Prop re-pitching

[SL Ethel]

Has anybody on the list successfully re-pitched his or her own prop? I have an 18x23 motor boat prop (bronze) with lots of blade area. Currently I have the engine geared up to spin it about 1.3x engine speed, but would much rather have more pitch at lower rpm, especially since I have all that blade area to use.

I have talked to two different prop shops, but nobody will even talk about taking it beyond 25" of pitch - I was hoping to take it out to 30" or even 32". In SBMSL, there are a few reference to people re-pitching their own wheels quite a bit more dramatically. Anybody have experience here? I'm thinking at for sub 500 rpm operation, perhaps there's more latitude in how it's done - I just don't want too break my prop in the process!

For a more extreme fix, could the blades be sliced off the hub and brazed back on at a higher pitch? Again, for this application, I'm under 10 hp and under 500 rpm.

Thanks,
Scott

[Akitene]

Good evening Scott,

I can't find any suitable prop on this side of the pond/chanel. I'm considering 2 alternative solutions:
1. Cut out the blades, milling the foot of each blade to a proper angle and braze them back together. I'm very doubtful about this one.
2. Get a larger prop with a suitable pitch then machine it to a smaller diameter while reducing the blade area. This latter one pleases me but I'd have to deal with the large shaft bore and the broader hub.

I'm very interested in your thread and I will follow it with attention.

Regards,

Christophe

[Dhutch]

I have heard of people reducing the diameter of the blade a little themselves, say taking a 26inch prop down to 24inch but I have never heard of someone re-pitching a prop themselves. Its hard enough to get anywhere near just straightening one slightly bent bat to match the other two, let alone changing the pitch of all three.

No doubt someone will now come up with an elaborate plan way in which they have achieved this with ease, but certainly anything other than a really quite small prop if going to need a lot of force.
Ours is a high DAR 26x32 and spec'ed for canal use which might be top-end for the average small steamboat but the forces needed to change its shape would be in multiple tons not kilograms I am sure!


Daniel

[Akitene]

Re-pitching a propellor can be done this way too:



The guy bends the blades by eye without great accuracy but it seems a good enough method if you're looking for but a couple more inches. I'll keep thinking about it.

[Dhutch]

...No doubt someone will now come up with an elaborate plan way in which they have achieved this with ease...

So there we have it, easy....

As you say, it appears he is doing the largely by eye, to and extend I expect by doing the same thing three times, and its not a huge prop, but none the less impressive.


Daniel

[DetroiTug]

Repitching a prop is something I've considered in much the same way the guy is doing it the video. As fast as these props turn on our Steamers, slight irregularities in pitch would be negligible. Balance would be somewhat critical.

There is a local steamer that makes props out of a piece of tube for a hub and the blades are just burned out of plate with a torch and trued up with a grinder, he welds it all together and they work just fine. Not much to look at, but they do the job.

-Ron

[johngriffiths]

The video shows techniques not much difference to the way inlet guide vans were fine pitched on aero engines in the late 1960s.
When I approached a UK propeller maker to re-pitch a prop for me he said that with a bronze prop, an extra 2" of pitch per foot of diameter was about the limit. Any decent prop re- pitcher forms each blade to a former and re-balances the prop afterwards.

JohnG

[Dhutch]

... with a bronze prop, an extra 2" of pitch per foot of diameter was about the limit. ...

That's an interesting figure, and presumably you could also decrease by an amount, maybe a similar allowance.

So our 26x36 could become a 26x38 or so but say 26x42 wouldn't go. Not that we have an issue, but just to flesh out the ratio with an example.


Daniel

[Bob Cleek]

Repitching a prop is something I've considered in much the same way the guy is doing it the video. As fast as these props turn on our Steamers, slight irregularities in pitch would be negligible. Balance would be somewhat critical.

There is a local steamer that makes props out of a piece of tube for a hub and the blades are just burned out of plate with a torch and trued up with a grinder, he welds it all together and they work just fine. Not much to look at, but they do the job.

-Ron

That seemed to work okay for Charlie Allnut, didn't it?

[DetroiTug]

That seemed to work okay for Charlie Allnut, didn't it?

Yes, it sure did. Him with Kate Hepburn on the bellows. He also did a slam bang job straightening that propshaft, only had about 1.500" runout. That movie is comical with it's technical inaccuracies. But, a masterpiece.

-Ron