The Thornycroft RY12 engines installed in the 67 ft Thornycroft MkI craft gave the maintainers never ending problems. Some improvements were made which included changing the propellers which resulted in the batch of craft designated MkIA and MkIB, but the speeds attained were never much more than about 24 knots. When sufficient Napier Sea Lions became available, the RAF switched further production of the type to these engines. Craft fitted with Triple Napier Sea Lion engines were thus designated MkII. Of the 104 boats built on the 67ft Thornycroft design, over half were of the MkII variant, and once again the lead builder built the first craft, HSL 2563, which entered RAF service on the 12th October 1942, having been ordered in March the same year. Other units were built by Walton Yacht, Shepperton and were ordered outright, i.e. they were not sub contracted as in the MkIB craft.
The layout of the main part of the craft remained as for the previous MkI variants, however the engine room became a lot more cramped due to the requirement for three engines as opposed to the two fitted in early craft. The outboard or wing engines were mounted aft and drove their respective propellers through "V" drives whereas the centre engine was coupled directly. This allowed the use of the centre engine for cruising and therefore extended the range of the craft. The significant result of the engine change was that a maximum speed of 25.5 knots, faster than the MkI craft. Because of the engine layout and the improved control systems, the seat for the fitter was not installed in the MkII craft.
Several of the Thornycroft MkII craft saw service in the Mediterranean area and post WWII, several were offered for disposal in that area. A few were transferred to the French Government, and some were transferred to the Civil Controller of Supplies at Gibraltar. One saw service in West Africa and five were operated in the Indian subcontinent, all being transferred to the Indian Government on the 15th August 1947. The remainder saw service in the UK areas, and like the MkI craft were sold off post war through various disposal outlets, which included Burmac Ltd, Marine Craft Constructors Ltd and Calshot.
Two craft were transferred to the Belgian Air Force. HSLs 2592 and 2651 were both transferred on the 1st March 1948, and from available records, at least one, then numbered V-01, was still in service with the Belgian Air Force in 1954.
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