Landing Craft

Overview

A landing craft is a flat-bottomed boat built to carry vehicles, cargo, and troops from a ship or staging port directly onto an unprepared beach. The flat bottom lets it run up onto sand and sit nearly upright while the load drives off over a Bow Ramp System that drops onto the foreshore. It then backs off, refloats, and returns for the next lift. The design trades open-water comfort and speed for the ability to deliver heavy loads where there is no pier, crane, or harbor. The utility classes described here cover the common LCU and LCM types, with the smaller LCVP personnel boat as a related variant.

Hull and structure

The Hull Structure is a welded steel box with a shallow flat bottom and near-vertical sides forming a long cargo well. The Bottom Plating is thicker than on a normal boat because it grounds on sand, gravel, and rock and must take the abrasion and point loads of beaching. Transverse strength comes from the Transverse Frame Set, and fore-and-aft bending is carried by the Longitudinal Stringer Set. Several Watertight Bulkhead walls subdivide the hull so that damage forward, where beaching impact is most likely, does not flood the engine spaces aft. A Keel Skeg protects the propeller and helps the craft track, and a Rubbing Strake / Fender along each side absorbs contact when working alongside a mother ship. Steel below the waterline corrodes fast in salt water, so a Sacrificial Anode Set of sacrificial zinc protects the hull and running gear.

The bow ramp and how beaching works

The bow ramp is what defines the type. At sea the Bow Ramp Door is raised and sealed by the Ramp Seal and held by a Ramp Sea Latch so it acts as the bow of the boat. On approach the coxswain runs the craft straight at the beach under power and grounds the flat bottom on the sand. Before grounding the crew may flood a Ballast Tank to trim the bow up, then pump it out to lift the bow once cargo is off. The Ramp Hydraulic Cylinder pair lowers the ramp about its Ramp Hinge Pin until the Ramp Plate rests on the foreshore, and vehicles drive down the Traction Strip cleats onto dry ground. A Ramp Recovery Winch provides a backup if the hydraulics fail. Power for the ramp comes from the Hydraulic Power Unit, an engine-driven pump and reservoir that also feeds the steering. To retract, the engines go astern, the bottom drags off the sand, the ballast is adjusted, and the ramp is raised for the next run.

Cargo deck

The open Cargo Deck is a flat well with Deck Plating strong enough for tracked vehicle ground pressure. Loads are secured against ship motion with two dozen flush Tie-Down Fitting rings and held in place during transit with a Wheel Chock at each wheel. A Side Coaming runs down each side to keep water and cargo within the well, and any sea or rain that comes aboard drains overboard through a Deck Scupper Drain. A typical utility craft carries one main battle tank or several loaded trucks; the smaller personnel variant carries an infantry section instead.

Propulsion and steering

Two independent Propulsion Line lines drive the craft so it stays controllable on one engine. Each line runs a turbocharged Marine Diesel Engine through a Marine Reduction Gearbox that reduces speed and selects ahead or astern, then out along a Propeller Shaft through the watertight Stern Tube to a fixed-pitch Propeller. Sea water drawn through the Engine Cooling System loop carries off engine heat, exhaust leaves through the water-cooled Exhaust System, and diesel comes from the Fuel System. Twin rudders in the propeller wash give the bite needed to steer at the low speeds used when working a beach. The Steering System turns each Rudder with a Steering Cylinder acting on a Tiller Arm, so the craft can twist its stern against wind and current while half aground.

Ship systems

Below and around the cargo well sit the support systems. The Bilge, Ballast & Firefighting clears water that collects in the hull, shifts ballast to set draft and trim for beaching, and pressurizes a seawater fire main. Electrical power comes from the Electrical System: a Diesel Generator Set feeds a Main Switchboard, and a bank of 12 V Battery units backed by a Battery Charger holds up engine starting and the DC services. Navigation and ship handling are run from the Wheelhouse, a raised steel box with the helm, engine controls, a Navigation Radar, a GPS Compass, and a VHF Radio. On longer passages the crew uses the enclosed Crew Accommodation, with seating, bunks, heating, and a Liferaft for abandonment.

Variants and use

The family spans several sizes. The LCVP is a small craft for an infantry section. The LCM is a medium craft sized for one vehicle or a platoon of troops. The LCU described here is a larger utility craft that carries a main battle tank or a cluster of trucks and can make coastal passages on its own bottom. All share the flat hull, bow ramp, and shallow draft. Beyond amphibious assault, the same craft move stores and vehicles to islands and undeveloped coasts, land relief cargo after disasters, and ferry equipment where ports are wrecked or do not exist. The trade is deliberate: the hull is slow and wet in a seaway, but it puts heavy loads on a bare beach, which a conventional ship cannot do.