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Power in reverse


Debs

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8 minutes ago, Athy said:

Thanks. So would those old Nationals, Listers, Gardners etc. have had their power measured by this dynamometer?

The French horsepower were fiscal, as you suggest, like the old R.A.C. Horsepower: something like 1 cv per 250 cc (the 2CVs made in the '50s were 425 cc, later ones had a "big" 602cc engine and in fact were taxed as 3cv even though they were still marketed under the original name because everyone knew what a "Deux Chevaux" looked like.) Actually, some of them may have been badged as "2CV6", just to confuse the enemy.

Cor, thanks. I never knew that temperature came into the calculation.

But that still doesn't explain (to me at any rate) why, for example, my 2LW is rated at 28 (b.) h.p. when it is (to me at any rate) obviously far more powerful than a modern buzzy 28 engine.

 

Blade size!

As I said earlier in the thread, put a big blade and a high reduction gear on a buzzy engine, and power-for-power the performances will match.

It's just that you don't generally get a buzzy engine in a hull suitable for a big blade. So a small, inefficient blade is compensated for by fitting a higher power buzzy engine.

Edited by Mike the Boilerman
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6 minutes ago, Mike the Boilerman said:

 

Blade size!

As I said earlier in the thread, put a big blade and a high reduction gear on a buzzy engine, and power-for-power the performances will match.

It's just that you don't generally get a buzzy engine in a hull suitable for a big blade. So a small, inefficient blade is compensated for by fitting a higher power buzzy engine.

Yes but (says he, his persistence matched only by his incomprehension) a Beta 43 is still a 43 h.p. engine whether it's driving an egg whisk or a roller mill.

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2 minutes ago, Athy said:

Yes but (says he, his persistence matched only by his incomprehension) a Beta 43 is still a 43 h.p. engine whether it's driving an egg whisk or a roller mill.

 

The egg whisk is incapable of communicating that 43hp of power into the water, resulting in poor performance. The answer to poor performance, in the mind of the uniformed boat buyer, is obvious - get a more powerful engine. So rather than argue about it, for an easy life the boatbuilders do what the customer wants. And keep taking the money.

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Just now, cuthound said:

And don't forget that a Beta 43 only develops 43 bhp at 2800 rpm. At a typical canal cruising speed of say 1200-1500 rpm it will develop a fraction of that, probably less than half.

A good point - but isn't that true of all engines?

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Just now, Athy said:

A good point - but isn't that true of all engines?

Yes, the manufacturers always quote maximum power, and power curves are always, well curved, unlike torque curves, where it is usual to tune an engine so that the torque curve is almost flat over the usable rev range.

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Brake horsepower is measured at the output.  Indicated horsepower is measure at the cylinder and will be higher.  Modern dynamometers don't work by measuring temperature but either by generating electricity or measuring the torque and rpm from which bhp can be calculated. 

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9 hours ago, alan_fincher said:

You seem to have overlooked the fact that OP has confirmed that it seems fine when in ahead gear, so clearly an Isuzu 25 on a 45 foot narrow boat is fine the vast majority oif the time she is using it.

It is not a big engine, but in my view should be adequate, and, unless the hull is very poorly shaped, it should be perfectly possible with the right prop and set up to get adequate reverse and stopping power.

 

I was/am making the assumption that the boat has a suitable prop for the engine/box combination, which would be something like 15".  

That would be fine for maintaining forward motion, you could probably move a 45 foot narrowboat forwards with an outboard motor, but when it comes to stopping what is maybe a 12 ton boat, even if the reverse ratio is higher, the prop is still the same size and i don't think 15" is big enough, especially if the swim is very blunt.  If the boat was shaped like a canoe it might have a chance.  

The PRM 80 came with two forward ratios, the higher one would allow for a bigger prop which might be ok for this boat.  

Our boat is 45' and has a Bukh DV24 which is a more powerful unit than the Isuzu.  With 2:5 and 2:4 reduction ratios she just about gets away with an 18" prop but Dragonfly has a very long swim and is probably around 10 tons.  She will stop very quickly.  When I consulted Crowthers they suggested a 16 inch prop which might theoretically be right to get maximum forward speed out of the hull, but in the real world of canals it would cause problems when trying to stop.  

Of course if the OP's prop is 17-18" it blows my theory right out of the water...    

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2 hours ago, cuthound said:

And don't forget that a Beta 43 only develops 43 bhp at 2800 rpm. At a typical canal cruising speed of say 1200-1500 rpm it will develop a fraction of that, probably less than half.

But you blast it in reverse when you want to stop.

I know my Beta 43/PRM150/blade combination stops particularly well (for a narrowboat).

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3 minutes ago, Machpoint005 said:

But you blast it in reverse when you want to stop.

I know my Beta 43/PRM150/blade combination stops particularly well (for a narrowboat).

Mine doesn't, but I think it is slightly under propped, with a 17" prop. Nevertheless, I don't rev it to anything like 2800 rpm when stopping. All that does is ensure I  end up across the cut! 

I rely on a combination of anticipation, observation and a "cadence" reversing technique when I have to stop quickly.

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On 19/07/2017 at 16:29, mross said:

Brake horsepower is measured at the output.  Indicated horsepower is measure at the cylinder and will be higher.  Modern dynamometers don't work by measuring temperature but either by generating electricity or measuring the torque and rpm from which bhp can be calculated. 

 

No but we were discussing brakes specifically, because Athy was asking about the 'b' in BHP, brake horse power. 

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On 19/07/2017 at 15:53, Athy said:

Yes but (says he, his persistence matched only by his incomprehension) a Beta 43 is still a 43 h.p. engine whether it's driving an egg whisk or a roller mill.

 

Ok let's do that reductio absurdum thing. Imagine a 43hp engine in a narrowboat but with a reallly teeny tiny blade, say half an inch in diameter like you would have had (have?) on the model boat in your bath.

How much of that 43 bhp will be available for stopping the boat?

 

It's still a 43 bhp engine, remember?

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5 hours ago, Athy said:

Whilst we're on horsepower, can anyone put me right on this point? Old engines' outputs are, as far as I know, measured in "horsepower", e.g. I think a JP2 produces about 25 h.p. and a 2LW about 28. Even the still sort-of-in-production DM2 has a similar rating. But these are obviously not the same sort of horsepower as, for example, the 25 quoted for the 1100 c.c. Isuzu. Is the latter a measure of brake horsepower, as it is for modern cars? If so, when did the transition take place, i.e. people started rating boat engines in b.h.p.? I wondered this when Alan mentioned his 30 h.p. B.M.C. engine, which, if I'm correct, is a power unit whose origins go back to the 1950s.

Horsepower rating is simply a measure of the amount of 'work' an engine can do in a given amount of time, and is no different whatever sort, or age, of engine it is applied to.

One (1) Horsepower = 550 ft.lbs of work per second, or 33,000 ft.lbs per minute .

The reason for older, slower revving engines apparent power advantages over modern high revving units is entirely and solely down to them being able to transmit more of the power they produce into the water as thrust due to them turning considerably more efficient, larger diameter and slower turning propellers.

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8 hours ago, Athy said:

Yes but (says he, his persistence matched only by his incomprehension) a Beta 43 is still a 43 h.p. engine whether it's driving an egg whisk or a roller mill.

An engine's horsepower rating is the maximum power it is capable of producing at maximum rated rpm. Any engine will only produce a horsepower output sufficient to cope with the load it is working against. For instance, if you run your boat's engine up to it's maximum rated revs in neutral, then it won't be producing the (horse)power it is capable of producing at those maximum revs, it will only produce sufficient (horse)power to overcome internal frictional losses in itself and in the gearbox, to draw in combustion air and expel exhaust gases, to heat up and operate it's own cooling system, and to drive the alternator.

Edited by PhilAtterley
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another perspective is that, it in simple terms, a traditional low speed diesel is designed for industrial uses (think Lister or Gardiner used to drive a pump, generator, compressor, fishing boat) and is normally operating at its governed maximum and optimum speed, and its BHP is calculated on that basis.

on the other hand a modern automotive based engine fitted in a narrowboat is normally operating at perhaps half its maximum speed, and if it has a fairly horizontal torque curve the BHP will be about half its quoted maximum.

so on that basis a suitably geared and propped 20HP Lister is equivalent to a suitably geared and propped 40HP Beta, at least in terms of usable power including stopping power.

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22 minutes ago, Murflynn said:

another perspective is that, it in simple terms, a traditional low speed diesel is designed for industrial uses (think Lister or Gardiner used to drive a pump, generator, compressor, fishing boat) and is normally operating at its governed maximum and optimum speed, and its BHP is calculated on that basis.

on the other hand a modern automotive based engine fitted in a narrowboat is normally operating at perhaps half its maximum speed, and if it has a fairly horizontal torque curve the BHP will be about half its quoted maximum.

so on that basis a suitably geared and propped 20HP Lister is equivalent to a suitably geared and propped 40HP Beta, at least in terms of usable power including stopping power.

No, this is completely wrong - rated horsepower is what an engine is capable of producing if it is loaded up sufficiently to induce it to produce it's maximum rated horsepower whilst running at maximum rated rpm. Normal operating speeds and loads don't come into the reckoning.

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2 hours ago, mross said:

Actually, the metric unit PS is   a horsepower unit equal to 75 kilogram-meters per second

Yes, metric horsepower, and in common with much else in the metric system we have had foisted on us, you get short changed with this too.

1 x PS (metric HP) = 542.5 ft.lbs per second - 7.5 ft.lbs short in comparison with 'proper' horsepower at 550 ft.lbs per second.

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16 hours ago, Mike the Boilerman said:

 

Blade size!

 

More correctly, the joint effect of both blade size and speed of rotation. 

On slow (most of today's so-called 'narrowboats struggle to achieve a V/L ratio much in excess of 0.8) displacement vessels, efficiency increases as diameter increases and rotational speed decreases -

Edited by PhilAtterley
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10 hours ago, PhilAtterley said:

No, this is completely wrong - rated horsepower is what an engine is capable of producing if it is loaded up sufficiently to induce it to produce it's maximum rated horsepower whilst running at maximum rated rpm. Normal operating speeds and loads don't come into the reckoning.

no, this is completely right.  I agree with you and it is exactly what I was saying.  The 40HP beta is never operated at its maximum, whilst the Lister is.

people think that a 40HP Beta is more powerful than a 20HP Lister in a narrowboat, but actually it ain't (when operated at normal canal speeds).

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8 minutes ago, Murflynn said:

no, this is completely right.  I agree with you and it is exactly what I was saying.  The 40HP beta is never operated at its maximum, whilst the Lister is.

people think that a 40HP Beta is more powerful than a 20HP Lister in a narrowboat, but actually it ain't (when operated at normal canal speeds).

It's the smoke and mirrors that modern engine manufacturers use, very few buyers know the difference between continuous and intermittent power and almost all manufacturers of (narrow) boat engines use the intermittent power figure (and without any ancillaries attached).  As a result I actually think a lot of modern narrowboats are underpowered, though the popular view is the opposite.

      

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At normal (breaking wash free) canal cruising speeds any engine will only be putting around 6-10hp into the prop, so it doesn't matter if you've got 20hp or 40hp -- remember at half maximum revs the prop will only absorb 1/4 maximum power if it's properly sized, and most modern engines cruise at about half maximum revs.

When it comes to stopping, *if* you push the engine to maximum revs (most people don't) it will dump its full rated power into the prop, in which case a 40hp engine will stop you faster than a 20hp one, regardless of intermittent or continuous ratings -- but only if the prop is correctly sized. A bigger slower-turning prop is more efficient, but the shape of the hull (and clearance between prop and deadwood) probably makes more difference than this -- and on many (especially modern) hulls the prop is too close to the hull and too far from the rudder, which helps explain both poor stopping and poor steering in reverse.

The advice for marine propeller sizing for slow displacement hulls (like narrowboats) suggests keeping prop speed below 1000rpm at full power, but with most modern engines a 2:1 gearbox is used meaning 1400rpm for a Beta 43 or even more for smaller engines, together with a relatively small diameter 3-blade prop. A better solution would be a higher ratio gearbox and a bigger diameter (maybe 4-blade or bigger BAR) coarser pitch prop, even with a modern shallow draft hull this would fit and would give better efficiency and better stopping power -- but would also cost more, which is probably why it isn't done very often...

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1 hour ago, IanD said:

 remember at half maximum revs the prop will only absorb 1/4 maximum power if it's properly sized, and most modern engines cruise at about half maximum revs.

 

Do you have a reference for that please?

I ask because Daslandia here (who designs propellers) said recently (IIRC) the power transmitted by a propeller, all other things remaining equal, rises with the fifth power of the shaft speed. Your statement suggests it rises only with the square of the shaft speed. 

Many thanks.

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1 hour ago, IanD said:

At normal (breaking wash free) canal cruising speeds any engine will only be putting around 6-10hp into the prop, so it doesn't matter if you've got 20hp or 40hp -- remember at half maximum revs the prop will only absorb 1/4 maximum power if it's properly sized, and most modern engines cruise at about half maximum revs.

When it comes to stopping, *if* you push the engine to maximum revs (most people don't) it will dump its full rated power into the prop, in which case a 40hp engine will stop you faster than a 20hp one, regardless of intermittent or continuous ratings -- but only if the prop is correctly sized. A bigger slower-turning prop is more efficient, but the shape of the hull (and clearance between prop and deadwood) probably makes more difference than this -- and on many (especially modern) hulls the prop is too close to the hull and too far from the rudder, which helps explain both poor stopping and poor steering in reverse.

The advice for marine propeller sizing for slow displacement hulls (like narrowboats) suggests keeping prop speed below 1000rpm at full power, but with most modern engines a 2:1 gearbox is used meaning 1400rpm for a Beta 43 or even more for smaller engines, together with a relatively small diameter 3-blade prop. A better solution would be a higher ratio gearbox and a bigger diameter (maybe 4-blade or bigger BAR) coarser pitch prop, even with a modern shallow draft hull this would fit and would give better efficiency and better stopping power -- but would also cost more, which is probably why it isn't done very often...

I don't know if cost is the issue, higher ratio boxes don't cost any more, so it's just the bigger prop that would increase the expense but not by much.  I think it's simple lack of thought, and the fact that middle market and budget boat builders know that most of their customers will be inexperienced and probably not at all concerned whether their boat is performing as efficiently as possible.  It's only when you do a lot of cruising on varied waters that you become aware of any shortcomings in your set up and let's be honest, these days most boats are either going nowhere or have a very limited cruising range.  

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On 19/07/2017 at 23:53, PhilAtterley said:

Yes, metric horsepower, and in common with much else in the metric system we have had foisted on us, you get short changed with this too.

1 x PS (metric HP) = 542.5 ft.lbs per second - 7.5 ft.lbs short in comparison with 'proper' horsepower at 550 ft.lbs per second.

You get the same amount of brake power, but the number is different. No short changing is involved: you actually get MORE metric hp from a given engine, not less! (Although metric hp aren't much used anyway, because it is usually more convenient to express power in kW).

By your argument you get "short changed" using yards instead of metres, because a metre is 39.36in but a yard is only 36in!

I did an engineering degree more than 40 years ago and imperial units were never used. I have little concept of the sizes of most of those obsolete units (except feet, inches and pints) and I have no intention of learning them now.

Refusing to use SI units is just another barrier to international commerce and co-operation. Just because the Americans are primitive, why should we imitate them?

Edited by Machpoint005
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