Purse Seiner

Overview

A purse seiner is a fishing vessel built around one method: surrounding a school of fish with a long wall of net, then drawing the bottom of the net closed like a drawstring purse so the fish cannot escape downward. The vessel carries the net, the winches that handle it, and a refrigerated hold large enough to store a full catch of tuna, sardine, mackerel, or anchovy. Everything on deck is arranged to set the net fast, close it before the school dives, and haul it back aboard with the catch concentrated in a shrinking pocket of mesh.

The hull is a welded steel displacement form, usually 35 to 75 m long, with a large insulated fish hold amidships and an open working deck. The main diesel drives a fixed-pitch propeller sized for bollard pull rather than top speed, because the vessel must tow a heavy net and hold station while pursing. A bow thruster gives the low-speed lateral control needed to keep the hull off the net during the set. The catch is preserved by a refrigerated seawater plant that chills the hold to near freezing, and the whole operation is run from a wheelhouse packed with sonar and radar.

Hull and structure

The hull is framed transversely with steel frames and longitudinal stringers stiffening the shell plating, which runs 8 to 14 mm thick depending on location. Watertight bulkheads divide the hull so that flooding in one compartment does not sink the ship. The dominant internal volume is the insulated fish hold, lined with food-grade aluminum over 120 to 200 mm of sprayed polyurethane foam. The hold is split by removable pen boards that stop a partial catch from sliding to one side and listing the vessel. Fuel rides in integral double-bottom tanks, and welded bilge keels along the turn of the bilge damp the rolling that would otherwise make deck work dangerous. Steering comes from a spade rudder turned by a hydraulic steering gear.

Propulsion

The main engine is a turbocharged inline-six diesel rated between 600 and 1200 kW. Combustion air is force-fed by the turbocharger, fuel is metered by the injection pump to six injectors, and the crankshaft delivers torque to the marine gearbox. That gearbox reduces engine speed to propeller speed through two gear stages and selects ahead, neutral, or astern using oil-actuated clutch packs. Power passes down the shaft line through the stern tube to the propeller. Engine heat is carried away by a closed freshwater loop that dumps into seawater through the plate heat exchanger, with the seawater pump drawing cooling water from outside the hull.

Deck machinery and net handling

The defining gear is the power block, a large rubber-lined sheave hung from the boom that hauls the wet net aboard without crushing it. Its sheave is driven by a hydraulic motor, one of many fed by the hydraulic power unit. The purse winch is the second key machine: its twin drums pull the purse line that runs through the rings on the footrope, closing the bottom of the net. Auxiliary net winches, an anchor windlass, and capstans round out the deck gear, all of them running on oil pressure from the same power unit so the crew can run several functions at once during the fast-moving set.

Net system

The net itself is a nylon mesh wall up to a kilometer long and 200 m deep. Its top edge is held up by the buoyant cork line and its bottom edge is pulled down by the weighted lead line, so the net hangs as a curtain in the water. Along the footrope hang dozens of bronze purse rings through which the purse line is threaded. To set the net the vessel circles a located school while a fast net skiff holds the far end, then the purse line is winched in, drawing the rings together and sealing the bottom. The fish are then concentrated against the hull and brailed aboard with the deck crane.

Refrigeration and preservation

Pelagic catch spoils quickly, so a purse seiner runs a refrigerated seawater plant. Two compressors circulate refrigerant through the chiller, which cools seawater that the circulation pumps push through the hold. Heat is rejected to the sea in the condenser. The hold sits just above freezing, holding the catch in chilled brine until landing. The plant is staged automatically by the refrigeration controller from hold temperature and refrigerant pressures.

Wheelhouse and electrical

Finding fish drives the electronics fit. A searchlight sonar pings horizontally to locate schools at a distance, while the echo sounder reads depth and fish density beneath the hull. A navigation radar, GPS chart plotter, autopilot, and twin VHF radios complete the bridge. Ship power comes from two diesel generators feeding the main switchboard, which distributes through molded-case breakers to the deck machinery hydraulics, refrigeration, and lighting, with a battery bank for emergency and electronic loads.