Roll-On Roll-Off Ship

Overview

A roll-on roll-off ship carries wheeled cargo that drives aboard under its own power or on a trailer: lorries, trailers, cars, buses, plant and military vehicles. There is no crane work and no container handling, just ramps and drivers, which makes RoRo the fastest cargo mode to load and discharge. The ship is a stack of long open Vehicle Decks linked by a Ramp & Lift System, sealed at sea by a Watertight Door System, and driven by twin medium-speed Medium-Speed Main Engines for fast schedules. RoPax variants add passenger accommodation on the upper decks. The hull comes from around 50 prefabricated Hull Blocks.

Hull and decks

The RoRo hull is structurally awkward. Cargo must drive the full length of each deck, so there are no transverse bulkheads and few longitudinal ones inside the cargo spaces. That removes the subdivision that makes most ships hard to capsize and concentrates strength in the side shell, the few Deck Pillar lines and deep Web Frames spanning the open decks. Each Vehicle Deck is a wear-resistant Deck Plate surface set with thousands of Lashing Points on a grid, so any vehicle position can be secured with Lashing Chains. Deck Deck Scuppers clear wash and firefighting water, and Deck Marking lanes guide drivers during the loading rush.

Ramps and lifts

Loading speed is the product. The Stern Ramp is the main access: a large hinged steel span, often quartered to one side so it can reach a quay at an angle, strong enough for the heaviest road trains. Its Ramp Girders carry the bending load, Ramp Cylinders raise and lower it, and a Ramp Flap toe bridges the gap to the quay as the ship moves on its moorings. Inside, Internal Ramps link the decks so traffic drives straight through, and Car Lifts move light vehicles between decks where a fixed ramp would waste lane metres. The whole system runs on hydraulics from Ramp Hydraulic Power Pack power packs.

Doors and survivability

The vehicle decks must be sealed watertight at sea, and this is the most safety-critical part of a RoRo ship. The Watertight Door System includes a Stern Door often combined with the ramp, Side Doors for simultaneous loading, and on some ships a Bow Door. Each is clamped to its seal frame by hydraulic Door Cleats against a Door Seal, with a Door Indicator confirming closure on the bridge before sailing.

The reason for all this rigour is grim history. The Herald of Free Enterprise capsized in 1987 after sailing with her bow door open; the Estonia sank in 1994 after her bow visor failed in heavy seas. In both, water flooded across the open vehicle deck, the free surface destroyed stability in minutes, and hundreds died. The disasters drove the SOLAS amendments and the Stockholm Agreement on damage stability that now govern every RoRo ship: door indication on the bridge, stricter residual-stability standards with water on deck, and design against the free-surface effect that makes these ships unforgiving once flooded.

Ventilation and fire safety

Enclosed decks full of running vehicles need huge ventilation. The Deck Ventilation System uses Ventilation Fans sized for many air changes an hour to clear exhaust during loading and on passage, with Gas Detectors watching carbon monoxide and fuel vapour. Fire is the other deck hazard: a single burning vehicle among tightly packed traffic can spread fast. The Fire Safety System covers the decks with a drencher deluge of Drencher Nozzles fed by Drencher Pumps and opened zone by zone through Drencher Section Valves, backed by Fire Detectors and Fire Dampers that seal the ducts to stop a fire travelling.

Propulsion and manoeuvring

Schedules are tight and berths are frequent, so the ship is built for speed and agility rather than ocean economy. Twin medium-speed Medium-Speed Main Engines of around 40 cm bore drive through reduction gears to twin Controllable-Pitch Propeller controllable-pitch propellers, giving 20-22 knots and full thrust control without stopping the engines. Each Cylinder Unit runs a Piston in a Cylinder Liner under a Cylinder Head, fed by the Fuel Injection System and kept turning by the Lubrication System. Twin Bow Thrusters and high-lift Rudders let the ship turn in its own length and berth fast at every call.

Accommodation and systems

The crew of around 25-30, plus passengers on RoPax ships, live in the Deckhouse above the upper vehicle deck. Crew Cabins sit on floating floors against engine and deck noise, served by an HVAC Plant, with the Bridge Console carrying ramp and door status alongside the usual navigation displays. Davit-launched Lifeboat Station boats meet SOLAS for the persons aboard. Four Auxiliary Gensets carry the heavy electrical load that ramps, lifts, ventilation and thrusters impose. A RoRo ship turns a port call into minutes rather than days, which is why it dominates short-sea trades, ferry routes and military sealift.