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Old 24 August 2006, 18:14   #1
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Prop diameter

Among the other bits and bobs I have just got, I ordered a 19" pitch prop for the main engine.

The old one was 14x17", the new one is 13.2x19".

The difference the pitch makes is obvious, but all other things being equal, is there likely to be any effect good or bad from the new one being a slightly smaller diameter?
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Old 24 August 2006, 19:49   #2
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It's not completely straightforward. If the blade shape and section have similar profiles then the larger diameter prop will load the engine more and, if the engine can power it, it will drive a bigger load in comparison to the original.

The tips are important because they are the fastest moving part of the prop. The shape at the tip will have some effect on the spillage of water from the back of the blade to the front and also the pattern of turbulence which is bound to be there.

In general but not without exception, the larger the blade area and the bigger the diameter the more load the prop can drive.

The thicker the blades the more drive can be produced at lower revs. Thinner blades are more efficient but this is misleading because it can be taken to imply that all blades should be as thin as possible. Very similar principles to aeroplane wings... a jet fighter wing would be no use on a Hercules and Hercules wings wouldn't be too good at mach II.

As usual, just my opinion, of course.

It'll be interesting to see how you get on with more pitch.
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Old 24 August 2006, 23:39   #3
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I haven't compared them side by side but it "looks" quite similar if you know what I mean, in terms of blade shape/thickness, a very similar looking 3 bladed ali prop, i.e. if you had showed me a picture of it and said "does that look like the prop off your boat" I would say yes only with more paint

I watched (for the nth time) the "Clarkson amphibious Toybota" edition of Top Gear tonight and one thing that struck me was that for a 225hp engine that looked positively huge (especially stood next to Richard Hammond) the prop on that looked tiny.

It will be interesting to see what happens. I am hoping that the change will give a better cruising speed at a given part throttle setting (say 3000rpm) which from what you say sounds likely. Hopefully the weather will be fit to have a fiddle on the weekend
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Old 25 August 2006, 03:27   #4
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Quote:
Originally Posted by BogMonster
Hopefully the weather will be fit to have a fiddle on the weekend
You should try getting out on the boat too if its nice weather!
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Old 25 August 2006, 03:43   #5
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Originally Posted by jwalker
Very similar principles to aeroplane wings... a jet fighter wing would be no use on a Hercules and Hercules wings wouldn't be too good at mach II.

As usual, just my opinion, of course.

It'll be interesting to see how you get on with more pitch.
ahhh, tell me do you know a bit about hercy birds then, have spent many an hour inside the wing of one of them
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Old 25 August 2006, 06:40   #6
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You should try getting out on the boat too if its nice weather!


Yes I hope to do that too, though the weekend weather forecast has gone from sunny to rainy to sunny to rainy to sunny to hail showers in the last 4 days. I wish they'd just put a thing on the website that says "really we have no idea whatsoever so just wait and see"...
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Old 25 August 2006, 08:26   #7
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I think with increased diameter you should see an improvement in grip (less ventilation) in the corners and less prop slip (leading to higher theoretical speed and fuel economy). The chap at prop revs told me this, and at the time I questioned the decrease in ventilation in the corners as I thought with the larger diameter, the blade tips would be nearer the surface resulting in an increased chance of sucking down air. He then however proceded to point out that the other blades would be deeper in the water, thus improving grip.
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Old 25 August 2006, 12:45   #8
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I think with increased diameter you should see an improvement in grip (less ventilation) in the corners and less prop slip (leading to higher theoretical speed and fuel economy). The chap at prop revs told me this, and at the time I questioned the decrease in ventilation in the corners as I thought with the larger diameter, the blade tips would be nearer the surface resulting in an increased chance of sucking down air. He then however proceded to point out that the other blades would be deeper in the water, thus improving grip.
OK, so with the 19in one being smaller, what it might mean is that I don't get much improvement in speed if there is more slip

Mind you I guess 0.8" smaller is not going to make that much difference? its not like I'm going from 14 down to 10 or something.

I get really bad ventilation in corners if the engine is trimmed out at all, though its ok when trimmed right in. Not sure if this is normal or not, I have been meaning to take a pic and post it to see whether people think the engine is at the right height or not.

Oh well I will whack it on and try it and see what happens
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Old 02 September 2006, 02:40   #9
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Back to the general rules for props again...

All are rough (sometimes really rough) approximations

Pitch: 1" = 200 rpm

Alum to SS: -100 rpm, for identically shaped props

Diameter: 1/4" = 100 rpm

Hope this helps (though trying it out is the only way to really know how it works.)

jky
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Old 02 September 2006, 03:12   #10
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ahhh, tell me do you know a bit about hercy birds then, (
they can't fly after 4.30 pm on Friday , always break down somehwere with posh hotels and sandy beaches and can't fly with passengers if there is a tin of paint on board as cargo. Spent 20 years being stranded around the world by them. Oh and they are feckin freezing. Now Singapore airlines with the cute air hostesses well thats how to run an airline

A Customer
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