Learning experiment with a ProPulse Prop... How to change blade shape for higher RPM

[Can'tFishEnough]

Background:
I have a '94 16.5 ft alumicraft V-16 Lunker with a '94 25hp Evinrude boat runs right at 5000rpm (about 23mph by GPS) with just me with an 11x11 (modified from 11x9 Johnson/Evinrude prop) by my local prop shop. I was going to have them lower the pitch just a bit so that I can run 5000 with a couple of adults and a kid or two since I hardly ever am in the boat on my own and then maybe closer to 5400 with just me)

When I bought the boat it had a 10x15 prop on it and would just slug through the water.
So I bought the ProPulse because I got it really cheap (about $25 with 4 extra blades for $5 each and about $8 s/h) on clearance (also came with one extra blade). Hoping that it would let me go from the 15 down notch by notch to experiment where to get the best RPM before buying a newer aluminum prop. Even at the lowest pitch 11in I couldn't quite get the boat up to speed (I guess the difference is the 4 blades on the PP vs. the 3 blades on the JE prop).

Problem:
So now that I have a decent prop, I'd like to use the PP as a spare (light weight and all) However, I'd also like to find a way I can make it get a little better performance out of it. (I don't remember the exact RPM of the PP at 11 but I think it was about 4200 with the boat lightly loaded with only me on board).

I talked to the prop shop and he suggested that trimming the diameter would be easy and might give better RPM but might not really add to the performance.

So I've been googling and haven't found much about how props are modified that would help me at my level of understanding...
Here are what I think are my options other than cutting the diameter. I think I can rig up a jig to modify the blades little by little but don't know which way would be best to start.

Other option is to run it in a higher pitch with 2 blades - I didn't think of that until after it was off the boat. But I'd like to learn a little bit and have an excuse to be out on the lake more :joyous:

thanks for your ideas and suggestions. I figure if I screw these up I still have the other set + one spare. I may try to get a few extra blade on clearance before they're all gone also... If I had 3 sets, maybe I'd just modify each one a different way but- gotta' start somewhere.

 

[steelespike]

I sure wouldn't want to try to balance a prop trimmed the way you want to try.
Try it with 2 blades Maybe more pitch.
2 blade are said to be the ideal number for speed.But they do struggle more with added weight
If your motor is a 2 stroke the rpm range is 4500 - 5500. So the PP doesn't even get within the rating.
If you make the diameter smaller you will lose thrust and gain rpm.
The Solas Amita props are squeeze cast making them thinner and stronger.
I think the PP are a little to thick.
Your 15" OMC prop is way too much unless your boat weighs 8 pounds.
9"pitch GPS , 23 @ 5000 indicates your tach may be off by a bunch, probably over 6000.

 

[Can'tFishEnough]

9"pitch GPS , 23 @ 5000 indicates your tach may be off by a bunch, probably over 6000.

To clarify, I said "...just me with an 11x11 (modified from 11x9". At 9 the 11x 9 pitch prop was at 6000 when I stopped turning the gas... so the local prop shop I bought it from modified it to an 11x11 for me.

So for diameter - that's pretty much the same thing the prop guy told me, I just wondered if there were other options...???
Yes, the 15 was way too much - which is why I got the PP in the first place - since it was cheap I figured it would be a good way to test different limits without haveing to buy over and over... it just still is too much at 11 pitch with 4 blades.

And, yes, it's a 2 stroke.

Guess I was just hoping to learn a little bit about how it all works. Would getting it balanced be that hard if each blade was modified the same way?

 

[Grub54891]

Any time you chip a blade or anything they are out of balance. It don't take much to knock them out. You could get a digital scale and weigh each one and match the weight. It would be better than guessing.

 

[Sea Rider]

Probably you order the wrong PP for that OB. With PP prop you can play with different pitches for several applications with same OB.

Happy Boating

 

[Can'tFishEnough]

no, it's the right one #4901 - the only one they make for a johnson/evinrude 25hp 2 stroke. All the other ones have a different spline counts.

It adjust from 11 to 17 going by the tick marks - I think my boat is just heavy for the motor. It's rated for up to 45 but going up right now is not a financial option unless I get a good deal somewhere. (and it is set up for a tiller which has been hard to find used in the 25+ hp range.)

Anyway, I was just hoping that if there was a way to test it out by carving a little off the blades leading or trailing edge (assuming I can get them equal weights using a digital scale for small things) and maybe even trying to rig up something like an old fashion tire balancing jig so the heavy part ends on the bottom...

Which side would be best to shave down some to see if there is any increase in performance.

I have no doubt that it would work fine to get me home if I busted the alum. prop and had to change it. I would just like to see if I could modify it a little to be able to get similar performance as the alum prop.

Our local lake is pretty small and (unfortunately in most cases- but fortunate if you need help) has enough traffic if I had to get a tow I probably could... But there is always that one time.

So before cutting I wanted to ask those who know much more than I do. I just see all these old motor props (online mostly) that have these larger sweeping paddle blades so I wondered if going to that shape might help. obviously there is a reason they've gotten away from that - but maybe a hybrid of sorts is what I was thinking.


Thanks.

 

[Sea Rider]

If your OB wot rpm is 4500-5500, what rpm was OB achieving at full grip while combo was running lightly loaded with the least 11 PP pitch available ? Is tach 100% healthy and in excellent reading order, is it an induction or OM tach ? Props are precise items to power boats, personally wouldn't dare modify any prop, surely will be out of correct blade balance giving vibration issues on prop shaft which are no good por lower case seals, A 9-8 pitch prop will surely better that combo's performance.

Personally like maximizing props while powering boats with underpowered OB's rated for much higher HP, but that's with a tach, lightly loaded and running on flat calm no wind water conditions.

Happy Boating

 

[steelespike]

Actually a prop with larger blades may work better for your setup. 10 or 9" pitch.
Your 25 on that boat is like a larger motor on a really heavy hard to push boat.
You need some blade area.If you look up props you may notice that usually as
pitch goes down diameter usually goes up.
More to follow.

 

[steelespike]

Actually a prop with larger blades may work better for your setup. 10 or 9" pitch.
Your 25 on that boat is like a larger motor on a really heavy hard to push boat.
You need some blade area.If you look up props you may notice that usually as
pitch goes down diameter usually goes up.
More to follow.

Then there is "slip" the measurement of actual distance compared to pitch.
Typical runabout prop slip is about 12%. Prop rpm affects slip as the prop slows slip generally goes up.
Hull weight and design affects slip.Of course prop design does as well.

 

[Can'tFishEnough]

Update:
Tach is an induction tach that you wrap a wire around the spark plug (I'd love to have an actual gauge but haven't found a way to set that up for this motor just yet.)

I had a chance to test out the ProPulse with only two blades on Sunday evening. Picked 13 pitch just me in the boat and it ran like a champ.
Motor was turning @ 4950 RPM with a GPS speed of 24.8 (roughly changed some depending on direction and wind but never dropped below 24 or got above 25.
Still need to check with a heavier load on the boat and might try again with PP pitch set at 12 with 2 blades to see if RPM comes up a little more.
(Side note: I may also try again with all 4 blades set at 11 and see how that works, I can't remember the RPM with just me on board but I do know that it didn't do very well with wife and kids also the last time I tried it last year - I vaguely remember it being around 4000 RPM and barely pushing at 16-17 Mph. )

The other thing I thought about with the PP was that with only 2 blades there are large (about 1.25 inch or so) holes on each side of the hub. I don't really understand exactly how that affects props with thruhub exhaust other than I know that high power performance props have vent holes but small ones for 25hp motors usually do not. The ProPulse did have a tendency to cavitate/ventilate pretty bad on turns at higher speed/throttle settings. But it seems like it would be ok still for "get you home" type set up if you had to use it with only two blades. I'm still curious what would happen performance wise if I shaved off a little bit of the cupping on the PP blades but for now will keep trying the 2 bladed approach in different water/load conditions to see how it does.

(side note)
While I'm here I'll go ahead and mention that I also had a chance to try out a Turning Point 10.5x11in pitch (3 blade aluminum). It was also pushing the boat just under 25 mph with just me and between 21 and 22 with 2 adults 180-200 lbs. (However, it did have a tendency to let the motor over rev some could get up to 5800 with just me and between 5600 and 5700 with 2). It did seem to hold the water well on turns and seems like it will work well at the top of RPM curve with my normally heavier load of more family and gear. Will know in a few weeks.

I got the Turning Point because the OMC 11x9 (modified to 11x11 got smashed into some rocks up the river and is at the shop.
As mentioned in earlier post -- at 11x11 it was running at 5000rpm pushing just me in boat at just above 23 mph. Shop is re-welding/balancing/and changing the pitch to 10 for me. Haven't picked it up yet.

 

[steelespike]

Your numbers with 2 13" blades are pretty good with a 11% slip.Rpm range is 4500 - 5500.
So your within specs but 12" may be better when adding weight.

 

[Can'tFishEnough]

Your numbers with 2 13" blades are pretty good with a 11% slip.Rpm range is 4500 - 5500.
So your within specs but 12" may be better when adding weight.

thanks for the feed back. Kids' baseball season and weather on my "free time" days/evening have been tough on the boating schedule... I've been hoping for a chance to try it set at 12 with just me, and with another adult or a few kids and full live well etc, to see how it does.

I think it's a great back up prop considering how light it is, (not that an aluminum prop for a 25hp is really heavy...) but also that it may offer a little bit of adjustment if needed and, if I can run it with two blades with light to moderate load with decent performance I'll have 7 spares (I bought one set of 4 with prop and one extra came with it).

I'll post some results after mid may for sure as we have a couple lake trips planned. So I should have a chance to get out and run around some with various set ups.