Stern Trawler

Overview

A stern trawler is a commercial fishing vessel that tows a large net behind the ship and recovers it by hauling the catch up a sloping ramp cut into the stern. This is the dominant design for deep-water trawling because it lets the gear be shot and hauled straight over the stern in line with the ship, which is safer and faster than the older side-trawler method of working the net over the rail. Larger stern trawlers carry a full processing factory and freezer hold so they can stay at sea for weeks, returning with a finished frozen product rather than fresh fish on ice.

The ship is built on a strongly framed Hull Structure with a Stern Ramp. The catch is taken by the Trawl Gear, worked by the Trawl Winch System through the Stern Gantry, and turned into product in the Fish Factory & Hold. The Propulsion Train train tows the gear and drives the ship, the Deck Machinery supports operations, and the crew live and navigate from the Deckhouse.

The trawl gear

The Trawl Gear is the fishing apparatus itself. The Trawl Net is a long cone of netting that narrows to the Codend, the closed bag of regulated mesh size that retains the fish. The net is held open in the water by two Otter Board doors, angled steel or composite panels whose hydrodynamic lift sheers them outward and spreads the net mouth. The upper mouth is lifted by Headline Float buoys and the lower mouth is kept on the seabed by a weighted Ground Rope fitted with rubber bobbins. Between the doors and the net, Bridle & Sweep sweeps herd fish into the path of the trawl. The whole assembly is towed by two steel Towing Warp ropes running from the deck winches.

Shooting and hauling

The Trawl Winch System handles the gear through the fishing cycle. To shoot, the net is paid off the wide Net Drum over the stern roller, the otter boards are streamed from the gallows, and the two Trawl Winch units pay out the warps to the towing length. While towing, an auto-trawl Winch Control keeps the two warps at matched length and tension so the net tows level and symmetric. To haul, the warps are wound back in, the doors are hung off on the gallows, and the net is wound back onto the drum; the heavy codend is then dragged up the ramp by a Gilson Winch and emptied into the deck receiving pound. All this machinery runs from a Hydraulic Power Pack power pack driven off the main engine or a generator.

The stern gantry

The Stern Gantry is the steel arch over the after deck and ramp. Its Gantry Frame carries the net-handling blocks through which the warps and net run, the Gallows from which the otter boards hang when the gear is out of the water, and the large free Stern Roller at the top of the ramp over which the gear passes without chafing. The gantry takes the full towing and hauling loads and is one of the most heavily worked structures on the ship.

Onboard processing

A freezer trawler carries a complete factory so the catch is preserved at its freshest. The Fish Factory & Hold receives fish from the deck pounds, runs it through a Heading Machine and then a Filleting Machine, and freezes the product in horizontal Plate Freezer units fed by a Refrigeration Plant. The frozen blocks are stored in the insulated Freezer Hold for the length of the trip. Offal and bycatch go to a Fishmeal Plant that cooks, presses and dries them into meal and oil, so almost nothing is discarded and the trip earns a second product.

Propulsion and towing power

Trawling is a towing job, so the Propulsion Train train is sized for high thrust at low speed rather than for top speed. A medium-speed Main Engine drives a Controllable-Pitch Propeller controllable-pitch propeller running inside a Kort Nozzle Kort duct, which boosts thrust at the slow speeds where the ship drags its net. The gearbox carries a power take-off feeding the deck hydraulics. Controllable pitch lets the skipper hold a steady tow against a changing load as the net fills or the weather builds, and the high bollard pull lets the ship keep towing in a head sea.

Navigation, electronics and variants

The skipper works the fishery from the raised Wheelhouse, whose Fishing Sonar search sonar and net sounder show where the fish are and how the trawl is fishing and filling. Stern trawlers range from medium wet-fish boats of fifty metres that land fresh fish on ice up to large factory freezer trawlers over a hundred metres with several processing lines and a crew of forty. The design covered here, a mid-size freezer trawler, is the workhorse of distant-water fisheries, combining catching, processing and cold storage in one self-contained ship that can fish far from port for weeks at a time.