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

[chad913]

I am currently running a 11-3/4 + 17 propeller on my 1975 50hp evinrude ob and it gets to around 6100 to 6300 r.p.ms and goes about thirty MPH..? I was wondering what prop would be better for more speed and less RPMS or what is maybe recommended on this motor. Ive seen i should be around 5500 Rpms to be safe. I throttle it wide open until i plane out and let back off a little to were i think its running at a good WOT and the RPMS sound good and not to much. Some help with this would be greatly appreciated .thanks

 

[nwcove]

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

best case scenario, try to borrow a prop with a bit more pitch.

 

[chad913]

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

I dont know many people with boats and with a 50hp at that.

 

[nwcove]

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

my opinion, look for a prop with a bit more pitch . you might be able to find a prop from a larger motor, with more pitch, or larger diameter....if it will swing under the cavitation plate. (id stay with the same diameter, and look to up the pitch)

 

[jayyyy]

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

In general every inch increase in pitch decreases RPM's around 200.

 

[chad913]

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

I am not fimilier with the size prop layout. Kinda like tires to some people. My current pitch is 17?

 

[nwcove]

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

yep, 17 is the pitch, ie. for one revolution of the prop your boat should move 17 inches forward, minus prop slippage ( 10%???....i used to have the formula for surfacing props, not sure if its the same tho) depending on your budget, up the pitch, but dont over do it.

 

[chad913]

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

What would you suggest?

 

[chad913]

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

were would you suggest to get one at a good price. The prop is about 1'' to a 1/2'' from the cavitation plate.

 

[boobie]

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

The first thing I would do is check the tach to make sure it's accurate.

 

[chad913]

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

It is. Its a teleflex with the adjustments on back. Its set to 5p. Little jumpy though. but wot every time it goes to around 6100 6200rpm. How would i check the tach to see if it right?

 

[Titanium48]

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

I think it should be set to 6p. See here: http://forums.iboats.com/showthread.php?t=290795
If it is indeed set wrong, it will read 20% too high, so 6000 RPM is actually 5000. In that case, you have the correct pitch and WOT RPM. That is a good thing, because 17p seems to be the highest pitch available for that motor (at least in the solas amita 3 and turning point hustler lines anyways).

 

[John_S]

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

I think it should be set to 6p. See here: http://forums.iboats.com/showthread.php?t=290795
If it is indeed set wrong, it will read 20% too high, so 6000 RPM is actually 5000. In that case, you have the correct pitch and WOT RPM. That is a good thing, because 17p seems to be the highest pitch available for that motor (at least in the solas amita 3 and turning point hustler lines anyways).

I'd bet he hit it on the nose, and you need to drop down in pitch(14-15P). Your top speed will probably not change much, but you will get into recommended rpm range, and you hole shot should be noticably better.

 

[steelespike]

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

I agree I think there is a tach issue.You may be able to borrow a shop tach or get an optical tach.That is a lot of prop for a 50 to turn up to 6,000+.
What kind of boat is it on.

 

[chad913]

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

Its on a 1975 AMF Crestliner stinger s440 14'. The one set as avitar and profile picture. I think the tach is right. When i floor it it picks up good then planes out and the rpms are WAY up there just by listening to it. At hole shot i floor it and then have to pull up on the throttle just a bit after planing because it sounds like it way up in the rpms.

 

[chad913]

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

And i had it set to 6p and the tach would pass 7,000rpms and sometimes pass 7000 rpm and zero out. i changed the rectifier and it was still doing the same thing. moved it to 5p and now its showing idle at 850 rpm and WOT at 6100 to 6200 rpms and the throttle response on tach is good.

 

[boobie]

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

For the teleflex tach it should be on 5 P. To be sure I would either substitute another tach or check it with a shop tach. I've seen tachs that are on at idle and get way out of wack at higher rpm's

 

[Silvertip]

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

For the teleflex tach it should be on 5 P. To be sure I would either substitute another tach or check it with a shop tach. I've seen tachs that are on at idle and get way out of wack at higher rpm's

12-1/4 x 15 and 11-3/4 x 17 are two of the most used props on 40/50 HP Johnny/Rudes. 5P is the wrong setting -- 6P is correct and your tach is wrong. Setting the tach to 6P will bring the "indicated" revs down, speed will not be affected and "actual revs should be in 5500 5800 RPM range but 6000 on that engine is not an issue. If the boat this engine is on is a 15 - 16 footer the prop you have is ok or very close to optimum.

 

[V153]

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

Mebbe buy one of those tiny tachs. Think you can get one delivered for under $30 bucks. Wrap one lead around the #1 spark plug wire, the others to positive & ground. Calibrate. And you've got yourself a tachometer. Hard to see in the sun but they're purty dang accurate.

 

[chad913]

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

When i put it on 6p it goes up and down and sounds like its idling really low but reading around 2200 to 2500 rpms? and when giving it alittle throttle it seemed to be right but when i would go full throttle it would come back down in the rpm around 2000 to 3000 rpms.Changed it to 5p and its seems to be better. I bought a 6p Faria tach and had it hooked up because i thought something was wrong with my teleflex that i also bought new, and the faria tach (fixed 6p setting) would spin all the way around and come back to 2000 to 3000 rpms and i was at full throttle or go in between 1000 rpms and 6000 rpms ? I have replaced the rectifier after installing the teleflex tach(the first one installed ) and i am now back to the teleflex on 5p. What could be making my tach read funny then?