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Jp3m prop


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It depends on how much space you have between the counter and the skeg, and how much space you have between the sternpost and the rudder. Then it depends on what you want from your boat.

 

Propellers are a bit of a 'dark art', but there are online calculators which can give an idea :captain:

Edited by pete harrison
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1 hour ago, bobl said:

This is the sort of question everyone will have a view on my best advice is contact Crowther Marine with your hull dimensions and engine / gearbox details and take their advice

 

With an overpowered engine like a JP3, there is a balance to draw between the blade loading the engine properly and not going along too fast at tickover. 

 

As you say, where the balance lies is always a matter of opinion. 

 

 

 

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7 hours ago, Mike the Boilerman said:

 

With an overpowered engine like a JP3, there is a balance to draw between the blade loading the engine properly and not going along too fast at tickover. 

 

As you say, where the balance lies is always a matter of opinion. 

 

 

 

Yes I agree a jp3 may seam a bit over powered ,, but ,, maybe I should have been a bit cleared about it all,

 

I,m looking into , taking on  a Bridgewater canal tug replica shell  so looking at  classic engines and everything else  to get her up to sail away status  , all options are still open , I would have liked to fit a Gardner but  cost will most probably rule that out , but I have  contacts for  jp2,s and  jp3,s that are available at a reasonable price , , I would be willing to consider other engines ,,

Also I would be grateful for any other help , I have worked as a marine engineer until I be came semi retired so I,m not a total novice ,,

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If you are doing a proper replica rather than a blinged up modern tug then the JP is good. Gardners are lovely engines but they are really an early design of modern engine whilst the JPs are more like a vintage engine. 2 cylinders always sound better than 3 (in my opinion) but the JPs are not the most powerful engines (for their capacity) so a JP3 is not a bad choice, its maybe a bit over powered but then the JP2 is maybe a bit low for a big deep boat on a river.

 

As said, fit the biggest diameter you can, dictated by boat design and sensible clearance, then select the pitch as a compromise between tickover speed and transmitting full engine power, but getting the boat speed at tickover correct is more important, you pass moored boats and do maneuvers every day, but only use full power on rivers in flood.

 

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

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1 hour ago, Spanners said:

Yes I agree a jp3 may seam a bit over powered ,, but ,, maybe I should have been a bit cleared about it all,

 

I,m looking into , taking on  a Bridgewater canal tug replica shell  so looking at  classic engines and everything else  to get her up to sail away status  , all options are still open , I would have liked to fit a Gardner but  cost will most probably rule that out , but I have  contacts for  jp2,s and  jp3,s that are available at a reasonable price , , I would be willing to consider other engines ,,

Also I would be grateful for any other help , I have worked as a marine engineer until I be came semi retired so I,m not a total novice ,,

Ignore all the technical pontification (sorry, advice).

 

Where choice of engine is concerned, it's a matter of what you want it to sound like.

 

If you want "narrow-boaty" sound, you want a two cylinder engine - JP2,RussellNewbery,or even Armstrong Siddeley - unless you're an absolute masochist and incline to a single cylinder semi-diesel.

 

I have experience of 3 cylinder JP and ArmstrongSiddeley;they sound like sewing machines.Very loud sewing machines!

 

Why not consider a modern engine and save yourself all that expense and heartache, trying to find spare parts when it breaks?  And you can silence it to your heart's content for enjoyable peaceful cruising. 

 

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10 hours ago, Mike the Boilerman said:

 

Not necessarily. 2:1 reduction box he said.  Hardly ideal for a really BIG blade.

Agreed if you are talking over 30" as being really big.  Crowther calculated mine as 28 x 20 hence the 80% blades as biggest I could swing was 23" . I still felt it was underpropped

Edited by Loddon
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1 hour ago, Spanners said:

One engine I,m considering is the jp3 that is listed on eBay  that has come out of wide barge , misterton  , 

The other is a jp2 but I,m a bit doubtful as to its history at present

 

A few points based on my own experiences of blade swapping: 

 

The bigger diameter the blade, the better the brakes in my experience.

 

Once you get to about 22" dia, this is big enough. Bigger is showing off and asking for trouble as you either need an unusually deep drafted boat and get stuck easily on shallow canals, or you have small tip clearances which in theory lead to prop jams with branches that get stuck in the small gaps top or bottom.

 

In practice, running with tiny tip clearances dos not result in more blade jams.

 

A blade that one person considers over-propped another will consider under-propped, on the same boat, so a lot is pure opinion.

 

So I would if I were you, start with measuring the swim height of the hull you are planning to buy, work backwards from there to establish the biggest diameter you can comfortably swing, then get Crowthers (if you are loaded) or Norris's in Isleworth (if not) to suggest a pitch and make it for you.

 

Finally, have a look back and find the thread Mike Askin posted about changing the blade on VICTORIA when he first bought the boat, and see what size he fitted. The boat has a JP2 and goes really well by all accounts. 

 

 

 

 

Edited by Mike the Boilerman
Add in the 'Norris' name
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1 hour ago, Mike the Boilerman said:

Once you get to about 22" dia, this is big enough. Bigger is showing off and asking for trouble as you either need an unusually deep drafted boat and get stuck easily on shallow canals, or you have small tip clearances which in theory lead to prop jams with branches that get stuck in the small gaps top or bottom.

Idleness (JP3m 2:1 blackstone) had a 22x22 on it when I bought it, it was like an egg whisk, 23x20 with 80% blades was a vast improvement.  22" is not anywhere near big enough for a JP3, if you can swing it a 26/28 would be better.

 

ETA blade clearance with the 23 was 1" to the skeg 5" to the counter. I did think of dropping the skeg to enable 26" blade but cost was to much.

Edited by Loddon
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1 minute ago, Loddon said:

Idleness (JP3m 2:1 blackstone) had a 22x22 on it when I bought it, it was like an egg whisk, 23x20 with 80% blades was a vast improvement.  22" is not anywhere near big enough for a JP3, if you can swing it a 26/28 would be better.

 

I'm curious how a propeller would know what engine is turning it. Surely a propeller that suits the hull doesn't care, provided the engine turns it fast enough!

 

I reckon a JP2 likes to run at similar speeds to a JP3. Or is this wrong? 

 

 

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6 minutes ago, Mike the Boilerman said:

 

I'm curious how a propeller would know what engine is turning it. Surely a propeller that suits the hull doesn't care, provided the engine turns it fast enough!

 

I reckon a JP2 likes to run at similar speeds to a JP3. Or is this wrong? 

 

 

exactly a 22x22 didn't suit Idleness at all ;)

 

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I think prop calc software can only be a rough guide because it is not designed for narrowboats, and narrowboats have a number of special features/requirements:

They are not very boat shaped.

They mostly operate in shallow water, often almost in the silt.

They mostly operate in a narrow channel.

Boat speed at tickover is extra important

Engine speed (and so noise) at cruising speed might be extra important.

 

Now, lets say you have a JP2 and a well matched prop but then swap to a JP3 (or other engine with more torque but a similar rev range), what happens to the prop?

To fully load the engine at maximum speed a bigger prop is needed, but to keep the tickover speed the same prop is needed. Cruising revs needs the same prop to keep it the same, but you might choose a bigger prop to drop the revs???? 

 

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

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  • 3 weeks later...

The suggested prop for mine was 28×20, i went for 27×22 to gain extra clearance. It works perfectly on a 2 to 1 blackstone. I used Fal props in Scotland and couldn't fault them, they also seemed well versed with narrow boats. 

  I would definitely go for a three, if only to avoid the possibility of catastrophic crankshaft faliure. Even though the Grand union boats were designed for a two cylinder, i quite enjoy having a bit in reserve!

 Another benefit i found from the larger engine was the additional weight allowed me to minimise ballast at the stern end.

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