In 1942 the RAF were looking for a suitable type of High Speed Launch to replace the then existing BPB 64ft and 63ft designs, both of which had a good speed, but were regarded as bad sea boats. The requirement was a craft that was suitable for operations in the Western Approaches in its particular severe winter weather conditions. Three designs eventually emerged, the Vosper 73ft design, although chosen, was only built in limited numbers, the successful BPB 68ft HSL and the Thornycroft 67ft HSL being the other two designs. In 1942 the firm of John I. Thornycroft produced HSL 191, Yard No 2561, at their Hampton Launch Works. After successful trials orders were placed with the company and sub contractors appointed.
The 67ft HSL was the first of the third "generation" of RAF HSLs and designed from the outset for rescue work. It had better accommodation for the crew and the sick bay was more capacious and capable of receiving more rescued aircrew in greater comfort. Speed was sacrificed for greater range and the physical strain lessened for crew as a consequence. It was particularly appreciated for its larger and well equipped galley. The first craft to emerge were powered by twin handed Thornycroft RY/12/I engines each of 650 bhp, the type gaining the designation Mk1. (Later versions were powered by triple Napier Sea Lion engines). As could be expected from a builder with such a renowned reputation for the construction of motor yachts, the Thornycroft 67ft HSL's hull lines had a "yacht" like appearance. It is recorded that the company were not at all happy with the "hard chine" principle of construction at that time, due to their long experience of round bilged stepped hulls dating back to their fast CMBs of WWI. A comparison with hull sections of other HSLs shows how in their design Thornycroft kept the underwater section rather "full". The effect of this in service was that the 67ft craft failed in heavy seas to lift her bows as well as the Power Boat craft and there are accounts of her shipping "green water" in adverse conditions. Nevertheless the type was liked by her crews and performed well with the units which received it, units based in the Indian Ocean as well as in the more sheltered waters of European coastal regions.
The Thornycroft RY/12 engine comprised two banks of six cylinders in a "V" arrangement. They were produced at the company's engine works at Basingstoke.
The general arrangement below deck comprised a fore peak at the bow which contained the anchor chain and spare lines and was accessed from the forecastle via a watertight door. Next aft was the forecastle , which was the crew's quarters, and which could only be entered through a hatch from the fore deck. The compartment contained seat bunks either side with additional double tiered cot bunks, four to port and two to starboard, above the seat bunks.
Underneath the seat bunks was ample room for stowage of personal gear, blankets etc. A folding table bench seat and paraffin heater were installed. On the starboard side was the crew's toilet and washbasin, aft of which was the well fitted galley containing an impressive array of storage racks for crockery, a good size sink and a pressure paraffin oven. Thermos containers of hot soup and tea etc, were always ready to hand.
Behind the forecastle bulkhead was a central passage way containing steps up to the wheelhouse. Opening from the passage way on the starboard side was a well equipped and roomy radio room with space for two operators. W/T, RD/F and IFF were standard equipment with VHF installed later. On the port side was a cabin for two NCOs, aft of which was the officer's toilet and wash basin. The passage way led aft via a door to the large sick bay which had room for four stretchers slung under the beams or lying on the floor. On either side were seat bunks giving extra space for rescued personnel if needed. The Medical Orderly had a large basin at the rear of the sick bay with hot and cold water and he also had plenty of storage space for the necessary medical equipment and supplies he would need in an emergency. In the forward starboard corner of this compartment was a cabin for two officers fitted out with bunks, a nest of drawers, wardrobe and table lamp.
Aft of the sick bay was the fuel compartment containing three 400 gallon petrol tanks. Behind this space was the large engine room giving space enough for the fitter to move freely around the two engines fitted. As well as the main engines, a Stuart 3½ h.p. Engine was provided for running an air compressor and for charging batteries and a Norman Lyon four stroke to drive a 1,250 watt generator to provide power when not under way. Compressed was required to start the main engines. On the port side forward of the engine room the fitter was seated in an armoured padded seat in front of a comprehensive instrument panel and in full view of the visual telegraph display controlled from the wheelhouse. He had levers to operate the gearboxes. Access to the engine room was via two hatches and ladders, one at each end. Aft of the engine room was the aft peak containing the batteries and storage for rescue equipment.
The arrangement above deck comprised the wheelhouse which was placed well forward in this type to provide the coxswain with a better view than was obtained in the BPB 63ft HSL. It contained the usual standard equipment (see plan of wheelhouse).
Leading aft from the wheelhouse were steps up to a wide bridge, a new and important feature which enabled the officer in command to have a much improved view and so greatly aiding the early sighting of an object floating in the sea. This addition of a bridge above the wheelhouse was fully developed in the BPB 68ft HSL which was to follow later into service. Ample armour plating was provided behind the officer's bridge position, at the rear bulkhead of the wheelhouse, including the door, and behind the coxswain's position in the wheelhouse.
Hull construction was of the hard chine principle, the planking being of double skin mahogany planks on closely spaced sawn frames, also of mahogany. Similar wood was used for the deck, hatches and exterior of the wheelhouse. To prevent slipping the deck was painted and sanded. To speed up production, the frames and bulkheads, as well as the sick bay cabin and wheelhouse "superstructure" were prefabricated.
In view of the difficulty experienced by crews of the BPB 63ft HSLs in trying to defend themselves with two single machine guns and the consequent need for additional guns to be mounted, the specification for the Thornycroft 67ft HSL called for improved defensive power. Three turrets were installed, each mounting twin 0.303 Brownings, one turret on each side of the bridge and the third aft. The aft turret on some of these craft was omitted, being replaced by a 20mm Oerlikon cannon. This arrangement was a standard called for on HSLs designed to this particular specification, for a similar arrangement appeared on the Vosper 73ft design and on the BPB 68ft design.
The majority of the Mk1 67ft HSLs saw service in and around the UK and Europe, with many being sold off after the war through the disposal agency. A great number were disposed of through Marine Constructors Ltd at Dumbarton.
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