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The LST (2) design was successful and production extensive but there was still a need for more LST's for British operations. As such it was decided to build a further 80 of them in UK and Canada to be available in the spring of 1945.
The British Staff drew up their own specification:
Two major problems made a redesign necessary. The locomotive type diesel engines were not available. Staff wanted more power and higher speeds if possible. The only engines available were very heavy steam reciprocating engines from frigates that had been cancelled. These delivered two and a half times the power of the diesels. So large were they that significant changes had to be made to accommodate them. Lack of welded construction facilities meant that the hull had to be riveted. This combination of heavy hull and heavy engines meant that speed was only 3 knots faster than the LCT(2)
At the same time some other improvements were made as well as simplifications needed to allow for rivetting most of the structure. The cutaway hard chine which had been dropped in the American version of the design of the Mark 2 vessels was restored. The tank deck, which was above the waterline, was made parallel to the keel, there was to be no round down to the upper deck, and the ship was enlarged to accommodate the more bulky machinery.
Provision was made for carrying the British Landing Craft Assault (LCA) in gravity davits, instead of American assault craft. Provision was also made for carrying Landing Craft Tank (LCT) and Landing Craft Mechanized (LCM), and NL pontoon causeways.
When the design was commenced, it was known that the beaches on which the ships were expected to be used would be very flat, but it was not possible to produce a satisfactory vessel with a 3 ft (0.91 m) draught forward, and very little keel slope, so the 1 in 50 keel slope was maintained. It was known that the 1:50 slope would often result in the LST grounding aft on a shallow beach, resulting in the vehicles being discharged into comparatively deep water.
Various methods had been investigated to overcome the problem, but heavy grounding skegs and the N.L. pontoon causeways were finally accepted as standard; the pontoon causeways were formed of pontoons 7 ft (2.1 m) × 5 ft × 5 ft (1.5 m), made up into strings and rafts. When offloading, the rafts were secured to the fore end of the ship, and the load discharged directly onto the shore, or towed on the raft to the shore.
The ships were fitted out for service in both very cold and tropical conditions. The accommodation provided for both crew and army personnel was greatly improved compared with "LST (2)". The main hazard, apart from enemy action, was fire on the tank deck. Fire sprinklers were provided, but the water drenching system installed in later American vessels could not be provided.
The bow door arrangements were similar to the "LST (2)", but the bow ramp was arranged in two parts in an attempt to increase the number of beaches onto which direct discharge would be possible. The machinery for operating the bow doors and ramp were electrical, but otherwise steam auxiliaries were fitted instead of the electrical gear on the "LST (2)".
The general arrangements of the tank deck were similar, but head room was increased, and a ramp fitted to reach the top deck, as in later "LST (2)"s. Provision was made for carrying LCA on gravity davits instead of the American built assault boats.
The arrangements were generally an improvement on the "LST (2)"s, but they suffered from their deeper draught, and to some extent from the haste in which they were built.
First orders were placed in December 1943, 45 with British builders, and 35 with Canadian builders. The first of these ships was delivered by Swan Hunter in December 1944. During 1944, follow up orders were placed in Canada for a further 36. These programmes were in full swing when the war ended, not all vessels were completed.
The ships were numbered numbers LST - 3001 to LST 3045 and LST -3501 to 3534. LST -3535 and later were cancelled.
Fifteen 40-ton tanks or 27 25-ton tanks could be carried on the tank deck with an additional fourteen lorries on the weather deck.
Steam was supplied by a pair of Admiralty 3-drum water-tube type boilers, working at 225 pounds per square inch. The main engines were of the 4-cylinder triple expansion 4-crank type, balanced on the Yarrow-Tweedy-Slick system, the cylinders being as follows;
The common stroke was 30 inches (760 mm). The piston and slide valve rods were all fitted with metallic packing to the stuffing boxes, and all pistons fitted with packing rings and springs. The high-pressure valve was of the piston type, whilst the remaining ones were of the balanced type. The main engines were designed to develop 2,750 hp (2,050 kW) at 185 rpm continuously.
With the ships being twin screw, the engines were fitted with a shaft coupling to the crank shaft at the forward end, allowing the engine to be turned end to end to suit either port or starboard side fitting.
 Modifications for landing craft
When the "LST (3)"s were being ordered the "LST (2)" programme was in full swing, and similar arrangements were made to enable the LSTs to carry the 112 feet (34 m) long LCT5 or LCT6 which were being built in America for the Royal Navy.
The LCT needed lifting onto the deck of the ship, being carried on wedge-shaped support blocks; at the time of launching she was set down on the "launch ways" by simply slacking off bolts in the wedge blocks, allowing the launch way to take the weight. To carry out a launch, the LST was simply heeled over about 11 degrees by careful flooding of tanks in the hull. The height of the drop was about 10 ft (3.0 m), and immediately after the launch the craft's engines were started and they were ready for operation.
This method was used for moving LCT5s from Britain to the far east, although there seems to be no reference to LST (3)'s being used, most being completed late in or after the war.
Even at the end of the war there was a need for more ships able to carry minor landing craft, and two of the LST (3)'s then completing were specially fitted to carry LCM (7). These craft, which were 58 ft (18 m) long and weighed about 28 tons, were carried transversely on the upper deck of the ship. They were hoisted on by means of a specially fitted 30-ton derrick; This 30 ton derrick replaced a 15-ton derrick, two of which were the standard fit of the LST (3). The 30-ton derrick was taller and generally more substantial than the 15 ton one.
The LCM (7)s were landed on trolleys fitted with hydraulic jacks. These ran on rails down each side of the deck, and were hauled to and fro by means of winches. The stowage was filled from fore to aft as each craft was jacked down onto fixed cradles between the rails. The ships completed to this standard were LST-3043/HMS Messina, and LST-3044/HMS Narvik. While these ships were able to carry LCMs, they were only able to carry out loading and unloading operations under nearly ideal weather conditions, and as such were not able to be used as such for assault operations; they also lacked the facilities to maintain the landing craft (which the Dock Landing Ships provided).
The LCAs, or Landing Craft Assault, were wartime developments. Wooden hulled, with a length of 41 ft 6 in (12.65 m) overall including propeller guards. Beam was 10 ft (3.0 m), and had a displacement of 10 tons, rising to 13 tons fully loaded. Draught was 2 ft 3 in (0.69 m), and normal load was 35 troops with 800 lb (360 kg) of equipment. A pair of Scripps marine conversions of Ford V8 marine engines, producing 130 bhp (97 kW) at 2,800 rpm, provided propulsion producing 11 knots (20 km/h) unloaded, 8 knots (15 km/h) service speed, 3 knots (5.6 km/h) on one engine. Range was 50–80 miles on 64 gallons. Armament was typically a Bren Gun aft; with two Lewis Guns in a port forward position.
The LCM (7)'s that were carried on the LST (2) were considerably larger, 60 ft 3 in (18.36 m) in length, 16 ft (4.9 m) beam, with a hoisting weight of 28 tons, full load displacement of 63 tons. Beaching draught was 3 ft 8 in (1.12 m), and propulsion was provided by a pair of Hudson Invader petrol engines, later replaced with Grays diesels, both sets providing 290 bhp (220 kW), giving a speed of 9.8 knots (18.1 km/h).
The main requirement of the design was to carry a 40-ton Churchill tank or bulldozer at 10 knots (19 km/h). 140 had been completed when the war ended, and some saw service through to the 1970s.
Some LST(3) were converted to LST(A) (A for "assault") by adding stiffening so they could carry the heaviest British tanks.
Two LST(3) were converted to command vessels, LST(C): LST 3043 and LST 3044. Post war they became HMS Messina (L112) and HMS Narvik (L114). They were better armed with ten 20 mm Oerlikons and four 40 mm Bofors.
Two LST(3) were converted during building into Headquarters command ships LST(Q) these were L3012 which became L3101 (and later HMS Ben Nevis) and LST 3013 which became LST 3102, and then HMS Ben Lomond. They acted as LST "mother ships", similar in most aspects to American ships based on the LST (2) hull. They had two Quonset huts erected on the main deck to accommodate 40 officers. Berths on the tank deck berthed an extra 196 men. A bake shop and 16 refrigeration boxes for fresh provisions augmented the facilities normally provided for the crew. Four extra distilling units were added, and the ballast tanks were converted for the storage of fresh water.
 Service in World War II
U.S. LSTs carrying the Australian 26th Brigade from Morotai Island to Tarakan Island in April 1945.
At the Armor Training School in Ft. Knox, KY, buildings were erected as exact mock-ups of an LST. Tank crews in training learned how to maneuver their vehicles onto, in and from an LST with these facilities. One of these buildings has been preserved at Ft. Knox for historic reasons and can still be seen there.
From their combat debut in the Solomon Islands in June 1943 until the end of the hostilities in August 1945, the LSTs performed a vital service in World War II. They participated in the invasions of Sicily (Operation Husky), Italy, Normandy, and southern France in the European Theater and were an essential element in the island-hopping campaigns in the Pacific which culminated in the liberation of the Philippines and the capture of Iwo Jima and Okinawa.
The LST proved to be a remarkably versatile ship. A number of them were converted to become landing craft repair ships (ARL). In this design, the bow ramp and doors were removed, and the bow was sealed. Derricks, booms, and winches were added to haul damaged landing craft on board for repairs, and blacksmith, machine, and electrical workshops were provided on the main deck and tank deck. Another successful conversion was the LST "Mother Ship". This version of the standard LST hull had two Quonset huts erected on the main deck to accommodate 40 officers. Bunks on the tank deck berthed an additional 196 men. A bake shop and 16 refrigeration boxes for fresh provisions augmented the facilities normally provided the crew. Four extra distilling units were added, and the ballast tanks were converted for storage of fresh water.
USS LST-310 (2nd LST from the right) along with other ships putting cargo ashore on one of the invasion beaches, at low tide during the first days of the Invasion of Normandy in June, 1944. Among identifiable ships present are USS LST-532 (in the center of the view); USS LST-262 (3rd LST from right); USS LST-533 (partially visible at far right); and USS LST-524. Note the barrage balloons overhead and Army "half-track" convoy forming up on the beach.
Thirty-eight LSTs were converted to serve as small hospital ships and designated LSTH. They supplemented the many standard LSTs which removed casualties from the beach following the landing of their cargo of tanks and vehicles. LSTs had brought 41,035 wounded men back across the English Channel from Normandy by D-Day+114 (28 September 1944). Other LSTs, provided with extra cranes and handling gear, were used exclusively for replenishing ammunition. They possessed a special advantage in this role, as their size permitted two or three LSTs to go simultaneously alongside an anchored battleship or cruiser to accomplish replenishment more rapidly than standard ammunition ships.
Three LST (2) were converted into British "Fighter Direction Tenders" swapping their landing craft for Motor Launches.
In the latter stages of World War II, some LSTs such as USS LST-906 were fitted with flight decks from which small observation planes were sent up during amphibious operations. It has been estimated that, in the combined fleets assembled for the war on Japan, the tonnage of landing ships, excluding landing craft, would have exceeded five million tons and nearly all built within four years.
Throughout the war, LSTs demonstrated a remarkable capacity to absorb punishment and survive. Despite the sobriquets "Large Slow Target" and "Large Stationary Target" which were applied to them by irreverent crew members, the LSTs suffered few losses in proportion to their number and the scope of their operations. Their brilliantly conceived structural arrangement provided unusual strength and buoyancy; HMS LST 3002 was struck and holed in a post-war collision with a Victory ship and survived. Although the LST was considered a valuable target by the enemy, only 26 were lost due to enemy action, and a mere 13 were the victims of weather, reef, or accident. A total of 1,152 LSTs were contracted for in the great naval building program of World War II, but 101 were cancelled in the fall of 1942 because of shifting construction priorities. Of 1,051 actually constructed, 113 LSTs were transferred to Britain under the terms of Lend-Lease, and four more were turned over to the Greek Navy. Conversions to other ship types with different hull designations accounted for 116.