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Author Topic: showing the file to propellers?  (Read 2162 times)

Hande

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showing the file to propellers?
« on: February 17, 2016, 07:47:29 PM »

I have twin screw, fixed Kort nozzle system.
I'm buying propellers, but cannot afford proper Kort props. Not to mention custom made ones.


The size of the propeller should be 54mm to be as close a shave inside the nozzle as possible.
Of course, no such thing among the cheaper products.


Should I buy 50mm props and live with a greater gap until I can afford proper props, OR


... is it feasible to buy 55mm more regular (146/149?) props and shave 1mm off the blades? (That would also make the blades look more Kort-like). I thought that I could achieve an even 1mm reduction in diametre with a dremel - fixating both the prop shaft and the dremel to make a balanced shave.



I understand that I would be running the risk of creating unbalanced propellers. In my case it would be a slowly turning propulsion system of a tug. If the propellers were plastic, the balancing problem is even less of a risk?


I tried to find a discussion on this matter, but didn't find the correct search terms or there hasn't been discussion about filing down the size of a propeller.





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morley bill 1

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Re: showing the file to propellers?
« Reply #1 on: February 17, 2016, 08:08:32 PM »

Hi Hande do you know anyone with a lathe who could skim them down to the correct size  Bill..
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Danny

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Re: showing the file to propellers?
« Reply #2 on: February 17, 2016, 08:09:46 PM »

Most racers lighten, balance and polish their CuBe or SS props using both hand and electric tools (Dremel).  There are many sites about it and youtube.
You will need a prop balancer - magnetic is best, and good safety wear.
Its quite easy but practice on an old prop first.

Robotnik

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Re: showing the file to propellers?
« Reply #3 on: February 17, 2016, 08:13:03 PM »

I made a jig once to trim down a 3 bladed aircraft propeller to fit in a hovercraft duct.
I screwed the brushless motor to a piece of wood, fitted the prop, then clamped the dremel with a cutting disc in such a way that when i turned the motor by hand the cutting disc would slice the right ammount off each blade.
If you make a dremel jig practice with lolly sticks or scrap pieces of wood before cutting the prop to be sure you have the right angles and length.
Don't try and cut too much in one go or the cutter will snatch at the prop, if you need to cut off much more than 1mm then do several cuts resetting the dremel eash time.
Take it slow.
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ballastanksian

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Re: showing the file to propellers?
« Reply #4 on: February 17, 2016, 09:43:21 PM »

You can do it in a lathe but just be careful not to take too deep a cut. I recently reduced a 25mm left hand plastic prop to 20mm in my little lathe and it worked quite well. The hand finishing  was quite easy as well.
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Peter Fitness

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Re: showing the file to propellers?
« Reply #5 on: February 17, 2016, 09:51:48 PM »

I skimmed a prop on my lathe so it would fit inside a home made Kort on a pusher tug I built for some of my grandsons. It was my first attempt at a Kort and I was very pleased with the result, the Kort and the tug work very well. As Ballastanksian said, don't take too much at a time, just gentle skims with the lathe will do the job nicely.


Peter.
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Shipmate60

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Re: showing the file to propellers?
« Reply #6 on: February 17, 2016, 09:59:07 PM »

If you are going to skim the prop it would be better getting a pair of larger props and reducing down to the correct size then you would have the correct profile props and not need expensive replacements.


Bob
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Hande

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Re: showing the file to propellers?
« Reply #7 on: February 17, 2016, 10:42:47 PM »

I'm grateful for all responses!
In summary:
- best to use a lathe (I think I can have access to one)
- take it slow, very little at a time (I can believe that!)
- it's not impossible, so I should try - practice with something first


Balancing I have to study on the Net. I think my local retailer can arrange something.


Is balancing still important with plastic props in my case? - Or to rephrase: Can I tell, if I have a prop out of balance, if I can honestly say that it spins smoothly by watching it?

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Danny

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Re: showing the file to propellers?
« Reply #8 on: February 17, 2016, 10:54:02 PM »

Any prop can turn smoothly with all three/four blades the same size but it could still be out of balance due to the thickness of the individual blades.
Personally, although I have a lathe, I do them by hand.  One chip too deep on just one blade and bang! Ruined!  The slower the revs, the less important balance is, it just helps the side load on the shaft bearings.  Plastic does not need balancing, its only the weight of the blades that matter, and plastic is light.  Aluminium is also light so usually just need a trim.


Although for speed, the principle is the same for any metal prop
https://www.youtube.com/watch?v=rO_mJVlq-Qk

Hande

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Re: showing the file to propellers?
« Reply #9 on: February 17, 2016, 11:07:55 PM »

Thank you Danny
Now I have all the pieces to my puzzle
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roycv

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Re: showing the file to propellers?
« Reply #10 on: February 17, 2016, 11:35:03 PM »

Hi all, just to chuck a spanner in the works, some years ago I was in email contact with the late Tow boat Joe (USA)  He was a tow boat Captain and also made many tow boat models. 

He said that originally the props in korts were close fitting as you might expect but in operation on the Mississippi there were logs in the water you could not see and if one went into the prop/ kort they were difficult to remove.  So they went back to ordinary rounded blades and these would either chop the logs or there was enough space around the prop to make un-jamming them much easier.
I must finish my Tow boat!
regards,
Roy
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