The problem of transporting heavy military equipment overseas and discharging it on beaches has long been a problem in military operations. The introduction of the Landing Ship, Tank during World War II was a significant milestone in the resolution of this problem. On 4 November 1941, a sketch of a proposed LST was prepared by John Niedermair of the Preliminary Design Branch of the then Bureau of Ships. The basic design embodied a ship with the capability of satisfactory operation in a deep draft condition, to give sufficient seaworthiness for long ocean voyages, and in a shallow draft condition, with proper trim for landing on relatively flat beaches with a full load of vehicles. Major problems which faced the designers were the means of operating the bow doors and ramp, and the means for lowering vehicles from the main deck to the tank deck to permit them to go ashore over the ramp. The magnificent performance record of the LST during and since World War II is a tribute to the ingenuity of the designers, planners, and production personnel involved in the early LST programs.
In recent years the significance of amphibious warfare has not waned. Amphibious operations have proven to be the most versatile aspect of conventional warfare because of their mobility and flexibility. The requirement to reach the world’s trouble spots at the right time and with the right amount of equipment and troops is very real and has been demonstrated time and time again in recent years. To meet this continuing requirement, considerable planning, design, and construction has taken place to modernize the U. S. Navy’s amphibious forces and to provide them with new 20-knot ships.
Perhaps the most difficult problem in realizing a balanced, 20-knot amphibious force has been the LST. Nothing has altered the LST’s need for a deep draft condition for seaworthiness and a shallow draft condition for landing on relatively flat beaches. The bluff hull form of the conventional LST in World War II does not lend itself to economical propulsion at speeds up to 20 knots, either through the use of conventional power plants or through the use of any power plant expected to be developed in the foreseeable future. In addition, the present LST hull form is far from the optimum with respect to seakeeping qualities of other ships of the Navy’s 20-knot amphibious force.
Innumerable studies have been made over the past 15 years to fulfill the seemingly incompatible requirements for a fast, oceangoing ship with a shallow draft beaching capability. Early schemes considered concepts such as catamarans with an offloading ramp between the hulls, bow propulsion ships which backed onto the beach, and ships with side ramps.
Finally, in 1962 the conclusion was reached that a bow beaching ship with a fine hull form was still the most practical solution for achieving 20 knots and maintaining good seakeeping qualities. Thus a 20-knot LST was designed with an over-all length of 525 feet, a beam of 68 feet, and a full-load displacement of 8,340 tons with a six-foot landing draft forward when carrying 500 tons of vehicles or cargo.
A fine hull form, however, does not lend itself to the use of bow doors and a ramp as in the conventional LST. To reach a point in the fine-hulled ship of sufficient beam to accommodate a 16-foot wide ramp somewhere below the main deck would mean cutting back into the ship some 75 feet amidships. Clam-shelling the bow for 75 feet would have required an excessive amount of heavy handling equipment to say nothing of the additional equipment for handling a suitable ramp for vehicular traffic. Thus, vehicles must be offloaded from the main deck over the bow and a ramp must be provided to bridge the gap between the ship and a causeway or between the ship and the beach should the beach gradient be suitable for conventional beaching operations. After having decided at what point the vehicles would be offloaded, the determination of ramp length became one of geometry involving the deck height above the ship’s six-foot beaching draft, a shallow beach slope, the degree of inclination which assault vehicles can negotiate, and the fording capability of assault vehicles. The minimum required length of ramp was determined to be 112 feet. Studies were also made of various ramp systems such as telescoping, folding, and one piece to arrive at the lightest weight installation compatible with good maintenance, reliability and ease of handling. One of the other major factors in determining ramp structure was the ability to handle successfully loads of 75 tons, either as a single load of one vehicle or as a combined load of several vehicles. It was decided that a one- piece, retractable aluminum ramp handled on rollers, and powered by electro-mechanical winches with wirerope cables was the most desirable system. Although each of the components had been employed successfully in other systems, it was decided in September 1963 to build a prototype ramp system and install it on a test ship to prove the concept prior to the actual construction of the 20-knot LST.
The Norfolk Naval Shipyard was chosen to prepare detail plans, fabricate the prototype ramp, install it in an existing LST, and evaluate the concept. The USS Mahnomen County (LST-912) was chosen for the trial installation and the Norfolk yard completed the installation in late July 1965. The Mahnomen County was engaged in causeway and beaching operations in the Little Creek, Virginia, area until early November 1965. Operations were conducted under both calm and heavy weather conditions. It was concluded that the ramp concept was sound.
Except for the significant departure in hull form, the 20-knot LST is not unlike previous LSTs. The controlling factor in the design as in previous LSTs is deck area, whereas in the case of the attack cargo ship, for example, volume is the controlling factor. The main deck and tank deck constitute the major parking areas for assault vehicles. The traditional problem of getting light vehicles from the main deck down to the tank deck and ashore via the ramp has been reversed. Heavy vehicles must now come up to the main deck from the tank deck to go ashore via the ramp.
Amphibious assault vehicles will be handled in a slightly different manner than in previous LSTs. Amphibious tractors will be launched and retrieved through a stern gate similar in some respects to the stern gate of LSDs and LPDs. The stern gate will be suitable for mating with an LCU as well as being strong enough to bridge the gap between the stern of the ship and a pier of suitable height for vehicle loading and unloading. In addition, the stern gate will provide a water-tight seal as protection against following seas and wave action against the stern.
The silhouette of the new LST, with the derrick arms protruding over the bow and the amidships superstructure, does not resemble that of existing LSTs. The derrick arms must be extended far enough forward of and above the bow to provide support for the hoisting and outhaul cable sheaves in order to extend and retract the 112-foot ramp. The principal advantage of locating the superstructure amidships is greater forward visibility. Other advantages include a simpler stack arrangement as well as a more suitable location for the helicopter landing area. Although the 68-foot beam of the new LST is comparable to previous LSTs, the 525-foot length can be considered as a significant increase in size.
Berthing, messing, and store spaces are located in the superstructure and wing walls outboard of the tank deck. The ship has a personnel requirement of ten officers and 160 enlisted men and will have a troop capacity of approximately 400 officers and enlisted men.
The achievement of a 20-knot sustained speed in an LST required either less carrying capability than existing LSTs or a growth in ship length and draft. Going to the latter solution has given the new LST a greater vehicle capacity than previous ships as well as a 20- knot speed.
The first of the new ships, the LST-1179, was ordered from the Philadelphia Naval Shipyard on 29 December 1964. She is scheduled to be laid down this fall and completed in mid-1968. Nineteen sister ships have been authorized through the Fiscal 1967 Shipbuilding Program.
The new 20-knot LSTs will provide the Navy with a key element of a 20-knot amphibious force and a quantum improvement in offensive capability.