[chad913]

Re: Propeller size for more speed and less RPM on 1975 50hp evinrude.

Oh yeah forgot to meantion after taking the teleflex out and putting the faria tach in i put ANOTHER new rectifier on it was still reading funny? with this teleflex on 5p it seems more steady on the rpms.

 

[steelespike]

Re: Propeller size for more speed and less RPM on 1975 50hp evinrude.

If my math is right a 2.42 ratio with a 17" prop @ 6100 rpm
10% slip should be about 36 mph.If your tach is right your prop is at 25% slip( a lot) to be making only 30 mph.Is the prop in excellent shape?Are you sure its a 17?

 

[chad913]

Re: Propeller size for more speed and less RPM on 1975 50hp evinrude.

The prop has 11 3/4 17 stamped on it. What should i be looking for inspecting it? I know it has alittle rough edge on the tip of one of the blades.?

 

[boobie]

Re: Propeller size for more speed and less RPM on 1975 50hp evinrude.

Silvertip, I tend to disagree as the teleflex instructions, which I have right in front of me say #5 position. This mtr has a 10 pole flywheel not 12 poles.

 

[steelespike]

Re: Propeller size for more speed and less RPM on 1975 50hp evinrude.

From your description it wouldn't seem you have a lot of prop slip. You could have a prop shop check it for size and pitch Also diameter
if its original it could be worn undersize especially if run in silty/sandy conditions.
I have an 18ft Crestliner utility, tiller steer it weighs 375 dry. with my 88 50, same ratio ,I run a 19" pitch.Never ran a tach and no spoeedo been like this for 23 years runs strong fast and powerful. I think your fiberglass 14 is probably 700 or 800 lbs. A 17 does seem reasonable.
A 19 would drop 300 to 400 rpm If we use your 6300 figure that would put you at about 6000. If my figures are right you would be going 40.14 at 10% slip.
It also appears you need to go to a 21 pitch to get to about 5200-5,500 rpm.depending on what present rpm we use.
I think that is unusual for a typical 14' runabout. If you have a regular marina see if you can borrow a shop tach or have a tech go with you to check rpm.If it checks out go for a 19" pitch.

 

[chad913]

Re: Propeller size for more speed and less RPM on 1975 50hp evinrude.

There isnt any prop shops around my little part of the world and as for that a marina with anyone that gives a damn other than trying to sell me something at twice the normal price. I think i will start looking for a good prop with this 19 pitch. I can use a 11 3/4 19 right?

 

[Howard Sterndrive]

Re: Propeller size for more speed and less RPM on 1975 50hp evinrude.

iboats has the best prop prices by far in my experience. used stuff on eBay often sells for more than new iboats prices

I would be tempted to disconnect that tach and just use one on a timing light or dwell meter that triggers off a spark plug lead. Once you figure out where you're at, go prop shopping if needed.

Trust me when I say, your ear cannot tell the rpm accurately - not even close. don't worry about what it sounds like it's revving.

 

[steelespike]

Re: Propeller size for more speed and less RPM on 1975 50hp evinrude.

You should take a look at www.evinrude.com under research tools performance report.
50 hp all boats.These new motors have a 2:62 gear ratio allowing even larger pitches than your motor yet most are the same pitch or even some lower. also note the rpm and speeds.
Not insisting tach is wrong but something is wrong.
Untill you figure out what it is prop selection is a shot in the dark.
Could cause poor performance and premature motor repair problems.

 

[steelespike]

Re: Propeller size for more speed and less RPM on 1975 50hp evinrude.

There isnt any prop shops around my little part of the world and as for that a marina with anyone that gives a damn other than trying to sell me something at twice the normal price. I think i will start looking for a good prop with this 19 pitch. I can use a 11 3/4 19 right?

If the prop is intended for your motor don't worry about diameter.