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Author Topic: WHAT DOES A PROPELLOR DO?  (Read 4158 times)

Roger in France

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WHAT DOES A PROPELLOR DO?
« on: November 21, 2008, 10:00:51 am »

In an idle moment it just occurred to me that we all know a propellor propels but does it push or pull, or both?

In other words does it push against the water behind or screw itself into the water in front?

Roger in France
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Proteus

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Re: WHAT DOES A PROPELLOR DO?
« Reply #1 on: November 21, 2008, 10:19:39 am »

Pulls

 Proteus
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sheerline

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Re: WHAT DOES A PROPELLOR DO?
« Reply #2 on: November 21, 2008, 10:25:13 am »

I'd say both, the negative angle in front causes a depression in pressure resulting in a pull whilst the positive angle at the rear deflects the water backwards resulting in a forward motion.
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tigertiger

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Re: WHAT DOES A PROPELLOR DO?
« Reply #3 on: November 21, 2008, 10:42:58 am »

I would have thought a perspective matter.

A truck can push or pull, on a draw bar. It just depends on where you consider the load is.
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boatmadman

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Re: WHAT DOES A PROPELLOR DO?
« Reply #4 on: November 21, 2008, 11:18:11 am »

Pulls.

As it spins for forward motion, it creates a low pressure side on the front of the blades, so, due to the pressure differential across the blade, the (relatively) high pressure water acts on the back face of the blade making the prop blade try and move into the low pressure area, thus pulling the prop and boat forward.

I think! :-))

Ian
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Bunkerbarge

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Re: WHAT DOES A PROPELLOR DO?
« Reply #5 on: November 21, 2008, 11:44:02 am »

Ian's got it, in exactly the same way as an aircraft wing, a rudder and an aircraft propeller, it generates lift, which is nothing more than the result of the pressure differentials across it's surfaces.
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Roger in France

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Re: WHAT DOES A PROPELLOR DO?
« Reply #6 on: November 21, 2008, 11:56:33 am »

Interesting though, is it not?

Bit like the chicken and the egg!

I should stop thinking so much! I seem to remember asking ages ago about how a space vehicle manouvres when there is nothing to push against!

My problem is I walk my dog for ages and while doing so all kinds of interesting/daft things float into my mind! Ah well, keep taking the tablets.

Roger in France
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sheerline

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Re: WHAT DOES A PROPELLOR DO?
« Reply #7 on: November 21, 2008, 12:34:31 pm »

The theory is correct but in practice the back portion of the blade is still doing work. If the hydrodynamics were perfect you should be able to fill the rearward portion in and not bother with the angle.
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Martin [Admin]

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Re: WHAT DOES A PROPELLOR DO?
« Reply #8 on: November 21, 2008, 12:55:51 pm »


As I run aground a lot.... mine usually break or throw a blade!  %)
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catengineman

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Re: WHAT DOES A PROPELLOR DO?
« Reply #9 on: November 21, 2008, 01:43:42 pm »


As I run aground a lot.... mine usually break or throw a blade!  %)

Or nearly chop your fingers OFF when you try to stop it on a bench test  <:(
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BarryM

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Re: WHAT DOES A PROPELLOR DO?
« Reply #10 on: November 21, 2008, 03:20:59 pm »

The propellor tranmits thrust to the hull via the thrust block. The latter consists of a shaft collar rotating within self-aligning bearings acting on the collar faces (ahead and astern). Going ahead, the thrust from the prop is trasmitted via the collar to the forward set of bearings and thence to the hull. Thus the prop definitely pushes when going ahead. Going astern, the thrust is transferred from the collar to the aft set of bearings and thence to the hull. Thus the prop both pushes and pulls; it just depends which way it's turning, i.e. ahead or astern.

Barry M
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sheerline

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Re: WHAT DOES A PROPELLOR DO?
« Reply #11 on: November 21, 2008, 03:48:48 pm »

Looks like you summed that up very nicely barry.
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Bryan Young

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Re: WHAT DOES A PROPELLOR DO?
« Reply #12 on: November 21, 2008, 05:05:35 pm »

A propeller is not called a screw for no reason. i.e; it screws itself into the water. The actual weight of the thing it is attached to is shoved along via the thrust blocks (as has already been stated).
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funtimefrankie

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Re: WHAT DOES A PROPELLOR DO?
« Reply #13 on: November 21, 2008, 07:55:59 pm »

I seem to remember a similar discussion on a (motor)bike forum...

Does a chain pull or push???
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andygh

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Re: WHAT DOES A PROPELLOR DO?
« Reply #14 on: November 21, 2008, 09:09:18 pm »

Every action has an equal and opposite reaction, that's what I was told at school, if that is the case then a prop must be pushing water backwards thus giving forward motion to the boat
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portside II

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Re: WHAT DOES A PROPELLOR DO?
« Reply #15 on: November 21, 2008, 09:18:56 pm »

true Andy ,if it is pushing the water backwards then it must be pulling as well otherwise wher would the water come from to be pushed , oh god now i have confused my self :D .
daz
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andygh

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Re: WHAT DOES A PROPELLOR DO?
« Reply #16 on: November 21, 2008, 09:22:46 pm »

true  %%
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boatmadman

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Re: WHAT DOES A PROPELLOR DO?
« Reply #17 on: November 21, 2008, 09:35:38 pm »

so a prop is really a push me/pull you?  :o
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catengineman

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Re: WHAT DOES A PROPELLOR DO?
« Reply #18 on: November 21, 2008, 10:37:05 pm »

A very pushy pulling, pulling pushy

I an just glad they do what they do  :-))

Its how I earn my living

R,
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tony52

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Re: WHAT DOES A PROPELLOR DO?
« Reply #19 on: November 21, 2008, 10:48:43 pm »

Whilst reading this thread, I came to the conclusion, that somebody should hurry up and perfect paddle driven boats.
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Martin [Admin]

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Re: WHAT DOES A PROPELLOR DO?
« Reply #20 on: November 21, 2008, 11:00:33 pm »

so a prop is really a push me/pull you?  :o

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BarryM

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Re: WHAT DOES A PROPELLOR DO?
« Reply #21 on: November 21, 2008, 11:06:59 pm »

true Andy ,if it is pushing the water backwards then it must be pulling as well otherwise wher would the water come from to be pushed , oh god now i have confused my self :D .
daz

The prop creates a low pressure area ahead of itself and thus the water rushes in to fill that area and Colin, see Bryan's posting. You must learn to pay attention.   :P

Barry M
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Bunkerbarge

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Re: WHAT DOES A PROPELLOR DO?
« Reply #22 on: November 21, 2008, 11:21:31 pm »

Propellors are also known as screws - so do they screw their way through the water?

Of course but if you multiply the pitch by the RPM you will only get the theoretical distance the prop should have travelled through the water.  If you then express this as a ratio of the actual distance travelled through the water you will get the percentage 'Slip'.

When we have a clean hull we can get as low as 16-17% but as the hull gets fouled, very quickly on our schedule, the slip can get up to 22%.  This is a very good indication of hull performance and we use it to determine when we need to clean the hull.
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garston1

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Re: WHAT DOES A PROPELLOR DO?
« Reply #23 on: November 21, 2008, 11:26:03 pm »

mmm yes mmm mmm yes
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malcolmfrary

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Re: WHAT DOES A PROPELLOR DO?
« Reply #24 on: November 21, 2008, 11:37:36 pm »

If you leave it running nice and fast while you lift the boat out of the water, it gives everybody nearby a bit of lake to take away.
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