Crab Fishing Boat

Overview

A crab fishing boat is a pot-fishing vessel. It does not tow a net; instead it sets baited traps, called pots, on the seabed, leaves them to soak while crab climb in, and then hauls them back up, empties the catch, re-baits and resets. The work is heavy and hydraulic: each steel pot can weigh over three hundred kilograms, a boat works a hundred or more of them, and the whole cycle happens on an open deck in cold, rough water. The defining feature of a modern crab boat is that it keeps the catch alive in circulating tanks so it can land a premium live product.

The boat is built on a beamy, high-bulwark Hull & Working Deck with a large open working deck. The gear is the Pot System, worked by the Hauling Gear, and the catch is kept alive in the Live-Tank System system. The Propulsion Train train drives the boat, the Deck Hydraulic System power the deck machinery, the Deck Machinery supports operations, and the crew live and navigate from the Deckhouse.

The pots

Each Crab Pot is a heavy welded-steel trap. A reinforcing-bar Pot Frame gives it shape and the weight to stay put on the seabed in current, covered by stainless or coated Pot Mesh. Sloped one-way Entrance Tunnel entrances let crab climb in but make leaving difficult, and a lashed Pot Door is opened to empty the catch. The door carries a biodegradable escape panel so that a lost pot stops fishing after a time, a rule meant to prevent ghost fishing. Each pot is baited with a Bait Jar, connected by a Pot Line to a surface Marker Buoy that marks it for retrieval. Pots are often set in long strings sharing a groundline, or set singly, depending on the fishery.

Hauling the gear

Bringing a heavy pot up from the bottom is the central deck task, handled by the Hauling Gear. The crew gaff the buoy, lead the line over the rail block to the Pot Hauler, a hydraulic V-sheave machine whose Hauler Sheave pinches and pulls the wet line at speed to raise the pot. As the pot breaks the surface, the Picking Boom lifts it clear of the water and swings it onto the Pot Launcher rail at the rail, where the crew empty it, sort the catch, re-bait it and tip it back overboard. The hauled line is flaked into a tub by a Line Coiler so it runs out cleanly on the next set, and a hydraulic Deck Crowder pushes the pot stack tight against the rail so the heavy traps can be moved by hand on a rolling deck.

Keeping the catch alive

What separates a crab boat from a simple trap boat is the Live-Tank System system. Live crab fetches a far higher price than dead or processed crab, but crab die quickly if held warm or in stale water. The boat therefore carries insulated Live Tank tanks below the working deck through which chilled sea water is kept moving by Circulation Pump units, cooled by a Sea-Water Chiller and kept oxygenated by Tank Aerator diffusers. A Tank Filter & UV cleans the water and a Tank Controller controller watches temperature, oxygen and flow and runs the plant automatically. Held this way, crab survive the trip and the days to market, which is the whole economic point of the boat.

Deck hydraulics

Almost every working machine on a crab boat runs on oil pressure, so the Deck Hydraulic System system is the muscle of the vessel. Engine-driven Hydraulic Pump units draw from the Hydraulic Tank reservoir and feed Valve Bank control stacks that operate the hauler, the picking boom, the crowder and the deck crane through high-pressure Hydraulic Hose runs. A Hydraulic Accumulator smooths the demand peaks when several functions move at once. Hydraulic drive is chosen because it gives smooth, controllable force, survives salt water and shock, and can be stopped instantly, which matters on a deck where crew work alongside heavy moving gear.

Propulsion and machinery

The boat is driven by the Propulsion Train train, a medium-speed Main Engine turning a Propeller through a gearbox that also carries a power take-off for the deck hydraulics. Propulsion is sized for thrust and sea-keeping rather than top speed, because the boat spends its time working slowly among its gear in bad weather. A Generating Set generating set carries the live-tank chiller and pumps and the ship electrical load, since these run continuously through the trip.

Variants and use

Crab boats vary widely by fishery. Small inshore boats of ten metres fish a few dozen pots a day trip with simple live wells; large offshore boats over thirty metres carry two hundred heavy pots and full circulating live tanks for trips of a week or more in exposed waters. The same pot-fishing layout is used for lobster, spider crab and whelk with changes only to pot design and bait. Across all of them the engineering job is the same: lift heavy traps safely off the bottom by the hundred and keep a fragile live catch in good condition until it reaches the market.