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Author Topic: props  (Read 1647 times)

colin-stevens

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props
« on: April 15, 2007, 08:03:50 pm »

ok, this is totaly basic, but little grey are not working as well as they used to.
what is the diference btween 3 and 4 bladed props, and how does size affect performance? and where does pitch come into it.
simple answers only please? cant handle pages of equastions
thank you, feeling stupid
colin
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Shipmate60

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Re: props
« Reply #1 on: April 23, 2007, 11:02:21 pm »

Colin,
3 blade props have 3 blades, 4 blade props have 4 blades,  :)
4 blade props deliver more "Torque" or transmit more power at lower rpm, 3 blades will go faster. Thats why Racing props are 2 bladed, it cuts down on the cavitation transmitting the thrust more effectively at higher revs.
The pitch is the amount of distance the propeller will move the water in one revolution.
A prop with a small pitch needs rpm to produce thrust, a larger pitch delivers thrust at far lower revs, and far more at the same revs.
The old steam tugs used to favour a large, 4 blade prop with lots of pitch as the steam engines were slower revving but powerful.
Fast Patrol Boats have smaller props with less pitch to allow for the gas turbines or high speed diesels.
Hope this helps without ANY calculations.

Bob
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Bunkerbarge

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Re: props
« Reply #2 on: April 24, 2007, 05:27:42 pm »

The pitch is the amount of distance the propeller will move the water in one revolution.

That is, of course theoretically assuming no slip.
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Bryan Young

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Re: props
« Reply #3 on: April 24, 2007, 05:41:43 pm »

The pitch is the amount of distance the propeller will move the water in one revolution.

That is, of course theoretically assuming no slip.
This, of course, is the reason our engineers always reckoned the engines got into port before the bridge appeared over the horizon.
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Shipmate60

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Re: props
« Reply #4 on: April 25, 2007, 08:28:33 am »

Gents,
This is a model boat question, not a Naval Arch question. I have done enough of them in the past as I am sure Bunker and Bryan have too!   :)
As we are aware the usual operating range for a ships propeller is 90 to 300 rpm.
Model props (direct drive) 2500 to 23000,so persinally I dont consider slip in models as the excess revs more than compensate for this.
Additionally most models are vastly overpowered compared to the real size ship.
How many real ships can accelerate from rest to full ahead in 2 or 3 ships lengths, or stop from full ahead in less that a ships length - None that I know of.
My answer was a generic one aimed at modellers, not at cost aware skippers who want to know how much diesel I use to clean up rusty bolts with?  :)
I always overpower my models as the medium we sail in (wind and waves) aren't scale, so we regularly sail in what would be considered hurricane conditions, so extra power is welcome.

Bob
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colin-stevens

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Re: props
« Reply #5 on: April 25, 2007, 08:38:49 pm »

nice concise answer. well one of them anyway.
cheers guys.
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