Tank v Tender Locos

In an idle moment I was pondering.......................

Tank engines have the obvious advantage of not needing to be turned.

Clearly you cant get the range of a tender loco, and - I imagine you must come up against weight limits if you want a big one. But in the immediate pre grouping period a number of large passenger & freight tank locomotives were built, such as the 'Rivers' & the GWR & LNWR 8 coupled freight locos.

From looking at the figures - and the performance of the Brighton tanks on the run to Rugby it seems as though range was not an issue which makes me wonder why greater use was not made of tank locos - for example why wasn't the SDJR 7F built as a tank rather than a tender loco?

 

I'd say range WAS an issue, along with route availability. Tank locos have less water and coal capacity than their tender equivalents. And with full water tanks would presumably exert a higher axle loading.

SFAIK large tank locos were rarely used for long-distance trains, passenger or freight, as they would have needed frequent stops for water.

An interesting side-question: Did any tank locos ever have water scoops?

 

Some certainly did, including the Standard 4's which had bi-directional scoops

 

I think L&Y radial tanks has scoops.
If you imagine a tank and tender equivalent of the same loco, the tank loco will have higher axle loading.
The example which springs to mind is on the Irish GNR, where the T class 4-4-2T were used on suburban routes, and the U class 4-4-0 was evolved from them, because its lower axle loading gave greater route availability.

 

Loading gauge has to be a factor here surely? Tanks limit boiler diameter and thus steam raising, limiting the load hauling capability surely??

 

Whilst in theory this is right, I don't think it is in reality. The higher power output locos are generally those required to run long distances and thus need to be tender locos to carry the coal and water. Racking my brains, I can't think of a tank loco of a power classification greater than 4.

Tank locos wouldn't have a higher axle loading because, like their tender counterparts, they were generally built to the maximum permitted axle loads. What does happen is that, as the coal and water is consumed, their effective axle load diminishes, whereas that of a tender loco remains constant (tender excluded.)
One great advantage of a tank loco over its tender equivalent is the fact that it doesn't have to drag a tender around and this is significant, generally being the equivalent of at least a coach and often more. It is certainly very noticeable on the NYMR where a Cl.4 tank has a significant advantage over its Cl.4 tender equivalents and is well ahead of a Black 5 or B1 in its ability to drag 8 coach trains up the gradient to Goathland.

 

42XX/52XX/72XX Rated 7F/8F depending on type.

 

A number of the early Churchward standard GWR tank engine classes had water scoops, but I think the gear had mostly been removed by WW1.

There was an amusing incident with the prototype of the Dean/Churchward transition 3600 2-4-2T. It picked up water faster than the air vents in the tanks could cope with and split the side tanks open! The GWR tank engine equivalents tended to have smaller boilers: weight limits were a definite issue. The 8 coupled tanks had Std 4 boilers and were red route (most restricted), whilst the 2800s had the larger Std1 and wider route availability. On the other hand the tank engines had more weight for brake power - perhaps why they were favoured on unfitted welsh coal trains.

By and large GWR crews seemed to have liked to turn tank engines and run chimney first if they had the opportunity. My guess is that apart from anything else controls were in the right place and the driver was on the right side for sighting signals.

 

were there also problems of stability with large tank locos on less than ideal track, it was this that lead to the Rivers being rebuilt as U class locos, a tender loco will always i think score higher on the operational side because of the increase level of water and coal carried and that the centre of gravity of the loco isnt as effected by the water level reducing as it would in te case of a tank engine

 

GWR 2-8-0T and 2-8-2T were 7F and 8F. Southern W class 2-6-4T was 6F, G-16 4-8-0T 8F and H16 4-6-2T 6F, GCR Q1 0-8-0T 5F, NER/LNER T1 4-8-0T 7F.

 

The Urie G16 4-8-0T were BR 8F; the H16 4-6-2T were 6F.

The Maunsell Z class 0-8-0T were BR 7F.

The Z was built as a shunting engine and as such didn't have the sustained boiler power to match the cylinder power, but the G16 and H16 did have boilers somewhat commensurate with their size, for example a grate area comparable to a Stanier 8F or a Urie S15.

Tom

 

I believe that the GWR 4-4-2 County Tanks had bi directional water scoops until they were withdrawn, when the 61xx class arraived on the scene

 

I don't have any data to back this up, but I would have thought that an Ivatt 2MT 2-6-2T, with full tanks, must have a higher axle loading than its 2MT 2-6-0 tender equivalent. But maybe they did something clever with weight distribution ....

 

Although everyone's pointed out all the classes that contradict @Steve 's suggestion about no tank engines over class 4, I can't help noticing they all have F suffixes, which rather goes to reinforce his original point about the lack of high power tank engines. Even the big LSWR tanks had a version of the D15 4-4-0 boiler, with a shallow firebox that probably didn't do anything for its sustained steam raising, yet those lovely engines were given an ignominious 3P power rating despite 27 sq ft of grate (I can't help but feel they were worth at least a 4P). The ones I had in mind as a possible exception were the Brighton Baltics, but even as rebuilt to N15X (with identical boilers) BR only gave them 4P. I don't think anything with a GWR No. 4 boiler got more than 4MT or 4P either.

I think it's fair to say that the axle load limitation due to carrying fuel and water on tank engines did restrict them - the standard SECR/SR Mogul boiler was designed to allow for the K-class tank, so even though the K-class was extinct after 1927 every SR Mogul before and since had slightly less boiler than it could have carried within the route availability restrictions. Holcroft thought this was a mistake, and maybe a few fireman shy for steam on a Mogul might have agreed with him here and there...

 

Never thought about this before, but all other things being equal, would a tank engine carrying a bunker full of coal and full water tanks get better adhesion than a tender version of the same locomotive?

If this was so, there must also be a decrease in adhesion as coal and water is consumed?

 

I was somewhat surprised by the N15X only being a class 4P, but I guess that for a small class, it would be as easy to set such a classification in relative terms as go to the trouble of measuring it. In other words, once you have decided a King Arthur is a class 5, then the most significant thing to know about the N15X is that it wasn't quite as capable, so calling it a class 4 is all a shed master otherwise unfamiliar with the type would really need to know.

With regard the moguls and weight saving: I seem to recall that as well as the boilers, the frames of the K class tanks were also marginally thinner than desirable in order to keep weight down. That was perpetuated not only in the class U rebuilds, but also those members of the U class built new as 2-6-0s, with the result that in later life they were prone to frame cracks and many had either partial or complete frame replacement towards the end of their lives.

Tom

 

I think its all about weight, as you say. With the GWR classes I'm familiar with, even given the addition of a set of trailing wheels and often a smaller boiler a tank engine is usually at least one weight class above the equivalent tender engine. As all large GWR express passenger locomotives were red route limited anyway, the only possibility for a high powered tank engine would have been a locomotive with the King route restrictions, which wouldn't have been very useful.

On weight and load BTW, a GWR standard 3,500 gallon tender with 7 tons of coal was about 40 tons. A GWR 3150 2-6-2T with 2,000 gals of water and 3.5 tons of coal was 81.5 tons, 19.5 tons heavier than a 4300, its very near equivalent, which was 62 tons. 14.45 tons of the extra weight was on the trailing wheels.

So the tender cost ~20 tons weight and 2 axles for approx double the range, water pickup capability and improved RA.

A 2-6-4T was presumably more extra weight on more extra axles, but BR "standardisation" being what it was a direct comparison is more difficult.

 

A number of issues here. Firstly, in theory a tank loco was a two-way machine; in practice the crew would always turn it to be chimney first if opportunity arose. Driving bunker first with the controls behind the driver was less than convenient, and coal dust blowing in from the bunker unpleasant. There will be many examples of tankies running bunker first produced, but where possible the crew would always turn it.

Large passenger tanks? The LT&SR, G&SWR and L&YR all had large 4-6-4 tanks for passenger work, there might be others. They were not necessarily good, and in the latter case far too big, but they did exist.

Range? a big tank might hold 2000 gallons of water and 2 1/2 tons of coal. The small Fowler tenders on the LMS held 3500 gallons and 5 1/2 tons - a big difference.

Size and weight. A tankie with full tanks would have a higher axle load than a comparative tender loco. However, part of this weight would be borne by a bogie or truck. And as water was used, any additional adhesion gained by the tanks disappeared. Additionally, if you had an unfitted goods, as most were, you wanted the extra braking power provided by a tender. When Thompson's L1 ran its trials, this was the major failing noted against it (other would materialise as wear and tear took its toll). There were also stability issues with big tank engines: the 'Rivers' on the SR are usually quoted here but they were not alone, and probably not the worst.

 

Must admit I was thinking about passenger train operations but I should certainly have remembered the 56XX tanks, even though I don't rate them as anywhere near a '5' classification.

Don't know about the Ivatt's but with the BR standard 2's the total adhesive weight is 0.75 ton more for the tank loco. (13t 15c/13t 12c/13t 3c v 13t 12c/14t 0c/13t 13c)

 

The 3150/4300 comparison is a good one - pretty much identical in tractive effort as well as driver diameter, cylinder size and configuration as well as boiler type and pressure. I have a adhesive weight of 58.2t for the mogul but I can't find one for the 3150 prairie. I would be interested in that comparison.


Interestingly:....

TE for the 3150 2-6-2t is 25,670 - BR power class 4MT
TE for the 3100 2-6-2t is 31,170 - BR power class 4MT
TE for Castle class is 31,625 - BR powers class 7P

What is going on here? Surely the 3100 has more grunt than a 4MT rating?