Another JP3 query - this time prop size

[starman]

After a few trial runs I have this inkling that my JP3 is under-propped. We're running with a 3:1 reduction Blackstone box in a pretty heavyweight 56ft tug and I think we're using quite a lot more revs than I would have anticipated to maintain low-ish (c. 2.5-3mph) canal speeds which, rightly or wrongly, suggests to me that there isn't enough pitch on the prop. To be fair, though, the canal round here is fairly shallow and we squat down a lot under load so maybe we're dragging along in the silt too.

 

I know the only way to tell is to have the boat out and take a look - trouble is we've done that and couldn't find any measurements stamped on the prop. All I know is it's 24 inch dia. I'd be interested to know what other JP3 owners run - next time we're out I'll pull the prop off and take a closer look.

 

 

[by'eck]

24" prop sounds a tad on the small size for a JP3 with 3:1 reduction.

 

I know Crowthers recommended 28" if you have room to swing one even with a JP2 and 2:1 reduction. I believe Mike Askin has one on Victoria with a pitch of 16".

 

FWIW Steve Hudson fits 24" x 22" Crowther props to his JP3 engined boats with 2:1 PRM box. He also fitted the same prop to my JP2 and even a recently completed JP4 engined tug!

 

To get a meaningful assessment of prop performance you really need to get on a river, since shallow canals will have the prop dragging the bottom if you have too much power on causing speed to reduce and steering to get heavier. I usually power up then keep backing off until I see a speed reduction on GPS.

Edited June 18, 2013 by by'eck

[GUMPY]

From memory Idleness had a 24x18 Crowther HE prop on a JP3 with a 2:1 box.

 

 

A GPS can be very helpful as regards speed in shallow canals,

wind the revs up to what you think is correct then back them down slowly until the speed starts to drop, that is as fast as you can go without dragging the bottom.

[martyn 1]

The best thing you can do is with the length, weight, HP at what rpm and max diameter of prop is speak to Crowthers. They will be able to tell you exactly what you need for you boat rather than a guess... Every boat is different so two boats with the same engine may not have the same prop if for example the weight is different

[Proper Job]

Ocelot has a JP3, 2:1 reduction and runs a 26 x 21

 

Dimensionally, she's 56', draws 3' at the stern (static), weighs in at a smidge over 20 tonnes (Hudson built)

 

I recently had the pitch increased to it's current 21" and she's still under propped. In deep water at 800 rpm I can pull three water skiers

 

Tickover speed is just about right now for passing boats, going in/out of locks etc.

 

I'd say the prop is now about right for the boat - slow speed, canal cruising, deep water cruising - but the engine is still not fully loaded (no black smoke and easilly reaches 1000 rpm max)

 

 

With a 3:1 box and a 24" prop it'd have to be over square and have a over size blade area

 

Is the 3:1 box standard? You only usually see that ratio on the higher speed Listers (1500 - 1800 rpm max) and they still carry a big blade

[starman]

 

Is the 3:1 box standard? You only usually see that ratio on the higher speed Listers (1500 - 1800 rpm max) and they still carry a big blade

Standard? Who knows, it was there when I got the boat! Apparently the engine was at some point on a working boat (of the same name) and presumably had the 3:1 and Blackstone back then. Maybe I need to be thinking of a 2:1 box...though if we were going that far I might be tempted to go to a PRM hydraulic box too. Sacrilege, I know.

[frangar]

I have a 22" dia Crowthers compensated prop which according to them should be the equivalent of a conventional 26" x 19". We draw 2ft 5in plus 3 inches for the drop skeg. Here on the weaver we can get about 4.4 mph downstream according to the GPS although the flow is slight at the moment. Not sure what the engine revs are but I think if I opened it up much more I might be catching the engine as it escaped! I'm sure we are still a bit underpropped for rivers & deep water but it gives us a good performance on canals. We have a jp2 with a 2:1 reduction. The prop is also v close to the counter ( poss 1/2" clearance) which I'm sure doesn't help the water flow but its all a bit of a trade off really.

 

Cheers

 

Gareth

Edited June 18, 2013 by frangar

[martyn 1]

Is the 3:1 box standard? You only usually see that ratio on the higher speed Listers (1500 - 1800 rpm max) and they still carry a big blade

 

Yes it is, The JP's were available in 1:1, 2:1 and 3:1. By'ecks JP2 when we rebuilt her had a 3:1 reduction on the Blackstone gearbox that was removed. I would think though that unless you have one hell of a draft with 3:1 it would be a struggle to get anywere close to the right diameter prop which would end up in a heavily compensated one being needed. I would have said that a 3:1 would not have been ideal.

 

If it proves to be horibly out the easiest thing to do may be to remove the reduction box and fit the needed gearbox end bearing and drive coupling and convert the whole thing to 1:1.

 

Maybe I need to be thinking of a 2:1 box...though if we were going that far I might be tempted to go to a PRM hydraulic box too. Sacrilege, I know.

 

If my memory serves I do have a set of new custom gears in the workshop that are for converting a 3:1 reduction box to 2:1 that might be easier than either the PRM or removing the reduction box as it wouldn't need all the prop shaft arrangement modifying.

 

Cheers

 

Martyn

[David Mack]

I agree that your gearbox ratio is wrong. For a deep draughted working boat style hull, your prop should be running at about 500rpm at full speed.

 

So that equates to a direct 1:1 drive for something like a Bolinder semi diesel, 2:1 for a JP, National or Russel Newbery, running at 1000 rpm, or 3:1 for a Lister HA or similar running at 1500-1800 rpm.

[alan_fincher]

What do you reckon is the biggest diameter prop you could swing?

 

Like several of the others, I think you will struggle to fully exploit a JP3 if the gearbox reduction is 3:1.

[starman]

 

Yes it is, The JP's were available in 1:1, 2:1 and 3:1. By'ecks JP2 when we rebuilt her had a 3:1 reduction on the Blackstone gearbox that was removed. I would think though that unless you have one hell of a draft with 3:1 it would be a struggle to get anywere close to the right diameter prop which would end up in a heavily compensated one being needed. I would have said that a 3:1 would not have been ideal.

 

If it proves to be horibly out the easiest thing to do may be to remove the reduction box and fit the needed gearbox end bearing and drive coupling and convert the whole thing to 1:1.

 

 

If my memory serves I do have a set of new custom gears in the workshop that are for converting a 3:1 reduction box to 2:1 that might be easier than either the PRM or removing the reduction box as it wouldn't need all the prop shaft arrangement modifying.

 

Cheers

 

Martyn

Custom made? That sounds expensive! Would another option be to find a 2:1 box or are we talking hens' teeth here? And if I did is it a straight swap (e.g. externally the same so prop etc) all made.

Certainly sounds from the opinions here that 3:1 is all wrong, especially as I don't reckon I could fit a significantly bigger dia prop.

[Proper Job]

What do you reckon is the biggest diameter prop you could swing?

 

Like several of the others, I think you will struggle to fully exploit a JP3 if the gearbox reduction is 3:1.

 

I don't think you'd ever be able to fully exploit a JP3 in a lesure narrowboat. If you managed to prop it up sufficiently enough to make the engine work hard, you'd end up with a hell of a ground speed at tick over and reach the terminal design speed for the hull well below full speed

[Proper Job]

Custom made? That sounds expensive! Would another option be to find a 2:1 box or are we talking hens' teeth here? And if I did is it a straight swap (e.g. externally the same so prop etc) all made.

Certainly sounds from the opinions here that 3:1 is all wrong, especially as I don't reckon I could fit a significantly bigger dia prop.

Obviously knowing the pitch of your current 24" prop would be useful, but I'd hazzard a guess that you could fit a 2:1 reduction and use it.

 

I'd even go so far as to say that I'd suspect that it was still under propped, unless your existing prop has a serious amount of pitch on it.

 

Lots of pitch on the prop will manifest itself in prop walk. This is especially apparent when going in to reverse and the stern swings to one side because of the paddle effect of the blades

[frangar]

I used to have a over square prop... 22" dia by 24" pitch....it was vastly under propped and not only was reverse very slow to happen the boat just went sideways when movement did occur.....hence the change to the crowthers compensated prop... I did very briefly toy with an axiom but despite all the promises they offered me decided that crowthers had a better track record with fitting "big" props in a small space.

 

I think fitting a slow revving engine to a modern boat will always result in some compromise and whilst it would be nice to go a bit quicker on rivers what I have now is a vast improvement on what was fitted...the engine is also happier...when we used to venture on rivers and open the engine up a bit we used to lose the roof under a layer of carbon....now the engine is having to work a bit harder ll the time we run lovely and clean on rivers.

 

Cheers

 

Gareth

[flatplane8]

Probably not useful, but we have a 26x18" prop on our 2:1 JP3. It is four bladed and on a 70 ton barge though.

 

PS its worth checking it's deffo 3:1 by putting it in gear and counting engine vs propshaft revolutions. My reduction box is labeled '3R' and a few of us have made the mistake of thinking that was the reduction ratio........

Edited June 18, 2013 by flatplane8

[starman]

 

PS its worth checking it's deffo 3:1 by putting it in gear and counting engine vs propshaft revolutions. My reduction box is labeled '3R' and a few of us have made the mistake of thinking that was the reduction ratio........

It's a good job you said that....

 

Yes, I thought 3R meant 3:1 – seems reasonable doesn't it – until I just went and checked.

And it's 2:1.

 

Thanks for putting this idiot right.

 

Still seems under-propped though but at least I don't have to nick one off an aircraft carrier to fix it.

[GUMPY]

I have this sitting in my garage, It was to much for my 6cyl ford with a 2:1 box, should load a 3 reasonably;)

 

Edited June 18, 2013 by Loddon

[Timleech]

It's a good job you said that....

 

Yes, I thought 3R meant 3:1 – seems reasonable doesn't it – until I just went and checked.

And it's 2:1.

 

Thanks for putting this idiot right.

 

Still seems under-propped though but at least I don't have to nick one off an aircraft carrier to fix it.

 

The '3' is the size of gearbox, goes on a 3G reverse gear.

 

Tim

[George94]

I think the clue to this is that you squat down as you are going along. I presume you mean that the stern goes down and the bow rises.

 

You are therefore trying to become airborne, which takes a lot of energy. You are also dredging the canal, ditto.

 

I used to the find I was doing the same until I got onto the Thames when suddenly everything seemed to come right.

 

Essentially you are too big for the canal, and there is nothing you can do about it.

[Davidss]

I assume we are talking about Tug Harry. (ETA, Ahh Yes. I initially followed the link in your posts here, which goes to the last post on the Star blog. I then neglected to go back to your first post in this thread. I still think my question is valid ;-))

The Harry blog has more white space than words, and unless you have hidden pictures and not linked them, there are no external pictures of the current state of the tug.

Which is a long-winded way of getting to the question, what is the boat trimmed like?

Static horizontal or static bow high?

You have told us the stern drops when under power, but this 'natural' tendency will be made worse if the starting attitude features a wedge under the flat hull base, with the wedge thicker at the front and thinner at the rear.

 

HTH

Edited June 19, 2013 by Davidss

[starman]

I assume we are talking about Tug Harry. (ETA, Ahh Yes. I initially followed the link in your posts here, which goes to the last post on the Star blog. I then neglected to go back to your first post in this thread. I still think my question is valid ;-))

The Harry blog has more white space than words, and unless you have hidden pictures and not linked them, there are no external pictures of the current state of the tug.

Which is a long-winded way of getting to the question, what is the boat trimmed like?

Static horizontal or static bow high?

You have told us the stern drops when under power, but this 'natural' tendency will be made worse if the starting attitude features a wedge under the flat hull base, with the wedge thicker at the front and thinner at the rear.

 

HTH

Sorry about the lack of content and pics in the blog - when we actually get cruising (hopefully very soon) there will be something more to talk about and some pics of the exterior too.

We are very, very slightly nose up when static; I can't ballast the front down any more for various reasons. I could un-ballast at the back but the uxter plate would then be well out of the water when moored which I can't think is an especially good idea for longevity.

 

Anyway, reflecting George94's point, we've been trying it in some deeper, wider water and it behaves a whole lot better - stops ok, reverses ok etc so I think I will stick with the prop I've got for the foreseeable and settle for pottering rather than ploughing on the shallow stuff.

[frangar]

The joys of a deep draught is you can't go fast on a lot of canals and even when you think it's deep it's probably not deep enough!

 

Cheers

 

Gareth

[MoominPapa]

I could un-ballast at the back but the uxter plate would then be well out of the water when moored which I can't think is an especially good idea for longevity.

 

 

Melaleuca's uxter plate is out of the water when stationary, and it doesn't seem to have caused a problem. Are you worried about corrosion? During the recent blacking the only problems I found were inside the weed-box, which are due to neglect of the blacking in there, in an area that would be above the water-line whatever. Being deep draughted, our ballasting is set up to keep the back as high as possible consistent with the uxter plate being underwater when we're under way.

 

MP.

Edited June 19, 2013 by MoominPapa

[alan_fincher]

Melaleuca's uxter plate is out of the water when stationary, and it doesn't seem to have caused a problem. Are you worried about corrosion? During the recent blacking the only problems I found were inside the weed-box, which are due to neglect of the blacking in there, in an area that would be above the water-line whatever. Being deep draughted, our ballasting is set up to keep the back as high as possible consistent with the uxter plate being underwater when we're under way.

 

MP.

Since we acquired her we have tended to run "Sickle" as she came, which had the back of the uxter plate a little out of the water until actually underway, althogh it then pulls well down once you have any speed on.

 

A number of people seemed to think she would benefit from being a bit lower, so in the recent re-ballasting exercise I actually arranged that the uxter was just touching the water when static.

 

The trip I did with her to get her to Stoke Bruenre last weekend has convinced me that even on the relatively deep GU this is probably not a good move. The boat crashed through far more bridge holes than I can ever recall in the past, even though I doubt we have added as much as 2" to the static draft at the rear, (probably more like 1.5").

 

I now intend to shift some ballast forward, to bring the front down, and the back up.

 

As the boat pulls like mad, both in forwards and reverse, even when more lightly ballasted at the back, I'm struggling to see much advantage in getting the uxter on the water when static, and have certainly now noticed disadvantages.

 

Maybe if I was trying the Ricky tug of war, or towing something heavy, but for the boat on its own, I'm persuaded what I've done has not helped, and I'm going to try reverting.

[David Mack]

If you find yourself digging in deeply and moving sluggishly in shallow water reduce the revs. The stern will rise, there will be less drag on the bottom, and you will actually go faster. And you will use less fuel and the engine will sound less laboured as well.