How to repitch your own impeller

[timmy_philson]

many thanks!

[saf]

Something that I have always wondered.
How is ti that you can use a wrench to change the impeller pitch BUT the water won't change it over time?

Ive wondered this aswell.

The blade of the impeller is cast and its on its own tension. If you bend it, it will surely have some memory to want to spring back and water pressure maybe just that factor to push it back as its now stretched.

Plus, your pulling it away from the pump on the trailing edge no ?. Clearance same before and after ?.

[PolarisNut]

Ive wondered this aswell.

The blade of the impeller is cast and its on its own tension. If you bend it, it will surely have some memory to want to spring back and water pressure maybe just that factor to push it back as its now stretched.

Plus, your pulling it away from the pump on the trailing edge no ?. Clearance same before and after ?.

You have to bend the metal past its yield point, then it will hold. Try bending an impeller once...you wont believe how much effort it actually takes. This isn't spring steel we're talking about here...It doesn't have much memory.

[saf]

You have to bend the metal past its yield point, then it will hold. Try bending an impeller once...you wont believe how much effort it actually takes. This isn't spring steel we're talking about here...It doesn't have much memory.

Hey there PN, Yes of course i understand this, i am a fabricator of raceparts for imports nowadays and work with stainless, CR-MO, alu, etc every day.

I realise it doesnt have much memory (some it will have) and i bet its tensile strength would be high in that it would rather snap a blade at a lot of stress than bend, but to bend a casting of sorts, or anything under a lot of tension (ie a wrap around blade) as you say past its yeild point, your stretching the metal on the outside and condensing it on the inside of the bend. Most metals will natrually with opposite resistance/load (water flow) will want to return the stretch under severe load to its former self with much less tensile strength.

Not that im doubting your method, as ive said, its made me think in the past. I guess you can prove the theory by bending one, feeling the diff .. leaving it and then after your very used to it, go back in with a new prop of the original pitch and see if you now feel the same difference. That would mean under pressure it would hold its corrected pitch.

But hey, ive not done it, if you say it works, ill buy that. If anything its made discussion

[Guest]

they won't bend back by themselves, once it is bent it stays that way unless of course you bend on it again

[Duke]

A couple tips on how to pitch your prop a little safer and easier.

http://www.greenhulk.net/showthread.php?t=20995

[32DegH2O]

Unfortunately bending the outside edge to a new pitch really isn't the best way to improve the power/thrust capabilities of an impeller. The outside edge area really is the weak area of an impeller relative to producing thrust. The real power band of an impeller is the inside edge where it connects to the hub...change that pitch then you'll have something. Of course not many people have the capabilities or the ability to cut the blade from the hub, reposition it and then weld it back to the hub. However, there is a second way that is much simpler and easy to do and gives a much better result than bending the outside edge. That's by bending down the back/bottom edge portion of the blade right where it connects to the hub.

[Duke]

Unfortunately bending the outside edge to a new pitch really isn't the best way to improve the power/thrust capabilities of an impeller. The outside edge area really is the weak area of an impeller relative to producing thrust. The real power band of an impeller is the inside edge where it connects to the hub...change that pitch then you'll have something. Of course not many people have the capabilities or the ability to cut the blade from the hub, reposition it and then weld it back to the hub. However, there is a second way that is much simpler and easy to do and gives a much better result than bending the outside edge. That's by bending down the back/bottom edge portion of the blade right where it connects to the hub.

Both are important and you have a window that can be altered on the outer verses the root. Solas has a few different castings for the Yami and SD but not sure about the Polaris.

Still the only way to fine tune it within reason is going to be the outside edge working all the way to the root and working it up or down to maintain decent flow between the blade.

[32DegH2O]

Still the only way to fine tune it within reason is going to be the outside edge working all the way to the root and working it up or down to maintain decent flow between the blade.

When you say fine tune it...what exactly are you after or what are you trying to enhance or improve by changing the outside edge? I just want to make sure first what we are talking about here before I add my comments.

[HU112BU]

I realize this post is not only over two years old, but a sticky as well so I hope you don't mind if I add my thoughts. I have to agree with Saf (post #19) that forming cold metal will induce fatigue. You are bending it one way and the forces applied under load are trying to force it back. Anyone who has bent a clutch lever on a motorcycle and tried to bend it back has learned first hand about metal fatigue. The "off" color of a Nu-Jet impeller is due to the tempering process. They aren't pretty but they are very strong. I'm 100% for repitching your blades, or even straightening out a bent one. I just feel that someone should mention that they should first be annealed (or softened) by heating them up to the point of glowing with a torch first and let them slowly cool down. Then do your work. After bending, tweaking, beating and cursing them into submission, the impeller then needs to be reheated and quenched in cold water to temper (or harden) the steel. The reheating relieves all remnant stresses from cold working, and the quenching realigns (or stabilizes) the molecules to form a stronger steel. There's more info here if you're interested. http://www.azom.com/article.aspx?ArticleID=1141

Tracy

[Fenderjack]

So what was the results of this ?? It has been dead for some time..