Prop diameter

BogMonster · 24 August 2006, 22:14

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?

jwalker · 24 August 2006, 23:49

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.

BogMonster · 25 August 2006, 03:39

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

benc · 25 August 2006, 07:27

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!

Hugh Jardon · 25 August 2006, 07:43

Quote:

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

BogMonster · 25 August 2006, 10:40

Quote:

Originally Posted by benc

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"...

slimtim · 25 August 2006, 12:26

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.

BogMonster · 25 August 2006, 16:45

Quote:

Originally Posted by slimtim

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

jyasaki · 02 September 2006, 06:40

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

Pete7 · 02 September 2006, 07:12

Quote:

Originally Posted by Hugh Jardon

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

BogMonster · 02 September 2006, 15:42

Right, prop changed.... more questions!

The prop appears to have a rubber insert - what is this for some sort of shock absorbing function I suppose? Both props are the same anyway.

On the old prop, this rubber bit looks a bit buggered - see the attached photos of the new and old for comparison.

The central splined part inside the prop doesn't seem to be loose or anything - is this still going to be OK to use if I want/need to reinstall it? The bushing looks a bit like rubber sometimes does when it has had oil on it (e.g. suspension bushes that have suffered from oil contamination). The effect seems to be only the outside layer, after I took the photos I pulled the flappy loose bit off and it seems ok underneath.

The new prop while slightly smaller diameter has slightly broader blades so I guess the overall blade surface area should be similar. Bit chilly for ribbing today but the forecast for tomorrow doesn't look too serious so I hope to be able to test the new prop and the new aux then, and see what we get

The overhaul manual says that the nut holding the prop on should only be torqued to 14nm does this sound right? I guess it shouldn't be too tight as there is a nylon bush under the nut which would obviously not stand up to being really really tight, but 14nm is not a lot... and it took a #kin big grunt to shift the old one even with lots of WD40 first (though it could have just been seized after 6 years on there).

Richard B · 02 September 2006, 15:55

Yes, the prop nut doesn't need much torque as you will lock it with whatever system Johnson outboards use.

The rubber bushing provides a degree of energy absorbtion in case your prop hits something. And a degree of isolation in case little dings put the prop out of balance. It is much bigger than the internal diameter of the hub, and shouldn't really be refitted if there's any sign of damage. Either use it as a spare or send it to a specialist like Propeller Revolutions or Streamlined Propellers for replacement. One key thing is that they use a water based lubricant to fit it... not oil or detergent based!

Poly · 02 September 2006, 16:09

Quote:

Originally Posted by BogMonster

The overhaul manual says that the nut holding the prop on should only be torqued to 14nm does this sound right? I guess it shouldn't be too tight as there is a nylon bush under the nut which would obviously not stand up to being really really tight, but 14nm is not a lot... and it took a #kin big grunt to shift the old one even with lots of WD40 first (though it could have just been seized after 6 years on there).

Not sure if it helps you or not but my Yamaha 20HP has a specified torque of 35 Nm.

[Edit sorry richardB had a much more intellegent answer which he posted as I was one finger typing!. ]

BogMonster · 02 September 2006, 16:31

Thanks. I'm only expecting to keep it as a spare anyway, unless there is something fundamentally wrong with the performance of the new one. Its got a few bumps and scrapes on the blades which can't do anything for the performance either, but it is sound enough I think.

Depending on the performance with the 19in prop I may even end up getting a 21in - but I'll worry about that when I see what difference this one makes as it isn't cheap having lots of props lying around and they are a bit hard to borrow round these parts

The nut is a castle type nut retained with a big split pin so there's no danger of it coming off. Guess who forgot to order any spare split pins.... oh well, another fax to Mr Oakley, ho hum...

Richard B · 02 September 2006, 16:41

Quote:

Originally Posted by BogMonster

The nut is a castle type nut retained with a big split pin...

...and you can bet that your torque wrench will click to a stop with the castellations covering the hole!

BogMonster · 02 September 2006, 16:46

I had a dial type torque wrench but not being sure of its accuracy (it was a cheapy Draper one) I elected to make it nearer 20nm - the book says that if you miss the hole you carry on tightening the nut till you can get the split pin in so it would have ended up being the same anyway if I had done it to 14 and then to the next hole

Bigmuz7 · 02 September 2006, 21:14

Quote:

Originally Posted by Richard B

...and you can bet that your torque wrench will click to a stop with the castellations covering the hole!

Nothing surer in life

BogMonster · 03 September 2006, 15:54

Testing complete

Bit of a windy day today (18-20kt and quite choppy in the harbour) so I might get a little bit more out of her on a calm day but I now get 38.8kt with the new 19in prop, and it cruises much better at part throttle - I didn't actually take readings before at 3000rpm 4000rpm and so on but its definitely noticeably better, I think 4000rpm (which is a decent cruise rpm on this engine) is now giving about 30kt which is pretty much what I was aiming for.

WOT now gives about 5100rpm which is perhaps a little low, so a 21" prop is definitely going to be too much for it, but this 19" one seems just right

For some reason it also ventilates much less in the corners when the engine is trimmed out for max speed, so that is an unexpected bonus too!

The Johnson 6hp four stroke aux gives me 5.5kt with a bit of a tailwind....