Confused by props

[barnacle2]

I have a 16' Tracker that I just put an Evinrude 40 2 stroke motor on. The prop that was on the used motor was a SS 12.25X 15. I could get 4900 rpms @ 30.5 mph (GPS) with it. Looking for a little more speed I installed an aluminum 12.5X 13. Now I get 4950 rpms @ 29mph! How could this be? Should I try an 11" pitch or be happy with the 15"?

 

[a70eliminator]

Re: Confused by props

If your looking for more speed your going the wrong way in pitch.
Keep your engine within specified rpm range and speed is what it is.

 

[Bondo]

Re: Confused by props

How could this be?

Ayuh,..... 'cause That's the way it Works.....

 

[barnacle2]

Re: Confused by props

Well thats what I thought but I was told by an engineer at a prop maker that the 13 would get the rpms up and increase the mph at the same time. You would think a company engineer would know what he is talking about. Guess not.

 

[Frank Acampora]

Re: Confused by props

IF--and that's a big if. If the lower pitch will unload the engine to deliver SIGNIFICANTLY higher RPM, THEN it will deliver more speed.

In your case, the prop diameter and area was increased so the load on the engine was about the same. Therefore RPM remained the same and speed went down because of the lower pitch.

Since pitch is a theoretical measure of how far a prop will travel through the water in one revolution: Just like the car guys say there ain't no substitute for cubic inches, you can not get higher speed without pitch! That, however, assumes that the engine has the ABILITY to turn a higher pitched prop at sufficient RPM. Conversly, if a high pitched prop is loading the engine down below it's horsepower range (Not your case), THEN a lower pitched prop of the same brand and diameter will get the RPM up and the speed will most likely increase also.

Sounds like your prop engineer was talking through his A**

Want to play around a little? With the original prop: Try raising the engine on the transom in about 1/4 inch increments until you get the anti-ventilation plate about one inch above the bottom of the hull or until the prop ventilates. Once it ventilates, then drop down a bit again. See what that does for your speed. Want to play some more? Try cutting the original prop diameter about 1/4 inch smaller. See what that does for RPM and speed.

 

[Silvertip]

Re: Confused by props

I would also make sure the engine is properly tuned and making full power. I happened to have owned a very similar rig and I can assure you a 12-1/4 x 15 prop is right for that boat and it should allow the engine to run in the 5500 - 5800 range. I therefore feel your tach is incorrect since the numbers you provide do not compute in a prop calculator. A 13P prop on that boat would have the engine well over 6000 RPM. How do I know that? I've tried it with a 13P stainless Michigan prop. I also tried an aluminum 11-3/4 x 17P "uncupped" and always ended up back with the 12-1/4 x 15.

 

[steelespike]

Re: Confused by props

Your present speed is pretty close to what a 40 would do in your combination.
my guess is about 30 to 32 mph.While playing with your setup don't forget trim, generally trimming out (prop away from boat)increases speed decreases hole shot.Trimming in improves hole shot decreases speed.

 

[barnacle2]

Re: Confused by props

Your present speed is pretty close to what a 40 would do in your combination.
my guess is about 30 to 32 mph.While playing with your setup don't forget trim, generally trimming out (prop away from boat)increases speed decreases hole shot.Trimming in improves hole shot decreases speed.

You are right about trim and speed. When I got the speeds reported I had the motor trimmed up till the boat started to porpoise then trimmed down till it smoothed out.

The tach has been switched out and both showed the same thing at WOT.

What gets me is I'm going from a 25 Merc to a 40 hp and only gaining about 4 mph. At that rate a 90 merc should be faster than a 175 Evinrude.

 

[steelespike]

Re: Confused by props

You are right about trim and speed. When I got the speeds reported I had the motor trimmed up till the boat started to porpoise then trimmed down till it smoothed out.

The tach has been switched out and both showed the same thing at WOT.

What gets me is I'm going from a 25 Merc to a 40 hp and only gaining about 4 mph. At that rate a 90 merc should be faster than a 175 Evinrude.

Generally the faster a boat goes the less the increase will be. Heres some stats from an old chart.Keep in mind speeds are for a stock motor on a standard light fast boat 10hp-21,18-27, 33-32,40-33,60-36,75-40,90,44 note that at 9 times the hp speed is only slightly over double.Also note thhat your speed is not that far off from the 40 on a light fast boat.By the way all bets are off when going from SS to Al. I think a 13" SS would be at least as fast with better hole shot.And get the rpm up around 5,200.BUT get your setup fine tuned before any prop changes.

 

[mthieme]

Re: Confused by props

While I'm not familiar with OMC's, that seems like a large diameter to me.
I am familiar with Mercs. My Mercs 25-35-40-50hp all use props in between 10" and 11". The 35-40-50 are interchangeable.

 

[MikDee]

Re: Confused by props

I have a 16' Tracker that I just put an Evinrude 40 2 stroke motor on. The prop that was on the used motor was a SS 12.25X 15. I could get 4900 rpms @ 30.5 mph (GPS) with it. Looking for a little more speed I installed an aluminum 12.5X 13. Now I get 4950 rpms @ 29mph! How could this be? Should I try an 11" pitch or be happy with the 15"?

I think the standard orig. equip. prop on my old 40hp Evinrude big twin was an aluminum 10.25"X13" Pitch for a 16' Thompson lapstrake runabout, and that boat was heavy but moved out fine, even with 4 people, & towing a skier.

I'm guessing your Tracker is a lot lighter, Why do you need anywhere near a 12" diam prop? Unless your carrying extra heavy loads? That large diameter seems like extra drag.

I think that's your problem.

 

[barnacle2]

Re: Confused by props

I think the standard orig. equip. prop on my old 40hp Evinrude big twin was an aluminum 10.25"X13" Pitch for a 16' Thompson lapstrake runabout, and that boat was heavy but moved out fine, even with 4 people, & towing a skier.

I'm guessing your Tracker is a lot lighter, Why do you need anywhere near a 12" diam prop? Unless your carrying extra heavy loads? That large diameter seems like extra drag.

I think that's your problem.

I agree that sounds a bit large but it is what is recommended by the prop wizzards.

 

[MikDee]

Re: Confused by props

I agree that sounds a bit large but it is what is recommended by the prop wizzards.

If you have a newer then 1984 40hp motor, I stand corrected, probably because you have a thru prop exhaust which would require a bigger diameter prop design, compared to my old shear pin design (non thru prop exhaust) motor.