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Maudesmaster
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Hi although not a Gardner question I would like to find out proper size for a new build 62' with 3LW AND 2:1 reduction box Distance between uxter plate and tiller Skeg is 24" 

I have had e mails from various companies who all seem a bit vague some would not advise at all, nearest being T Norris who advise either 22 X 16 or 20 X 17 or 18 i would really like to get this right first time so looking to you good folk for your experience  Thank you 

Richard 

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I also run a 1.5:1 PRM 500 with my 3LW and doubt you will work yours hard enough if you stick with the 2:1 ratio you are quoting.  Believe my prop is 20x17 3 bladed but it feels like I am slightly under sized if anything but it is close enough for me.

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I'd call Crowther marine if you haven't already. If anyone can help it will be them. Seems like you are limited in diameter for that 2:1 reduction ratio. You really need at least a 24" prop with that set up. My arrangement for example is a 4lk with 2:1reduction, 24x19 3 blade prop on a 65 foot trad hull. The shaft speed will be similar to yours, say 200 to 800 rpm. Your 3lw has more torque than my 4lk, I think, without looking it up, also. Can you not change the gearbox? I know prm boxes come in alternative ratios.or do you have the original gardner box? 

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I am not a prop or Gardner expert, but offer the following observations.

With 24 inch clearance the prop diameter should not really be any bigger than 21 inch.

Having more pitch than diameter is not the done thing, but I have no real experience of what the disadvantages are.

We have a Beta JD3 with a 21 by 20 prop and that is just about spot on.

The 3LW produces quite a lot more torque than the JD3, and works over a roughly similar speed range, though different Gardner specs show quite different maximum speeds.

I reckon 21 by 20 would be about right but you will likely not get to use the full power of your 3LW unless its a higher revving version.

You could investigate 4 blades and/or compensated options to give you a "bigger" prop for the 21inch diameter but you will need to think carefully about boat speed at minimum revs, any more than 21 by 20 and you might need to go for a tickover quite a bit less than 400rpm which some people would not recommend.

............Dave

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  • 2 weeks later...

Sorry for late reply I have been working away Thank you for your input it seems I have a lot to research ie 3 or 4 blade change gearbox (which is new) as I said before prop manufacturers seem a bit vague and 1 wouldn't advise at all 

Richard 

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I've got a JP2 and can swing a maximum of 22 inches. The tip clearance to the counter is about 1 inch. I don't suffer too much with cavitation but have had a folding chair stuck between prop & counter. Mines one of Crowthers compensated props with overlapping bats..fitting one was one of the best things I did.  It's equivalent to a 24 x 19 according to Crowthers. 

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On 15/06/2017 at 16:40, dmr said:

I am not a prop or Gardner expert, but offer the following observations.

With 24 inch clearance the prop diameter should not really be any bigger than 21 inch.

Having more pitch than diameter is not the done thing, but I have no real experience of what the disadvantages are.

We have a Beta JD3 with a 21 by 20 prop and that is just about spot on.

The 3LW produces quite a lot more torque than the JD3, and works over a roughly similar speed range, though different Gardner specs show quite different maximum speeds.

I reckon 21 by 20 would be about right but you will likely not get to use the full power of your 3LW unless its a higher revving version.

You could investigate 4 blades and/or compensated options to give you a "bigger" prop for the 21inch diameter but you will need to think carefully about boat speed at minimum revs, any more than 21 by 20 and you might need to go for a tickover quite a bit less than 400rpm which some people would not recommend.

............Dave

 

I'd say Dave's comments are spot on and are much the same as I was thinking. When I had a Beta BD3  (similar engine to Dave's) and a 2:1 reduction gearbox the blade was 21 x 19 and again felt spot on for the engine. So as the power transmitted into the water rises with the fifth power of the shaft speed, the higher torque 3LW isn't going to get fully loaded until near the top of its rev range, if ever. I would however say my BD3 in a 68ft boat had power in spades, more than I ever needed even on fast flowing rivers so I doubt your 3LW will ever get to work properly hard unless you're in the habit of towing a butty up the Thames in flood conditions.

Second point is an 'oversquare' blade i.e. one with a pitch exceeding its diameter is said to be more likely to suffer from 'turbine effect', as are four-blade propellers. This is where the propeller 'grips' the water rather than slicing through it, and gets a cylinder of water rotating roughly the diameter of the blade, with the blade, instead of pushing it out backwards. So as you increase the pitch of a blade from nothing at first the thrust increases, but as you pass 'square', at some point the blade thrust begins to decrease again as you increase the pitch still further. (Imagine a propeller with blades almost at 90 degrees to the plane of rotation, this would give no thrust at all on a narrowboat).

Third point is Axiom probably have ther perfect blade for you if you ask them...

.

.

(Sorry, only kidding with that last point ;) )

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2 hours ago, frangar said:

I've got a JP2 and can swing a maximum of 22 inches. The tip clearance to the counter is about 1 inch. I don't suffer too much with cavitation but have had a folding chair stuck between prop & counter. Mines one of Crowthers compensated props with overlapping bats..fitting one was one of the best things I did.  It's equivalent to a 24 x 19 according to Crowthers. 

The JP2 produces a lot less torque than the 3LW but then again has a lower maximum speed, so its not straightforward to make a direct comparison between the two.

Less torque = smaller prop.

Less speed = bigger prop.

............Dave

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2 hours ago, Mike the Boilerman said:

 

Third point is Axiom probably have ther perfect blade for you if you ask them...

.

.

(Sorry, only kidding with that last point ;) )

I worked on a boat recently with an Axiom prop. The (new) owners weren't sure why as it seemed to be much the same as any other boat/prop combination

Richard

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On 26/06/2017 at 12:49, RLWP said:

I worked on a boat recently with an Axiom prop. The (new) owners weren't sure why as it seemed to be much the same as any other boat/prop combination

Richard

 

I recently worked out a way to make your own Axiom prop. 

Take your existing propeller and an angle grinder. Square off the ends of each blade with the grinder and BINGO, your very own Axiom propeller at no cost at all!

 

(I should write for VIZ magazine...)

  • Greenie 1
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2 minutes ago, Mike the Boilerman said:

 

I recently worked out a way to make your own Axiom prop. 

Take your existing propeller and an angle grinder. Square off the ends of each blade with the grinder and BINGO, your very own Axiom propeller at no cost at all!

 

(I should write for VIZ magazine...)

 

It's quicker to stick the lids from ice cream containers onto your existing blades.*

Richard

*Bizzard doesn't have a monopoly on engineering creativity

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I have a 50ft narrow boat  with a 3LW PRM 2:1 and  23" X 23" prop. The B.A.R of the prop is around the 60% mark, this is more commonly known as a compensated prop. A standard being 45-50% Max revs are limited to about 1100rpm, normal running speed is 475/500rpm on narrow canals 6-700rpm on Northern waterways. 7-750rpm on tidal rivers. anything more is purely just moving water for no great gain. Slow running i.e passing moored boats is at 350-375rpm. I originally had a 24" X 18" 50% B.A.R blade fitted, normal running was 600rpm and stopping was poor.
All LW's can have there tick over reduced to 330rpm by fitting a different rack spring. This was detailed in a service bulletin A.I 160 issued in February 1952

Gardner's recommended prop dia for a 3LW with 2:1 gearbox is 27"

Steve

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On 26/06/2017 at 12:58, RLWP said:

 

It's quicker to stick the lids from ice cream containers onto your existing blades.*

Richard

*Bizzard doesn't have a monopoly on engineering creativity

 

You clearly haven't seen the latest and newest Axiom propeller design. It really does look as though they are using my technique...

IMG_1331copy.jpg

(Photo from the Axiom website. Looks like they are glueing them on now instead of using a nut.)

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34 minutes ago, dmr said:

The JP2 produces a lot less torque than the 3LW but then again has a lower maximum speed, so its not straightforward to make a direct comparison between the two.

Less torque = smaller prop.

Less speed = bigger prop.

............Dave

It was more to comment on the tip clearance in practice with a vintage engine and the reference to a post about 2LW

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On ‎15‎/‎06‎/‎2017 at 11:26, crossley said:

I'd call Crowther marine if you haven't already. If anyone can help it will be them.

Yes indeed.

Mel Davis, who built our 'Trojan' and a man whose word I will take as gospel when he's talking about boats, told me "I've never known Crowther get it wrong yet".

As I am not a technical sort of person, I don't know exactly what info he gave them to enable them to make the correct prop. No doubt boat length (in our case 45 feet), engine type (2LW) and gearbox type (PRM 260) come into it but there are doubtless other factors. I think ours is 22 X 18, and it continues to give good service.

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On 26/06/2017 at 13:51, Athy said:

Yes indeed.

Mel Davis, who built our 'Trojan' and a man whose word I will take as gospel when he's talking about boats, told me "I've never known Crowther get it wrong yet".

As I am not a technical sort of person, I don't know exactly what info he gave them to enable them to make the correct prop. No doubt boat length (in our case 45 feet), engine type (2LW) and gearbox type (PRM 260) come into it but there are doubtless other factors. I think ours is 22 X 18, and it continues to give good service.

 

Ahem. Crowther got it badly wrong designing a blade for my K1.

Three whole boat lengths to come to a halt with the blade they made was not acceptable braking performance!

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42 minutes ago, Mike the Boilerman said:

 

Ahem. Crowther got it badly wrong designing a blade for my K1.

Three whole boat lengths to come to a halt with the blade they made was not acceptable braking performance!

Not the sort of braking news which you want to hear.

Exception...proves...rule...?

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1 hour ago, Athy said:

Not the sort of braking news which you want to hear.

Exception...proves...rule...?

I don't follow the logic. How does an exception prove a rule?

Surely an exception is evidence that the rule is flawed and cannot be relied upon.

What if there are two exceptions? Or three? Or ten?

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1 hour ago, Mike the Boilerman said:

I don't follow the logic. How does an exception prove a rule?

 

Because there has to be a rule in the first place for an exception to occur. So the exception proves that there is a rule.

 

There is another interpretation of the expression, which I don't care to give ear to, but which could even be right as well.

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7 minutes ago, Athy said:

Because there has to be a rule in the first place for an exception to occur. So the exception proves that there is a rule.

 

I still don't see any logic. Ok so there is a rule which can be demonstrated to not fully explain the observed facts. How does that prove the rule is correct?

I accept it proves the rule exists, but I hold it disproves the rule rather than proves it. This is the stage at which your logic fails in my opinion.

Any philosophers here able to cast some educated light on this please?

Edited by Mike the Boilerman
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2 hours ago, Mike the Boilerman said:

 

I still don't see any logic. Ok so there is a rule which can be demonstrated to not fully explain the observed facts. How does that prove the rule is correct?

I accept it proves the rule exists, but I hold it disproves the rule rather than proves it. This is the stage at which your logic fails in my opinion.

Any philosophers here able to cast some educated light on this please?

If there is a sign saying "No mooring between 9a.m. and 10a.m.", that is the exception which proves the rule that mooring is permitted at all other times.

 

7 hours ago, Mike the Boilerman said:

You clearly haven't seen the latest and newest Axiom propeller design. It really does look as though they are using my technique...

It is interesting to note how the Axiom propeller design is developing in a very similar way to historic propeller design. They have now passed the the SS Great Britain stage, and I predict that in 10 years or so an Axiom and a Crowther will be virtually indistinguishable.

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On 26/06/2017 at 20:34, Iain_S said:

If there is a sign saying "No mooring between 9a.m. and 10a.m.", that is the exception which proves the rule that mooring is permitted at all other times.

This is logical rubbish.

Why does the rule that mooring is permitted between 10am and 9am need 'proving'?

And surely the rule is 'no mooring between 9am and 10am'. Why would it need an exception to 'prove' it? 

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