propellor question

[Johnjo]

Hi all
bought a new two bladed propellor today, was giving it a good polish
in readyness to coat it with Lanolin,
Noticed the leading edge of the blades are round, while the trailing edges are
champhered, any particular reason for this?

mike

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[snowleopard]

an efficient aerofoil/hydrofoil shape has a rounded leading edge and sharp trailing edge. the trailing edge may be blunted a bit for safety.

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[Cornishman]

Propeller design is a notoriously complex subject, but think of it being similar in section to an aircraft wing in the vertical plane instead of the horizontal and you will begin to see why the leading edge is thicker than the trailing.

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[vyv_cox]

A sharp trailing edge often produces an annoying ringing noise. The chamfer is there to overcome this. It's usually enough to run a file over the sharp bit to produce a flat about 2 mm wide. Subject comes up on PBO on a regular basis. Expect a few in April when boats start going back in the water.

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[Johnjo]

Yes Vyv
This is exactly what I was talking about,
As if someone had taken a file to it, all makes sense now thanks,

regards mike

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[clyst]

<Propeller design is a notoriously complex subect> couldn't agree more ---Aircraft
wing profile and prop profile similar??? Don't think so! One is designed to give lift the other to screw . shant labour the fact but you can see my meaning???

Regards

Terry

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[qsiv]

Early props were simply screws = everything produced since the Wright brothers is absolutely an aerofoil = the 'lift' the blade produces is what drives the boat through the water. You'' notice this most on a surface piercing prop where the 'front' face of the blade is close to flat, the rear is distinctly cambered.

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[Twister_Ken]

Propellor question question

>the 'front' face of the blade is close to flat, the rear is distinctly cambered.<

Wouldn't that produce 'lift' astern?

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[spannerman]

Sorry but aircrft wings and boat props both work on exactly the same principle, just in different mediums, hence the difference in section and shape. A prop blade generates hydrodynamic lift in font of the blade which is a reduction of water pressure due to the fact that the water passing over the front of the blade has to travel further then the water passing behind the blade, (due to the profile) The water molecules have to meet up at the same time at the rear edge so the ones on the front travel faster. Now if you go back to basic physics if you increase speed you reduce pressure, thats how your fridge\air-con\ carburettor work. So the blade advances forward due to the water pressure being lower on the front than the rear the blade. This is also known as 'slip', if there was no slip there would be no thrust. for example the only time a prop has no slip and therefore no thrust is on rapid deceleration or if the boat is being towed and the prop is windmilling. Hope this helps to clarify it.
Steve

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[Twister_Ken]

Ah but

Yes a prop can have an aerofoil section, but they can surely still work if they consist of flat, angled surfaces. prime example would be those ceiling fans that you see in indian retaurants, or turbine blades in aeroengines.

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[qsiv]

Re: Propellor question question

Thanks!

Read what I mean not what I say. I'll put that down to an 04:00 wakeup from the younger boy. Must engage brain before fingers in future, and infuse more caffeine.

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[Jacket]

Re: Ah but

But a flat angled surface can act as an aerofoil- after all thats all a kite has to provide lift. Its just a question of the efficiency of the foil, and the fact that a flat plate tends to stall very easily, which would mean that a prop made with flat sections would only work reasonably over a small range of speeds.

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[Cornishman]

Thank you, Steve. couldn't have put it better myself!

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[snowleopard]

Re: Ah but

a flat blade will work but very inefficiently. the fan in the curry house is there to stir up the air & look pretty.

on the other hand a gas turbine compressor blade is anything but a flat simple surface, it's an extremely sophisticated aerofoil.

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[spannerman]

Re: Ah but

Couldn't agree more, aero engine blades are very complex in design as a look inside any jet engine will confirm, they have incredible twist and camber and some compressor stages also have variable pitch. As they have to work in widely varying pressure densities and temp ranges, something our humble boat prop doesn't have to contend with.

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