HOWITZER ANCHOR SPADE

HOWITZER ANCHOR SPADE

The present invention relates to towed howitzers, which are sometimes referred to as field or artillery guns. The invention provides a means for increasing the stability of towed

howitzers and reducing the extent of rearward movement of such howitzers under the

recoil forces when the howitzer is fired. Howitzers generally have a recoiling ordnance

(barrel and breech mechanism) mounted on a trail assembly and having a firing platform

and/or one or more rearwardly extending trail legs, and it is known to fit anchor spades to

the rearward ends of such trail legs which engage or dig into the ground under the recoil forces when the gun is fired. Such anchor spades may also be fitted to firing platforms.

The present invention is intended to provide an improved design of anchor spade

particularly for lightweight howitzers that have a recoil management system.

Many modern conventional towed howitzers generally have an ordnance with a barrel of

105mm or 155mm internal diameter, and the substantial weight of the trail assemblies in

these howitzers helps stability. For example the typical weight of a conventional 155mm

howitzer might be 18,000 pounds with a recoiling mass of 7,000 pounds, and excessive

movement of the trail assembly is usually avoided because of the significant energy

required to overcome the inertia of the mass of the howitzer which is high in relation to the

firing impulse. Also the suitable design and management of the recoil braking system will id stability. However modern armies now require howitzers that are significantly lighter than

conventional howitzers, but without loss of performance. Lightweight howitzers can be

deployed more rapidly, towed by lighter motorised vehicles and can be more readily transported by military aircraft or slung under military helicopters. These are all important

benefits for rapid deployment forces, which play an increasingly important role in modern

armed conflicts.

The requirement for lightweight howitzers, for example for 155mm howitzers weighing

less than 9000 lbs but with the same ordnance and firepower performance, creates

considerable design challenges to maintain stability of the howitzer in recoil. Much of the weight saving is to be made in the trail assembly and with a trail assembly weighing only 4,900 lbs and a recoil mass of 4,100 lbs, much less energy is required to overcome the

inertia of the lightweight howitzer. Such a lightweight howitzer, if of conventional design

would not be sufficiently stable during firing and there would be a much greater tendency

for the howitzer to move rearwardly. Also the movement of the recoil forces pivoting

about the point where the spade engages the ground is not, in the case of lightweight

howitzers, balanced by the counter-balancing moment of the weight of the howitzer acting

through the centre of gravity of the howitzer. Accordingly the howitzer will tend to jump

or bounce. An improved anchor spade design that will readily dig in, stay dug in, and

which does not cause excessive rearward movement, will help to reduce this jumping and

bouncing. This instability is unacceptable if excessive. For example, excessive rearward movement

of the firing platform and/or the trail assembly and howitzer may endanger the safety of

the gun crew, some of whom during firing stand to the rear of the howitzer, between the trail legs if present. Also excessive movement, particularly if the howitzer is fired at a high

traverse angle and low elevation angle, can cause the howitzer to rotate about one or other

of the trail legs if present. This can make it difficult and time consuming to re-emplace the howitzer for firing the next round on target. For operational reasons sustained accurate

firing is essential.

PCT application WO89/06778 describes a means of managing the recoil of a lightweight howitzer by mounting the recoiling mass, which includes the ordnance, on a curvilinear recoil track to produce an increased stabilising moment.

The present invention has as its object the provision of a lightweight anchor spade fitted to

the rear end of each of the trail legs or to the firing platform, which for most soil types

will, under the force of recoil, readily dig into the ground and not dig itself out under

further recoil force. It is also an object of the present invention to reduce rearward

movement of the howitzer during recoil, and to provide a howitzer that can be readily

pulled out of the ground by the gun crew after firing.

According to the present invention there is provided an anchor spade for fitting to the rear end of a trail leg of a howitzer or to the firing platform for engaging and digging into the

ground under the rearward recoil forces of firing, which anchor spade comprises a rearwardly facing blade plate having a tip digging into the ground under the forces of

recoil and a rearwardly facing top plate mounted above the blade plate so as to be

generally parallel with the level of the ground when the anchor spade is embedded in the

ground, wherein the spade blade angle (as defined below) lies in the range 35° to 55° and

the spade forward angle (as defined below) lies in the range of 80° to 100°.

The blade plate is preferably planar.

For the purposes of this patent application the "spade blade angle" means the acute angle of the blade plate (or of that part of the blade plate adjacent the tip) to the general level of

the ground, and the "spade forward angle" means the angle between the rearward direction

of the general level of the ground and the line between the tip of the blade plate and the

rearward extremity of the top plate.

It has been shown that the anchoring effect of the anchor spade is a function of the vertical

penetration of the tip of the blade plate below the ground surface (referred to herein as

"the effective depth"). Until the anchor fails, for example by digging itself out under

further recoil forces, the greater the effective depth the greater will be the anchoring

effect, and in general, further rearward movement of the anchor will be reduced for greater

effective depths. Experiments with some anchors (particularly those in use with

lightweight towed howitzers having a pair of trail legs) have shown that for a given area of

spade blade, the effective depth has a greater anchoring effect than the width of the spade

blade. It has been found by theoretical analysis and confirmed empirically that the optimum spade blade angle for a range of soil types is about 45°. At angles significantly above 45°

the effective depth is increased but there is a tendency, when the anchor spade is first put

in place and the howitzer fired, for the tip of the blade plate either to scrape along the top

of the ground or to plough up the top soil in the ground and thereby produce unacceptable rearward movement of the howitzer. If the anchor spade is already fully embedded there is

a possibility that the greater the spade blade angle the greater the possibility of the anchor

digging itself out of the ground. Also at higher spade blade angles there is a tendency for

the anchor plate to rotate about the tip of the blade instead of digging into the ground. This

can put excessive loads on the blade plate and cause failure.

With spade blade angles of less than 45° the effective depth is reduced which means that a longer and hence heavier spade plate is required for the same anchoring effect. Also, when

the anchor is first put in place and fired, a lower spade blade angle is likely to cause

greater rearward movement of the howitzer to achieve the same effective depth and

anchoring effect.

The top plate acts to limit the extent to which the anchor spade will dig into the soil during

recoil so that the anchor spade can be readily removed by the gun crew during

displacement of the howitzer. Some existing anchor spades for 155mm howitzers have a long top plate to ensure rapid displacement, which produce a spade forward angle of about 135°. Experiment has shown that for some howitzers the magnitude of the spade forward angle

is more significant to the mode of failure of the soil than is the spade blade angle. In some

soil conditions with a spade forward angle significantly above 90°, the anchor spade will

readily dig into the ground during recoil but when the top plate rests on the ground surface

further recoil forces (for example resulting from subsequent firings) can result in the

anchor spade digging itself out of the ground. Not only will this produce unacceptable additional rearward movement but will also decrease the effective depth and hence

accentuate the failure which can become catastrophic. Analysis of the means of failure in this case (i.e. when the anchor spade digs itself out of the ground) shows that a shear

failure plane is set up between the compacted soil in the anchor plate (i.e. between the top plate and blade plate) and the looser soil to the rear. This shear failure plane is

approximately along the line between the rearward end of the top plate and the tip of the

blade plate, hence the importance and significance of the spade forward angle. For some

soil characteristics the spade forward angle may be critical. Ideally it should be the maximum possible to ensure ready removal of the anchor spade but less than the critical

value producing shear failure.

Experiment has demonstrated that for typical soil condition the optimum spade forward

angle is about 90°. For lightweight towed howitzers a spade forward angle in the range

90°-110°, or preferably 90°-100°, will operate satisfactorily if the soil is the character of

clay and digging-out of the howitzer may be difficult. On the other hand in loose soil conditions, where the anchor spade can be more easily

removed by the gun crew, a spade forward angle in the range 70°-90°, or preferably

80°-90°, will operate satisfactorily. With a spade forward angle of less than 90° the shear

plane acts to increase penetration of the blade plate.

Initial spade dig-in is improved by increasing the pressure between the tip of the spade

blade and the ground. This can be achieved by sharpening the blade tip and/or by reducing

the width or line of action of the edge of the blade plate that engages the ground. The width or line of action of the blade plate can be reduced by removing an area of the tip of the blade plate, either centrally along the tip edge or symmetrically either side at the tip of the spade blade angle, or by otherwise shaping the tip of the blade plate.

Many modern towed howitzers have at least two rearwardly extending trail legs and an anchor spade according to the present invention may be fixed to the rear end of such trail

legs by any convenient means. A single anchor spade according to the present invention

may be fitted to howitzers that have a single trail leg assembly, for example having a pair of bowed trail legs joined at the rear. Some howitzers dispense with trail leg assemblies

and may instead rely on the firing platform which engages the ground during firing to

transmit recoil forces to the ground. One or more anchor spades according to the present

invention may be fitted to the firing platform.

The blade plate preferably extends at the blade plate angle to a height close to the plane of

the top plate, so as to provide a maximum length of blade plate at the design spade blade angle. In this arrangement the blade plate extends at the design spade blade angle for

virtually all the effective depth.

However in another arrangement, the blade plate extends at the design spade angle for between one half and the whole of the effective depth, in which case the blade plate may

be fixed to or positioned in close proximity to the top plate by means of an approximately vertical plate, which may comprise an integral part of the blade plate or top plate.

An arcuate plate may be substituted for the vertical plate, having a radius varying from as

much as a half to a third of the blade plate length to a small fraction thereof.

It has been found that for a lightweight howitzer having a pair of trail legs and a recoil

management system of the kind described in PCT Application No WO89/06778, and

which weighs approximately 9000 lbs with a recoiling mass of 4, 100 lbs, satisfactory

anchor spade performance can be achieved for all magnitudes of charge and most soil

conditions with a spade width of approximately 28 inches, and a maximum effective

depth of 25 inches, and with a spade blade angle of approximately 45° extending for most

of the effective depth. Optimum spade forward angle is 90°.

Normally a howitzer will be more stable when fired directly ahead (i.e zero traverse) and

if the trail legs are provided, these are often splayed out symmetrically to the rear of the

howitzer. The recoil forces in each trail leg would in this case be approximately equal. However when fired in maximum traverse, unequal recoil forces are transmitted through the trail legs to the ground, and one of the trail legs will be subject to greater recoil forces

than for similar firings at zero traverse. There is therefore a tendency for each leg to move

backwards to a different extent, and for the howitzer to tend to rotate about one of the trail

legs. It has been found that fitting one or more ribs, known a grousers, on the rearward

facing surface of the blade plate extending into the volume between the top plate and blade plate will reduce sideways movement of soil across the blade plate. The grousers are preferably vertical, and act in a similar manner to groynes on beaches to help prevent

lateral movement of sand across a beach. Also the fitting of stiffeners on the forward or

outside face of the anchor spade helps reduce sideways movement of soil across the blade plate as well as strengthening the anchor spade. The use of grousers has been shown to

reduce the rearward movement of the anchor spade and trail legs at most traverse angles

including zero traverse. At maximum traverse the grousers also reduce the tendency of the

howitzer to rotate about one of the trail legs.

The blade plate and top plate (and vertical plate if present) may be formed from one single

metal plate or from separate plates. A single metal plate may be bolted or welded to the end of a trail leg or to the rear of a firing platform. In one arrangement two anchor spades

according to the present invention are welded or bolted to the rear of a firing platform, and are disposed symmetrically either side of the rearward direction.

In another arrangement of the anchor plate suitable for fixing to the rear of a howitzer trail

leg, the top plate and blade plate are separately and independently fixed to the trail leg.

The blade plate is pivotal ly connected about a point at or near to the forward end of the blade plate, and fixing and locking means are provided for fixing and locking the blade

plate relative to the top plate and trail leg, the top plate and blade plate are separately and

independently fixed to the trail leg. The blade plate is pivotally connected about a point at

or rear to the forward end of the blade plate, and fixing and locking means are provided

for fixing and locking the blade plate relative to the top plate and trail leg, either in a lower

operational (i.e. firing) configuration or in an upper stowage configuration with the blade

plate directly under, substantially parallel to and adjacent the top plate. The fixing or locking means may comprise a removable retention pin arranged to fit a hole in the trail

legs which can be aligned with cither a first hole in the blade plate for operational (i.e.

firing) configuration or a second hole in the blade plate for the stowage configuration.

In this arrangement, and when displacing the anchor spade after firing, the retention pin

may be removed from the first hole to allow the blade plate to pivot to facilitate removal

of the blade plate from the ground. The pivot pin may also be removable so that if the anchor blade cannot be readily removed from the ground it can be unfixed from the trail

legs by removing the pivot pin.

A rearwardly extending towing arm. known as a lunette, may be removably fixed to the top plate of the anchor spade. This allows the howitzer to be towed in the rearward

direction, and when the blade plate is in its stowage configuration adequate ground

clearance is provided. In order to more fully understand the present invention the following embodiments are

described by way of example with reference to the drawings in which:

Figure 1 is a schematic drawing of a 155mm lightweight towed howitzer having two linear

trail legs symmetrically positioned, in operational configuration, one either side of the

rearward direction, and to which anchor spades according to the present invention may be

fitted.

Figure 2 is a simplified schematic cross-section drawings of an anchor spade in its operational configuration for fitting to the rear of a pair of trail legs of a lightweight towed howitzer. Figure 2 illustrates the geometrical characteristics of an anchor spade according

to the present invention but excludes much of the detail such as the means for stowing the

anchor spade and the stiffeners and grousers.

Figure 3 is an isometric projection of one embodiment of an anchor spade according to the

present invention suitable for fixing one each to the rear of the trail legs of a 155mm

lightweight howitzer having two trail legs, and which shows the rearward face of the

anchor spade.

Figure 4 is another isometric projection of the embodiment of Figure 3 showing the forward face of the anchor spade. Figure 5 is an isometric view of an anchor spade similar to the embodiment of Figures 3 &

4 for fixing to the rear of the trail legs of a 155mm lightweight towed howitzer, but

showing detail of the fixing of the anchor spade and lunette to a trail leg.

Figure 6 is an isometric projection of a different embodiment of anchor spade according to

the present invention for fixing one each to the rear of the trail legs of a 155mm lightweight towed howitzer, showing the rear face of the anchor spade.

Figure 7 is another isometric projection of the anchor spade shown in Figure 6.

Figure 8 is a side elevation of the anchor plate shown in Figures 6 & 7.

Figure 9 is a drawing of 105mm light howitzer having a pair of bowed trail legs that are

joined together at the rear of the trail legs, and to which a single anchor spade according to the present invention may be fixed.

Figure 10 is an isometric projection of an anchor spade according to the present invention

which is suitable for fixing to the joined trail legs of the light howitzer shown in Figure 9.

Figure 11 is a top view of the anchor spade shown in Figure 10.

Figure 12 is a side view of the anchor spade shown in Figure 10. Figure 13 is a bottom view of the anchor spade shown in Figure 10.

Figure 14 is a schematic isometric projection of the firing platform of a lightweight

howitzer, which may not have trail legs, showing a pair of anchor spades according to the

present invention.

Figure 15 is a side view of the firing platform and anchor spades shown in Figure 14.

Figures 16, 17 and 18 show three different isometric projections of an anchor spade

according to the present invention fixed to the trail leg of a different 155mm lightweight

towed howitzer.

For the purposes of the description of the embodiments of the invention, the direction in

which a howitzer is fired when at zero traverse is referred to as the 'forward ' direction. Recoil is therefore in the 'backward' or 'rearward' direction and the blade plate and top plate face rearwardly. In practice, with a howitzer having a pair of trail legs, the legs are

positioned rearwardly symmetrically and either side of the line of recoil at zero traverse

(i.e. rearward direction). The angle of offset from line of recoil can vary from 20° to 35°,

and is always less than the angle of traverse. For the purposes of this description the direction of the forces transmitted along the trail legs and the direction of movement of the

trail legs under the recoil forces is also referred to as "rearward" or "backward". As illustrated in Figure 2 the anchor spade 1 comprises a blade plate 2 connected via a vertical plate or portion 7 to a top plate 5 at the forward end of the anchor spade 1. The

anchor spade is fixed to the rear end of a trail leg (not shown in Figure 2) of the howitzer. The spade blade angle is shown as angle μ in Figure 1 and is approximately 45°. The

blade plate 2 has a sharpened blade plate tip 3 for easier digging-in into the ground under recoil forces. In Figure 2 the anchor spade 1 is shown resting with its sharpened spade

blade tip touching the ground 4. This is typical of the position of emplacement of the

anchor prior to initial firing or manual dig-in. To improve initial and subsequent

penetration of the blade plate 2 into the ground, the width or length of line of action of the tip 3 in the transverse direction in contact with the ground 4 is reduced. This has the effect of increasing the pressure between the blade plate tip 3 and the ground 4.

The top plate 5 is approximately horizontal to the ground when the spade anchor in the

emplacement position prior to initial firing. The rearward extremity 6 of the top plate 5 is

located directly above the spade blade tip 3, so that the spade forward angle β is 90°.

In operation, and prior to the first firing from a given howitzer emplacement, the spade

anchor fixed at the rearward extremity of the respective trail leg is positioned generally as

shown in Figure 2 with the top plate horizontal, the spade blade angle μ at 45°, and the

spade forward angle β at 90°. It is usual in practice for the spade anchor to be dug in to the ground to a limited extent prior to initial firing. Generally a hole is dug (not shown)

perhaps to a depth of about 6 inches into which the sharpened spade blade tip 3 is positioned. The positioning of the spade blade tip 3 in this hole does not significantly

affect the orientation of the spade anchor relative to the ground.

Upon firing of the howitzer, recoil forces are transmitted through the trail legs to the anchor spade 1. The magnitude of the recoil force transmitted through the trail legs will

depend on many factors, such as the elevation and traverse of the howitzer, the extent of

the firing charge, the mass of the recoiling parts and trail assembly, the recoil management

system, stiffness of the trail legs and the extent to which recoil energy is transmitted through a base plate (not shown) upon which the howitzer is mounted when in its firing configuration. In general maximum recoil energy is transmitted through one of the trail

legs when the howitzer is fired at minimum elevation, maximum traverse and maximum charge.

The recoil forces act to drive the spade blade tip 3 into the ground. The effective depth of

the anchor is the vertical distance of the spade blade tip 3 under the ground surface, and

this effective depth increases as the anchor spade is driven into the ground under the

action of recoil forces. The extent to which the effective depth is increased during any one

or a number of firings will also depend on the soil conditions. Digging-in may continue until the top plate 5 comes into contact with ground surface 4. The reaction of the

horizontal top plate 5 engaging the ground 4 will resist further digging-in of the anchor

spade 1. In this way the top plate 5 only comes into full effect when the effective depth

approaches its maximum. An anchor spade having a spade forward angle β of approximately 90° will minimise the

risk of the spade digging itself out under further or additional recoil forces, consistent with

making the anchor spade readily removable from the ground by the gun crew. A longer top plate 5 may facilitate easier removal of the anchor spade from the ground, but a spade

forward angle β of greater than 90° and for certain soil conditions a shear failure plane will be set up along the line from the spade blade tip 3 and the rearward end 6 of the top

plate 5. Failure along this shear plane causes the anchor spade to dig itself out of the

ground, which will reduce the effective depth and accelerate the failure.

The embodiment illustrated in Figures 3 and 4 is a specific howitzer anchor spade

designed and manufactured for a 155mm lightweight towed howitzer having a pair of trail legs. In this embodiment the blade plate 2 has a generally vertical plate 7. In the

arrangement shown, the vertical plate 7 and the blade plate 2 are formed from an integral sheet of metal. The top plate 5 is rigidly fixed to rear extremity of the trail legs (not

shown). The spade blade angle is 45° and extends at this angle for about two thirds of the

perpendicular distance between the spade blade tip 3 and the top plate 5. This embodiment

of the invention has a pair of stiffeners 8 fixed to the forward face of the blade plate 2 and

vertical plate 7. Although in this embodiment the vertical plate 7 extends for about one

third of the maximum effective depth of the anchor spade, it may be of any convenient

size and need not be vertical. It is not essential to the operation of the invention and as will

be described later in relation to, for example, Figures 6, 7 & 8, may be dispensed with.

Also the vertical plate 7 may be replaced by an arcuate plate with any convenient radius.

The presence of the vertical plate 7 reduces the length of the spade blade 2 and hence may limit the digging-in effect of the blade plate, but on the other hand the presence of the

vertical plate 7 may reduce the rearward movement of the anchor spade and trail legs.

Two parallel fixing plates 15, located on the forward face of the anchor spade, are fixed one each to stiffeners 8. These plates 15 also extend vertically above the top plate 5 and each have two holes, a firing retention hole 10 and a pivot hole 11. Also shown is a

semicircular stowage retention hole 12. The rear extremity of the trail leg (not shown) fits

between the pair of parallel plates 15 and a pair of pivot pins (not shown) allow the blade plate 2 and vertical plate 7 to be pivotally mounted on the rear extremity of the trail leg. It

will be understood that in the embodiment shown in Figures 3 & 4 the top plate 5 and

blade plate 3 (with vertical plate 7) are separately and independently mounted and fixed to the trail legs.

When in its operational or firing configuration as shown in Figures 3 & 4, the firing

retention holes 10 align with corresponding fixing holes (not shown) in the rear extremity

of the trail leg. Two retention pins (not shown), one each in association with each fixing plate 15, fix the blade plate 2 and vertical plate 7 to the trail legs, so that there is no

relative movement between the top plate 5, blade plate 2 and trail leg when in this

operational configuration.

In order to remove the anchor plate of Figures 3 & 4, when fully embedded in the ground,

the retention pins which pass through the firing retention holes 10 of the fixing plates 15

are removed. This allows the spade blade 2 and vertical plate 7 to pivot about the pivot pins located in the pivot holes 11, and hence allows the blade plate 2 to be moved relative

to the trail legs and thereby aid removal of the blade plate 3 and vertical plate 7 from the ground. In extreme soil conditions, such as wet clay it is sometimes not possible readily to

remove the blade plate 3 and vertical plate 7 by this means. In these circumstances the

pivot pin may be removed thereby detaching the blade plate 3 and vertical plate 7 from the

trail legs and top plate 5. In this way the howitzer may be moved clear of the anchor spade(s), which will allow room for the anchor spades to be dug out separately if

necessarv.

When the anchor spades have been successfully removed from the ground, the blade plate

3 and vertical plate 7 may be placed in the upper stowage position generally parallel to and under the top plate 5 by rotating the blade plate 2 and vertical plate 7 so that the

semicircular stowage retention holes 12 are positioned below the fixing holes in the trail

legs. The retention pins arc then inserted in the fixing holes in the trail legs, and the

semicircular stowage retention holes 12 engage the retention pins to retain the blade plate

2 and vertical plate 7 in the upper stowage position.

In the embodiment of Figures 3 & 4 the blade plate tip 3 is both sharpened and shaped by

cutting away a proportion in the centre of the cutting edge and chamfering the edges either

side of the blade plate tip 3.

Grousers (not shown in Figures 3 and 4) may be fitted to the anchor spade shown in

Figures 3 and 4. Figure 5 shows an anchor spade very similar to that of Figures 3 and 4, but showing detail

of the trail legs 16 to which the anchor spade 1 is fixed in a manner similar to that described for the embodiment shown in Figures 3 & 4. In the embodiment of Figure 5 the

stowage retention holes 12 are complete holes and not semicircular as in Figures 3 & 4. To

fix the blade plate 2 in the stowage position the retention pins are removed from the firing

retention holes 10 and from the lined fixing holes in the trail legs, and the blade plate 2 rotated upwards until the stowage retention holes 12 are in line with the fixing holes.

Insertion of the retention pins will then fix the blade plate 2 in its stowage position.

The embodiment of Figure 5 shows a lunette or towing arm 17, which has a towing eye 20

and is fixed to the top plate 5. A pair of anchor handles 18 are fixed to the upper surface of the top plate 5 to assist in removing the anchor spade from the ground for displacement of

the howitzer. The trail leg 16 shown in Figure 5 also has a pair of trail leg handles 19 used for lifting and locating the trail leg 16.

Figures 5 and 6 show a pair of parallel grousers 9 fitted onto the internal rearward facing

face of the blade plate 2 and vertical plate 7. The grousers extend longitudinally generally

in the rearward direction. The grousers have the effect of increasing the moving functional force required to cause soil to move laterally across the blade plates when dug into the

ground. This acts to help prevent rotation of the howitzer about one of the trail legs which

could happen when firing the howitzer on uneven ground or at high traverse angles. The

grousers shown in Figures 5 and 6 may equally well be fitted to the embodiment described with reference to Figures 3 and 4. Any convenient number of grousers 9 may be fitted to

the blade plate 2 and vertical plate 7. They may extend over the whole length of the blade

plate and vertical plate, or may be limited to any lesser length on one or other or both of

the blade plate and vertical plate. The grousers shown in Figures 5 & 6 may be of any

convenient height above the surface of the blade plate 2 and/or vertical plate 7, sufficient

to prevent lateral movement of the soil over the blade plate 2 and/or vertical plate 7.

In another embodiment of the invention (not shown) the stiffeners 8 on the forward face of

the blade plate 2 and/or vertical plate 7 may be dispensed with, or one of much smaller

size employed. Instead the grousers 9 arc strengthened so that they can also act as

stiffeners.

The embodiment shown in Figures 6, 7 and 8 have a pair of parallel grousers 9 positioned on the blade plate 2 longitudinally and symmetrically about the centre line of the anchor

spade .

In the embodiment of Figures 6, 7 and 8 the blade plate 3 has an arcuate portion 21 in

place of a vertical plate 7 of other embodiments, at its forward end. The arcuate portion 21

has a radius which is a small fraction of the length of the blade plate 2. This allows blade

plate 2 to extend at the optimum spade blade angle of 45° for most of the effective depth

of the anchor spade. Figures 10-13 show an example of an anchor spade according to the present invention

suitable for fixing as a single howitzer anchor spade to a 105mm light towed howitzer

having a pair of bowed trail legs that are joined at their rear end. The anchor spade is

preferably wider than the embodiment shown in Figures 6, 7 and 8, and the role of the

three grousers 9 may be more significant than the grousers of the embodiment of Figures

6, 7 and 8. Because there is only one anchor spade for the howitzer the grousers 9 together

with the friction force of the soil acting on the width of the compacted soil between blade

plate 2 and top plate 5 act to prevent rotation of the howitzer particularly at high traverse

angles of firing.

In some arrangements of light towed howitzers, trail legs are not present and the recoil forces are transmitted through a firing platform.

Figures 14 and 15 show a schematic view of a pair of anchor spades according to the

present invention fitted to the underside of a firing platform 22.

In the embodiment shown the top plate 5 of the anchor spades 1 are fixed by any

convenient means to the underside of the firing platform 1. The anchor spades face

rearwardly and arc symmetrically located either side of the rearward recoil direction. The

anchor spades are each fitted with four vertical grousers 9. 2

Figures 16, 17 and 18 show an anchor spade similar to that shown in Figures 10 to 13

fitted to a different form of trail leg 16. The top plate 5 of the anchor spade 1 is fixed by

any convenient means to the underside of the rear extremity of the trail leg.

Preferably the means for fixing the anchor spade to the firing platform or trail leg, in the embodiments shown in Figures 14 & 15 and in 16, 17 and 18 respectively allows the

anchor spade 1, or at least the blade plate 3 thereof, to be disconnected from the firing

platform 22 or trail leg 16 respectively.

In all the embodiments of the invention described above the blade forward angle is 90° and the spade blade angle is 45°.