The Tinian invasion was a shore-to-shore operation from Saipan by landing craft. It began on 24 July 1944, when an advanced party of officers and enlisted personnel from the 121st and 18th Naval Construction Battalions (NCB) landed with the Fourth Marine Division on two narrow beaches on the northwest coast of the island. The assault troops were tasked to install landing ramps, used to expedite transportation of supplies and equipment over coral lined beaches at Tinian Island.
The Seabee assault organization on Tinian differed from the usual landing configuration for construction forces assigned to Marine Divisions. Assault patrols were trained, prior to the occupation, to perform certain specific tasks by patrols that could be activated or inactivated according to the operation requirements in the field. Seabees were organized into assault patrols under the Construction Officer on staff. These patrols worked in close harmony and coordination with the Marine Division engineer unit during the assault, and did not become part of the construction brigade until the island was secured. Twelve assault units were set up, as follows:
LVT-ramp maintenance Traffic Circulation
Beach Access Railroad Demolition
Road Reconnaissance Railroad Construction
Road Construction Airfield Rehabilitation
Road Maintenance Civil Affairs Construction
Water Supply Reserve
The landing at Tinian presented the Seabees with a unique problem: landing men and supplies on the Tinian invasion beach, which consisted of jagged coral cliffs up to 15 feet high that flanked the narrow landing beaches. Commodore Paul J. Halloran drew rough sketches of a plan to overcome the cliff obstacle, and described what he wanted built to mechanics of two Seabee battalions on Saipan, who then designed the apparatus using materials from an abandoned Japanese sugar mill. This apparatus became known as the “Doodlebug”. Seabees converted a LVT (landing vehicle, tracked) to supply the flotation and mobility necessary for putting the ramp itself in place. Ten of these assault ramps were built by the battalions in a short time, and enabled combat personnel and supplies to land across the ramp onto the Tinian shoreline.
The ramp consisted of two 10-inch I beam side-rails, supporting a 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, and sloped to permit their forward ends to clear the top of the bank. The first 10 feet of the timber mat was supported by the rails and the remainder by slides built over the vehicle’s cargo well. At the landing point, the forward ends of the rails were released and 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, allowing the vehicle to go ashore over the ramp.
A group of Seabees and Civil Engineer Corps (CEC) officers were assigned to ramp detail, and were responsible for maintaining the LVT ramps and ensuring that they were installed properly. The work lasted three days, as harbor facilities were non-existent and everything had to come across the barrier reef, to be unloaded and transported inland. As ramps were put into place and made ready, men and their equipment streamed onto the island. Seabee ingenuity was key to providing a tactical advantage that enabled the Marines to capture the beachhead and place Tinian under American control.