Tinian

Tinian, the third of the three largest islands of the Marianas, lies just southwest of Saipan and is separated from it by a strait only 3 miles wide. The island is about 12 miles long, from north to south, and about 6 miles wide, with a generally flat and terraced terrain. The Japanese had developed extensive sugar plantations; the island's industry was confined to sugar-refining.

The northern part of Tinian contains an extensive plateau ideally suited for a large airdrome, a circumstance that had not been overlooked by the Japanese. During the months just prior to our Marianas offensive, they had done considerable work there on two airstrips, one a coffin-shaped field, 4,700 feet long, and another of more orthodox shape, 3,900 feet long. In addition, they had completed a 5,000-foot strip about midway of the west coast, and had begun a small runway near the center of the island. Japanese construction on the northern plateau formed the nucleus of the great airfields the Seabees built on Tinian during the succeeding months.

In view of the magnitude of the development planned for Tinian, the Sixth Construction Brigade was formed and was assigned responsibility for all construction work on the island. It comprised, originally, the 29th and 30th Construction Regiments; later, while base development was in progress, the 49th Regiment was formed and became part of the Brigade. Two of the battalions, the 18th and the 121st, participated in the assault on Saipan and were again given an assault assignment in the Tinian invasion. As was customary, during the assault period these battalions were under Marine Corps control and did not formally become part of the construction brigade until the island was secured.

The invasion of Tinian was principally a shore-to-shore operation from Saipan by landing craft. On July 24, 1944, the Fourth Marine Division landed on two narrow beaches on the northwest coast of the island. They were accompanied by theforward echelons of the 18th and 121st Construction Battalions. The remaining personnel of these battalions went ashore on Tinian on July 27.

The 18th and 121st had a different organization than was customary for construction forces assigned to Marine divisions. The men had been especially trained, prior to the occupation, to perform certain tasks that could be activated according to operational requirements in the field. Twelve assault units were set up, as follows: LVT-ramp maintenance, beach access, road reconnaissance, road construction, road maintenance, traffic circulation, railroad demolition, railroad construction, airfield rehabilitation, civil affairs construction, water supply, and reserve.

One of the more significant tasks of the Seabees in the Tinian assault was the maintenance of LVT ramps - especially developed services to allow LVT's to climb the steep banks that flanked the narrow landing beaches.

The ramp consisted of two side-rails, made of 10-inch I beams, which supported an articulated mat of 6-by-12-inch timbers, and was carried to the landing point by the vehicle itself. The side rails were suspended from the sides of the LVT, with enough upward slope to permit their forward ends to clear the top of the bank. The timber mat was supported for its first 10 feet by the rails and for the remainder of its length by slides built over the vehicle's cargo well. At the point of landing, the forward ends of the rails were released so that they came to rest on the top of the bank; the LVT then backed away a few feet, allowing the after ends of the rails to rest on the bottom. Further backing permitted the timber mat to come to rest upon the rails for its entire length, and the vehicle could then go ashore over the ramp thus formed.

The first echelon of the 18th battalion installed and maintained these ramps, the work lasting three days. Harbor facilities were non-existent and everything had to come across the barrier reef, to be unloaded and transported inland.

Upon arrival of the remainder of the battalion, road-repair and construction details were assigned, emphasizing the construction of special roads for cleated vehicles. The terrain was level, and access roads were easily built from the beaches to the existing Japanese roads.

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