William Foster’s Flotilla Leader

United Kingdom (1916)
Tank – Design Only

Prior to World War One, the Lincolnshire-based firm of William Foster and Co. Ltd. had been a manufacturer of primarily agricultural equipment and heavy tractors. It was the frightful slaughter of that war that brought ideas of using modern mechanical traction machines based on wheels, tracks, or both, to the fore. The British had got their first tanks just ahead of the French, but with very different machines. The French had based their vehicles on modified agricultural tractors. The British had started this way as well, but, by Autumn 1915, had moved from repurposed existing tractors to a new type of track system from the pen of Sir William Tritton and Major Walter Wilson. That track system was used on the first British tanks, consisting of large flat steel plates riveted to a steel shoe and running around the outside of the tank, producing one of the most distinctive vehicle shapes in warfare. Those early quasi-rhomboidal tanks were not the only tracked designs from William Foster and Co. Ltd., which continued to experiment with ideas about tracked vehicle layouts, protection, firepower, and moving back to concepts of armored personnel carriers. The weaknesses of these first designs were ones of mobility and protection. Their armor, whilst able to protect against enemy rifle fire, left the tanks vulnerable to enemy artillery. Even before they had first seen combat, there was a desire to try and move to a level of being ‘shell proof’. In April 1916, this heavily armored and armed tank was drafted as the ‘Flotilla Leader’.

Conceptualization

The design of the Flotilla Leader has to be considered in the context of other designs from the pen of William Foster and Co. Ltd. and specifically of two key men. The first was the Consulting Engineer and later the Managing Director of the firm, Sir William Tritton, and the second was the lead draughtsman, William Rigby.

Sir William Ashbee Tritton (left) and Major Walter Wilson (right).
Source: William Foster Archives

Both of these men had been engaged in work for the Landships Committee from its origins in February 1915. The Committee was the coordinating body set up to bring together experts and ideas to try and resolve substantial military problems hindering progress on the Western Front by the Army through the use of an armored vehicle. The primary problem to be overcome was protection – specifically for men.

Unprotected infantry assaults across open ground, especially broken ground covered by enemy machine gun fire and strewn with barbed wire belts, led to enormous casualties and little progress. Even if the infantry could get across the ground and storm the leading enemy trench, they would be so worn down by casualties and exhaustion that a counterattack could displace them. They could not be resupplied easily, as these too would have to cross the open ground between the opposing sides and any success could be quickly lost. If, however, a party of men could be brought across this ground, protected from enemy fire and then dropped off at the enemy trench, they would arrive in full force and fresh. Subsequent vehicles could transport men and supplies to sustain the advance and press it forward to new trench lines. The carrying vehicle would need to carry at least enough armor to protect the contents (the storming party) from enemy machine gun and rifle fire and would need two other substantial features. The first of these was a means of carriage, for which there were only two options, wheels or tracks.

The second was firepower – the offensive or defensive armament the vehicles carried. Ideas about armaments varied greatly in the early concepts of armored vehicles, ranging from machine guns to large caliber guns to flamethrowers or even acid sprayers, but the role was to be the same. The firepower was there to support the infantry attack by suppressing or knocking out enemy machine gun positions and then to provide enfilading fire down the enemy line. With no enemy vehicles to worry about, the firepower would not be concerned with penetrating armor other than that of a gun shield of a field gun or machine gun, smashing a concrete bunker, or spraying shrapnel at an enemy. The long-barrelled (40 caliber) Naval 6 pounder (57 mm Q.F. Hotchkiss) gun had been an ideal choice for this role, providing shell options from solid steel shot to a high explosive shell and a shrapnel round.

That first British tank, the Mk.I, came in two forms: a ‘Male’ version armed with a pair of 6 pounder guns mounted in sponsons along with machine guns, and a ‘Female’ version armed with just machine guns. Major Walter Wilson and Sir William Tritton had been the brains behind that design and its unique shape back in October 1915. At 28 tons (28.4 tonnes) for the ‘Male’ version, this 8 man vehicle could achieve just 3.7 mph (6.0 km/h) on good ground courtesy of the 105 hp 6 cylinder Daimler sleeve-valve petrol engine. It could, however, cross a 10’ (3.0 m) wide trench, climb a vertical step (or parapet) 4’ 6” (1.4 m) high and could bring substantial firepower to a fight. Protected by armor up to 12 mm thick, this was the first true tank of the modern era and, despite its many problems, proved to be a success when first deployed in battle in September 1916.

Even before the combat debut of the Mk.I, design work, and development had been continuing. Perhaps concerned over the position of the armament being focussed on the sides and not the front, Wilson and Tritton continued to consider other options. If, for example, the track on the front of the tank could still be kept high, it would assist greatly with climbing and crossing obstacles. Moving the armament forward would then mean the best features of the Mk.I could be kept whilst reducing the vulnerability of the design, notably the tracks over the roof of the tank. With a ‘depressed’ type of track run, where the track returning from the front was pushed down low, it would allow substantial freedom over the design of the upper structure of the tank. If they went one step further, and made the tank longer, then not only could it cross even greater gaps than the Mk.I, but it could do something the Mk.I could not, and which had been the original goal all along – it could carry troops. Such a large tank would need heavier armor on the front.

The Germans had a good number of 77 mm field guns in service that were a serious concern. Testing had shown that between 1 and 2 inches (25 to 50 mm) of armor would be needed to protect against these guns and this would substantially increase the weight beyond that of the Mk.I. Thus, the vehicle would be larger, better armed, better armored, substantially heavier, and require more tractive power than that provided by the 105 hp engine of the Mk.I. Not just that, but more weight meant it would sink more into soft ground, so the emphasis in the design was on a long track length, actually projecting past the body of the vehicle. The resulting design bore a resemblance to the Mk.I, particularly in the sponsons and track plates but was, to all other extents, a completely different breed of tank.

Design work began with a sketch in April 1916 labeled incongruously as the ‘Suggested Flotilla Leader’. Flotilla is a naval term for a fleet of ships and this, perhaps more than anything else, made clear some of the thinking about the potential use for the vehicle – literally leading a fleet of other vehicles – likely ‘normal’ tanks, in an attack. Quite how that tactical deployment was to take place is not clear and likely was never fully considered. A simple explanation may be that this vehicle would lead an attack and take the brunt of enemy fire, deliver its cargo and then have the supporting tanks (Male and Female) follow it to exploit the breakthrough. If that is correct, then this vehicle would form a type of heavy assault APC.

The Flotilla Leader

The Flotilla Leader design embodied these ideas and was the first evolutionary step from the Mk.I with the armament moved forwards. An angular casemate projected above and ahead of a lower-body and featured a pair of large sponsons projecting wider than the hull at the front. A terrifying array of machine guns would project from the front, with positions for no less than 5 machine guns across the front of the tank arranged in a curve to cover up to 180 degrees of fire. A further machine gun was added into the outer end of each sponson, behind the 6 pdr. main gun.

Layout

The dominating features for the front of the tank, if it was driving directly towards an observer, would be the large sponsons projecting from either side of the casemate and completely concealing the rest of the vehicle’s body behind them. Between these two sponsons was what would have appeared to be a short vertical track running from just below the level of the main guns in the sponsons to just above ground level between the two main tracks. This was, in fact, a tertiary supporting track located at the front and would only be in contact with the ground when crossing or climbing an obstacle.

The fighting chamber was contained at the front of the tank, with the primary armament and machine guns on the front of the hull. Behind this was a long and lower hull with vertical sides and an angled rear end. The hull space was clearly allotted for troops along each side and, as suggested by the plans for the ‘Battletank’ which followed, would have had covered loopholes from which troops within could fire their rifles and doors to get in and out of the tank. These doors would lead to a 2’ (0.6 m) wide gangway running the full length of the hull from the back of the casemate to the rear and projecting out over the sides of the tracks. Troops would thus be able to get out of the tank to assault the enemy by exiting through those side doors and jumping down. Presumably, some ladders or steps would be added on the sides at the rear to assist with entry prior to use. The step was 3’ 6” (1.1 m) above ground level and, in soft ground, a soldier would have difficulty climbing up that high unassisted. At the back of the hull, above the angled rear end, was a short step where the roofline dropped down vertically to the rear end. In this space, 6 more fighting positions were shown.

The powerplant for the tank would be located just aft of center. It was positioned centrally in the width of the hull, with the transmission and gearing going backward from it towards the rear end. No separation between the engine and the troops is shown, suggesting an unpleasant driving experience for the men sat next to a noisy, smelly, and hot set of engines. At around the level of the rear step lay the final gearing, taking the power to the sprockets on each side. These drove a chain which, in turn, drove the drive sprocket for each track in the same manner as that used on the Mk.I. At the rear of the tank would be a pair of trailing wheels.

The dimensions for the Flotilla leader, as drawn, would be a hull width of 9’ (2.7 m) which, including the 2’ (0.6 m) sidestep over the skirting armor, widened to 13’ (4.0 m). The hull height was 7’ (2.1 m), which included a ground clearance of 18” (0.45 m). The casemate was slightly higher than the rest of the hull, reaching a height of just under 8’ 6” (2.6 m). It was also slightly wider as it angled outwards from the front, reaching a maximum width of 14’ (4.3 m). From the front of the leading track section to the rear of the track of the vehicle, the length was to be 32’ (9.8 m) and, including the trailing wheels, a total of 39’ 6” (12 m).

This was not the end of the track system either. A faint outline of a second set of driven tracks can be seen at the back of the design, acting as a supplement to the original tracks.

Suspension

No speed was specified or recorded for the Flotilla Leader on the April 1916 sketch. The only note of assistance was the weight – some 45 tons (45.7 tonnes). This weight would be borne by a pair of track running along a very low and flat track run from an idler at the front, under the sponson main gun position, to the rear. The track measured 26’ (7.9 m) from front to rear along the length, of which only 9’ (2.7 m) would be in contact with the ground when on a hard surface. If the vehicle began to sink into soft ground, the amount of track in contact with the ground would increase proportionally to spread the load. Each of these primary tracks was to be 2’ (0.6 m) wide. The tertiary track measured about 5’ 6” (1.7 m) long at the front, on the nose of the machine, and was to be 50% wider, at 3’ (0.9 m).

This was not the end of the track system either. A faint outline of a second set of driven tracks can be seen at the back of the design, acting as a supplement to the original tracks. The lines on the original sketch are faint but discernible.

Difficult to see on the surviving sketch is the outline of a second set of driven tracks as supplemental units at the rear of the design. Original (left) and false-colour highlight (right). Source: Stern Archive.

If there were lingering doubts over the double rear supplemental tracks, a close examination of the front aspect of the vehicle shows this hazy shading is visible on the inside of each of the primary tracks.

Seen either side of the tertiary track on the nose of the tank and laying between it and the primary tracks, the supplemental tracks at the back can be seen. Original (left) and false-color overlay of all three sets of track with primary tracks blue, tertiary track yellow, and the pair of powered supplemental tracks red (right). Source: Stern Archive

No system of springing for shock absorption was provided – none of Tritton’s designs featured springing suspension but cushioning from vibration was a function of the rollers in Timken bearing running on the track. A relatively slow speed, perhaps not more than matching the Mk.I’s at 3.7 mph (6.0 km/h), did not necessitate the complexities of adding springs into a system.

Extra Tracks

Those supplemental tracks would become a feature of the Flotilla Leader and subsequent heavy vehicles and are described by Tritton explicitly as an “auxiliary driving system”. In other words, these tracks were not merely for assisting in obstacle crossing, like the tertiary track on the nose, but were powered. In November 1916, William Tritton (he was knighted in 1917 for his work on the development of the tank) submitted a patent application for “Improvements in and relating to Transport Vehicles Propelled by an Endless Moving Chain Track or Tracks”. Euphemistically written to avoid mention of ‘tanks’, the explanatory description and drawings within the patent make it clear as to why these tracks were designed and what they were meant to do.

The tracks themselves were essentially just smaller and shorter versions of the primary tracks fitted onto a centrally pivoting point and driven by chains from the primary gearbox. These tracks measured about 7’ 6” (2.3 m) long. The driving chains for these tracks, would, in fact, be the same ones driving the primary sprockets for the tracks, sharing a common axle as the point around which these two auxiliary tracks could rotate.

All four tracks (2 long primary and 2 short auxiliary tracks) would usually be driven at the same time. Tritton, in his patent, describes a potential future modification to drive them together or separately. However, as drawn, they would run at the same time and speed as the primary tracks. On a good surface, like a road or hard ground, it might be supposed that these tracks, not being in contact with the ground, would simply rotate in thin air, but this is not correct. Being pivoted around their central axle and driven by a sprocket attached to and projecting past the center point of the axle would induce a rotational moment on the whole unit, causing them to tip forwards and down until they reached the ground. The opposite would be true in reverse, where the rear end of the track would pivot down until it reached the ground. Thus, even on a good surface, at least one end of this auxiliary unit would be in contact with the surface, providing additional traction.

This dynamic of how the unit would move when subjected to a driving force was particularly valuable when considering the vehicle crossing very rough ground. In the case of climbing a bank, the long primary unit would, as it reached the crest, progressively lose ground contact and thus traction. Not only that, but, with a reduced bearing surface for the vehicle’s weight on the ground, the vehicle would be more and more liable to sink or slide backward as it climbed a slope, severely handicapping its mobility. The same would be true of crossing a ditch or wide trench, as large portions of the primary track would lose contact with the ground because it was inherently fixed to the inflexible body. Both of these situations would be resolved in theory by this auxiliary drive unit which, courtesy of its ability to move independently of the vehicle’s body and always being forced down in contact with the ground, ensured that it would act as both a foot to prevent the vehicle sliding backward and also that traction could be delivered at all angles, improving the obstacle crossing ability of the machine.

The final element of note on the Flotilla Leader’s automotive elements was the pair of trailing wheels at the rear. Made from simple steel wheels with no rubber or wooden tire, the wheels were fixed to the rear of the tank and fitted with springs to push them down onto the ground. The purpose of these wheels is usually described as steering but they actually served two purposes; the first was to assist in steering, as the wheels could move left or right by means of a chain pulling on them from the tank’s steering system. While this was not an important factor when going off-road, when on-road, this offered the vehicle an alternative and far more energy-efficient way of doing light turns, as it did not require breaking any of the tracks.

In practice, these wheels were of little use on a tank for steering, but they were also there for a second often overlooked reason; they helped with crossing obstacles. As a vehicle crosses a wide gap and reaches the other side, there is a moment when the tail of the vehicle leaves one side and becomes unsupported over the gap. Due to the weight of the machine, the rear end will have a tendency to sink into the gap which, if the gap is too wide or the opposite face too soft, will stop the machine from crossing the gap. Placing an extension over the back end of the vehicle to maintain contact with the ground would reduce this sinking tendency and assist with crossing the gap. Whilst the wheels ultimately proved unnecessary for this purpose, the descendant of this idea stayed around in the form of an unditching or ‘trench tail’ on tanks in British service even into 1940, on the A.12 Matilda.

Engine

The Mk.I was to use a simple 105 hp Daimler sleeve-valve petrol engine to provide the motive force for the 28-ton tank, delivering an anemic 3.75 hp/ton. With the Flotilla Leader coming in at nearly twice the weight (45 tons) of a Mark I, a single 105 hp would be insufficient, as it would deliver just 2.3 hp/ton. The solution was simple – if the weight doubles, then double the power and, if the engine only produces 105 hp, you simply add a second engine. Thus, the design would be fitted with a pair of those 105 hp engines for a total of 210 hp. At 45 tons, this meant a power to weight ratio of 4.7 hp/ton. With a greater length of track on the ground per unit length than the Mk.I and with this higher power to weight ratio, it is a reasonable assumption that it would be at least as mobile in speed terms as the Mk.I, if not a little faster, and certainly more capable of crossing obstacles.

With two engines located side by side just aft of the center of gravity of the tank, there is no indication of where the petrol tank would be, but the exhaust would almost certainly have been vented upwards out of the roof.

Each engine is shown with a separate drive to the primary gearing at the back of the tank and, as with the later Medium Mark A Whippet and ‘Flying Elephant’ designs from the firm, steering would be affected by varying the throttle on each engine, which altered the power to the track on that side. Thus, if the driver wished to slew the tank to the left, all he would have to do was to reduce the throttle to the left engine, meaning that the right side would drive faster than the left, and turn the vehicle in the desired direction. With that simple and effective method in place, it would show why the trailing wheels at the back would be of little utility for steering and were an unnecessary feature.

Armor

The Mk.I was meant to protect against German bullets, which meant armor up to ½ inch (12 mm) thick. However, as was proven by the German 13.2 mm Tankgewehr M.1918, which entered development soon after tanks were first used in September 1916, that level of protection was insufficient. Indeed, concentrated machine gun fire, and the use of steel-cored bullets could prove a substantial threat in their own right, not to mention the use of field guns in both direct and indirect fire roles. Many British tanks would also fall prey to the German infantry when they became stranded and then assailed by troops. Ensuring that all-around protection could be maintained would also be a consideration, as well as a safe means of discharging the troops carried inside while covered from direct enemy fire.

Armor for the design was outlined on the April 1916 sketch as focussing heavy protection 2” (50 mm) thick across the entire front of the casemate and lower front hull. Thus, all of the armor facing the enemy was 2” (50 mm) thick. The rest of the hull, including the back of the casemate, hull slides, rear of the hull, track, and plating suspended over the tracks was ½” (12.7 mm) thick. The armor hanging over the sides of the tracks and on the hull sides, however, consisted of two thicknesses of this plating, meaning not less than 25 mm of protection if taken as a uniform plate in two parts or a little more if there was to be a small air gap between them. The weakest part of the armor on this design, at the back of the machine, would therefore be as good as the thickest part of the Mk.I design.

The nose of the tank was to be made from 5 separate flat plates connected at an angle to form a semi-circular section between the two sponsons.

Armored with 2” (50 mm) heavy protection on the front and this double skin system over the whole sides, including the tracks, would render the vehicle effectively immune to even concentrated machine gun fire. It is possible that this double skin armor was done in order to protect against shell fire.

Armament

The Flotilla Leader was meant to carry a fearsome amount of firepower. The primary firepower came in the form of what appears to have been a short-barrelled 6 pounder gun in each of the side sponsons. This was an interesting departure from the long-barreled 6 pounders in the Mk.I, which were later found to be problematic and switched for a shorter barrel. On the Flotilla Leader, with the barrels 6’ 6” to 7’ (2.0 to 2.1 m) off the ground at the front, there would be little chance of the long barrel fouling on the ground, as happened occasionally on the Mk.I, so the decision to draw them with shorter barrels was clearly not for that reason. They are clearly labeled as 6 pounder guns, so a shorter barrel may simply have been a means to reduce the weight, as the longer barrel was not needed anyway.

For machine guns, the Flotilla Leader was provided with no less than 13 separate machine guns or machine gun mounting points. Three were located in each of the rear corners of the main hull, one each in the back end of each sponson on each side, and the remaining 5 arrayed across the front. Positioned in this way, the tank ensured complete coverage across the front 180 degrees of fire for the machine guns. The rear-firing positions provided the same across the back and, although there is no dedicated machine gun position in the side, the overlapping arcs of fire intersect about 10’ (3.0 m) from the side, providing excellent coverage. Protection across the sides would be supplemented by the troops inside who, using loopholes, would be able to fire rifles or other weapons at any enemy forces.

The Mk.I needed 8 men to crew it and this was a substantially larger machine. Each of the 6 pounder guns would require at least one man to operate it and, if a loader was used, then that would mean at least 4 men to service both guns with two men moving the shells and the other firing. Although there were 13 machine gun positions, it is unlikely to have had 13 guns and operators at the same time. Three or four men would be able to operate the machine guns in the front and, although each rear corner had another 3 positions, likely one man and a portable machine gun would have been able to cover all of those aspects. With a commander located in the casemate to drive, it is likely there would be no room for a driver up there, who would probably have to have sat behind the casemate in front of the engines and rely upon the commander for information on which way to turn. That would be an estimated crew complement of 11 men at least, although those rear machine guns may well have been intended for operation by the troops carried on board.

Space inside the main hull behind the casemate was limited to the space between the outer hull walls and the powerplant in the center. Measuring 9’ (2.7 m) wide by 21’ 6” (6.6 m) long, this was 193.5 sq. ft. (17.8 m2) of space, although the engine and gearing occupied at least 150 sq. ft. (13. 9 m2) down the center, allowing just 43.5 sq. ft. (4.0 m2) of space in which to carry soldiers.

Back in February 1915, one of the earliest ideas for solving the trench problem had come from Colonel Crompton. He had proposed an armored wheeled machine to carry 48 men as a trench raiding party. Those men were to be carried in a platform measuring some 250 sq. ft. (23.23 m2), meaning an estimated space per man of 5.25 sq. ft. (0.49 m2). Assuming roughly the same allowance here, would mean that the 43.5 sq. ft. (4.0 m2) of space inside the Flotilla Leader would be able to carry a party of around 8 to 10 men at most – well below the large storming party size wanted in Spring 1915.

Conclusion

The Flotilla Leader did not get built. The extra track on the front was certainly an idea considered by other designs, perhaps most notably for the French St. Chamond, but it was never a practical solution. Vulnerable to fire, heavy, and perhaps not even necessary when the solution of something like the French Schneider CA was not much more complex than a raised steel section fastened to the front of the tank to prevent it digging in.

The Flotilla Leader was not much more than a concept and, as it was refined and reconsidered, it concluded in July with a design better in almost every way but following a very similar form. That vehicle, known as the Battletank, embodied a series of improvements but would not enter production either.

The features of both which did, however, see service in WW1 with the British were perhaps the two most understated ones. These were the low-slung track concept that was used on the Medium Mark A Whippet and on the Gun Carrier Mk.I and II, and the twin-engine system also used on the Whippet.

Both of those vehicles showed, perhaps more than anything else, that what was needed was not increasingly larger and heavier tanks, but a variety of tracked vehicles to support and exploit attacks and move supplies and guns. The British learned this lesson quickly in these first early years of tank warfare. By 1919, the idea of a Flotilla Leader type tank must have seemed a whole generation away from the realities of war.

Sources

British Patent GB126070 Improvements in and relating to Transport Vehicles Propelled by an Endless Moving Chain Track or Tracks, filed 28th November 1916, granted 8th May 1919
Mechanical Warfare Department. (1925). ‘Tanks and Accessory Vehicles used in Great War’.
Hills, A. (2019). Col. R. E. B. Crompton. FWD Publishing, USA

Comparison of Data from Flotilla Leader to Battletank April 1916 – July 1916 (Imperial / Metric)
		Flotilla Leader	Foster’s Battletank
Date		April 1916	July 1916
Crew		11 crew + 8/10 men	9 crew + 12/15 men
Armament		2 short-barrelled 6 pdr Up to 13 machine guns Small arms	2 long barrelled 6 pdr Up to 5 machine guns Small arms
Engine		2 x 105 hp Daimler sleeve valve petrol
Tracks	Primary	2’ (0.6 m) wide / 26’ (7.9 m) long
	Secondary	2’ (0.6 m wide) / 7’ 6” (2.3 m) long
	Tertiary	3’ (0.9 m) wide / 5’ 6” (1.7 m) long	None
Length	Hull	32’ 6” (9.8 m)
	Overall	39’ 6” (12 m)	43’ (13.1 m)
Width	Hull Body	9’ (2.7 m)
	Over Tracks	13’ (4.0 m)
	Maximum	14’ (4.3 m) at casemate	13’ (4.0 m)
Height	Hull Body	7’ (2.1 m)	est. 7’ (2.1 m)
	Overall	8’ 6” (2.6 m)	9’ (2.7 m)
Armor	Casemate Front	2” (50 mm)
	Casemate Sides	2” (50 mm)
	Hull Sides	Double skin ½” + ½” (12.7 + 12. 7 mm)
	Roof	½” (12.7 mm)
	Over Tracks	Double skin ½” + ½” (12.7 + 12. 7 mm)
	Over Front	–	2” + 2” (50 mm + 50 mm)
For information about abbreviations check the Lexical Index