Anchor Windlass Product

Overview

An anchor windlass is a motorized winch mounted on a ship's forecastle deck that hauls or lowers the anchor chain using powered rotation of a gypsy drum. The gypsy drum—a ribbed, sprocket-like spool—grips the chain through a segmented stripper band, transmitting continuous torque without slip even under shock loads. Hydraulic anchor windlasses are the marine industry standard for vessels above 30 meters because they deliver high pulling force (30–80 tonnes) with smooth proportional control, fail-safe load-holding via spring-applied brake, and compact integrated power.

The hydraulic drive path begins at a variable displacement piston pump (25–40 cc/rev) fed by a 150–200 liter reservoir. The pump supplies pressurized fluid (280–350 bar) through proportional directional control valves to a bent-axis reversible motor. The motor spins at low speed (~50–200 rpm) but high torque due to its geometry and a integral planetary gearbox (typically 10:1 to 20:1 ratio), multiplying the haul force applied to the gypsy drum.

Load holding is passive: a large spring-applied multi-disc brake is energized via pilot pressure while hoisting; when hydraulic pressure decays (motor de-energized), heavy preload springs lock the brake statically, preventing any drift. This fail-safe design is mandatory for moored vessels where unintended anchor slip creates collision risk. Deck-mounted controls—wireless or hardwired proportional pendant—allow the mate to smoothly modulate haul speed and instantly halt the load.

Modern units integrate accumulator damping (2–5 liter bladder accumulator) to absorb shock when the drum suddenly engages a slack chain and is also used for emergency lower if main pump fails. All components are rated for salt-spray and UV exposure per DNV, ABS, or RINA marine class requirements.

How it works

Chain engagement. The gypsy drum rotates, and the segmented chain stripper—bronze jaw segments held in a band brake—grips each chain link at its shoulders. As the drum turns, friction pulls the chain link over the stripper jaws, advancing the chain without slipping. The ribbed drum geometry ensures smooth engagement even if the chain has slack or shock loading.

Motor and gearbox. A bent-axis hydraulic motor (8–15 cm³/rev, swashplate type) converts high-pressure fluid flow into mechanical rotation. Because the motor alone would spin too fast (~500–1500 rpm) for adequate pulling force, a compact planetary gearbox (10:1 to 20:1) reduces speed and multiplies torque. The combination delivers 30–80 tonnes pulling force at the gypsy drum.

Haul and lower control. The ship's mate holds a proportional control pendant with a joystick. Pushing the stick forward energizes pilot solenoids that open the proportional directional valve spool, routing pump pressure to the motor. Motor displacement (spool position) is proportional to joystick deflection, so pulling speed is smooth and operator-controlled. Reversing the joystick (or pressing a lower button) switches fluid routing to rotate the motor backward, paying out anchor chain.

Brake hold. The spring-applied brake is a large multi-disc unit clamped shut by heavy preload springs. During haul, a solenoid pilot valve opens an orifice to the brake piston, routing pump pressure behind the piston. This pilot pressure pushes against spring force, retracting the piston and releasing the brake discs (unclamped). When the motor is de-energized, pump pressure decays, pilot pressure drains away, and springs slam the brake piston back, re-clamping the disc stack on the motor output shaft. The friction coefficient of sintered bronze discs (>0.4 wet) ensures the entire static load is held indefinitely.

Accumulator function. A bladder accumulator (2–5 liters at 350 bar) is charged to roughly 80% of system pressure. When the drum engages slack chain and the load surges, system pressure spikes. The accumulator bladder collapses, absorbing energy and dampening shock oscillations. Conversely, if main pump pressure is lost (engine shutdown), a small charge of accumulator energy can power the solenoid pilot valve and emergency lower solenoid, allowing the brake to be momentarily released so the anchor can pay out slowly under load without sudden free-fall.

Power loop closure. Return fluid from the motor and brake flows back to the reservoir through a 10 micron filter. Any entrained air rises to the top of the tank; baffles slow fluid velocity so particles settle. A return relief valve (cracking at ~10 bar above inlet) protects against reverse-flow shocks. The pump inlet draws from the settled, clean pool at the reservoir bottom, maintaining fluid purity and motor seal life over years of service.

Subsystems and maintenance

Gypsy drum service. Chain strippers wear because they rub the chain. Periodic inspection (every 500–1000 haul cycles) checks stripper jaws for cracking or flattening; if friction drops below specification, the entire band is removed and rebuilt with new bronze segments. The drum barrel itself is durable ductile iron and rarely requires resurfacing unless the ship has endured extreme shock loads or grounding.

Gearbox flushing. Every 2 years, hydraulic fluid is sampled and analysed for wear particles and water content. If particle count exceeds ISO 4406 16/14/11, or water exceeds 500 ppm, a complete flushing and fluid change is mandated. Planet bearings and gears are precision-lapped and depend on film thickness; contamination causes accelerated wear.

Motor and seals. The bent-axis motor swashplate has high-precision surfaces that can pit if contaminated or rust if hydraulic fluid degrades. Annual visual inspections check for external leakage at shaft seals; if a oring set is replaced, the work includes flushing the motor case and replacing all internal seals to prevent re-contamination.

Brake disc inspection. Sintered bronze discs are inspected during drydock; if worn below a minimum thickness (typically 2 mm) or if friction has declined due to glazing, the entire disc pack is replaced. Spring preload is checked with a specialized gauge; if springs have lost set, the brake housing is sent to a specialist for re-springing.

Hydraulic fluid. ISO VG 46 anti-wear hydraulic fluid is standard. For tropical deployments, water separator additives are used. Fluid change interval is 3–5 years or based on laboratory condition monitoring (particle count, viscosity index, acid number). Contamination is the primary failure mode of hydraulic motors, so fluid discipline is critical.

Standards and certification

All deck-mounted machinery is built to Anchor Windlass specifications from classification societies:

• DNV-GL: Propulsion, steering, and anchor handling machinery must comply with DNV-RP-C205 (Marine operations at low temperature), and all pressure equipment meets PED (Pressure Equipment Directive) 2014/68/EU.
• ABS: American Bureau of Shipping requires windlass brakes to hold 125% of static anchor load, pressure relief at 110% of rated working pressure, and a secondary emergency lower circuit.
• RINA: Italian classification specifies shock load testing (sudden chain engagement at 2× design load) and fatigue cycling to 50,000 load reversals.

Hydraulic systems are inspected annually for leakage, system pressure verification (static and dynamic), brake holding force (pull test at rated load), and electrical pendant functionality (wireless communication range, emergency stop response time). Documentation of all maintenance and fluid changes is part of the vessel's class file.