Propeller for a Historic Boat with a Lister JP2

[andywatson]

Hi Folks,

 

I've just asked Crowther Marine Engineering in Oldham for a propeller for Victoria, but they've come back with an unexpected response. They suggest a 30" 3 bladed prop (which costs a fortune). Now I've asked a few people with JP2's and most are swinging much smaller diameters (in the 24" sort of range).

 

Are there any advantages or disadvantages with such a big prop, and what do other people recommend?

 

Victoria's JP2 develops 22BHP at 1200 RPM, with a 3:1 reduction box with Right Handed rotation onto a 2" shaft. I think Victoria is 3'3" deep, so should give around 36 inches space to spin a prop.

 

Cheers,

 

Mike

 

Coincidentally I am facing a similar problem concerning a prop for a new boat.

IE highly respected boatbuilder and highly respected propeller maker disagree over the best prop for:

 

62 ft boat, 28" draught, enough room for 24" prop.

 

Gardner 3LW, 42 hp @1300, 1.5:1 gearbox.

 

Crowther recommend 24x16

Builder recommends 22 and possibly 17 pich

 

From what I can gather, the 24 will be more efficient (probably due to 19% more swept area) but the 22 will give a better response to rudder input and stopping (smaller but faster moving "jet"?)

 

Other thoughts:

-Given the 2" on Diameter, 1" pich equivalance these two are not miles apart.

 

-The 22 will allow more debris clearance and can be installed deeper in the water.

 

-The 22 will allow a slighly lower back cabin floor.

 

-Prop design and sizing seems to be something of a black art and I wonder if predictive programmes and tables take enough account of the unusual conditions in which narrowboats operate (the ability to go very slowly, bottom interference, swim design etc).

I suspect that the 22 will, despite its greater pitch but lower efficiency, result in a slightly faster engine which will maybe not sound so good but be better for the engine and give faster charging.

 

I am at present inclined more towards the builders recommendation based on his long experience and knowledge of his own boats.

I look forward to the comments of the experts, particularly if I need any correction before I splash out on the prop.

Edited October 23, 2009 by andywatson

[Timleech]

Coincidentally I am facing a similar problem concerning a prop for a new boat.

IE highly respected boatbuilder and highly respected propeller maker disagree over the best prop for:

 

62 ft boat, 28" draught, enough room for 24" prop.

 

Gardner 3LW, 42 hp @1300, 1.5:1 gearbox.

 

Crowther recommend 24x16

Builder recommends 22 and possibly 17 pich

 

From what I can gather, the 24 will be more efficient (probably due to 19% more swept area) but the 22 will give a better response to rudder input and stopping (smaller but faster moving "jet"?)

 

Other thoughts:

-Given the 2" on Diameter, 1" pich equivalance these two are not miles apart.

 

-The 22 will allow more debris clearance and can be installed deeper in the water.

 

-The 22 will allow a slighly lower back cabin floor.

 

-Prop design and sizing seems to be something of a black art and I wonder if predictive programmes and tables take enough account of the unusual conditions in which narrowboats operate (the ability to go very slowly, bottom interference, swim design etc).

I suspect that the 22 will, despite its greater pitch but lower efficiency, result in a slightly faster engine which will maybe not sound so good but be better for the engine and give faster charging.

 

I am at present inclined more towards the builders recommendation based on his long experience and knowledge of his own boats.

I look forward to the comments of the experts, particularly if I need any correction before I splash out on the prop.

 

As you say, the two should be pretty much equivalent in terms of loading on the engine. The 16" pitch should be plenty with your 1.5:1 ratio, my instinct is always to go for the biggest diameter which can be properly accommodated with good clearances, provided that the pitch is about right.

To be honest the two are close enough that you might not notice much difference if you were able to swap between one and the other.

 

Tim

[flatplane8]

Hi,

 

Just a quick thread hi-jack here. I was looking at Misterton's JP3M reduction box the other day, and it is 'Type 3R' serial number 584B. Does this sound like a 3-1 reduction box?

 

Just curious, it seems to go well enough, but a 3-1 reduction would give quite a multiplication in torque.

 

Thanks,

 

Simon

[Timleech]

Hi,

 

Just a quick thread hi-jack here. I was looking at Misterton's JP3M reduction box the other day, and it is 'Type 3R' serial number 584B. Does this sound like a 3-1 reduction box?

 

Just curious, it seems to go well enough, but a 3-1 reduction would give quite a multiplication in torque.

 

Thanks,

 

Simon

 

Probably the physical size (to match 3G gearbox) rather than the reduction. Put a chalk mark on the coupling, and one on the flywheel. Put it in ahead gear and wind it over by hand, count how many times the flywheel goes round before the mark on the coupling returns. Easy.

 

Tim

[mykaskin]

Hi,

 

Just a quick thread hi-jack here. I was looking at Misterton's JP3M reduction box the other day, and it is 'Type 3R' serial number 584B. Does this sound like a 3-1 reduction box?

 

Just curious, it seems to go well enough, but a 3-1 reduction would give quite a multiplication in torque.

 

Thanks,

 

Simon

 

Mine also has the same on it:

 

 

I've just found out it is a 2:1 box (though it does run off a little over a couple of turns, so must be 2.1/1.9:1 !!)

 

BTW: I cleaned a joint on the coupling on the shaft, and used the TDC 2 slot on the flywheel.

 

Cheers,

 

Mike

Edited October 26, 2009 by mykaskin

[mykaskin]

Next problem: How do I know what sort of taper and keyway it is? I know the shaft size is 2", but not sure how the measure the others in the water!

 

Mike

[Dorman]

As you say, the two should be pretty much equivalent in terms of loading on the engine. The 16" pitch should be plenty with your 1.5:1 ratio, my instinct is always to go for the biggest diameter which can be properly accommodated with good clearances, provided that the pitch is about right.

To be honest the two are close enough that you might not notice much difference if you were able to swap between one and the other.

 

Tim

Hi Tim

 

You seem to have a good following on all things that spin under water and wondered if you could help.

 

I have a 57 foot nb with a vintage 3 cylinder Dorman. It currently runs on a 22"x22" with 2.1 reduction gearbox. I spend a lot of time on the Avon and Severn and the engine seems to run a bit fast. The previous owner towed a butty with it and a 27"x17" prop (29" draught) but changed it for "Cruising"

Im wondering if this is under proped as its a bit slow off the mark and revs up ever so easily and slow off the locks etc. It stops well however. The engine ticks over at 400-450 rpm and l do have to slip out of gear occasionally but mainly due to tight manouvers before engaging reverse. Max hp is 48hp at 1400 rpm.

 

Out of interest l enclose some Axiom correspondance as l am considering one-rash devil l know.

 

Many thanks

Simon

 

Hello Simon,

Diameter is king, be it marine screw or Axiom. Screw makes will recommend a marine screw of 28'' by 17'' for your engine to give you max % efficiency ( 35% )!. The problem being that when you reduce the Dia. you have too increase the pitch , so the marine screw becomes what is know as a ''square wheel'' ( Dia. the same as pitch i.e. 24 X 24 ) this drops the efficiency of the screw to the low 20's and also give very poor stopping power , very high ''prop walk'' ( paddle wheel effect ) high cavertation ( noise inside the boat and disturbed wake) poor control , tiller pulsing and increased wake.

The blades of an Axiom propeller are different to that of a marine screw and propel the water smoothly with non of the problems listed above. A 24 inch by 28 Dig. Axiom will lower your RPM and give you good control i.e. low prop walk , less tiller pulsing ,at least a 2Db drop in noise and reduced wake. The Axiom will also improve your fuel efficiency by 5-7% by loading your engine correctly.

If it do's not we will buy it back from you, giving you a full refund .

[Laurence Hogg]

Hi Folks,

 

I've just asked Crowther Marine Engineering in Oldham for a propeller for Victoria, but they've come back with an unexpected response. They suggest a 30" 3 bladed prop (which costs a fortune). Now I've asked a few people with JP2's and most are swinging much smaller diameters (in the 24" sort of range).

 

Are there any advantages or disadvantages with such a big prop, and what do other people recommend?

 

Victoria's JP2 develops 22BHP at 1200 RPM, with a 3:1 reduction box with Right Handed rotation onto a 2" shaft. I think Victoria is 3'3" deep, so should give around 36 inches space to spin a prop.

 

Cheers,

 

Mike

 

 

Hi Mike,

We had a 27" x 19" on the JP2 in "Neptune" and it seemed near perfect.

[nb Innisfree]

Hi Tim

 

You seem to have a good following on all things that spin under water and wondered if you could help.

 

I have a 57 foot nb with a vintage 3 cylinder Dorman. It currently runs on a 22"x22" with 2.1 reduction gearbox. I spend a lot of time on the Avon and Severn and the engine seems to run a bit fast. The previous owner towed a butty with it and a 27"x17" prop (29" draught) but changed it for "Cruising"

Im wondering if this is under proped as its a bit slow off the mark and revs up ever so easily and slow off the locks etc. It stops well however. The engine ticks over at 400-450 rpm and l do have to slip out of gear occasionally but mainly due to tight manouvers before engaging reverse. Max hp is 48hp at 1400 rpm.

 

Out of interest l enclose some Axiom correspondance as l am considering one-rash devil l know.

 

Many thanks

Simon

 

Hello Simon,

Diameter is king, be it marine screw or Axiom. Screw makes will recommend a marine screw of 28'' by 17'' for your engine to give you max % efficiency ( 35% )!. The problem being that when you reduce the Dia. you have too increase the pitch , so the marine screw becomes what is know as a ''square wheel'' ( Dia. the same as pitch i.e. 24 X 24 ) this drops the efficiency of the screw to the low 20's and also give very poor stopping power , very high ''prop walk'' ( paddle wheel effect ) high cavertation ( noise inside the boat and disturbed wake) poor control , tiller pulsing and increased wake.

The blades of an Axiom propeller are different to that of a marine screw and propel the water smoothly with non of the problems listed above. A 24 inch by 28 Dig. Axiom will lower your RPM and give you good control i.e. low prop walk , less tiller pulsing ,at least a 2Db drop in noise and reduced wake. The Axiom will also improve your fuel efficiency by 5-7% by loading your engine correctly.

If it do's not we will buy it back from you, giving you a full refund .

 

I notice they make no mention of blade area, basically blade area replaces diameter, although pitch is affected also. Blade area though a very important element is so often ignored.

[mykaskin]

I have a 57 foot nb with a vintage 3 cylinder Dorman. It currently runs on a 22"x22" with 2.1 reduction gearbox. I spend a lot of time on the Avon and Severn and the engine seems to run a bit fast. The previous owner towed a butty with it and a 27"x17" prop (29" draught) but changed it for "Cruising"

Im wondering if this is under proped as its a bit slow off the mark and revs up ever so easily and slow off the locks etc. It stops well however. The engine ticks over at 400-450 rpm and l do have to slip out of gear occasionally but mainly due to tight manouvers before engaging reverse. Max hp is 48hp at 1400 rpm.

 

Hi Simon,

 

48 hp is some beast, which is why I'm not surprised it can happily swing a 27" x 17". Cirtainly your 22 square prop is way to small to maximise the power out of the engine, but you have to decide if you want to go faster with slower revs! I'd go for a bigger prop if I were you, especially since you are doing a lot of river work.

 

Hi Mike,

We had a 27" x 19" on the JP2 in "Neptune" and it seemed near perfect.

 

Hi Laurence,

 

I've now got my 28 x 16 prop on Victoria (Lister JP2, 22HP@1200 RPM, 2.9 litre swept volume on a 2:1 box for comparison with the dorman). I think she is ever so slightly under prop'ed as I still do get to full revs, but boy does she shift now! If you use the rule of thumb of 2" on the pitch is the same as 1" on the diameter Neptunes prop would be 28 x 17 which isn't far off. My prop is equivelent to a 24 x 24 which is a popular size for historic boats, but more effecient and without the prop walk! Tom Hill on Archimedes uses a 24x24 with a 2:1 reduction on a HR2, but has more revs to play with - that is well prop'ed but suffers from a good paddle wheel effect!

 

(Other videos showing the dry docking, and Archimedes as well are also available on my videos).

 

I notice they make no mention of blade area, basically blade area replaces diameter, although pitch is affected also. Blade area though a very important element is so often ignored.

 

Blade area can make up for lack of diameter if you can't swing a bigger prop, but at the loss of some efficiency over the correct sized prop on a normal BAR. Crowthers will make a "high effeciency" large blade area prop for any boat and calculate the correct size also on placement of an order.

 

Cheers,

 

Mike

Edited March 27, 2010 by mykaskin

[Timleech]

Hi Tim

 

You seem to have a good following on all things that spin under water and wondered if you could help.

 

I have a 57 foot nb with a vintage 3 cylinder Dorman. It currently runs on a 22"x22" with 2.1 reduction gearbox. I spend a lot of time on the Avon and Severn and the engine seems to run a bit fast. The previous owner towed a butty with it and a 27"x17" prop (29" draught) but changed it for "Cruising"

Im wondering if this is under proped as its a bit slow off the mark and revs up ever so easily and slow off the locks etc. It stops well however. The engine ticks over at 400-450 rpm and l do have to slip out of gear occasionally but mainly due to tight manouvers before engaging reverse. Max hp is 48hp at 1400 rpm.

 

Out of interest l enclose some Axiom correspondance as l am considering one-rash devil l know.

 

Many thanks

Simon

 

Hello Simon,

Diameter is king, be it marine screw or Axiom. Screw makes will recommend a marine screw of 28'' by 17'' for your engine to give you max % efficiency ( 35% )!. The problem being that when you reduce the Dia. you have too increase the pitch , so the marine screw becomes what is know as a ''square wheel'' ( Dia. the same as pitch i.e. 24 X 24 ) this drops the efficiency of the screw to the low 20's and also give very poor stopping power , very high ''prop walk'' ( paddle wheel effect ) high cavertation ( noise inside the boat and disturbed wake) poor control , tiller pulsing and increased wake.

The blades of an Axiom propeller are different to that of a marine screw and propel the water smoothly with non of the problems listed above. A 24 inch by 28 Dig. Axiom will lower your RPM and give you good control i.e. low prop walk , less tiller pulsing ,at least a 2Db drop in noise and reduced wake. The Axiom will also improve your fuel efficiency by 5-7% by loading your engine correctly.

If it do's not we will buy it back from you, giving you a full refund .

 

Simon

 

Axiom may have a worthwhile product (I really don't know, my guess is it has plus and minus points), but they do themselves no favours with their approach which sets off bullsh*t alarms all over the place! Also you might think that someone who was serious about selling props would learn how to spell cavitation plus I don't know whether you would achieve true cavitation (boiling under reduced pressure) with your setup.

Your current prop does sound rather small, and the relatively small dia & high pitch will work against you for starting (and stopping), the high pitch will also mean the boat is 'trying' to go faster than you want on tickover.

If you can't accommodate the diameter, you can compensate to some degree by increased blade area or more blades, either of those would be better than increasing the pitch.

 

Hope that helps

Tim

Edited March 27, 2010 by Timleech

[mykaskin]

Someone with a four bladed prop didn't like it at all, kept catching lots of rubbish. Also, they tend to be less effecient than a large BAR three bladed prop.

 

Re: the Axiom... Not sure at what sort of speeds cavitation starts at, but there could be a small amount on the tips. It's not normally a problem with low power boats like ours. Ventilation is the biggest problem, and modern boats with shorter swims and shallow draft can suffer badly from this at high powers especially on choppy water.

 

The bigger prop I now have on does everything Axiom says theirs will. It has lowered engine revs for given speeds, reduces wash and prop walk, and has got rid of tiller shake. The prop doesn't turn fast enough for noticable caviation effects, and since it's 1/3 bigger than my old prop ventilation is worse as the tips are closer to the surface of the water, but I am running with the Uxter plate 2" out of the water!

 

Now I have a suitable prop on Victoria, I'd like to take Axiom up on their promise of money back if you are not satisfied, but I can't afford it if I am!! :-)

 

Cheers,

 

Mike

[Timleech]

Someone with a four bladed prop didn't like it at all, kept catching lots of rubbish. Also, they tend to be less effecient than a large BAR three bladed prop.

 

Collecting more rubbish could easily be caused by a different blade shape, not sure why more blades would of itself be a cause?

 

I'm interested that you say a four bladed prop would be less efficient than a 3-blade with big bats. I'm not doubting it, but genuinely interested and would like to know the source.

 

The bigger prop I now have on does everything Axiom says theirs will. It has lowered engine revs for given speeds, reduces wash and prop walk, and has got rid of tiller shake. The prop doesn't turn fast enough for noticable caviation effects, and since it's 1/3 bigger than my old prop ventilation is worse as the tips are closer to the surface of the water, but I am running with the Uxter plate 2" out of the water!

 

Now I have a suitable prop on Victoria, I'd like to take Axiom up on their promise of money back if you are not satisfied, but I can't afford it if I am!! :-)

 

Cheers,

 

Mike

 

A lot of Axiom's claims seem as though they could be met more cheaply with a better matched (for the purpose) conventional prop.

 

Tim (with four blades )

[nb Innisfree]

Blade area can make up for lack of diameter if you can't swing a bigger prop, but at the loss of some efficiency over the correct sized prop on a normal BAR. Crowthers will make a "high effeciency" large blade area prop for any boat and calculate the correct size also on placement of an order.

 

Cheers,

 

Mike

 

Yes I do agree our first engine, 2.5 litre, 33bhp @ 2000 rpm on a 2:1 box needed to turn a 17" prop and Crowthers made us a beauty, 3 bladed 17x14 with somewhere in the region of 75% BAR IIRC with swept blades, absolutely no cavitation very little prop walk and engine pulled just under max revs.

[nb Innisfree]

I'm interested that you say a four bladed prop would be less efficient than a 3-blade with big bats. I'm not doubting it, but genuinely interested and would like to know the source.

 

Tim (with four blades )

 

Just guessing here but perhaps up to a certain BAR the three blader will have more clearance between each blade and lessen the tendency for them to 'interfere' with each other though above that certain BAR a 4 blader may become less of an issue than a three blader.

Edited March 27, 2010 by nb Innisfree

[casper ghost]

I've now got my 28 x 16 prop on Victoria (Lister JP2, 22HP@1200 RPM, 2.9 litre swept volume on a 2:1 box for comparison with the dorman). I think she is ever so slightly under prop'ed as I still do get to full revs, but boy does she shift now! If you use the rule of thumb of 2" on the pitch is the same as 1" on the diameter Neptunes prop would be 28 x 17 which isn't far off. My prop is equivelent to a 24 x 24 which is a popular size for historic boats, but more effecient and without the prop walk! Tom Hill on Archimedes uses a 24x24 with a 2:1 reduction on a HR2, but has more revs to play with - that is well prop'ed but suffers from a good paddle wheel effect!

 

My bother's HR2 has a 22x22.5 prop, maybe not the prop size we would have chose but it was cheap. We thought it was a bit over-propped as it is too fast on tick-over but his isn't in a deep draughted hull.

 

I've just fitted an 18x19 on my boat ready for the Ruston 2VTh i'm fitting tomorrow. I had to cruise 4 hours with that prop with my little Sabb engine. I can tell you the Sabb did not like it very much and wouldn't rev high. Crowther said I should ideally have a 22x16 and that they could make me an 18" x16 with increased blade area to give the same blade area. Again I got the 18x19 cheap so it seems a good comprimise.

Edited March 27, 2010 by casper ghost

[Chertsey]

Hmm, interesting reading given that we were looking at Chertsey's prop yesterday and various people were sucking their teeth and saying it looked a bit small. Suspect it might be the National blade, and the engine's now a PD2 (and the one it's about to be replaced with is a Petter McLaren PD2 governed at 1500, if I've expressed that right) with a 2:1 reduction.

 

Now my problem is that I have no experience of how the boat runs or steers. It's out of the water now but I'm loath to make any (expensive!) changes without having tried it first. Presumably it was good enough for BW and for Richard Barnett, albeit with the other PD2 (is that marginal 300 rpm significant or would it have been outside its normal range anyway?)... So am I being sensible saying 'wait and see'?

 

PS. I like being able to stop and go backwards better than I like going fast. What difference does that make?

[Timleech]

Hmm, interesting reading given that we were looking at Chertsey's prop yesterday and various people were sucking their teeth and saying it looked a bit small. Suspect it might be the National blade, and the engine's now a PD2 (and the one it's about to be replaced with is a Petter McLaren PD2 governed at 1500, if I've expressed that right) with a 2:1 reduction.

 

Now my problem is that I have no experience of how the boat runs or steers. It's out of the water now but I'm loath to make any (expensive!) changes without having tried it first. Presumably it was good enough for BW and for Richard Barnett, albeit with the other PD2 (is that marginal 300 rpm significant or would it have been outside its normal range anyway?)... So am I being sensible saying 'wait and see'?

 

PS. I like being able to stop and go backwards better than I like going fast. What difference does that make?

 

Petter-McLaren PD2 engines were used as 'drop in' replacements for Nationals using the same blade, but they had 3:1 reductions with a special 3-wheel box to give the correct (left hand) prop rotation.

A Nationals blade will be too big for you with a 2:1 reduction on a PD2, also if it's a mechanical (Parsons?) box the rotation will likely be wrong (never seen a 3-wheel 2:1 Parsons box, that doesn't mean they never made them though!)

Having said all that, I've seen Nationals blades which are quite significantly worn down, also heard that in later working days one thing which was tried was cutting them down deliberately to allow the Nationals to rev a bit higher and get a couple more hp.

 

Tim

[Dorman]

Simon

 

Axiom may have a worthwhile product (I really don't know, my guess is it has plus and minus points), but they do themselves no favours with their approach which sets off bullsh*t alarms all over the place! Also you might think that someone who was serious about selling props would learn how to spell cavitation plus I don't know whether you would achieve true cavitation (boiling under reduced pressure) with your setup.

Your current prop does sound rather small, and the relatively small dia & high pitch will work against you for starting (and stopping), the high pitch will also mean the boat is 'trying' to go faster than you want on tickover.

If you can't accommodate the diameter, you can compensate to some degree by increased blade area or more blades, either of those would be better than increasing the pitch.

 

Hope that helps

Tim

 

Hi Tim

 

Agree with the bullsh*t as they seem desperate for me to buy at over £1000!!

 

I think lm beginning to understand some principles here.

 

Larger diameter is best for efficiency but if l drop the pitch to 17" as Crowther suggest will this not make the engine run faster at the same speed---or as the props is so much for efficient it will do the opposite when on the move but let me travel slower past moored boats etc?

 

So greatest dia as l can prop leaving clearance with a 3 blade with high surface area to makeup for any short fall in dia is best compromise?

 

29" draught is it best to leave 11/2" clearance aside say a 26" best in the real world.

 

Cheers

Simon

 

Can anybody explain "propwalk" and "paddlewheel effect"--sorry to be dim

Edited March 27, 2010 by Dorman

[nb Innisfree]

Hi Tim

 

Agree with the bullsh*t as they seem desperate for me to buy at over £1000!!

 

I think lm beginning to understand some principles here.

 

Larger diameter is best for efficiency but if l drop the pitch to 17" as Crowther suggest will this not make the engine run faster at the same speed---or as the props is so much for efficient it will do the opposite when on the move but let me travel slower past moored boats etc?

 

So greatest dia as l can prop leaving clearance with a 3 blade with high surface area to makeup for any short fall in dia is best compromise?

 

29" draught is it best to leave 11/2" clearance aside say a 26" best in the real world.

 

Cheers

Simon

 

Can anybody explain "propwalk" and "paddlewheel effect"--sorry to be dim

 

They are both different names for the same thing, when a prop rotates on a shallow drafted boat the top half is in less dense water than the bottom half which therefore grips the water more and results in the prop trying to act like a wheel and pull the back end of the boat in the direction of rotation i.e. if prop rotates anticlockwise as viewed from the rear (normally in reverse) then the stern will pull towards the port side.

 

But having said that I think another force is in action which has a more pronounced effect.

Edited March 27, 2010 by nb Innisfree

[Chertsey]

Petter-McLaren PD2 engines were used as 'drop in' replacements for Nationals using the same blade, but they had 3:1 reductions with a special 3-wheel box to give the correct (left hand) prop rotation.

A Nationals blade will be too big for you with a 2:1 reduction on a PD2, also if it's a mechanical (Parsons?) box the rotation will likely be wrong (never seen a 3-wheel 2:1 Parsons box, that doesn't mean they never made them though!)

Having said all that, I've seen Nationals blades which are quite significantly worn down, also heard that in later working days one thing which was tried was cutting them down deliberately to allow the Nationals to rev a bit higher and get a couple more hp.

 

Tim

Hmm. Not too small then. Whatever the set up is, it's obviously worked with regard to the rotation etc - so maybe it's not the National blade after all?. Yes, it is a Parsons box.

 

For next time I go up there, how does one go about measuring it?

 

Hang on, here's a pic, fwiw.

[alan_fincher]

I may be missing something here, but I think Tim has said something very pertinent.

 

If you previously has a 3:1 3-wheel box, on a PD2, and the plan is to replace with a 2:1 2-wheel box, then what size your current prop is will surely become an irrelevance, as (assuming both engines had "standard" rotation) the prop will be wrongly handed for the new box.

 

If you had a prop that was correct for a 3-wheel box, it must need replacing if the only change is to move to a 2-wheel box. That's because of "handedness" long before you consider size.

 

Do you have the original 3:1 box, and is it believed to be viable ? If so, why not swap out and use instead of a 2:1, when you'll then be forced (I believe) into an expensive new prop.

 

However, if I've got my wires completely crossed, simply tell me, and I'll go away!

[RLWP]

Hang on, here's a pic, fwiw.

 

Oh my, that swim is very beautiful

 

Richard

[Chertsey]

I may be missing something here, but I think Tim has said something very pertinent.

 

If you previously has a 3:1 3-wheel box, on a PD2, and the plan is to replace with a 2:1 2-wheel box, then what size your current prop is will surely become an irrelevance, as (assuming both engines had "standard" rotation) the prop will be wrongly handed for the new box.

 

If you had a prop that was correct for a 3-wheel box, it must need replacing if the only change is to move to a 2-wheel box. That's because of "handedness" long before you consider size.

 

Do you have the original 3:1 box, and is it believed to be viable ? If so, why not swap out and use instead of a 2:1, when you'll then be forced (I believe) into an expensive new prop.

 

However, if I've got my wires completely crossed, simply tell me, and I'll go away!

AFAIK the replacement engine has exactly the same box as the old one, which I had always understood to have been 2:1 but I may of course be wrong about that and I will make further enquiries...

 

 

Oh my, that swim is very beautiful

 

Richard

I know

[Timleech]

Hmm. Not too small then. Whatever the set up is, it's obviously worked with regard to the rotation etc - so maybe it's not the National blade after all?. Yes, it is a Parsons box.

 

For next time I go up there, how does one go about measuring it?

 

Hang on, here's a pic, fwiw.

 

That looks at first sight as though it has more pitch than a usual Bruntons' National blade, I've got one buried somewhere I might try to dig it out to compare. Definitely left hand though, so a PD2 will definitely need a 3-wheel box to drive it.

 

Tim