50 hp Evinrude 1000rpm drop

[55Crestliner]

I'm running a 1972 50hp Evinrude on a 1956 Crestliner aluminum runabout.
This motor is a recent addition to this boat.
The prop that came on the motor is a 13.25 X 17p that was pretty chipped up on all the blade edges. I shaved down the chips to smooth them out. Reducing the diameter by about 1/4 inch per blade.

This prop was producing 5900 RPM with a light load, and very close to that, 5800 RPM with four people on board. I bought a new prop. It is a 13.25 x 19p with cup. It dropped my RPM by a full 1100 to 4800 light load. It was my understanding that 2 inch of pitch would only drop about 400 RPM.

RPM goal at WOT is 5500.

Am I simply dealing with such a drastic change in prop design between the older reduce diameter prop and the new one? (As in the new one is a much more aggressive design?)

Would it be the logical solution to get a 17p prop of the same brand and design as the 19p that I bought?

 

[Sea Rider]

The rule of thumb that accounts for an increase or decrease of 200 wot revs per one +,- pitch is not cast on stone. Sometimes works, some doesn't. If previous 19 prop was producing 5900 RPM with a light load, and very close to 5800 RPM with four people on board and you aim to achieve 5500 wots revs. Will need to up pitch for at least a 20 one for engine to reduce wot rpm towards 5500 wot rpm.

Happy Boating

 

[55Crestliner]

If previous 19 prop was producing 5900 RPM with a light load, and very close to 5800 RPM with four people on board and you aim to achieve 5500 wots revs. Will need to up pitch for at least a 20 one for engine to reduce wot rpm towards 5500 wot rpm.

Happy Boating

original prop is 17. The new one was 19. 1000 RPM drop.

 

[ahicks]

<<<Am I simply dealing with such a drastic change in prop design between the older reduce diameter prop and the new one? (As in the new one is a much more aggressive design?)

Would it be the logical solution to get a 17p prop of the same brand and design as the 19p that I bought?>>>

1. That's about the only way you can explain what you experienced, in my mind anyway.
2. That's how I would do it - unless the 19" prop cannot be returned for credit. If that were the case, I'd take it to a prop repair facility and explain your story to them. They should be able to help you for a little less than the price of another new prop.

 

[Sea Rider]

Sorry missed the buy same prop model as of current 19 pitch with at least one pitch up to reduce wot rpm range from 5800 to 5500 wot revs if that's what you're aiming for. Only a water test will confirm if dialed a 300 wot rpm reduction with a 20 pitch prop.

Happy Boating

 

[Silvertip]

19P is a lot of pitch for that engine. And yes -- you can buy a prop of given diameter and pitch from several different manufacturers and all three will perform differently. Why? Because designers may bias their designs toward hole shot, top end, or cruise but physics dictates that you cannot have all three optimized. There are lots of prop characteristics not just diameter and pitch. Remember, an outboard has only one gear (the prop) so it it needs to get you moving, cruise comfortably and yet provide reasonable top end. Too much prop simply puts the engine up against a wall that it cannot climb.

 

[jimmbo]

Your engine was rated 50hp@5500 rpm. WOT range is 5000 - 6000. It is recommended to prop in the upper half of that range.

Was the original 17 in prop cupped? If not, a switch to a 19 cupped is more than a 2 in increase. Also if the 17 was blowing out from chewed up blades, the new prop will grip the water better, so the 1000 rpm drop might well be valid

 

[Lightwin 3]

19P is a lot of pitch for that engine. And yes -- you can buy a prop of given diameter and pitch from several different manufacturers and all three will perform differently. Why? Because designers may bias their designs toward hole shot, top end, or cruise but physics dictates that you cannot have all three optimized. There are lots of prop characteristics not just diameter and pitch. Remember, an outboard has only one gear (the prop) so it it needs to get you moving, cruise comfortably and yet provide reasonable top end. Too much prop simply puts the engine up against a wall that it cannot climb.

Agree, 19" is a lot for a 50. Once you start monkeying with filing props, etc., you change the design. Go back to what you know worked.

 

[55Crestliner]

Your engine was rated 50hp@5500 rpm. WOT range is 5000 - 6000. It is recommended to prop in the upper half of that range.

Was the original 17 in prop cupped? If not, a switch to a 19 cupped is more than a 2 in increase. Also if the 17 was blowing out from chewed up blades, the new prop will grip the water better, so the 1000 rpm drop might well be valid

Nope, the the original 17p is not cupped. I have ordered a 17-pitch of the same design as the new 19 pitch. This should put me right where I'm trying to get.

This is the first season I have run this boat with this motor. For 10 years I've been running a 1958 35 horse. The boat would do 30 mph with just me in it. The new 50 horse has only seen 35 mph, but typically is doing 33 mph. (GPS). My speedometer stops at 35 mph, I was really hoping the new motor would just put me out of the realm of the speedometer. LOL.
But honestly I am still very very happy with it. The acceleration is outstanding, it will be great for pulling up a skier and pulling a big tube. The 35 really struggled to get an adult up.

 

[Sea Rider]

If propped at middle 5500 wot rpm range when ading more load or passengers wot rpm will drop accordingly, if too much load is added, engine will rev towards the min lugging side of the wot rpm range which is bad besides reducing darmatically best hole shot while revving near max wot rpm range.

Propping towards the max wot rpm range will be much better if boating with a variable load which isn't same as propping an engine for a fixed load to run middle or max wot rpm range, whichever range you prefer.

Have experienced myself increasing + 1000 wots revs going per 2 less pitch props from factory delivered one while maintaining same number of blades, diam and brand prop, so it's noy unusual to achieve way more wot revs that the rule of thumb near cast on stone states.

Happy Boating

 

[jimmbo]

It must be a very light boat if you got 30 mph out of a 1958 35hp. Normally a 5 hp increase gives a 1 mph speed increase. But there were significant advances in engine design between the 58 35hp and the 72 50hp, so you get more than 3 mph. Much better gearcase and propeller designs. The 71- 74 50 HP use the same props as the V4s, which gave the 50 very good acceleration. Getting rid of unneeded weight(Wife, kids, Mother-in-law, especially Mother-in-law) will help you get higher speeds. Girlfriends and Mistresses can remain aboard.

 

[55Crestliner]

It must be a very light boat if you got 30 mph out of a 1958 35hp.

The original brochure says it weighs 235 pounds. (and yes, in case anyone is nervous, it is rated for 50hp - 1957 was rated for 60hp, with no changes).
It would do 30, occasionally flickering at 31 with just me in it. With wife and 2 kids, it would do 26/27.
The new 1972 50hp is doing 32/33 fully loaded, and about 33/34 with just me. With very brisk acceleration, either empty or loaded. That is why I figured it needed more pitch. But as several of you have helped me confirm, a difference in design can really equate to much more than just 2 inches of pitch.
BTW, the speed was pretty much the same with either the old 17P prop doing 5800rpm, or, the new 19P prop doing 4800rpm. And even with that 19P, the acceleration was still much stronger than the 35hp.

All the techies out there will enjoy those numbers, but we shall see sometime this week when the new prop gets here. I think I'm going to be quite happy with it.

Thanks for all the replies!

 

[55Crestliner]

I put on a new 17P prop, same design as the 19P.
(btw, I'm talking about the Turning Point Hustler.)
I was very excited to try this new 17 out, with all of the numbers that I had previously on the other two props. I thought for sure this one was going to be The Sweet Spot. I am very frustrated to report that the new 17-pitch prop will only turn 4900 RPM, barely over what the 19 pitch prop was doing. I am still a full thousand rpm lower than the stock 17-pitch prop.

Top speed does not seem to be affected much by either three of these props. It's still anywhere from 32 to 34 miles an hour.

Throwing my hands up here, really not sure where to go at this point. I can go and get a bone-stock Evinrude/Johnson prop, like what the motor came with, except not have the chips and the reduced diameter. Or I suppose I could go to a prop shop, and give them the 17-pitch prop that I just put on, and have them tone it down or something.

What is so odd to me about this whole experience is that the boat accelerates so hard with either the new 17 pitch or the 19 pitch prop. It just sort of gets to top speed very quickly and just shuts off.