A cable ship is a specialized vessel built to lay, repair, and maintain the submarine cables that run along the ocean floor. These cables carry over 95% of intercontinental internet traffic and increasingly deliver offshore wind power to shore. Cable ships are floating factories equipped with massive storage tanks, precision machinery, and positioning systems that allow them to unspool thousands of kilometers of cable across ocean basins with remarkable accuracy.
What Cable Ships Actually Do
Cable ships perform three core jobs: installing new submarine cables, repairing damaged ones, and maintaining existing routes. During installation, the ship follows a pre-planned route at a slow, steady pace while feeding cable from onboard storage tanks over the stern or bow and down to the seabed. The work can take weeks or months depending on the route length. A transatlantic crossing, for example, covers roughly 6,000 kilometers.
When a cable breaks or develops a fault, a cable ship heads to the affected area, locates the damage, and performs what amounts to underwater surgery. The ship drags a cutting grapnel along the seabed to sever the cable at the fault point, then picks up one end and tests it back to shore to confirm no additional faults remain. Any damaged section gets cut away, and a fresh piece of spare cable is spliced in to bridge the gap. Both splice joints are tested before being lowered carefully to the seabed. If conditions allow, a remotely operated vehicle (ROV) buries the repaired section for protection. The whole process can take days, and in deep water or harsh conditions, considerably longer.
Specialized Machinery Onboard
What sets a cable ship apart from other vessels is the equipment packed into its hull and deck. The most important component is the cable tank: large circular holds below deck where cable is coiled in tight, flat layers. A single ship can carry thousands of tons of cable in these tanks. The world’s largest cable-laying vessel, the Fleeming Jenkin launched by Jan De Nul, holds 28,000 tonnes of cable, double the capacity of any other ship in the fleet.
To control how cable feeds off the ship, cable ships use a linear cable engine, a machine typically made up of 21 pairs of wheels arranged in parallel. Each pair of wheels can grip the cable with about 2 tons of holding force, giving the crew precise control over the speed and tension as cable pays out. This matters because too much slack causes tangles on the seabed, while too much tension can snap the cable or pull it off course.
Sheaves, which are large grooved wheels about 3 meters in diameter, guide the cable overboard. These are split so one half can feed cable out while the other half recovers cable during repairs. Towing winches hold over 2,000 meters of high-strength steel wire, used to pull sea plows that cut trenches in the seabed for burial. A separate spool carries a command cable that controls the plow remotely from the ship.
Slack control systems tie all of this together. They monitor exactly how much cable is being paid out relative to the planned route, integrating data from the ship’s navigation system and its dynamic positioning controls. Getting slack right is critical: the cable needs enough extra length to conform to the contours of the ocean floor without piling up or stretching tight.
How Cable Ships Stay in Position
Laying cable on the ocean floor demands pinpoint positioning, sometimes in deep water with strong currents. Cable ships use dynamic positioning (DP) systems that combine GPS, thrusters, and computer controls to hold the vessel on a precise track without anchors. Most modern cable ships carry DP Class 2 or Class 3 systems. A Class 2 system ensures the ship won’t lose position even if a single active component fails. Class 3 takes that further, maintaining position through any single fault or failure in the system, including fire or flooding in one compartment.
This level of redundancy matters because drifting even slightly off course during a splice operation could damage the cable or compromise the repair. During laying operations, the DP system keeps the ship moving along its planned route at a steady, controlled speed, often just 1 to 2 knots.
Burying Cables for Protection
In shallow coastal waters where fishing nets, anchors, and storms threaten exposed cable, ships bury cable beneath the seabed. The primary tool is a sea plow, towed behind the ship, which lifts a wedge of sediment so the cable can be inserted below. The trench is typically less than 1 meter wide. In areas where a plow can’t operate, water-jet machines cut a narrow trench instead.
In deeper water, ROVs handle burial and inspection work. These remotely piloted robots descend to the cable and can jet-trench, inspect for damage, or assist with splice burial. For cables buried more than a meter into the seabed, repair crews sometimes need multiple passes with a cutting grapnel just to locate and retrieve the cable.
Telecom Cables vs. Power Cables
Most cable ships were historically built for telecommunications work, laying fiber optic lines that are relatively thin and lightweight. But the growth of offshore wind energy has created demand for a different kind of cable: high-voltage power cables that are thicker, heavier, and stiffer. Power cables require ships with greater deck strength, larger cable tanks, and more powerful handling equipment. Some vessels specialize in one type, while newer ships are designed to handle both. The Fleeming Jenkin, for instance, was built specifically to serve the offshore energy market with its massive carrying capacity.
A Brief History of Cable Ships
The first cable ships weren’t purpose-built at all. In the mid-1800s, operators converted ordinary merchant vessels by adding cable tanks, brakes, and basic handling gear. The most famous early cable ship was the SS Great Eastern, originally a passenger liner that proved too large for commercial success. Converted for cable work, the Great Eastern attempted to lay the first transatlantic telegraph cable from Ireland to Newfoundland in 1865. That attempt failed when the cable snapped and couldn’t be recovered from the deep Atlantic. But in 1866, the Great Eastern returned, successfully completed a new cable, and then recovered and finished the 1865 cable as well. In 1869, she laid a French transatlantic cable from Brest.
Since then, cable ships have evolved into highly specialized vessels. Modern ships bear little resemblance to those converted merchant hulls, though the fundamental job is the same: get cable from the ship to the seabed reliably and precisely.
The Global Fleet Today
The International Cable Protection Committee tracks roughly 63 cable ships operating worldwide. That number is small relative to the scale of the global submarine cable network, which includes over 500 cable systems spanning more than 1.4 million kilometers. The limited fleet size means scheduling can be tight, especially for urgent repairs. When a major cable breaks, the nearest available ship may be days or weeks away, which is why some cable operators keep dedicated repair vessels stationed in strategic locations.
Demand for cable ships is growing on two fronts. Tech companies are investing heavily in new subsea fiber routes to handle surging internet traffic, while offshore wind developers need power cables connecting turbines to the grid. Both trends are driving orders for larger, more capable vessels that can operate in deeper water and carry more cable per voyage.