Appendix IV
In 1939, most destroyers had Type 128 asdic sets, sloops had Type 127 and trawlers Type 123, though there were probably a few older sets in use. Brief particulars of the more important sets follow. (For more detail see Hackmann.) Note that there were frequent updates to sets while in service; for example, the ultimate 128 was virtually identical to 144. These updates took place when the ship was in hand for other work. Information on ship-fitting is scarce and often unreliable. While this note was written with care, its complete accuracy cannot be guaranteed.
Type 121. Prototype tested in Woolston in 1931. Fitted in D, E, F and G class destroyers, some cruisers and the sloop Enchantress. First production retracting dome.
Type 123. Trawlers and other auxiliaries. Introduced 1934, replacing earlier Type 122. Detachable dome.
Type 124. Updated 121. In 1934–7 fitted to C, H, J, K and Tribal class destroyers, coastal sloops and a few older destroyers. First with standard range recorder.
Type 127. Designed for sloops but very widely fitted from 1937 in older destroyers, frigates and in allied ships. Dome as 122 and electronics as 123. Some had Q (qv).
Type 128. From 1937 in destroyers; prototype in Acheron, then A (retro-fit), L and Hunt classes. Dome and directing gear as 121, electronics as 127. There were at least nineteen wartime variants, with improved recorders, helmsman display, etc. Type 128 XE became 144. Some had Q attachment.
Type 141. US set QCJ/QCL found in forty-seven flush-deck destroyers, fitted with RN range and bearing recorder. It had no dome and a few were given British domes as Type 141A.
Type 144. Started May 1941. Introduced in 1942 into destroyers and major escorts after trials in Kingfisher. It was the first set specifically intended for ahead-throwing weapons such as Hedgehog. It was a complete redesign, although many of its features were worked into later updates of 127/128.
Type 145 was similar to 144 but had portable, rather than retractable, dome for slower escorts.
Type 147. Sea trials in Ambuscade in May 1943. Very much part of a weapon system – Squid. Depth measurement.
Q attachment. 1943. Wedge-shaped beam only 3º wide in horizontal plane. Could measure depths 300–700 feet. Fitted to Types 127 and 145 without the need for docking the ship; vessels with Types 128 and 144 needed docking. Fitting took two to six days. Production from April 1943. In an attack, the ship would switch to Q at about 1,500 yards.
Hadleigh Castle was the first ship with the complete system of 144Q, 147B and Squid (single). She visited the USA in September 1943, creating a major impression. The USN received forty sets from the UK and a further forty-five from Canada.