Author Topic: Propeller design  (Read 1236 times)

Offline Robert Hornby

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Propeller design
« on: October 27, 2022, 04:52:44 AM »
I am remaking the prop for Columbine and giving it a bit more thought this time. Can someone help me with the pitch angle of the blades. 30 deg looks right but I am only guessing.
Age and treachery will always overcome youth and skill

Offline Jo

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Re: Propeller design
« Reply #1 on: October 27, 2022, 08:04:17 AM »
It is not that easy :disappointed:

The pitch angle should be thought of as being like a machine screw it indicates the distance the propeller would “drive forward” for each full rotation. What actually drives forward is the boat not the prop and the distance it moves forward is actually less than the full rotation distance (nominal pitch vs actual distance is known as slip). So the blade pitch converts torque of the propeller shaft to thrust by deflecting or accelerating the water astern.

We also need to know the diameter of the prop because the larger diameter of the prop the more water it is trying to move astern. This allows us to take into account the power of the engine. A standard pitch/diameter ratio is normally between 0.8 and 1.8.


Or if you have bags of power available is comes down to: how fast do you want it to go?

Jo

« Last Edit: October 27, 2022, 10:51:55 AM by Jo »
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Online Jasonb

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Re: Propeller design
« Reply #2 on: October 27, 2022, 08:39:01 AM »
It's really the helix angle derived from the pitch that you want so a prop with 200mm or 8" pitch would have a lesser angle that one with 300mm or 12" pitch.

And just like a screw 200mm pitch may be a fine thread on a large diameter but very coarse on a small dia, think 2mm pitch on a M20 fixing vs the same pitch on a M5 fixing which would give very different helix angles.

From this it is also possible to see that the angle will change along the length of the blade

Offline Bluechip

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Re: Propeller design
« Reply #3 on: October 27, 2022, 09:33:49 AM »
I used to have this book .... MAP (?)

https://www.amazon.co.uk/Model-Boat-Propellers/dp/0852427123

Not here now though.  :headscratch:

Dave


Offline Jo

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Re: Propeller design
« Reply #4 on: October 27, 2022, 10:48:52 AM »
I used to have this book .... MAP (?)

 :thinking: I haven't got that one. This is the degree briefing paper we used to use in those days when I had to Work  :toilet_claw: for a living.

Much easier to use the 0.8 to 1.8 figure on the ratio and probably go for the 0.8 for slow running.

Jo
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Offline MJM460

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Re: Propeller design
« Reply #5 on: October 27, 2022, 12:05:25 PM »
Hi Jo, that’s an interesting paper with some useful concepts not often published.  Thanks for posting it.  I have enjoyed reading it.

Robert, that paper will give you some idea of what is involved in designing a propellor from scratch.  For an easier answer to your question, I suggest that you refer to your signature panel, and visit your local hobby store, or favourite on line outlet, and buy a few ready made model boat propellors, and see which one works best.  Then you can make a similar propellor from your preferred materials, knowing that it is based on tried and true model size propellor designs.  You will also see the blade profiles and outline shapes selected by the model propellor manufacturers.

Select the largest diameter that will fit your model while still leaving a sensible clearance between the propellor blade tips and the hull.  About 10% of the diameter is usually suggested for full size sail boats, but a little less will still be ok on a model where the hull stiffness is usually relatively higher than a full size hull.  As Jo’s paper states, the larger diameter is normally more efficient, meaning more thrust for a given engine power.  For your specific model, I would expect a four blade propellor might be a better choice than two blades, but if not too expensive all the available options are worth a try.  Once you have a good selection, you might even consider one of the commercial brass propellors which will look more in keeping with your model than the usual red plastic ones, but more expensive so better to work out a suitable size and pitch first.  This leaves only engine rpm to experiment with.

I have enjoyed following your build almost since the start, exciting to see it ready for a propellor.

MJM460

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Offline john mills

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Re: Propeller design
« Reply #6 on: October 27, 2022, 12:19:47 PM »
the propeller is made to dia and pitch when a pattern maker makes the pattern the angle changes from the boss out to the out side dia.
it can be laid out on the drawing but i cant remember how i did it it was a few years ago i have made some patterns for larger propellors
20 " dia.    for a steam engine the pitch would be fairly high for a reasonable speed at slow engine rpm.the on for a launch with a stuart star engine 2 1/2"dia 8 " pitch the engine 5/8 bore 2 cylinder single acting 5/8" that was a stuart turner launch in a old catalog .
John

Offline john mills

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Re: Propeller design
« Reply #7 on: October 27, 2022, 08:13:30 PM »
Hi I have 2 comercial bronze cast propellors 1 is 2 1/2" dia and 3 1/2" dia  three blades and the blades would be 45 deg .

       John

Offline Robert Hornby

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Re: Propeller design
« Reply #8 on: October 28, 2022, 05:32:24 AM »
Great John, that is what I was hoping to hear. I will make mine with a 45 deg pitch. Picture of the original prop with 30 deg pitch 62mm dia. the new one will, be 85mm dia. I have also increased the rpm from 160 to 260. (It is now 12 volt electric motor driven (I gave up on the steam plant)
Bob
Age and treachery will always overcome youth and skill