West Somerset Railway General Discussion

Thanks to all who contributed to the locomotive physics lesson - very educational!

 

Spending one's formative years working in at least three systems of units still has advantages.

 

Yes, apologies for the mixed units: I was writing in a hurry so didn't tidy up; and I suspect that for most people saying the speed was e.g. 20mph may be more meaningful, even though 8.9m/s is what went into the calculation ...

Incidentally, one odd conversion that always sticks in my mind is that 25mph is about 11 metres per second. In the context of a heritage railway, saying you run at 25mph always seems quite slow, but when you realise it is 11m/s, it shows how far you could travel in a short time if for example you see an obstruction. Essentially rather less than two seconds to travel a carriage length.

Tom

 

The legislation which we run to has the cut-off at 40 kmph, which is near enough 25 mph.

When it comes to calculations and a sense of whether the answers are realistic, I can cope with primary metric but have to convert it all to imperial to get any sense out of it.

 

I tend to flit fairly comfortably between imperial and metric when it comes to mass, length, volume etc (though I can't imagine acres: I have to convert mentally to a box of certain dimensions laid out on the ground). When it comes to slightly more abstract concepts like energy, I normally think exclusively in SI units - probably a consequence of having learnt thermodynamics at University entirely in that system.

There is also the question of finding a set of units that makes sense for the kind of calculation you are performing, not least to sanity check the answer. If you want to know how long it will take to drive to some distant location, miles (or kilometres) per hour makes sense. If you want to know how long you've got to e.g. compose a photo of an approaching fast-moving object, metres (or feet) per second is a more natural unit!

Tom

 

When the WSR re-opened to Williton in the late 70s one of the xPwllheli water tanks was installed at Williton, there was the usual 'panic' of the time and the tank was erected very quickly after some repairs. the location of the tank was as per the (then) loco departments instructions. There was no budget for the project, the WSRA paid for the concrete base, the repairs cost nothing and the water feed pipe to the tower was donated. What surprises me is that at some point (after the installation of the water supply at BL?) the Williton tank was removed, given the comments here I would have thought that keeping the then present water tank at WN would have been a good insurance policy for tank engines and even tender locos with 'problems'.

 

At Arley at the SVR Gala I was looking at a porters trolley with 2C 1Q 3LB on it and trying to work out how many pounds that was. Off the top of my head I came up with 255. If I am wrong, can anyone help?
I'm not very good on roods, perches and acres though.

 

I'm pretty sure that 255 lb is correct.

 

In operational terms the Williton tank was a nightmare. Exactly where you would not want it to be, so an up train would block the north end points taking water, preventing the down train getting away.

The concrete base is still there.

Robin

 

Bring back 'net ton train miles per engine hour' I say!

Robin

 

Excuse my predeliction for Imperial units, it's what I remember from BR steam days (I am old enough to have been educated in Imperial, introduced to SI at Uni, then worked in the US to add to the confusion etc)

At 20 and 40 mph I estimate the EDHPs with a Manor are 475 and 1006 (IHPs are always difficult, some have derived quadratic equations but I remain doubtful) probably based on the Swindon tests IHPs 580 and 1250.

Non stop Williton Crowcombe times at these speeds I think will lead to start to stop times of 20 & 11 minutes. (Over the last 20 years there have often been non stop runs between the two at Galas.) The 20 is based on actual performances, I have never travelled from WN to CH in 11 minutes !

An alternative back of a 'fag packet' approach to estimating total power requirements is to assume the figures quoted above appertain to the whole run ie extra energy involved in accelerating the train to line speed is recovered in the slowing down for CH as per Steve (I will ignore the complication of the 10 mph psr into the Up platform at CH)

Taking 20 and 11 minutes this gives an increased energy requirement for running at 40mph compared with 20mph of 18% if based on IHPs, 16% if based on EDHPs

This I think highlights another issue when considering accelerated schedules. Operating locos beyond their optimum efficiencies.

The 20mph would require (based on Swindon test figures for Manors on Blidworth coal at 30mph) 1300 lbs coal/hr

The 40 mph 3200 lbs/hr at least ie an increase in coal consumption of 35% for 40mph vs 20 mph running between WN and CH..

This assumes that the Manor could even sustain such an evaporation rate ie to run six coach trains at 40 mph uphill reliably from WN to CH would require a Class 5 loco at a minimum.

"It is the gradient not the speed that is the killer" At our speeds yes.

At higher speeds things change ie the gradient of course remains a linear function but speeds:

Figures from the very detailed Mark 1 tests gave:
20 mph - 4.1 lbs/ton, 40 - 6.5, 60 - 9.7, 80 - 13.8, 100 - 20
That is to double the speed at 20mph increases the resistance by 58%
To increase the speed by 25% at 80 mph increases R by 45%.

As you go faster gradients are of course less percentage wise a killer, hence 1/40 gradients and steeper on HS1

Aldfort. When I quoted 4160 operating at 225 psi I was referring to that stage of its Preservation life rather than latter day 200 operation. I assume the steam chest pressure would be 200-210 max with RFO and valves simmering ie ideal firing !

Michael Rowe

 

Sorry 241 LB 2 X (14 X 8) 112 = 224 + 14 = 238 + 3 = 241 LB

 

Does not one quarter = two stones ie 28lbs. The answer is 255 lbs ?

Michael Rowe

 

In your calculation the terms 224 and 3 are correct but 14 should be 28; one quarter is 2 stone or 28 lbs.

 

Quite so.

The "Quarter" is a quarter of a hundredweight (cwt) so 112/4 lb.

 

Sorry Tom , next time can you not convert to metric! Especially when talking about steam engines, as I, & a reckon a lot of others have no idea what you're talking about !

 

It does make calculations easier without having to put in a bunch of constants though! (Especially when sitting on the train home doing the calculations away from any references...)

But the key point is really the relative numbers: for a climb comparable in length and gradient to Williton - Crowcombe, the interesting point is that most of the work is done against gravity at the kinds of speed run on heritage railways: speeding up even to 40mph only adds about 14% more energy requirement. (The original contention was that, broadly the amount of coal and water used was only slightly dependent on speed, which is true up to speeds faster than we can run on a heritage railway, but not at very high speeds).

As for power: 0.7MW is 940hp; and 0.33MW is 440hp. Those figures aren't a million miles away from those arrived at from presumably a different methodology by @Maunsell907: given the approximations inherent in what was a back of the envelope calculation, probably largely insignificant difference. Also interesting that Michael came up with an energy difference of about 16% between 20mph and 40mph, which again is pretty close to the 14% difference I came up with on a notional bit of 1 in 100 gradient.

Tom

 

Looks right 2 x 112 lbs per cwt - plus 28lb [1quarter of cwt] plus 3 lbs.
I worked all my life for a coal merchant in c.q.lbs

 

You'll have to come and show me how to do that one day Michael.

 

Someone please tell me what time BL station shop closes 2moro please thanks