W Bombs were early aquatic mines.
A type of drift mine in practice.
From
http://www.etherit.co.uk/articles/level1/w_bomb.htm
Quote:
W Bomb
These so-called bombs were in fact mines, designed to be dropped in Germany’s inland waterways to destroy barges and bridges. The origin of the bomb is obscure but it seems that some of the responsibility must go to a manufacturer of pipeless organs.
From the end of 1939 when a plan to attack Germany’s inland waterway traffic was devised by the War Cabinet, designs had been submitted and experiments carried out. Despite numerous experimental failures a meeting of the Air Ministry on December 29th 1939, gave the contract to a firm to produce 10,000 of the ‘bombs’ and containers.
By April 1940 a supply of the weapons had been sent to 18 Bomber and 8 Coastal Command stations. The ‘bomb’ was designed to be dropped between 300 and 1,000 feet at speeds of up to 250 mph. These height and speed restrictions were imposed to minimise the possibility of the bomb shattering on impact. It consisted of a cylindrical casing 16 inches high by 10 inches in diameter and carrying an explosive charge of 20 pounds of TNT, the total weight being 35 pounds. The main body, which had negative buoyancy, was maintained at a pre-determines depth by means of a number of corks attached to it by lines. A contact firing device, which was fitted to the top of the main body, was protected together with the corks and lines by a lid. On entering the water a soluble plug freed the lid after a lapse of two to three minutes and armed the bomb. The main body sank to a depth of two to three feet at which it was maintained by the floats and was carried downstream by the river current. Impact with the firing device completed an electrical circuit and fired the bomb. A soluble sinking plug was fitted, which operated in six, 18 or 48 hours according to which plug was fitted.
Volume I of the Air Historical Branch monograph ‘Armament, ‘W’ Bomb’ produced after the war states:
“The ‘W’ bomb is an example of a weapon conceived in great haste and developed so rapidly that weak points in design were inevitable. Its outstanding disadvantage lay in the fact that it could not be stored. Small batteries and soluble plugs deteriorated rapidly and were intended for immediate use. The bomb, too, was never completely safe. Dissolution of soluble plugs which formed part of the safety device and the possibility of electrical’ shorts’ made it a constant source of danger during carriage, storage or under preparation.”
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So if Blenheims were dropping W Bombs as noted, they were engaged in mining sorties.
Rudimentary, not very effective and haphazard but still mining sorties.