Reducing the pitch of my prop

[bargemast]

I have a 19" x 13" prop on my 57' x 12' widebeam, powered by an Isuzu 55 and PRM 150 gearbox with a 2:1 reduction ratio (or more accurately if I've remembered correctly, a 2.09:1 reduction ratio).

 

Anyway, after talking to Crowthers and others I had planned to get an inch taken off the pitch of the prop because in gear it only revs up to 2000 rpm. (3000 rpm out of gear). I'm running out of time to get this done before I go onto the Bristol channel in a couple of months, but I'm still in two minds as to whether I'd actually get any more power from this modification.

 

The torque curve peaks at around 2000 rpm so in theory I wouldn't get any more power, but it's a shallow torque curve so perhaps another 200 rpm (which is all I'm likely to get from taking an inch off the pitch), might not go amiss.

 

What does the panel think?

 

 

 

I talked to the owner of another LB widebeam (60' x 12') that went up the Bristol channel with the same engine, gearbox and prop as mine and he said the pilot remarked it could do with a few more revs.

 

 

 

 

Hello Mike,

 

as far as I can see in the power curve, your 55Hp engine has become a +/- 40Hp engine, as you can only get 40Hp at 2000n.

 

So you're leaving 15 unused Hp to not be able to give you their, maybe sometimes needed, extra power, which is a shame.

 

Allan's (Keeping Up) idea of changing the gear ratio to 3 : 1 may be a good one, if your actual prop would be better suited to that reduction, and your engine to reach 800n to 1000n more.

 

It may be worth enquiring.

 

Good luck with making the right decision,

 

Peter.

[nb Innisfree]

Bottom line is that any prop configuration is at best a compromise, I do think a best solution is a variable pitch prop, perhaps not suitable for grotty canal water, or a two speed or more box. It would be quite handy if boxes came with an extra gear, one for pootling along on min revs and one for max revs and power, and maybe an extra one in the middle for max torque. Yeah three ratios.

[Keeping Up]

Bottom line is that any prop configuration is at best a compromise, I do think a best solution is a variable pitch prop, perhaps not suitable for grotty canal water, or a two speed or more box. It would be quite handy if boxes came with an extra gear, one for pootling along on min revs and one for max revs and power, and maybe an extra one in the middle for max torque. Yeah three ratios.

 

 

Only three?. You could take it even further; I once saw a converted Springer that had a huge American truck engine fitted, complete with a 10-speed gearbox. The owner had fitted it out specifically for use in a tug-of-war competition, which he won in spectacular style. Just when everyone thought he had been beaten, as he was towed backwards towards the line, he just changed up by a couple of gears and pulled the other boat backwards as if it was a toy.

[blackrose]

Hello Mike,

 

as far as I can see in the power curve, your 55Hp engine has become a +/- 40Hp engine, as you can only get 40Hp at 2000n.

 

So you're leaving 15 unused Hp to not be able to give you their, maybe sometimes needed, extra power, which is a shame.

 

Allan's (Keeping Up) idea of changing the gear ratio to 3 : 1 may be a good one, if your actual prop would be better suited to that reduction, and your engine to reach 800n to 1000n more.

 

It may be worth enquiring.

 

Good luck with making the right decision,

 

Peter.

 

So I need a 3:1 ratio gearbox? That sounds like a much more expensive option.

[Keeping Up]

So I need a 3:1 ratio gearbox? That sounds like a much more expensive option.

 

 

More expensive than a prop change, but probably cheaper than a dry-docking plus a prop change.

[blackrose]

Surely having a prop which limits the engine to the speed of its maximum torque, is akin to driving a car up a hill in too high a gear; you can go faster by engaging a lower gear. This made me think that possibly a cheaper option than taking a boat out of the water and changing or re-shaping the prop, could be to replace the gearbox by a 3:1 version (or have the existing gearbox re-cogged if that's the correct term).

 

I wouldn't take the boat out the water just to remove the prop - or if I did then it would be done on the tidal Thames, so replacing or re-cogging the gearbox is probably a more expensive option.

[bizzard]

To gain that extra few hundred RPM's you could rig up an intravenous drip ''hospital style'' on a stand above the air intake with a rubber tube led into the middle of the air cleaner and drip Ether into it carefully regulated for accuracy with clothes pegs squeezing the rubber tube. And there's also the Turbo-charger if that engine could stand one without blowing up.

[GUMPY]

changing the gearbox wont work the difference in ratio will be to much

As an example my NB for 6knots with a 2:1 needs a 18x17 at 3:1 it will need an 18x26!

 

The limiting factor on all displacement boats is hull speed, on the barge I could reach hull speed at under 1400rpm the engine would rev to 2000+ did it go any faster, not really but used to burn over twice the fuel.

First thing to do would be to check the the boat is reaching hull speed at or below present maximum revs if it is there is nothing worth doing.

Easiest way to check you are at hull speed is to look for a double bow wave best pic I have is this one

https://www.dropbox.com/s/iu23zpj4yhi50is/DSCF0440.JPG

you see the second wave just back from the bow.

[bizzard]

To gain that extra few hundred RPM's you could rig up an intravenous drip ''hospital style'' on a stand above the air intake with a rubber tube led into the middle of the air cleaner and drip Ether into it carefully regulated for accuracy with clothes pegs squeezing the rubber tube. And there's also the Turbo-charger if that engine could stand one without blowing up.

And to supplement the intravenous Ether drip a mains hair dryer blowing at max heat into the air cleaners intake powered by your generator would provide a make shift Turbo. Both these aids working in unison should easily encourage the engine to reach if not exceed your sought after revolutions per minute.

[TNC]

And to supplement the intravenous Ether drip a mains hair dryer blowing at max heat into the air cleaners intake powered by your generator would provide a make shift Turbo. Both these aids working in unison should easily encourage the engine to reach if not exceed your sought after revolutions per minute.

 

Is this the way the teeny weeny engine in Mrs TNC's new Ecoboost Focus works?

[Keeping Up]

changing the gearbox wont work the difference in ratio will be to much

As an example my NB for 6knots with a 2:1 needs a 18x17 at 3:1 it will need an 18x26!

 

The limiting factor on all displacement boats is hull speed, on the barge I could reach hull speed at under 1400rpm the engine would rev to 2000+ did it go any faster, not really but used to burn over twice the fuel.

First thing to do would be to check the the boat is reaching hull speed at or below present maximum revs if it is there is nothing worth doing.

Easiest way to check you are at hull speed is to look for a double bow wave best pic I have is this one

https://www.dropbox....is/DSCF0440.JPG

you see the second wave just back from the bow.

 

If the prop rotates at 1000 rpm when driven through a 2:1 box by an engine doing 2000 rpm, that same prop would rotate at the same speed of 1000 rpm when driven through a 3:1 box by an engine doing 3000 rpm - therefore the boat would do the same speed.

 

In the OP's case you are right there is no speed gain by changing the ratio, apart from the ability to travel more slowly at tickover, because the engine could not use the new gearing to increase the prop speed; if the engine cannot go beyond 3000 rpm, a ratio of 2.5 would be better - but is not available on the PRM

[bizzard]

Is this the way the teeny weeny engine in Mrs TNC's new Ecoboost Focus works?

You'd be surprised, maybe a little more refined though but at a cost where mine is cheap, just an old whiskey bottle, bit of tube, pegs, the Ether and hairdryer would be the most expensive items, Mike already has a generator.

My power boosting devices could both be made to operate automatically by rigging them up to either the throttle linkage or Tachometer wire from the alternator so that with his generator already running and waiting for the moment when Mike whacks the throttle wide open they both come into play and cause such a powerful boost that he stumbles over backwards, or forwards if in astern gear.

[GUMPY]

If the prop rotates at 1000 rpm when driven through a 2:1 box by an engine doing 2000 rpm, that same prop would rotate at the same speed of 1000 rpm when driven through a 3:1 box by an engine doing 3000 rpm - therefore the boat would do the same speed.

 

In the OP's case you are right there is no speed gain by changing the ratio, apart from the ability to travel more slowly at tickover, because the engine could not use the new gearing to increase the prop speed; if the engine cannot go beyond 3000 rpm, a ratio of 2.5 would be better - but is not available on the PRM

 

What I was trying to say ius that swapping to 3:1 without changing the prop will, give you a boat that is so severly underpropped as to be almost unusable.

I did it the other way round on an underpropped 3:1 and changed to 2:1 thinking it would save me a prop change, it was so overpropped the engine wouldn't rev above 1100rpm.

ended up dropping the pitch from 21 to 16 and then it was OK.

Its not as simple as people think.

Propking.xls is a good calculator for props google is your friend to find a copy

[bizzard]

You'd be surprised, maybe a little more refined though but at a cost where mine is cheap, just an old whiskey bottle, bit of tube, pegs, the Ether and hairdryer would be the most expensive items, Mike already has a generator.

My power boosting devices could both be made to operate automatically by rigging them up to either the throttle linkage or Tachometer wire from the alternator so that with his generator already running and waiting for the moment when Mike whacks the throttle wide open they both come into play and cause such a powerful boost that he stumbles over backwards, or forwards if in astern gear.

Of course the hairdryer could be run from Mikes inverter but with the added load put on the alternator the engines extra big boost might be forfeited to just a medeocore boost.

[bargemast]

changing the gearbox wont work the difference in ratio will be to much

As an example my NB for 6knots with a 2:1 needs a 18x17 at 3:1 it will need an 18x26!

 

The limiting factor on all displacement boats is hull speed, on the barge I could reach hull speed at under 1400rpm the engine would rev to 2000+ did it go any faster, not really but used to burn over twice the fuel.

First thing to do would be to check the the boat is reaching hull speed at or below present maximum revs if it is there is nothing worth doing.

Easiest way to check you are at hull speed is to look for a double bow wave best pic I have is this one

https://www.dropbox.com/s/iu23zpj4yhi50is/DSCF0440.JPG

you see the second wave just back from the bow.

 

 

So you think it's better to keep the 2 : 1 reduction, and limit the 55Hp engine to deliver a maximum of only 40Hp at the maximum revs it can do now 2000n ?

 

Peter.

[Timleech]

changing the gearbox wont work the difference in ratio will be to much

As an example my NB for 6knots with a 2:1 needs a 18x17 at 3:1 it will need an 18x26!

 

The limiting factor on all displacement boats is hull speed, on the barge I could reach hull speed at under 1400rpm the engine would rev to 2000+ did it go any faster, not really but used to burn over twice the fuel.

First thing to do would be to check the the boat is reaching hull speed at or below present maximum revs if it is there is nothing worth doing.

Easiest way to check you are at hull speed is to look for a double bow wave best pic I have is this one

https://www.dropbox.com/s/iu23zpj4yhi50is/DSCF0440.JPG

you see the second wave just back from the bow.

 

Surely that wave is mainly a result of the boat having a bluff bow?

Hull speed as I understand it is when the boat is going fast enough for that second wave to be at the stern! A 38m barge will have a very high 'hull speed', one it could never achieve in reality.

Yes a bluff bow will limit the speed, but not in the same way or as dramatically as what is usually understood by hull speed.

 

Tim

[GUMPY]

So you think it's better to keep the 2 : 1 reduction, and limit the 55Hp engine to deliver a maximum of only 40Hp at the maximum revs it can do now 2000n ?

 

Peter.

If the boat is reaching hull speed that there is no point in changing anything as the extra power wont do any good.

If not then a possible repitch may be in order however it could go the other way as its very close.

A gearbox ratio change is for me a non starter been there done that its just to much of a change when all that may be needed is a 1" pitch change

 

Surely that wave is mainly a result of the boat having a bluff bow?

Hull speed as I understand it is when the boat is going fast enough for that second wave to be at the stern! A 38m barge will have a very high 'hull speed', one it could never achieve in reality.

Yes a bluff bow will limit the speed, but not in the same way or as dramatically as what is usually understood by hull speed.

As I understood it was double wave at either bow or stern but I bow to you on that one. I was also using the picture to illustrate the wave form not saying that that particular boat was at "hull speed"

 

A final thought Theoretical hull speed is usually way higher than a barge will travel at.

Maybe I should have used the words "practical hull speed" which in Parglena's case was 5 knots despite the theoretical hull speed being twice that.

Edited February 23, 2013 by idleness

[bizzard]

A variable pitch prop that adjusts itself automatically to fuel, water, pump out tank and peoples weight alterations and shifts. Similar to an aeroplanes variable pitch propellers that adjust for grip, speed and load.

[bargemast]

A variable pitch prop that adjusts itself automatically to fuel, water, pump out tank and peoples weight alterations and shifts. Similar to an aeroplanes variable pitch propellers that adjust for grip, speed and load.

 

 

Yes, variable pitch would be the best solution as you can adjust the pitch exactly to get the best results with a given number of revs, only on a canalboat thats cruising on canals with almost more shopping trolleys than water under her keel, it wouldn't last long, as it's a bit more delicat than a fixed prop.

 

With a variable pitch prop on bigger ships you adjust the pitch while watching a pyrometer in the exhaust manifold to avoid the engine getting overloaded, like you get with an overpropped engine.

 

Peter.

[blackrose]

If the boat is reaching hull speed that there is no point in changing anything as the extra power wont do any good.

 

Perhaps my boat is already reaching its maximum hull speed - this might be why my friend's 60' x 12' LB widebeam with an Isuzu 70 (65hp) doesn't go any faster than mine? We had a "burn up" on the Thames last summer. Also when we travelled together downstream on the tideway he struggled to turn into Limehouse lock against the current the same as me.

 

He has the PRM 260 (I don't know what reduction ratio), with a 19" x 15" prop and also can't get more than 2000 rpm in gear.

 

Here are the curves for the Isuzu 70 which show that at 2000 rpm he is achieving about 54 hp compared to my 40 hp.

 

If 54 hp can't go any faster than 40 hp does that point to the maximum hull speed being reached for this design of boat?

 

Edited February 23, 2013 by blackrose

[Timleech]

Perhaps my boat is already reaching its maximum hull speed - this might be why my friend's 60' x 12' LB widebeam with an Isuzu 70 (65hp) doesn't go any faster than mine? We had a "burn up" on the Thames last summer. Also when we travelled together downstream on the tideway he struggled to turn into Limehouse lock against the current the same as me.

 

He has the PRM 260 (I don't know what reduction ratio), with a 19" x 15" prop and also can't get more than 2000 rpm in gear.

 

Here are the curves for the Isuzu 70 which show that at 2000 rpm he is achieving about 54 hp compared to my 40 hp.

 

If 54 hp can't go any faster than 40 hp does that point to the maximum hull speed being reached for this design of boat?

 

 

Am I right in thinking that yours is a Liverpool Boats 'fat narrowboat'?

If so, speed may be effectively limited by water flow to the propeller?

 

Tim

[blackrose]

Am I right in thinking that yours is a Liverpool Boats 'fat narrowboat'?

If so, speed may be effectively limited by water flow to the propeller?

 

Tim

 

Yes, it is.

 

Why would water flow to the prop be limited any more than any other fat NB?

 

Perhaps you just meant that water flow to the prop is limited on any fat NB, but I've heard people on this forum say that LB's have short swims. Generally these people turn out to have no experience of LB.

 

I can't find a decent pi cture of the swims of my boat so this will have to do. I'm not quite sure how long the swims are but this is a 57ft boat so you can judge for yourself. I don't think they are any shorter than most other canal boats (although any fat NB will always have shorter swims relative to its beam than a thin NB). Anyway, at the sort of speeds we're talking about I really can't see any reason why water would have any trouble getting to the prop.

 

 

I've also heard people say that you can't drill LB hulls because the steel's too brittle (nonsense), and last week a "boat engineer" told me that LB's were made of tin cans recycled by the Chinese... Recycling is exactly what a lot of people do. They just hear this sort on nonsense and then blindly repeat it.

Edited February 23, 2013 by blackrose

[JDR]

I'd recommend contacting Clements (based near Bedford) when you decide to get the prop off.They supply the RNLI amongst others.I found them excellent to deal with, willing to find solutions if there was nothing off the shelf to fit, do refurbs, repitching etc and costwise are reasonable.01234378814

Edited February 24, 2013 by JDR

[Dalslandia]

If you do 6 mph now with 40 hp you will do 6,67 mph with 55 HP

 

Jan

[RLWP]

If you do 6 mph now with 40 hp you will do 6,67 mph with 55 HP

 

Jan

 

How? By that reasoning, if he had 400 HP he would do 60MPH

 

Richard