prop sizing

[Admiral]

Does anyone have a list/program to work out blade sizes for engines? I am trying to work out the correct size blade on a JP2 with a 3:1 reduction on a Josher motor. Any help would be appreciated.

[Chris-B]

Does anyone have a list/program to work out blade sizes for engines? I am trying to work out the correct size blade on a JP2 with a 3:1 reduction on a Josher motor. Any help would be appreciated.

 

 

Lister original recommendation for that setup is 31 x 26 3 blade 50% bar

 

Hope this helps

Chris

[alan_fincher]

Does a typical Josher have space to swing a 31" prop without problem, though ?

 

Sounds very large ?

[Liam]

Admiral is a Steamer so I'm guessing it'll have room to swing that.

[boatyboy]

Lister original recommendation for that setup is 31 x 26 3 blade 50% bar

 

Hope this helps

Chris

Hi Chris.sorry to hijack the topic but Whilst we are on the subject.I've always thought our narrowboat had a propeler that was too smal. Can you help me please.I have a 57'semi trad,with a seventeen inch prop,i don't know or understand what the other measurements mean.My engine is a Vetus MR 1.7.Is that enough info ? Do i need to change it for a larger one?

Dave

[alan_fincher]

You do need to know the second dimension of a propeller, the "pitch".

 

One explanation could be this is how much it would move forward through the water in a single turn, if it could cut through it without "slippage".

 

So a 12" pitch propeller would move forward a foot in a single revolution, if you envisage something like it driving itself through a big block of cheese!

 

So from this you can appreciate that a 17" (diameter) by 17" (pitch) propeller is a very different beast from (say) a 17" by 12"

 

One might be perfect for your boat, but if so the other would be quite unsuitable, probably.

 

A third factor is the physical size of the actual blades, so how "wide" they are at the fattest point, in simple terms.

 

All these thing go together when sizing a prop.

Edited October 26, 2010 by alan_fincher

[Chris-B]

Hi Chris.sorry to hijack the topic but Whilst we are on the subject.I've always thought our narrowboat had a propeler that was too smal. Can you help me please.I have a 57'semi trad,with a seventeen inch prop,i don't know or understand what the other measurements mean.My engine is a Vetus MR 1.7.Is that enough info ? Do i need to change it for a larger one?

Dave

 

If you would like to give me a ring I can do a prop calc for you on the phone

Chris

[Proper Job]

Does a typical Josher have space to swing a 31" prop without problem, though ?

 

Sounds very large ?

The 'issue' is the 3:1 reduction box. It gives shed loads of torque but a very low prop speed. Hence big prop.

 

Try to find a 2:1 reduction box? Or go for a higher DAR (70%?)

[classicstove]

You may also like to try looking at a Crowther propellor who have a good reputation

 

Where the space under a counter is limited, they can also advise the prop required to compensate

 

http://www.crowthermarine.co.uk/

[Admiral]

Lister original recommendation for that setup is 31 x 26 3 blade 50% bar

 

Hope this helps

Chris

 

 

Chris

Thanks for the info. I managed to get a formula for the blade which is coming out at 32 X 23 so looks about right. I have space to turn a 35" blade but dont know how many problems that would give with picking up rubbish?

Thanks

Steve

[Proper Job]

I always though that the general rule of thumb for clearance was 10% top and bottom?

 

The main thing is clearance from blade tip to bottom of uxter plate. You don't want debris (logs etc) jamming up there and stopping things dead.

 

Mechanical bits tend to give way when that happens . Trust me..... I know

[Timleech]

I always though that the general rule of thumb for clearance was 10% top and bottom?

 

The main thing is clearance from blade tip to bottom of uxter plate. You don't want debris (logs etc) jamming up there and stopping things dead.

 

Mechanical bits tend to give way when that happens . Trust me..... I know

 

2" clearance is another figure to work from.

Unless the OP really wants to be swinging a steamer-sized prop, which will certainly make his life harder, I'd go along with the 2:1 gearbox suggestion, even though a spare one for a JP might be hard to find now.

 

...and this comes from someone who generally advocates decent-sized props on canal boats

 

Tim

Edited October 27, 2010 by Timleech

[mykaskin]

Hi Steve,

 

From a very simple calculation going from my prop:

 

JP2 on 2:1 = 28 x 16

JP2 on 1:1 = 28 x 8

JP2 on 3:1 = 28 x 24

 

Not sure if it really works like this though. Also I understand 2 inch on the pitch is roughly the same as 1 inch on diameter, so:

 

28 x 16 = 27 x 18 = 26 x 20 = 24 x 24

 

Victoria is prop'd for static pull rather than high speed in open water, if you want speed working from a 28 x 17 (or maybe even 18) may be a better starting point.

 

I did find some nice tables on the internet somewhere for sizing against engine but can't now find it.

 

BTW - My prop is 2" from the skeg, it can pick stuff up if you are not careful, but it doesn't seem really much worse than the too small prop I had on before.

 

Mike

Edited October 27, 2010 by mykaskin

[alan_fincher]

2" clearance is another figure to work from.

Swinging a prop of anything like that size, I'd certainly want at least 2" clearance top and bottom.

 

There is simply too much floating woodwork around that is easily that big.

[soldthehouse]

You do need to know the second dimension of a propeller, the "pitch".

 

One explanation could be this is how much it would move forward through the water in a single turn, if it could cut through it without "slippage".

 

So a 12" pitch propeller would move forward a foot in a single revolution, if you envisage something like it driving itself through a big block of cheese!

 

So from this you can appreciate that a 17" (diameter) by 17" (pitch) propeller is a very different beast from (say) a 17" by 12"

 

One might be perfect for your boat, but if so the other would be quite unsuitable, probably.

 

A third factor is the physical size of the actual blades, so how "wide" they are at the fattest point, in simple terms.

 

All these thing go together when sizing a prop.

Would it travel further through say Red leicester than it would through a cream cheese

[Proper Job]

Chris

Thanks for the info. I managed to get a formula for the blade which is coming out at 32 X 23 so looks about right. I have space to turn a 35" blade but dont know how many problems that would give with picking up rubbish?

Thanks

Steve

I just noticed the space that you have to turn the blade. For what it's worth, that's similar to Ocelot.

 

I'm throwing a 26" blade. On one occasion it has been stopped by a lump of wood (and it was a real bug#er to get it free!)

 

Supprisingly (as it spends most of its life spinning through the bottom silt), it picks up no less than previous 'egg whisk' boats I have owned. I think that's more to do with its size and the speed it revolves.

[Chris-B]

Would it travel further through say Red leicester than it would through a cream cheese

 

oy you leave creamcheese out of it mate !

[soldthehouse]

oy you leave creamcheese out of it mate !

OOOPS!

[Timleech]

Hi Steve,

 

 

Not sure if it really works like this though. Also I understand 2 inch on the pitch is roughly the same as 1 inch on diameter, so:

 

28 x 16 = 27 x 18 = 26 x 20 = 24 x 24

 

 

Mike

 

Not sure about that Pitch/Diameter ratio, I had it in mind that it's the other way round.

Anyway, 24 x 24 is about right for an RN, 18 bhp @ 1000 rpm with 2:1 reduction (so 500 rpm).

Load goes up with roughly the cube of rpm. JP is normally governed to 1200 rpm, so 600 at the prop with a 2:1 Reduction. (600/500)cubed x 18 = 31 bhp, whereas a JP2 is normally rated at 21 bhp @ 1200 rpm.

I think some of the early CS/JPs were governed to 1000 (and a lot of industrial/genset engines were), your 24 x 24 might well be about right for 1000 rpm top speed.

 

Tim

[mykaskin]

Not sure about that Pitch/Diameter ratio, I had it in mind that it's the other way round.

Anyway, 24 x 24 is about right for an RN, 18 bhp @ 1000 rpm with 2:1 reduction (so 500 rpm).

Load goes up with roughly the cube of rpm. JP is normally governed to 1200 rpm, so 600 at the prop with a 2:1 Reduction. (600/500)cubed x 18 = 31 bhp, whereas a JP2 is normally rated at 21 bhp @ 1200 rpm.

I think some of the early CS/JPs were governed to 1000 (and a lot of industrial/genset engines were), your 24 x 24 might well be about right for 1000 rpm top speed.

 

Tim

 

Unloaded the engine does reach 1200 (or at least did last year before a lot of work on it), under load it reaches around the 1000 mark, but not actually measured it.

 

Mike

[paulcatchpole]

oy you leave creamcheese out of it mate !

 

Chris: Creamcheese's defender...

 

She'll like that, when she sees it.

 

Mind you, that'll be when she gets back from the two mile walk to find the car.

 

PC

 

Mmm, coffee, choccy biscuits and TV. Boat-guarding is better than walking.

[Chris-B]

Chris: Creamcheese's defender...

 

She'll like that, when she sees it.

 

Mind you, that'll be when she gets back from the two mile walk to find the car.

 

PC

 

Mmm, coffee, choccy biscuits and TV. Boat-guarding is better than walking.

 

Do I get a preview of the bikini pictures as a reward

Edited October 27, 2010 by Baldock

[Mick_B]

Remember you promised to share Chris....

[paulcatchpole]

Do I get a preview of the bikini pictures as a reward

 

Ya know what? After a week like this, and now finally having the stove lit - I'll parade round Calcutt in the bikini

 

(Luv creamcheese using Paul's 'puter)

[casper ghost]

Not sure about that Pitch/Diameter ratio, I had it in mind that it's the other way round.

Tim

Me too, at least that's what I was told. 1" pitch equivalent to 2" diameter.

Casp'