More prop woes

1996 alumacraft dominator cs powered by a 1996 evinrude 70 2 stroke mounted on the 2nd hole from the top . there is also a hydrofoil on it. I have been told 5500-5800 is the ideal wot RPM. Not sure if that’s true

Boat came with a 13 pitch and it over reved to around 6400rpm. Powerhead had to be rebuilt. My guess I because the previous owner was running around wot at 6400.

Jump up to a 17 pitch and the RPM drops to 5200. 5000-5100 if I have a partner. Ran that for a half season. I’d also have to trim the motor up a lot to get the RPM up to 5200

Picked up a 15 pitch today and took it out. Hole shot so much better but the RPM at WOT is running 5900-6000. I could trim it out so it’s 6000-6100. I’m worried that’s too high and that it’s going to lead to problems down the road.

What would you do in my situation? Should I get a prop re-pitched to a 16? Remove the hydrofoil? Raise the motor more?

Second question. I’m missing paint off my new 15 prop. Seems kinda uniform any ideas what could of caused it

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My personal short opinion and I can say I am 100 % right or what I would do is best for you:
I am guessing that the prop was not new but a rebuild and the paint is worn off from poor paining after a fix , or you hit something.
I also personally would not spent the $ on a custom prop for a 20 year old boat with a 70 hp motor and just be cautious to back off wot a bit to not get it over 6000. That is just my opinion and it might not be the best, just all depends on how much $ you want to spent, what performance you need vs want out of the boat and how long you wan tto keep it.

What brand prop? Aluminum or stainless steel? Looks to me you hit something or the prop is flexing. The paint job looks poor with runs in the paint. I wouldn’t waist the money on a custom prop on that boat/motor combo. I don’t think that the hydrofoil is causing your problems, if installed correctly.

Hi. If you want to get the most from your motor you will need accurate tach and speed information. What source are you using for speed? I hope it is GPS and not your speedo. Tachometers are notorious for being inaccurate. You need a tacho that reads directly from the sparkplug wire or the engine module. I found info that says you should be reading RPMs in the 5800-6000 range optimally. Once you verify that you have accurate speed and RPM data, you can plug them in to a prop slip calculator and at WOT it should be 5-10%. A Stainless prop will give you better results–just don’t hit any rocks with it.

As for you paint problem–could be a poor paint job. If the paint was in more consistent locations on each blade it would be some kind of obstruction ahead of the blades such as the skeg or rivets on the hull. Still, it could be damaged blades with cavitation occurring at different spots on each blade.

Brand of prop will make a huge difference also. You can get a 15 pitch with wide.micky mouse ears or a performance tuned with more aggressive rakes. I agree on the age and recovery time for rig with going custom. But try to find a prop guy that will let you test run a couple different styles.

The white marks are cavitation burns which can be caused by a disturbance in getting of the prop creating air bubbles. Google “cavitation burns” and you’ll learn more. I know Evinrude’s prop selection guide talks about it in more detail.

The prop is a rebuild from my local shop. It is an Evinrude prop(brp) same brand and model as my 17 pitch prop.

My speed ratings are GPS off of my Elite 7 and also a speed app on my phone. I am getting the RPM reading off my factory tach in the dash.

Would an increase in RPM’s cause cavitation burns? I had no such issues with my 17P prop. I always thought it was caused by a defect in the hull or a poor transducer mounting.

I also have yet to find anything official but doing some deep google searches I am seeing a recommended WOT range of 5800-6000.

The problem is in my area of the 3-4 marinas around I have yet to find one that will let me test the prop. It’s why I bought this 15 rebuilt. I was told I could return it as long as it wasn’t scratched…Well the burns took care of that haha.

I did take my son out later tonight and I really like the way the boat runs with the 15. I lost 1-1.5 MPH but the hole shot and staying on plane at lower speeds is nice.

I also took the hydrofoil off just to try it out. What is nice about having it off is that my big butt 290#s would cause a slight list and I would have to trim the motor up high to get the starboard side foil out of the water.

Unless you are really cavitating that prop a lot I am guessing it is not prop burn, just the paint falling off the reapir area. Aluminum can be tricky to repair and most times repair guys dont focus on a great quality pain job, but raterh the metal work being adequate… I have to admit, I dont knwo a lot about prop burn but ran rebuilt aluminum props growing up and have seen the paint flake off before.

Also the bare spots are not in the exact same spot, alsomost like each blade had a bit of a differant amount of damage before the reapirs were made.

5900 6000 is fine for rpm’s for the little googling I did on the rpm range for that motor. The paint looks like its just not sticking right. Ask the prop shop you got it from if the paint is a sign of a problem. IT is far worse to over prop and have lower rpms at full throttle. Its the equivalent of trying to pull a bobcat with a yugo. LOL motor will not handle the stress. You will just have to be careful of over revving the motor when its just you in the boat with a light load.
I blew a motor because it was over propped it was only getting 4100 rpm’s wide open when the motor recommended up to 6200 rpm. I have refused to buy from any dealer who would not go to the lake with me with several props and test drive to assure it was in the right range.
Mwal