Cupping a prop

Looking for videos on how put some cupping on my prop(s).
Maybe I'm not typing it in right on YouTube but can't seem to find anyone actually doing it themselves.
It's only going to be on aluminum prop so heat won't be needed but I'm trying to understand exactly where to bend the tip.
Most of my props have been dinged by rocks in the past so not to concerned with messing them up. I'm high enough now with my motor that the prop can sometimes blow out when turning and plan on putting a manual Jack plate on to go even higher.
Also if I were to buy a new prop, should i be looking at a 4 blade or just some cuppi g on a 3 blade?
Current motor is a older 30hp Mariner(Yamaha),2cyl, 10 spline , 11 ish pitch...get 27mph....I say 11 ish because I've welded new tips to the prop a few times.

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Aluminum should be annealed before any bending or it can crack.

I think it will be very difficult to get all 3 blades the same. Net result will be vibration.

If you seen the props I'm using...lol
Dinging rocks is a weekly occurrence,one has had new welded on blades 4/5 times.Wondering which edge and how far down to bend...video would be great but guess I'll see if I can find a tutorial.

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FWIW I’ve only seen or heard of cupped props making a difference on high HP OBs, like 150hp and above. And even then it is trial @ error and not an exact science and no 2 prop shops define or measure it the same.

Yet those good prop shops have 3-axis measuring tools to ensure repeatable results for each blade, and more importantly, know where it ‘was‘ vs. where they ‘moved it to’, in order to ascertain any performance difference.

As a DIY’r, I highly doubt you’d ever see any appreciable difference, never mind on a 30hp no less, with likely worse performance if anything, but hey ... if you have the time to spare/expend/waste ... have at it ...

If you go for it, either buy or make a pitching block.

I'll look into a pitching block...
As far as them " working" on smaller outboards, alot of tunnel hull boats and reg. Jon boats use them, ...flats guys and duck hunters. I mean I know I probably won't gain speed but the ability to run high and not blow out, is fine with me.
Thanks for the replies.

Good article here Zum:

https://www.mercuryracing.com/prop-school-part-4-blade-cup/

Thanks again
I planned on taking it slow, seeing as I have know real idea on what I'm doing.
Might just try to tip portion first , then the outer edge....32 or 16th at a time.
Still trying to find a video, seen some on repairing props and the pitching blocks...no cupping yet. Like I say my motors as high as I can go without blowing out all the time, plus I want to go higher with the manual jack plate....although there's also a compression plate I'm reading about that might help me.
5 inches back might get me cleaner water also...so many variables...

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cupping on the leading edge does one thing an cupping on the trailing edge does another

I forget which does what, I quit doing it many years ago and enjoy fishing more than working on stuff nowadays, thus I hire the prop stuff out. Generally if I call the guy and tell him what it is doing and what I'd like to achieve, he fixed it up for minimal cost and sends it back with expected results. Adding cup on aluminum prop costs more to ship it there/back than to have it done...stainless is a little different (higher of course), and keeping in mind it's been a number of years since I've sent any off.

Just curious, Zum, why so intent on running with the outboard raised as much as possible?

I'm not criticizing, I wish I could run with mine up higher. I'm on my second prop this duck season.

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Only 3 props and almost a lower unit last year.



This was the last time,had to use hammer to straighten it out so I could make it out acouple days later. Shallow lakes,big boulders and lack of rain lately makes navigation "tricky" at times. "If you run with the bulls sometimes you get the horns",fits me pretty much. If I were to get a newer outboard maybe I'd be more careful but somehow i doubt it.

Pappy, did the skeg get mig welded or tigged?

eeshaw said:

Pappy, did the skeg get mig welded or tigged?

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Used a mig on the skeg(and prop)above but a tig will work as well.
Thanks Pappy in regards to the cupping, I have acouple props I can play with.

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something worth mentioning is that if you plan on running really shallow, and you add cupping so you can run higher on the transom, you might be wise to watch the cooking water pressure if you aren't already. It's not common but on some outboards they don't like to run shallow--just depends on how the water intake is set up. If you are gonna run really high, you can also try a 4 blade. It helps.

CedarRiverScooter said:

I think it will be very difficult to get all 3 blades the same. Net result will be vibration.

Click to expand...

I’ve been following this as I plan on trying it on one of my props when I get my jack plate built.

What I plan on doing is using one blade as a master and cup it until it looks correct, then making an imprint of it either with bondo or similar, then using that as a guide for cupping the other two blades.

After that’s done it will get sharpened and then balanced.

The sharpening is more to help it cut through weeds rather than any performance advantage.

Is your jack plate going to be a fixed or adjustable one?