The Admiralty had received intelligence in early 1944 about new U-boats which were reported to be able to achieve a top speed of around underwater, compared to the of the fastest existing U-boats. As these new XXI-class U-boats were considered to pose a major threat, "Seraph" was modified at Devonport as a matter of urgency to have a high underwater speed so that trials and exercises could be carried out against a submarine having a similar underwater speed; for example in developing new tactics.