Coast Guard Cutter

Overview

A coast guard cutter is a medium patrol vessel used for search and rescue, fisheries protection, customs and immigration enforcement, anti-smuggling patrols and pollution response. The term cutter covers a wide size range, but a typical offshore patrol cutter is 40 to 60 m long, displaces 350 to 900 tonnes, and carries a crew of 20 to 40 for patrols lasting up to three weeks. The design balances three competing demands: enough speed to intercept fast craft, enough range and endurance to stay on station far from base, and enough deck space and boat-handling capacity to recover people and board other vessels.

The cutter is built around a welded steel Hull and Superstructure with an aluminium superstructure to keep weight high in the ship low. Twin diesel Propulsion System drives controllable pitch propellers for fast acceleration and precise low-speed handling. Power for ship systems comes from a diesel-generator Electrical System plant. An integrated Integrated Bridge and Navigation and a Communications Suite suite let a small crew run the ship and coordinate with shore. The mission equipment is the Search and Boarding Gear, the Deck Gun Mount, and the Firefighting and Damage Control systems that keep the ship safe.

Hull and structure

The Hull and Superstructure is a welded steel monohull. The bottom and side shell are Hull Plating stiffened by a Frame Set of transverse and longitudinal frames, all carried on the Keel that runs the length of the hull bottom. The hull is divided by eight Watertight Bulkhead walls into watertight compartments, so flooding from a collision or grounding stays contained and the ship keeps enough buoyancy to stay afloat. A Bilge Keel runs along each side at the turn of the bilge to damp rolling. The Superstructure sits on the Main Deck and is aluminium to save topweight. Twin Rudder blades give the ship a tight turning circle.

Propulsion and steering

The Propulsion System system uses two medium-speed diesels. Each Main Diesel Engine is rated at 2000 to 4000 kW and drives a Reduction Gearbox that steps the engine speed down and provides the clutch and astern function. Power passes through a Propeller Shaft that exits the hull through a Stern Tube sealed against seawater by an Oil Seal, supported inside the hull on a Shaft Bearing line. Each shaft turns a Controllable Pitch Propeller with controllable pitch, so the ship can change thrust from full ahead to full astern without changing engine speed. This is valuable when holding station alongside another vessel or maneuvering in harbor. The Steering Gear is a hydraulic ram unit that turns the rudders on bridge command. Engine exhaust leaves through an Exhaust Uptake in the funnel, and a Seawater Cooling Pump circulates seawater through the engine heat exchangers.

Power, bridge and communications

The Electrical System plant runs three Diesel Generator sets, normally two online and one on standby, feeding a Main Switchboard at 440 V. A Distribution Transformer supplies 230 V for lighting and small loads through local Distribution Panel boards. In port the ship draws from a Shore Connection so the generators can be shut down. If main power is lost, an Emergency Generator starts automatically to feed steering, navigation and communications, backed by an 12 V Battery bank.

The Integrated Bridge and Navigation is an integrated console layout. The watchkeeper steers from the Helm Console using an Autopilot for long passages. Two Navigation Radar sets, an X-band for close detail and an S-band for long range in rain, feed target data alongside ECDIS charts and the GPS Compass for position and heading. An AIS Transponder shows nearby traffic, and trainable Searchlight units illuminate targets at night. The Communications Suite suite carries VHF Radio for short range, HF Radio for over the horizon, and a SATCOM Terminal for data links, all served by antennas on the Antenna Mast. An EPIRB transmits the ship position by satellite if it is abandoned.

Search, rescue and boarding

The cutter earns its living with the Search and Boarding Gear. A fast Rigid Inflatable Boat is the workhorse for intercepting and boarding other vessels and for plucking people from the water. It is launched over the side by a RHIB Davit or, on faster ships, run off a Stern Launch Ramp while the cutter is still moving. A Rescue Net is rigged over the side so survivors can climb aboard, and a Towing Winch lets the cutter tow a disabled vessel to port. A Deck Crane handles stores and the boat, and several Liferaft canisters cover the cutter own crew. For law enforcement the cutter carries a Deck Gun Mount: a stabilised Gun Barrel of 25 to 30 mm on a Gun Cradle, trained and elevated by Servo Motor drives under a Fire Control Unit sight, used mostly for warning fire and to disable an engine or steering.

Habitability and safety

For patrols of two to three weeks, the Accommodation and HVAC matters. Crew sleep in cabins fitted with Crew Berth bunks, eat from a Galley in the Mess Room, and a Watermaker desalinates seawater to fill the Fresh Water Tank. The Sewage Treatment Plant treats black water before discharge. Safety rests on the Firefighting and Damage Control system: a Fire Pump pressurises the Fire Main feeding hydrants around the ship, a CO2 System floods the engine room to smother a machinery fire, a Bilge Pump clears flooded spaces, and a network of Fire Detector and Pressure Sensor units reports trouble to the bridge. Cutters serve long lives, often 25 to 40 years, and are reengined and refitted several times to keep their sensors and machinery current.