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Author Topic: 3 and 4 blade props  (Read 1738 times)

boat captain

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3 and 4 blade props
« on: October 11, 2010, 08:21:22 PM »

Hi

Which gives more thrust for the same diameter 3 or four blades.

Joe  :-)) :-))
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sailorboy61

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Re: 3 and 4 blade props
« Reply #1 on: October 11, 2010, 08:29:25 PM »

Would depend on the pitch too, but I assume the more the merrier since most large ships today use 5/6 blades rather than the traditional 4.
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Pirate

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Re: 3 and 4 blade props
« Reply #2 on: October 11, 2010, 08:33:11 PM »

That would depend on the pitch of the blades, the steeper the angle, the more water is moved but the more energy is needed to turn the prop. It also depends on the blade shape (scallop, fan,scimitar etc) each has a particular use/ efficiency and its ability to drive a craft depends on the hull shape.
The simple answer is the four bladed as it has in theory a greater surface area and thus moves more water per revolution.

If this is a specific question, ie; what prop is best for the particular model I am making, then more detail would enlighten us all.

Regards
Pirate
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derekwarner

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Re: 3 and 4 blade props
« Reply #3 on: October 11, 2010, 08:33:56 PM »

Joe...it all comes down to a few variables  %%  >>:-(

blade surface area, blade shape & pitch ....in a perfect world we think that adding an extra blade would increase the thrust as long as the input power was capable of handling the additional drag created ....

one of the options available is to use a prototypical scale propellor from the full sized vessel if posible.....after all the designers probably spent a substantialm sum of $ incorporating the optimal size/shape prop....Derek
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Derek Warner

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Illawarra Live Steamers Co-op
Australia
www.ils.org.au

boat captain

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Re: 3 and 4 blade props
« Reply #4 on: October 11, 2010, 09:58:01 PM »

Hi

The prop is for ESK (Whitby harbour grab dredger) which would not be very fast maybe 8 to 10 knots, she is 920mm loa, 245mm breadth and draught 40 mm.  The drawing prop looks like a 30mm 3 blade, I might just be able to fit a 35mm.

Joe :-))

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amdaylight

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Re: 3 and 4 blade props
« Reply #5 on: October 12, 2010, 03:17:39 AM »

Here is something that I learned from my RC Aircraft days, some times a two bladed prop is more efficient that a three bladed prop which is more efficient than a four bladed prop and the reason is that in the size that we work in the blades are operating in disturbed water / air from the proceeding blade. Every thing has a wake and if the blade is running in the wake of the proceeding blade that blade will loose efficiency. Some of the early rubber duration planes had only one propeller blade with a counter balance weight opposite the blade for maximum efficiency. I hope this makes some sence.

Andre :embarrassed:
over yonder in Portland Oregon
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Peter Fitness

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Re: 3 and 4 blade props
« Reply #6 on: October 12, 2010, 03:37:46 AM »

I can't give the technical reasons, but I do know that I replaced the 3 blade props on my pusher tug with 4 blade ones of the same diameter, and the increase in pushing power was quite dramatic.

Peter.
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fatcat123

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Re: 3 and 4 blade props
« Reply #7 on: October 12, 2010, 09:48:04 PM »

I'd be interested to see a thread of this build since i live in Whitby.

No idea on the spec of the barge though   :((

Dan
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