The Holman Projector must be in with a claim.
Early in WW2 the British Merchant Navy was seriously short of anti-aircraft guns and the German Luftwaffe sunk a lot of ships. Any proper AA guns were reserved for larger vessels which meant that smaller vessels were defenceless under Luftwaffe attack.
The Royal Navy had impressed a lot of trawlers for vital work like mine sweeping, submarine patrols etc. and the British fishing fleet was very important in providing food from the North Sea. Fish and chips were never rationed because of the effect it would have had on public morale!
So an anti-aircraft defence for small ships was desperately needed and the Holman machine tool company of Cornwall came up with low cost solution. Their Holman Projector was effectively a mortar run off compressed air or steam. It featured a 4½ foot cylindrical steel barrel and a rather primitive sight. Rounds were dropped down the barrel from the muzzle and the pneumatic system triggered instantly upon the round striking the base.
The rounds themselves were made from an open-topped metal container, holding a Mills bomb, the standard British hand grenade – fitted with a 3.5 second fuse – that was simply dropped into the barrel like a mortar round. When the container hit the bottom of the barrel it triggered a switch and steam or air pressure propelled the can out the tube into the path of the oncoming aircraft. Air resistance would then pull the can from the grenade thereby releasing the firing mechanism; about three seconds later, the grenade was supposed to explode in front of the aircraft. The rate of fire could reach thirty rounds per minute in the hands of an experienced crew.
High-pressure air bottles were able to supply enough power to fire fifty rounds each, with a maximum height during trials of around 600 feet (180 m). Later on steam from the ships boilers replaced the air bottles making the whole device even simpler.
The Projector was highly inaccurate when fired at distant moving targets: only a dozen or so aircraft were confirmed to have been downed by the weapon in its first year of service but it succeeded in convincing many more pilots that the target vessel was equipped with an effective anti-aircraft weapon, confirmed by the large number of reports made about Luftwaffe aircraft turning away from an attack after salvos were fired from ship-mounted Holman Projectors.
This was because the bombs fired by the projector displayed an unexpected property — the explosion would leave a large puff of black smoke – not seen in ground-based explosions from similar grenades. Firing a large number in quick succession gave the impression to incoming bomber pilots that the target vessel was armed with something far more deadly than the Holman Projector, deterring or disrupting attacks, or convincing the aircraft’s crew an attack at greater range would be prudent, with a resulting decrease in accuracy.
And since the projector would fire anything that would fit down the barrel, it was often used to fire potatoes at more friendly targets!