Propeller for a Historic Boat with a Lister JP2

[alan_fincher]

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...

I'm starting to doubt my own memory here.....

 

I firmly thought the PD2s in the BW fleet used a 3:1 reduction.

 

However, looking at some old documentation I have for the Parson's Type F box, where it shows oil capacities it actually says....

 

"F" 2:1 Reduction Gear (2 wheel type) 5/8 pint

"F" 2:1 Reduction Gear (3 wheel type) 7/8 pint

"F" 3:1 Reduction Gear (2 wheel type) 3/4 pint

 

So it only lists a 3-wheel box in a 2:1 ratio, which is a surprise.

 

A 2:1 box looks visibly distinguishable externally from a 3:1, so even if it doesn't bear any labelling on it, if you post pictures, someone should be able to advise you what you have.

[Chertsey]

I've just been looking at various lovely drawings, in the Petters manual for the PD2 and in a Parsons manual which I've just been lent. Gearboxes have always been a bit of a closed book to me, so this might not make any sense at all, but the Parsons manual refers to a 'Type F reduction gear 2:1 ratio 2 wheel' and a 'Type F reduction gear 3:1 ratio 2 wheel' - the thing being that both the 3:1 and the 2:1are described as 2 wheel. The Petters manual refers to the reduction gears by different numbers (as part of the reversing gear) but the drawings are exactly the same. This would suggest that neither the 2:1 nor the 3:1 was 3 wheel... where does that leave us? Or have I missed something entirely? I notice it also says that 'With reverse and 3:1 or 2:1 reduction gear... For standard rotation engines use a right-handed propeller'. So I'm even more confused now.

 

Edited to note that Alan has noticed something similar.

Edited March 27, 2010 by Chertsey

[Timleech]

I've just been looking at various lovely drawings, in the Petters manual for the PD2 and in a Parsons manual which I've just been lent. Gearboxes have always been a bit of a closed book to me, so this might not make any sense at all, but the Parsons manual refers to a 'Type F reduction gear 2:1 ratio 2 wheel' and a 'Type F reduction gear 3:1 ratio 2 wheel' - the thing being that both the 3:1 and the 2:1are described as 2 wheel. The Petters manual refers to the reduction gears by different numbers (as part of the reversing gear) but the drawings are exactly the same. This would suggest that neither the 2:1 nor the 3:1 was 3 wheel... where does that leave us? Or have I missed something entirely? I notice it also says that 'With reverse and 3:1 or 2:1 reduction gear... For standard rotation engines use a right-handed propeller'. So I'm even more confused now.

 

Edited to note that Alan has noticed something similar.

 

The 3-wheel box was made to reverse the prop rotation, probably mainly for twin-screw vessels so that the two props were of opposite hands but BWB ordered them specifically to allow the PD2 to be used as direct replacements for Nationals (which went round the other way).

The GU Nationals always had 2:1 boxes and ran at 1000 rpm max, the PD2 ran at around 1500 rpm for only a couple more horsepower so needed the 3:1 reduction to give the same prop rpm.

 

The '3-wheel' configuration has nothing to do with the reduction ratio, but I've only ever seen them in 3:1 ratio.

 

Tim

Edited March 27, 2010 by Timleech

[alan_fincher]

I know the three wheel reduction box, (which is not illustrated in either a Parsons or Perkins version of the manual for a "Type F" box), is a completely different appearance externally from a standard two wheel box, (in either reduction ratio).

 

Post some pictures, and it should be easy to sort out.

[Chertsey]

I'm sure I've got better pictures than these, but they're the best I can find at the moment. In fact, the old gearbox is at home now and I can go and look at it in the morning. I am pretty sure they are identical externally.

 

Old one:

 

New one:

 

Doesn't look that different from the one in the drawing to me but it is rather hard to compare.

[alan_fincher]

Both are 3-wheel boxes, unless my eyes are really playing tricks.

 

Without seeing a "frontal view" of each I'd not like to guess at reduction ratios, though.

 

At least new and old need the same "handedness" of prop, so that's a good start!

 

Tim's explanation about the relative speeds of a National and a Petter is fairly compelling. I think it would be normal for a PD2 to use a 3:1 box.

 

If you think the new one is only 2:1, do you know where it originates from, please ?

[Chertsey]

Both are 3-wheel boxes, unless my eyes are really playing tricks.

 

Without seeing a "frontal view" of each I'd not like to guess at reduction ratios, though.

 

At least new and old need the same "handedness" of prop, so that's a good start!

 

Tim's explanation about the relative speeds of a National and a Petter is fairly compelling. I think it would be normal for a PD2 to use a 3:1 box.

 

If you think the new one is only 2:1, do you know where it originates from, please ?

Now I think about it, I don't know where I got the 2:1 from - someone said it at some point but I don't recall who or when. As I can be pretty certain that the 'old' engine was the one fitted in Chertsey by BW in 1960, and the 'new' one also came out of a (former) GU boat it would make sense that they were the 3:1 ones as described by Tim.

 

So, assuming that it is 3:1, how does that prop look for size?

 

Just thinking, if you had told me five years ago that I'd be having this conversation....

Edited March 27, 2010 by Chertsey

[Dorman]

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.

 

I think lm beginning to understand some principles here current prop 22"x22" would appear small from replies lve had so.....................

 

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?--hopefully

 

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.

 

Anybody got any comments???

Edited March 31, 2010 by Dorman

[magnetman]

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.

 

I've never quite got my head round this theory as I am under the impression that a 'unit' of water is of the same density whatever depth it is at.

I know that propwalk does occur and pulls the boat to one side or the other specially when reversing but also when going forward but I don't think its got anything to do with depth of water unless ventilation is part of the equation.

 

 

 

 

just found it on wikipedia

 

http://en.wikipedia.org/wiki/Propeller_walk

[WotEver]

just found it on wikipedia

 

http://en.wikipedia.org/wiki/Propeller_walk

I read it. I read it again. I still don't understand it. I don't think reading it again will help me any.

[magnetman]

I read it. I read it again. I still don't understand it. I don't think reading it again will help me any.

 

"Upward oblique flow at the propeller location"

 

it makes sense. honest.

[ymu]

I've never quite got my head round this theory as I am under the impression that a 'unit' of water is of the same density whatever depth it is at.

Density depends on temperature and pressure. Water has a weird density/temperature relationship - the highest density is at 4 degrees C. It responds pretty much like anything else when it comes to pressure, AFAIK. Put more stuff on top of it, and it will get compressed, and thus denser.

 

So, the densest water sinks to the bottom and then gets even denser because of the weight of stuff on top of it.