Although the aerial defence of coastal convoys would have been best met by shore based fighter aircraft, many circumstances and conflicting priorities prevented this policy from being fully implemented. Therefore, provision had to made for suitable vessels to be converted to purely coastal A.A., vessels in addition to the A/S cover given by coastal escorts, mostly of limited A.A., fire-power.  Coastal convoys frequently found themselves harrowed in estuarial waters [such as the Thames estuary] where once committed to the navigational deep water channel, they were unable to take any form of avoiding action and thus simplified the task of the attacking aircraft.  Fortunately, the steadily mounting minesweeper construction programme enabled practically all the requisitioned paddle minesweepers to be released for this purpose, and together with other paddle vessels not taken up for minesweeping duties and some miscellaneous small vessels, they were converted, or adapted as coastal auxiliary A.A., vessels.

The paddle vessels enjoyed the additional advantage that their shallow draught did not wholly restrict them to the deep water channels and they virtually provided A.A., cover from harbour entrance to harbour entrance.

Their armament was of a makeshift nature and consisted of numerous close range weapons as their light construction did not permit heavier pieces being mounted. Generally, it consisted of single 2 pdr A.A., gun fitted forward and aft, a few 20mm A.A., guns before and abaft the paddle-boxes, which usually had quadruple 0.5 inch machine-guns on top them, quadruple Boulton & Paul aircraft turrets on the bridge and superstructure, several single or twin vintage machine-guns disposed overall, and one or more 4-barrelled rocket projector of capricious nature. The latter were ultimately replaced by 20mm guns as they became more readily available.

At close range they were able to confront attacking aircraft with a considerable, but ill-directed volume of A.A., fire [there were no H.A., [high angle] control arrangements] and their true measure of success was more in the number of air attacks they broke up than in the actual number of aircraft brought down.