The present invention relates to a practice sea mine.
Presently, real mine cases filled with an inert substance, e.g. sand/cement,
instead of explosive, are used as practice mines (see e.g. US-A-3,086,464). These
may be deployed from a submarine or ship or are sometimes deployed from an aircraft.
As real mine cases are costly items an effort is made to recover these which in
itself is a costly operation and the recovery rate is low. Real mine cases clutter
up the sea bed and can cause damage to fishermen's nets.
The use of biodegradable materials for the casing of a practice missile
has been proposed in FR 2,364,425.
The present invention consists of a practice mine comprising a body
shell containing an inert substance, characterised in that the body shell is provided
with an internal tubular member sealed at one end by a removable plug for allowing
passage of water on deployment of the mine, and in that both the body shell and
tubular member are fabricated from material which decomposes on exposure to water.
Preferably, the mine is designed to degrade within a predetermined
time period after deployment, for example, between one and six months. The mine
may comprise means for initiating degradation on deployment such as a lanyard and
Preferably, the body structure of the mine is made from wood, e.g.
plywood and the inert filler may be a soluble cement/plaster mix.
Particular embodiments of the present invention will now be described,
by way of example, with reference to the accompanying drawings, in which:-
- Figure 1 is a view in longitudinal cross section of a practice sea mine according
to the present invention;
- Figure 2 is a view in transverse cross section of a second embodiment of the
- Figure 3 is a view in transverse cross section of a third embodiment of the
Referring to Figure 1, a practice sea mine is indicated generally
at 10 and comprises a tubular casing 12 made from plywood (14/15 laminations)
which is formed from two vacuum moulded halves jointed and bound and fastened with
low water resistant glues. A central tube 14 of spun cardboard extends almost the
entire length of the casing 12. The casing 12 is filled with a hydrodegradeable
concrete/plaster mix indicated at 16. An annular plywood block end cap 18 provides
a filling access aperture 19 and bolted thereto is a nose piece 20 which may be
a metal mine nose piece or may be made from hydrodegradeable material. At the other
end of the casing 12 a block 22 of cardboard material is inserted to adjust the
centre of gravity of the mine 10 and a plywood block end cap 24 abuts the end wall
of the casing 10 to which a tail piece 26, of metal or of wood for example, is
attached. A lanyard 28 is attached to a pull-off patch 30 initially sealing the
end of the central tube 14. A top lug 32 is provided for keying the mine 10 in
the launch tube of a submarine.
The length of the practice mine 10 is approximately 2.5 metres, its
weight approximately 800-900kg. and it simulates the shape of a specific real mine.
In use, the practice mine 10 may be deployed from a submarine or
an aircraft and is arranged so that, on deployment, the lanyard 28 is tugged to
release the pull-off patch 30 (simulating weapon arming) thus allowing water to
enter the central tube 14 to initiate degradation.
Initially, the central tube 14 is degraded, then the concrete/plaster
mix 16 and finally the casing 12. It is envisaged that the mine 10 will be designed
so as to degrade almost totally within between one and six months on the sea bed.
The practice mine described above simulates a real mine whilst being
fabricated of low cost materials using low cost production techniques. The mine
can be suitably coated to help prevent unwanted external degradation prior to deployment
and/or to provide a realistic sonar image for mine hunting practice. The practice
mine is structurally sufficiently strong to withstand transport and deployment,
even from an aircraft, but degrades to a required standard during a predetermined
time after deployment.
An alternative configuration is shown in Figure 2 which shows a practice
mine 50 which is generally the same in external appearance as the mine 10 of Figure
1. Internally, the mine 50 comprises a plywood casing 51 containing seven tubes
52-58 of soft plywood which may be filled with sand sausages vacuum packed in polythene.
Runners 60-65 which are triangular in cross section are positioned around the
periphery of the mine casing 51 to maintain the tubes 52-58 in position. Six softwood
poles are positioned around the central tube 58 as shown.
The tubes 52-58, runners 60-65 and poles are strapped together and
glued with water soluble glue prior to fitting of the casing 51. This embodiment
has the advantage of being structurally relatively strong and furthermore the tubes
52-58 provide several passages to allow water to circulate the start dissolving
glue and initiating the degradation process.
A further alternative is illustrated in Figure 3 in which a practice
mine 80 comprises an outer casing 82 of plywood and three laminated, soft plywood
struts 84, 86 and 88 defining a cruciform structure. The struts 84, 86 and 88 extend
along the length of the casing 82 and are held in place relative to the casing
82 by channelled wooden runners 90-93. Quarter pieces 94-97 of softwood are glued
in place as shown.
The struts 84, 86 and 88 are apertured as indicated at 98 to allow
keying of a filling 100 e.g. a salt and plaster mix.
This embodiment has the advantage that the struts 84, 86 and 88 lend
extra strength in all three dimensions to the mine 80 and in that load spreading
is achieved by the keying of the filling 100 through the apertures 98 in the struts
84, 86 and 88.
Another possible construction would be to fabricate the mine casing
in the form of a barrel with interlocking stays held by hoops e.g. of aluminium
It is envisaged that very many hydrodegradeable materials could be
chosen for incorporation in a practice mine according to the present invention
including corrodable materials such as Swedish Iron, Aluminium, Magnesium, bi-metal
combinations giving cathodic effects, sodium etc. All sorts of fillings might be
used such as soluble salts, salt and plaster mixes, clay and gravel mixes, salt
and sand mixes etc. Organic materials for attracting marine organisms to consume
them may be used and it is also envisaged that the practice mine may incorporate
biological organisms or materials such an enzymes to ensure degradation.