A matter that's been at the back of my mind for a while is the lack of unditching rails on Mks V** and VIII. Obviously they arrived too late for combat, thus didn't need them fitted, but I'm wondering if the change in the nature of battle in 1918, with increasingly open battlefields and the ground not being torn up so much by shelling lead the rails to be abandoned as unnecessary for the new designs that were on the way, or if they would have received them had the war continued long enough for these types to be used.
I note the vestigial rails over the engine compartment of the Mk VIII, and the fact that these would be pretty useless for unditching gear because the tank would have to reverse to use an unditching beam, then drive forwards tobring the beam back to rest on the rails; this would either mean driving back into the spot where you ditched, or reversing with the beam in use, then pivoting the tank to put it on a different bearing prior to driving forwards - something impossible with the beam chained to both tracks!
Obviously if unditching gear was intended to be carried by these marks (if they had been used in combat), it would have needed to be tall enough and long enough to clear the superstructures - which on a Mk VIII would have made it ridiculously high.
Possibly with trenches, but the photographic evidence suggests that embankments and shell holes and the like caused tanks to ditch too. Some ditched by sideslipping into holes, such as Fray Bentos 1. Wouldn't long tanks still be susceptible to ditching in these manners?
Not to forget the Mk V*, which was the same length as the V** and not much shorter than the VIII - yet it was fitted with unditching gear.
-- Edited by TinCanTadpole on Monday 4th of June 2012 02:41:50 PM
My point is that plenty of V*s had rails fitted, so they must have thought it handy. You're quite correct in what you say, PDA, but that's not what I'm asking.
However, you may be pointing in the right direction, whether intentionally or not: since many V*s were equipped - but as you point out, a good number were not - perhaps earlier examples received rails on the assumption that they would be fighting in similar conditions to those previously encountered by tanks, whereas examples arriving in the field later in the last hundred days might have had the rails omitted because experience showed that the fighting had moved into more open country with more favourable ground conditions in which tanks were less likely to get stuck.
In such a case I'd presume that it was decided not to bother fitting rails to V** and VIII, since V* was no longer being fitted with them.
An interesting debate. Just taken a scan through my collection of Mark V* photos and I notice that every photo I have of a Mark V* in US or French service shows unditching rails. The evidence is very mixed for British Army machines in the UK or France. Worth looking into a bit more, IMHO.
I've found Mk V*s with and without rails. Hadn't thought about it before. What was deemed unnecessary on the V* onwards was fascines or cribs - the greater length meant they weren't needed for trench-crossing. I wonder if the extra length of track in contact with the ground had something to do with it. On the V* the ratio to overall length was almost 33%, but it was reduced to about 18% on the V**, close to that of the Mk I - V, Whippet, etc. It caused considerable steering problems, but is it possible that the lower ground pressure also produced some marginal reduction in the V*'s tendency to bog down, and they thought they could get away with it some of the time, but the problem returned with the Mk V**?
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I don't know if the problem recurred with the V**, nor if that is what you intend to suggest, James, but I'm doing some quick calculations, and it looks like the reduction in ground pressure may have been large enough to cause more than a marginal reduction in tendency to bog down.
Mks I-V contact patch is 4ft 7in as I recall from an old thread;
Mk V* 11ft
Mk V** 6ft 7in if memory serves.
Assuming, for the sake of simplicity, that our tanks are sitting on firm ground with just enough give for the full surface of a track plate to settle in and support the tank (rather than the weight resting only on the raised ridges), and ignoring the ridges to imagine the contact patch as a simple rectangle:
Mks I-V (narrow track) 20.5 x 55 = 1127.5 square inches contact patch per track = 2255sq in total.
Mk V (wide track) 26.5 x 55 x 2 = 2915 sq in.
Mk V* (early example, narrow track) 20.5 x 132 x 2 = 5412 sq in.
Mk V* (wide tracks) 26.5 x 132 x 2 = 6996 sq in.
Mk V** 26.5 x 79 x 2 = 4187 sq in.
Now to weights; here I admit that I'm not totally sure - I know the approximate figures for Mk IV and V, but it varies a half-ton or so from one source to another, with Mk IV females around 28-28.5 tons (males half a ton heavier, I think - though it may be a full ton). 29 tons rings a bell for Mk V, probably female as I've read about 30 tons for the male?
Anyway - I'm taking 29 tons as an average figure for a standard rhomboid, and since the V* was 4 tons heavier, that makes 33 tons; I think the V** was similar, or only a little heavier.
29 x 2240 lb (I'm assuming imperial tons, not metric tonnes) = 64960 lb.
33 x 2240 = 73920 lb.
Thus for a narrow track rhomboid we get ground pressure of 64960/2255 = 28.8 psi (pounds per square inch)
For Mk V with wide tracks - 64960/2915 = 22.3 psi
Mk V** - 73920/4187 = 17.7 psi
Mk V* (narrow tracks) - 73920/5412 = 13.7 psi
Mk V* (wide tracks) - 73920/ 6996 = 10.6 psi
Obviously these figures are approximate because of the guesswork over the weights, but they give a good idea how radically that 6ft extension reduced the ground pressure.
I've often wondered about the talk that surrounds the steering of the V*: modern writers always seem to repeat that the ability to turn was poor because of the extra length of track dragging on the ground, yet an account closer to the period (1930), which I think is quoted somewhere on Landships, uses the word "unwieldy"; thinking about this, I've wondered whether that might be a more accurate word.
Naturally, soft mud would add a lot of drag to a tank trying to turn, so if a V* encountered the sort of morass that Mk IVs had to face in 1917, then indeed it probably couldn't have turned with anything except great difficulty.
Pics from 1918 however, tend to show V*s on firm ground, often with short grass, because the worst of the mud was left behind when the Germans went into retreat.
I suggest therefore, for your pondering, that the V* was not necessarily hard to turn, being used on more favourable ground, but rather the extra length increased the radius of swing on a turn, making it easier to clout unfortunate bits of scenery, ie "unwieldy".
In addition I also suggest that the dramatic reduction in ground pressure may actually have REDUCED resistance to turning (on the firmer ground often pictured) to the extent that drivers used to having more of a battle to steer their tanks and new to the Wilson steering might have exacerbated the problem by throwing the newer machines around as though they were shorter and dragging more - leading to collisions with trees etc.
-- Edited by TinCanTadpole on Thursday 7th of June 2012 09:05:16 PM
Just for consideration, I quote what is quoted on the Landships II article on the V*, which quotes F Mitchell's Tank Warfare, 1933 (I mistakenly said 1930 before):
"The heavy machines showed a tendency to slip their tracks and also to slide backwards if resting on a slight incline, and owing to this slipping about, their great length and weight, and their quickness in turning, they wrought much havoc, knocking down and crushing many a slender tree, until it looked as if a herd of clumsy elephants had stampeded through the wood"
I have highlighted the phrase which intrigues me. Since the last post it has occurred to me that the complaints which lead to the redesign of the track run for the Mk V** may not represent what we think - that the contact patch was too long and dragging on the ground, making the tank hard to turn.
Perhaps, in light of the much lower ground pressure of the V* and the eyewitness description of "quickness in turning", the problem of the V* was not long tracks dragging on the ground and causing resistance, but low ground pressure reducing resistance and making the V* respond too sharply to steering input.
I suggest that the redesign to shorten the contact patch was not to reduce track drag, but to increase ground pressure sufficiently to tame the speedy turn and make the tank more manageable.
Both reasons for shortening the contact patch could make sense; it's the same change made to the tank, but for opposite reasons. Has anyone read anything that says that eyewitnesses considered the tanks hard to turn? Or have we - and possibly plenty of historians - been jumping to wrong conclusions about the nature of the tank's problems?
edit - it also occurs that the increased length moved the driver further ahead of the pivot point, so he would move further laterally when turning the tank. If the problem was that the tank turned TOO quickly, the larger spatial distance the driver was moving would mean that more of the countryside outside his narrow vision slit would pass by, and it would do so at a faster rate - which I'm sure would be difficult and disorientating for drivers, hence their complaints.
-- Edited by TinCanTadpole on Friday 8th of June 2012 12:48:58 AM
-- Edited by TinCanTadpole on Friday 8th of June 2012 12:50:38 AM
Should point out that the bottom run of the tracks on these tanks was slightly curved so that as the track sank into soft ground the contact area increased and lowered the ground pressure. I've read (somewhere) that the Mark V** had an increased curvature on the bottom run to improve its handling by reducing the contact area on dry ground. This was a response to complaints about the handling of the Mark V*.
Yes, I've heard that, and it may well prove me wrong. Nonetheless, I don't think it harms to ponder and try to dissect the information we have available, just in case it improves our understanding.
Pictures I've seen and drawings seem to corroborate that the curvature was increased. I think this involved making the flanks of the V** slightly deeper than earlier models - effectively lowering the ground level relative to the rest of the tank, then filling in the gap, but with a shorter horizontal section and stronger curve to the forward rise, whilst the rear rise becomes (I think) a bit more angular - but that may not be right, as photos of the surviving tank "Ol' Faithful" do not suggest an angular transition (ie, kink) from horizontal to the rear rise. I'm not certain that the V** had deeper sides, it's a case of observations I've made.
Coming back to the V*, I'm not sure that it's quite as simple as accounts make out. That doesn't mean I'm looking to start a great debate or argument, I just think that the vast drop in ground pressure would have had a noticeable effect on handling - which may or may not have been properly understood by the drivers who were experiencing problems. One of the difficulties in considering the matter is that there are certain conditions where the tracks will tend to dig in and find more resistance - if the back of the tank is in a hollow, for example.
It's possible that even with the tiny ground pressure that the radius of the swing may have had a significant effect, or that something to do with the relationship betwixt contact length and tank width may have been the cause. All I can say is that the V** with it's shorter contact patch seems light enough on it's feet in the video (available in the Landships II article), and that I've been forgetting that the muddy conditions normally encountered by the shorter tanks tended to make them sink a bit into the mud which increased contact length and must have reduced their ground pressure. We often tend to assess the shorter tanks according to their firm-ground contact patch, but drivers would be accustomed most to driving the things on softer ground with more of the track in contact for a lower ground pressure. It may be that this reduced the pressure enough for the apparent advantage of the V* to be nullified by the increased swing radius.
So yes, I concede that I seem to have been on a wild goose chase, but it would nonetheless be interesting to how Mk V and V* compared on hard flat ground, where the full difference in ground pressure applied; would the shorter tank still turn more easily when there is effectively twice the weight carried on that 4ft 7in patch? I might try to think up an experiment that would represent the difference, just for my own interest.
Hang on. Everyone is partly right. I was obliged to discuss this with a ****head on Wikipedia, who took his info on this from Landships of Lincoln, which, unfortunately, is substantially wrong.
The experimental Mk V was a Mk IV with three 2ft panels amidships. The Mk IV, like all the previous rhomboids and the Gun Carrier and Whippet, had a "fishbelly" lower track profile, so that only a short section of track rested on the ground - the "point of balance". Lengthening the Mk IV increased the contact length by six feet, and that feature was carried over into the production vehicles. (The Tadpole didn't have this effect because it was added to the rear of the fishbelly)
TCT's calculations are entirely pertinent. The ground pressure would certainly have decreased as a consequence, and I am pleasantly surprised to see the extent to which it did.
Unfortunately, the problem it caused was a different one - the LC ratio. The short track contact produced a higher ground pressure but made turning easier. The longer the contact, the more lateral resistance is produced. In other words, the more track has to slide sideways over the ground during turns. So the extra six feet might have helped when the vehicle was moving in a straight line, but were a drawback when steering. It's an unavoidable trade-off.
Wilson realised this and designed the V** with the fishbelly reintroduced as far as possible. As TCT's figures show, the contact length was reduced, but the ground pressure increased.
If you have D. Fletcher's British Tanks 1915-18 handy, there's a drawing on p119, superimposing the profiles of the Mk I-V, Tadpole, and Mk V*. You can see the difference in the points of balance.
Whether the pressure reduction was by itself enough to reduce ditching on the V* to the point where a beam could be considered optional was a bit of speculation on my part. Could be wrong. Maybe it was a question of availability or time. Gwyn's point is an interesting one. And maybe the less broken ground encountered after Amiens was a factor. I'd have to look into the details of the actions in which the V* was involved.
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"Taming the turns" makes some sense TCT. It was possibly rather more serious than disorientation though.
"Quick" turns in an armoured vehicle = crew injuries. In (much) later times the Bren carrier was somewhat renowned in that department. I know of one craftsman who was so injured while taking one of the things on a test run after repair that he had to take medical retirement (it didn't roll or anything, just slung him about he and wrenched a leg very severely). Regular crew would be more prepared for the behaviour, even so it was something of a hazard.
Good point Steve; I remember a few years ago the Daily Mail printed a photo of every British soldier lost in one of the recent conflicts up to that date, with a brief statement of how each one died. What was shocking was how large the proportion killed in vehicle accidents was - a needless waste of lives.
A quick further point on the turning issue: we have been looking at the extra length dragging on the ground as though it is the full difference between 4ft 7in and 11ft that counts; this is not quite so, as these tanks were incapable of rotating upon their own axis - with one track driven forwards, the other in reverse. The closest they could come to that was to stop one track and pivot upon that; it may seem that it doesn't make much difference, but a quick calculation shows this is not so. When pivoting on one track, the pivot point will be at the intersection of the longitudinal centre line of that track and an imaginary lateral line drawn through the centre of gravity (CoG). The radius of the turn as far as track drag is concerned, is the distance from that pivot point to the back (or front) outer corner of the contact patch of the outboard track; agreed? To simplify matters we can consider the distance to the centreline of the outboard track rather than the outer edge, as it is easier to work out the length of that radius than go a foot (give or take) further to the outer edge and have to draw two triangles to scale to work out that distance. Obviously this won't be as accurate, but it still gives the basic idea; viewing the drag radius as the hypotenuse of a triangle between the pivot point, the point at the end of the contact patch and the opposite pivot point (for turns made in the other direction), knowing the distance between track centrelines to be 7ft, and halving the contact lengths of 4ft 7in and 11ft, you can work out that the radius for the simpler measurement is 7.3656ft for the standard length tank and 8.9ft for a V*; despite the large difference in contact length (6ft 5in, making 3ft 2.5in extra either side of the CoG), the position of the pivot point lessens the potential turn radius considerably, so that the increase is halved to about 18.5in. Although the full difference of 3ft 2.5in each end still has to drag over the ground, it doesn't have to move as far as the difference might suggest, making the turn easier.
On a different note, I've not got round to James' query about ground pressure and propensity to bog down; I wonder if the low pressure might have made the tanks skate a bit on wet mud - float over the surface rather than sink in, at least in comparison with their shorter peers. I'm thinking of the way the V** in the video (Landships II article) gets part of the way up a rather muddy slope before losing traction and sliding back a bit, when the less buoyant Mk IV/V might have sunk deeper into the mud and perhaps had enough pressure on it's tracks to improve grip a little? Any ideas?
I think it is long established that the line between the physics that I understand and that (those?) that I do not is a slim one. But I cannot be persuaded that there was any danger of a crewman being catapulted out of a Mk V* during a sharp turn. The V* weighed 4 tons more than the V, with the same engine. Both had a top speed of 4.6mph on level ground. Changing direction could only be a ponderous exercise.
The point of the fishbelly was, as someone says above, that the track would sink until the ground pressure stabilised, bringing us back to square one.
D. Fletcher and J. Glanfield are ambiguous on the matter, both saying that there were "steering difficulties"; Smithers says the V* was "a pig to steer", but is also slightly misinformed about other aspects of the vehicle. However, I find it hard to believe that the V* was over-responsive.
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"Sometimes things that are not true are included in Wikipedia. While at first glance that may appear like a very great problem for Wikipedia, in reality is it not. In fact, it's a good thing." - Wikipedia.
Tend to agree with the sentiment - it's hard to see anything occurring quickly driving the rhomboid tanks. The Carrier exemplified for sensitive handling was a bit of a pig to steer - it had track warping for large radius turns, once the steering wheel was turned more than a small amount the track brake on the inside of the turn was engaged. The brakes weren't progressive so tended to go on max. braking - with a short, light vehicle this could give an "interesting" ride. As with most things, with experience and training the Carriers could be chucked around like a sports car - admittedly a sports car with a boring paint job.
As with most things, with experience and training the Carriers could be chucked around like a sports car - admittedly a sports car with a boring paint job.
And a V8! Even if it was just a flathead.
On the Mk V*, perhaps we'll just have to agree to disagree. It looks to me that there's a good case for thinking the machine was too responsive; it may simply have been a relative matter - train a man to handle the Wilson gears in a Mk V then send him into action in a tank with a lot less ground pressure and friction, and any steering input that gave a particular response in the short tank would be met with a quicker reaction in the long one because it's not scraping so hard against the ground.
After getting used to IVs and maybe briefly Vs, I expect any driver would find the V* like driving on a greasy surface - it's simply got so much less friction that it would need a gentle hand to respond smoothly.
I've found a document held at The National Archives, Kew that shows that ten Mark V* Males and one Mark V* Female arriving at Central Stores in France during September 1918 from the factory had their unditching rails removed before issue to troops. The document lists various "defects" found in the tanks as received and the alterations undertaken to correct them. The list of alterations made includes "unditching gear removed - all tanks". As a result a memo was sent to the Mechanical Warfare Department by DDSD (Tanks) (i.e. Deputy Director Staff Duties Tanks) drawing their attention to the matter. I suspect that there must have been a decision by the Tank Board that unditching gear was no longer required to be fitted to the Mark V* but that the instruction hadn't been acted upon by the manufacturer as quickly as the Army expected. Unfortunately I don't have minutes of the Tank Board here so I can't check without visiting either The National Archives or the Tank Museum, which I won't be anytime soon.
Also, I suspect the reason that all the French Army Mark V*s have unditching rails is that they were all supplied from stocks held in Britain, presumably of tanks built before the (presumed) order to stop fitting the rails at the factory was implemented.
None of this answers the question why, but at least I now have a place to search for an answer.
Gwyn, that's a very useful find - excellent! As you say, it does leave a question over whether or not the Tank Board decided that unditching gear was unnecessary, but one can certainly see that it would make some sense.
Experience throughout 1918 from the start of Operation Michael in March, up to that date, and operational experience of the V* in August, would have shown that battle had changed in nature and whether or not ditching was the problem it had been in 1916 and 1917. I suspect that the semi-open-field warfare was one factor, as the terrain would be less churned up and shell holes fewer and further between. James' query about ground pressure and propensity to ditch is probably also a factor - albeit one unlikely to be credited - as V*s wouldn't bog down so easily if they did find softer mud.
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