Boat Davit Crane Product

Overview

A lifeboat davit is a hydraulic crane mounted on a ship's side or stern that lowers lifeboats, rescue craft, or fast rescue craft to the water surface. The davit consists of a tapered steel boom that pivots at a luffing axis (controlled by a hydraulic cylinder), a slewing gear bearing allowing the boom to rotate toward the water's edge, a dual-fall winch system that independently controls port and starboard wire ropes, and a hydraulic power pack with proportional controls.

The critical design feature is the "lifeboat descent" sequence: the luffing cylinder angles the boom downward to swing the lifeboat clear of the ship's hull, then the winch is played out under proportional speed control so the boat descends at a steady rate (typically 15–30 m/min) until the boat bottoms in the water. The dual-fall system ensures balanced lowering—if one fall runs out faster than the other due to rigging differences, the dual-drum winch automatically compensates, preventing the boat from tilting.

Davits are subject to the International Convention for Safety of Life at Sea (SOLAS), which mandates that lifeboats can be lowered in adverse conditions (wind to 60 knots, sea state 4) and that the entire descent and launch be reversible if the boat becomes entangled. Modern davits feature emergency lower valves that maintain powered descent even if main hydraulic pump power is lost, a critical safety requirement for rescue operations in power-loss scenarios.

How it works

Boom geometry and luffing. The davit boom is a tapered welded steel tube, typically 8–15 meters long, pivoting at a luffing axis 8–12 meters above the normal waterline. A double-acting hydraulic cylinder (bore 50–80 mm, stroke 500–800 mm) is connected between the boom and the pedestal base frame. When the operator moves the control joystick forward, proportional directional valves open pilot lines to the luffing cylinder. Pump pressure pushes the piston rod, extending the cylinder and rotating the boom downward from a stowed near-horizontal position (10°) toward vertical (90°). This action swings the suspended lifeboat clear of the hull, preventing collision with the ship's side during descent.

Slewing and position. A separate slewing drive—either a hydraulic motor coupled to a gear pinion or an electric geared motor—rotates the entire boom-and-pedestal assembly around a large slew bearing (turntable bearing, 800–1200 mm pitch diameter) embedded in the pedestal base. The operator manually slews the boom 180–360 degrees to position the lifeboat over the water. The slew bearing is sealed with primary and secondary dynamic seals to prevent saltwater intrusion.

Winch and dual-fall descent. The dual-fall winch is powered by an electric AC motor (7–15 kW) or hydraulic motor through a parallel-axis gearbox (10:1 to 15:1 ratio). The gearbox has dual independent output shafts, each driving a separate ductile iron drum (250–350 mm diameter). Wire rope is wound on each drum in a split configuration: the first few wraps are common (wound together), but then the wraps separate so that each fall can unspool independently.

When the operator presses the "lower" button on the control pendant, the motor spins the drums at a speed proportional to the button pressure (proportional hydraulic controls or VFD for electric motors). As the drums rotate, wire rope pays out from both port and starboard falls at nominally the same rate. If one fall has slightly more inertia or friction, a mechanical differential or load-sensing valve (not shown in basic description) ensures the other fall doesn't advance faster, maintaining lateral balance of the lifeboat.

Emergency lower. If the main hydraulic pump loses pressure (engine failure, system rupture), an accumulator charged to 150 bar supplies pilot pressure to an emergency lower solenoid valve. When the solenoid is energized, it opens a ported path that allows proportional descent of the winch using residual accumulator energy. This ensures the lifeboat can be lowered safely to the water even if the ship's main power is lost.

Speed control and load feedback. The proportional directional valve governing winch speed includes load-sensing logic that prevents uncontrolled free-fall. If a fall becomes slack (boat touches water), the fall pressure drops and the proportional valve automatically reduces pump flow to that fall, preventing violent jerk-loading. Pressure transducers on each fall provide feedback to the deck officer, displaying real-time load on the control panel display.

Water contact and rescue pick-up. Once the lifeboat bottoms in the water, it floats and crew unfasten the shackles connecting the davit falls to the boat lift lugs. The boat drifts away, and the winch operator spools the empty falls back to the stowed position. Reverse operation hauls the lifeboat back aboard using the same proportional winch and luffing controls.

Design requirements and testing

SOLAS and IMO standards. Every lifeboat davit must be certified to IMO LSA Code (International Life-Saving Appliance Code, Chapter 6.1), which specifies:

• Dual falls capable of independent operation; if one fall ruptures, the other must hold the lifeboat to prevent catastrophic drop.
• Winch capable of lowering at 15–30 m/min without creeping (unintended descent).
• Hydraulic system must maintain full descent capability for 10 minutes after primary pump failure, using only accumulator energy.
• Boom luffing must prevent any "swing-out" motion that could collide the boat with the ship's hull during deployment.

Proof load testing. Before commissioning and then every 5 years during class survey, the davit is subjected to proof load testing: a static load of 125% of the maximum lifeboat weight is suspended from both falls (using purpose-built test bags) and held for 10 minutes. All mechanical and hydraulic components are visually inspected for deformation, leakage, or crack initiation. Wire ropes are proof-tested by pulling the test load through a simulated descent cycle (lower and raise) 20 times, checking for strand breakage or outer wire distress.

Endurance testing. Some classifications (DNV, ABS) require endurance testing: the davit is cycled (lower to water, raise to stowed) 100 times with a nominal 50% load. This verifies that the mechanical brakes, gearbox bearings, and hydraulic seals can withstand repeated duty without accelerated wear or seal extrusion.

Operational safety and limitations

Weather constraints. SOLAS mandates that lifeboats cannot be launched if wind speed exceeds 60 knots or if the ship is in a beam sea (waves perpendicular to hull) with height greater than 5 meters. In these conditions, the lifeboat must be secured and the crew must transfer via helicopter or await calmer seas.

Crew training. Every crew member involved in launching or securing lifeboats (typically 2–4 deck crew and 1 officer) must hold a valid certificate in lifeboat and life-raft operation. Training includes donning immersion suits, entering the davit-lowered lifeboat while it swings, releasing shackles under dynamic load, and operating the lifeboat engine for sea-anchor drift control.

Mechanical hazards. The rotating slew bearing, extending luffing cylinder, and spooling winch drums are serious crush and entanglement hazards. Work areas around the davit are marked with warning signs and yellow and black paint. Crew must never lean over the boom or reach into the gap between boom and pedestal when power is on. An emergency stop button must be within 2 meters of the control pendant operator.

Corrosion and maintenance. All davit components exposed to saltwater (boom, pedestal, slew bearing seals) are hot-dip galvanized or stainless steel. The hydraulic system is flushed and fluid changed every 2 years or if particle count analysis shows contamination. Wire rope is inspected monthly by visual examination and is replaced if more than 6 broken wires are found in any 6-meter length, or if the rope diameter has reduced by more than 10% due to corrosion.

Types of davits

Gravity davits: Older designs rely on the weight of the suspended lifeboat to pull the davit boom outboard. They are simpler but less controllable and have fallen out of favor.

Slewing davits: The boom rotates on a turntable bearing (as described above), allowing the lifeboat to be positioned anywhere along the ship's side or stern. Most modern commercial ships use slewing davits.

Telescopic boom davits: The boom extends and retracts hydraulically, useful for large vessels or when deck space is limited.

Articulated davits: A multi-jointed boom (similar to a knuckle-boom truck crane) allows precise positioning and is preferred on very large container ships where the davit must deploy craft a significant distance from the hull to clear the cargo.

Each type uses the same basic dual-fall winch, emergency lower, and proportional control philosophy, but the boom mechanics differ according to ship size and deployment envelope.