A strange question

Discussion in 'Props' started by BertKu, Dec 29, 2020.

  1. BertKu
    Joined: May 2009
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    Location: South Africa Little Brak River

    BertKu Senior Member

    Gonzo, my problem is that Diesel and Petrol engines has a narrower area of torque in the rpm range. While a brushless motor is the torque from 1 to maximum roughly the same. Thus when I place that in a software program like PropCalc and keep the power the same , one get the most rediculous answers, because it is developed for fuel engines. If I go to a shop , what can a salesperson sell me? the right prop? I doubt it. Bert
     
  2. BertKu
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    BertKu Senior Member

    Hi Gonzo, actual I should have a variable pitch/size propeller, then one can adjust the pitch and the size to the rpm of the motor. But it is not worth for me to invest in those expensive propellers. Does those propellers actual exist? I don't think so. Bert
     
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  3. gonzo
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    gonzo Senior Member

    There are variable pitch propellers in the larger sizes . A propeller with the same power curve as the input is the holy grail of designers. However, the problem of mismatched curves are not particular to electric motors. As everything else in life, we choose an acceptable compromise.
     
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  4. BertKu
    Joined: May 2009
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    BertKu Senior Member

    Gonzo, If you wonder how I balance the prop and check the pop, it is very simple. I have a two point system where between the propeller is turning, If it stops all the time roughly at the same place, I correct this. Also I have a Jacuzzi made long time ago and I use that also to put the propeller on a very thin string, mark one blade and drop it in the water. I count then the number of turns the blade has made from top of water to bottom ( 1 meter) and this I convert into expected performance of the prop, because I know the pitch roughly . If you have a better idea, I am listening. Bert
     
    Last edited: Mar 7, 2021
  5. fallguy
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    fallguy Senior Member

    I am curious why your blades look rather thick on the ends?

    In my small brain, there would be cavitation occuring here.

    Asking more than telling..
     
  6. BertKu
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    BertKu Senior Member

    Hi Fallguy, You probably totally right. But probably not at the speed I am going, below 5 knots. I hope I understand you properly, you mean at the 3 mm edges of the blades, or at the center? At the end of the blades, I still have to grind it down during balancing. Look Fallquy, it costed me 12 dollar per propeller in material as I use offcuts from a company who uses 316 sheets and pay scrap value for it. Any improvemnt proposed, I will try to make new propellers according to suggestions. Bert
    P.S. or do you mean when I made the the 2 propellers on the lathe, to ensure that they both are the same? Thus I placed 2 round sheets on top of each other.
     
  7. BertKu
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    BertKu Senior Member

    Fallquy and all others. I still haven't received a reply whether the two propellers must be parallel. pointing towards inside or other way. The reason is I am 80, got a year ago a pace maker and now I am no longer allowed to weld, due to the interferance from the currents in the cables with my pace maker. The person who welded it for me, made one side not parallel, but 5 mm outwards. It is a very big job to take it apart again in the hope that something else is not going wrong. Any views what the consequence is, is greatly appreciated. Bert
     
  8. fallguy
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    fallguy Senior Member

    Well, I would not worry about 5mm for low speeds.

    I assumed your question was geared more toward wave effects and such. The 5mm error is only going to cause a bit of drag as I see it.

    yes, referring to 1/8" thick edges
     
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  9. BertKu
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    BertKu Senior Member

    Thank you Fallguy, Yes I still have to finish the propellers off. I will grind it during balancing. Thanks, pleased to read, I don't have to be worried,
     
  10. BertKu
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    BertKu Senior Member

    Gonzo to come back on balancing, don't laugh, when one is restricted with traveling and some companies closed down, a man has to come up with a solution for a problem. Here is a photo of my simple balancing method. on the left a very low friction ball bearing and on the right, clamped in a vise on top of the table from the table drill, a sharp pointed pin. It works like a dream.


    Balacing.jpg
     
  11. gonzo
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    gonzo Senior Member

    As they say, necessity is the mother of invention.
     
  12. BertKu
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    BertKu Senior Member

    I fully agree with you. Here his my water probing test in my Jacuzzi before I put very thick gearbox oil with added 17, 5% extreme low friction substances with it, in the gearboxes. The testing to see whether my blades are all aligned, that was easy. I had it in the table drill and then it was easy to see which one blade was out. The reason for very thick gearbox oil, I used stainless steel 316 and that is softer then hardened metal gears and was advised as such, by an mechanical engineer
     

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  13. BertKu
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    BertKu Senior Member

    What a disaster, after 24 hours in the water, both gearboxes were filled with water. Although the one I used marine silicone, to seal the rubber against the metal and the other one just plain rubber against metal tight with bolts and nuts. Now I have a nightmary to figure out were the leak is in both gearboxes. What would you do when you stood in my shoes, a) blow compressed air in the inlet and see where it leaves the gearbox. b) fill up with water and try to see where it is leaking out c) redo the sealing with another water resistance silicone d) try to blow air with a color in the inlet. Any other idea?
    The shaft entrance is sealed and has two rubber seals on propeller side and the other side is inside in an inside bearing mounting, very unlikely
     
  14. gonzo
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    gonzo Senior Member

    If there is water in the gearboxes, you need a vacuum check not a pressure one to see where the water comes from. How are the seals installed? A seal installed backwards will let water in. The seals should be double and each installed with the lips in opposite direction and back to back . That means you should see the springs on both of them, so that pressure will tighten the lip of the seal against the shaft. Hope I make sense with the explanation. Otherwise I can post a diagram. Good luck.
     
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  15. BertKu
    Joined: May 2009
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    BertKu Senior Member

    Thanks Gonzo, I do have 2 seals against each other, but upon checking they don't have the springs in them. Will buy new ones. But I don't think it comes from the seal.
    But if the spring is outward, it will see the water and it will rust, it will mean on long term, again a disaster. With the brushless motor I was able to get the seals with a spring, but with covid19 I have restrictions in choice of suppliers. No I think it comes from the gearboxes I made. Here I use 3 mm thick mineral oil withstanding rubber sheets with 1/2 Inch apart bolts , 140 of them. between the metal and rubber I used marine silicone, but I will now use an English product, which can even be applied under water to seal a leak. I am quite impressed with that silicone. I don't have a vacuum chamber, thus I have to find another way. Thanks for the suggestions made, I will shop around and see what I can get.
     
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