Background
The IACS (International Association of Classification Societies) Equipment Number (EN) calculation determines minimum anchor and chain requirements for every classed merchant ship, ensuring consistent anchor capability across the world fleet. This calculation, refined over decades of operational experience and casualty analysis, balances the wide range of conditions ships might encounter at anchor: wind loads on superstructure, current loads on the underwater body, wave action, and the holding capacity of various seabed types. Modern bulk carriers, tankers, container ships, and passenger vessels all carry anchors and chains sized to provide adequate holding under typical anchorage conditions, with the equipment designed to withstand the dynamic loads of windlass operation, chain catenary effects in deep water, and the impact loads associated with anchor deployment and recovery.
Regulatory Framework
The regulatory framework for ship anchoring equipment combines IACS unified requirements, individual class society rules, SOLAS provisions, ILO conventions, and various flag state requirements.
IACS Unified Requirement A1 establishes the Equipment Number formula and minimum anchor and chain requirements applicable to all IACS member class societies. The formula combines ship displacement, beam, and exposed surface area to produce an Equipment Number used to determine required anchor weight, chain diameter, chain length, and windlass power. The formula has evolved over decades, with the current version (Rev. 7, 2018) reflecting accumulated operational experience and addressing larger ship sizes including very large container ships and ULCCs.
Class society rules (DNV, Lloyd’s Register, ABS, Bureau Veritas, ClassNK, RINA, KR) implement IACS UR A1 with detailed engineering requirements for anchor design and certification, chain construction and testing, windlass design and testing, hawse pipe and chain locker design, and survey requirements. Each class society publishes specific rules for these components, with mutual recognition agreements ensuring consistency across the IACS members.
ISO 1704 specifies stockless anchors, which are the dominant anchor type on commercial ships. The standard covers anchor design, construction materials, proof load testing, and identification marking. ISO 1704 anchors are marked with their weight, type designation, and certification details.
ISO 7892 specifies stud-link chain (the chain type used in ship anchor and mooring service), covering chain construction, materials, manufacturing tolerances, proof and breaking load testing, and identification marking. Stud-link chain has integral cross-pieces (studs) that prevent kinking and locking under load.
SOLAS Chapter V (Safety of Navigation) Regulation 19 requires ships to be equipped with adequate anchor equipment for the trade in which they engage. SOLAS Chapter II-1 (Construction) addresses structural requirements for anchor handling equipment installations including hawse pipes and chain lockers.
ILO Maritime Labour Convention (MLC 2006) addresses crew safety in anchor handling operations, with emphasis on hand and finger protection during deck work near operating windlasses, chains, and anchors.
Flag state regulations may impose additional requirements through national maritime administration rules. Most flag states adopt IACS requirements as the basis but may add specific operational requirements for ships under their flag.
Anchor Types
Several anchor types are used in marine service, with different geometries optimised for particular holding mechanisms and operational characteristics.
Stockless anchors (sometimes called “patent anchors”) are the dominant type on modern commercial ships. The anchor consists of a shank with a crown bearing two flukes that pivot on a horizontal axis at the crown. When deployed, the flukes pivot to bury into the seabed, with the angle of fluke penetration determined by the seabed type and the angle of pull. ISO 1704 stockless anchors are tested for required holding power per unit weight, with modern designs achieving holding power ratios of 5 to 12 times anchor weight in firm soil. Common ISO 1704 designs include the Hall (the original stockless design from the 19th century), the Spek, the Pool, the Byers, and various proprietary designs from anchor manufacturers.
High Holding Power (HHP) anchors achieve greater holding power per unit weight than standard stockless anchors, with class society approval allowing reduced anchor weight (and consequently reduced chain weight) for the same holding capability. HHP designation typically requires demonstrated holding power of 1.4 to 2.0 times standard ISO 1704 anchors of equivalent weight, with formal testing under class society supervision. Common HHP anchor designs include the AC-14 (a development of the Spek), the Pool TW, the Stevpris (more common on offshore vessels), and proprietary designs from manufacturers including Vryhof, Ankertown, and SHM. HHP anchors are common on modern large commercial ships where their higher holding power per weight reduces overall equipment mass.
Super High Holding Power (SHHP) anchors achieve even higher holding power, typically 2.0 to 3.0 times standard ISO 1704 anchors. SHHP is most relevant for ships with high windage (large container ships, car carriers, passenger ships) or for vessels with operational anchoring requirements (offshore support vessels, drillships).
Stocked anchors (admiralty pattern, fisherman’s anchors) have a stock perpendicular to the shank that ensures the anchor lies on its side and that the flukes engage the seabed correctly. Stocked anchors offer high holding power per unit weight, particularly in soft mud, but are bulky and require special stowage arrangements. They are rarely used on modern commercial ships but appear on some specialist vessels.
Drag anchors (also called drag embedment anchors or fluke anchors) are specifically designed for offshore mooring rather than ship anchoring. The Stevpris, Stevin, and similar designs achieve very high holding power for given weight when properly set, and are used for permanent or long-term moorings of floating production units, drilling rigs, and similar applications.
Plate anchors and suction anchors are alternative offshore mooring technologies, with plate anchors deployed by being driven into the seabed and suction anchors using suction generated by removing water from a buried caisson. These technologies are not used for ship anchoring but appear in offshore mooring engineering.
Anchor materials are typically cast steel for the body and forged steel for the shank and crown components, with high-strength low-alloy steels providing the necessary strength-to-weight ratio. Anchors are surface-treated for corrosion protection (typically galvanising and coating) and marked with identification including weight, type designation, manufacturer, and certification details.
Anchor Chain
Anchor chain is the strong flexible connection between anchor and ship, with the chain’s weight and friction at the seabed providing essential damping and load distribution that significantly enhances holding capability beyond what the anchor alone could achieve.
Stud-link chain is the standard for ship anchor and mooring service, with each link incorporating a horizontal cross-piece (stud) that prevents the link from collapsing under load and minimises chain kinking. Stud-link construction provides higher strength per unit weight than open-link chain and superior handling characteristics in tight spaces (hawse pipes, chain lockers).
Chain grade designations (Grade 2, Grade 3, ORQ - Oil Rig Quality, R3, R4, R5) reflect material strength with higher grades having higher minimum yield and breaking strengths. Grade 2 (sometimes called “general purpose”) has a proof load of about 50 percent and breaking load of about 70 percent of Grade 3 (sometimes called “high tensile” or “K3”). Most commercial ship anchor chains are Grade 3, with Grade 2 used on smaller vessels and Grade R3, R4, R5 used on offshore mooring service where extreme loads occur.
Chain diameter (the cross-section diameter of the chain bar) determines breaking strength along with grade. Common chain diameters for commercial ships range from 50 millimetres on small ships to 130 millimetres on the largest ULCCs and bulk carriers. The IACS Equipment Number formula prescribes minimum chain diameter for each Equipment Number range.
Chain length is specified in shackles, with one shackle being 27.5 metres in metric measurement (15 fathoms in old British measurement). A typical merchant ship carries 12 shackles (330 metres) per anchor, with totals of 24 shackles when both anchors are deployed. Larger vessels and those operating in deeper anchorages may carry 14 to 16 shackles per anchor.
Chain markings identify each shackle joint along the chain length, allowing the deck officer to count out shackles during anchor deployment and accurately know the length of chain payed out. Traditional markings include painted bands on adjacent links (one band for first shackle, two for second, etc.) plus wire seizings or zinc loops at the joining shackles. Modern ships often supplement with electronic chain counters that read directly from the windlass operation.
Joining shackles connect chain segments at shackle intervals, allowing the chain to be made up from manageable sections during installation and replaced section-by-section during overhauls. Kenter shackles, Lugless shackles, and other proprietary designs provide flush profiles that pass through hawse pipes and chain pipes without snagging.
Swivels at the anchor end (between the anchor and the chain) prevent chain twist when the anchor moves on the seabed. End shackles connect chain to anchor at the swivel and to the chain locker bitter end at the other end of the chain run.
Bitter end attachment (the chain locker connection at the inboard end of the chain) typically uses a bitter end shackle attached to a slip arrangement on the chain locker structure. The slip allows the chain to be released (“slipping the cable”) in emergencies where the ship must depart immediately and recovery of the anchor is impractical or dangerous.
Chain certification includes manufacturer’s certificates documenting material composition, mechanical properties, proof load testing, and breaking load testing. Each shackle (length) of chain has its own certificate, with the chain’s history maintained throughout its service life.
Windlasses
The windlass is the deck machinery that hoists and pays out the anchor chain, providing the mechanical power to raise the anchor from depth and the controlled release function for anchor deployment.
Windlass machinery design centres on the cable lifter (gypsy), a wheel with chain-pocketing whelps that engage the chain links and convert windlass rotation into chain motion. The cable lifter is typically cast steel with hardened wearing surfaces, sized to match the specific chain diameter. The whelp profile and depth must precisely match the chain link geometry to ensure proper engagement; mismatched lifters can damage chain or cause it to ride out of the lifter under load.
Windlass drives are typically hydraulic or electric, with both technologies having strengths and limitations. Hydraulic windlasses use hydraulic motors driving through reduction gearboxes to the cable lifter, with hydraulic supply from an HPU in the engine room or local fore deck pump units. Electric windlasses use electric motors driving through reduction gearboxes, with variable-frequency drive (VFD) speed control increasingly common. Hydraulic windlasses offer smoother control and better stall characteristics, while electric windlasses are simpler to maintain and avoid hydraulic infrastructure on the foredeck.
Windlass capacity is specified by the IACS Equipment Number formula in terms of the continuous lift force at constant speed, typically 150 percent of the weight of one anchor plus four shackles (110 metres) of chain hanging vertically in water. This continuous capacity must be sustained at the rated lifting speed (typically 9 metres per minute), with ratings demonstrated by physical testing during construction.
Windlass overload capacity (the short-duration lifting capability used when the anchor is lodged in difficult seabed) typically allows pull of 1.5 times continuous capacity for limited durations, with stall capability of 2.0 to 2.5 times continuous capacity (the static load that the windlass holds against the brake when not running).
Brake systems on windlasses include the band brake (a steel band wrapped around a brake drum) which is the primary load-holding mechanism with the windlass not running. Band brakes are manually applied via mechanical screw arrangement and provide high holding capability without continuous power consumption. Disc brakes on some modern windlasses provide more controllable application and easier maintenance.
Combined windlass-mooring winches integrate anchor lifting and mooring rope handling in a single machine, with the cable lifter and drum on a common shaft or with multiple drums on independent shafts. This arrangement is common on smaller ships where deck space is at a premium, while larger ships often use separate dedicated windlasses and mooring winches.
Windlass control includes local controls at the windlass position, deck-level controls allowing operation while a deck officer observes the cable, and bridge-monitored controls on some installations. Emergency stops and overload protection are mandatory class requirements.
Hawse Pipes
The hawse pipe is the structural tube through which the anchor chain passes from the deck to the ship’s side, allowing the anchor to deploy outward and downward while protecting the deck and hull from chain damage.
Hawse pipe design accommodates the chain’s curvature as it transitions from horizontal motion on deck to vertical motion alongside the hull, with the bend radius selected to allow smooth chain motion without binding or excessive friction. Hawse pipe diameter is sized for chain plus shackles plus typical seabed mud and debris, with minimum clearances specified by class rules.
Hawse pipe construction is typically welded steel pipe with reinforcing flanges at top (deck level) and bottom (ship’s side), integrated into the surrounding shell plating and main deck structure. The hawse pipe transmits significant loads from chain tension and motion into the surrounding hull structure, requiring careful attention to local reinforcement and welding quality.
Anchor stowage at the hawse pipe top (when the anchor is fully recovered) places the anchor against the hull with the shank entering the hawse pipe and the crown and flukes pulled tight against the hull surface. Anchor stowage faces (sometimes called “anchor pockets”) are flush-fitting recesses in the hull plating where the anchor crown rests, ensuring secure stowage during sea passage.
Wash plates around the anchor stowage area prevent water from washing aboard through the hawse pipe in heavy weather. Wash plates close to the chain when the anchor is stowed and open to allow chain motion during deployment.
Anchor recess on some modern ships extends the anchor pocket into a fully recessed configuration, with the anchor flush with or slightly inside the hull line. This arrangement reduces parasitic drag and improves hydrodynamic efficiency on long sea passages, though at the cost of more complex hawse pipe geometry.
Chain Lockers
The chain locker is the compartment that stores the anchor chain when not deployed, providing space for the chain to coil and self-stow as it is hauled aboard during anchor recovery.
Chain locker design provides sufficient volume for the full chain length to coil without binding or jamming. Chain stowage volume is typically 1.0 to 1.2 cubic metres per shackle (per 27.5 metres of chain), depending on chain diameter. A ship with 12 shackles per anchor and two anchors requires chain locker volume of approximately 25 to 30 cubic metres total, divided into separate compartments for each anchor.
Chain locker geometry is typically cylindrical or rectangular with rounded corners, with sufficient depth for the chain to drop and coil without piling. The chain pipe (a vertical tube from deck level to chain locker top) directs chain into the locker as it comes off the windlass, and the locker geometry ensures the chain coils within the space rather than piling under the chain pipe.
Chain locker drainage removes water that accumulates from anchor seabed mud, salt deposits washed off the chain by rain, and condensation. Drainage typically discharges to a sump that pumps overboard, with strainers preventing chain debris from blocking pump suctions. SOLAS and class rules require positive separation between chain locker drainage and other ship drainage to prevent cross-contamination.
Chain locker access for inspection, maintenance, and chain handling is typically through a manhole in the deck or a watertight door from an adjacent space. Access is constrained by the requirement that chain locker boundaries form watertight barriers within the ship’s overall watertight integrity scheme.
Chain locker bitter end attachment provides the attachment point for the bitter end of the chain (the inboard end opposite the anchor). Bitter end attachment includes a slip arrangement that allows the chain to be released in emergency, sending the entire chain overboard if the ship must depart anchorage immediately.
Watertight integrity of chain locker boundaries is critical to overall ship safety. Chain pipes through the deck typically include watertight closures (chain pipe covers) that close after anchor stowage to prevent water ingress through the chain pipe in heavy weather.
Anchor Handling Operations
Anchor handling operations comprise anchor deployment, anchored watchkeeping, and anchor recovery, each requiring careful procedure and crew competence.
Approach to anchorage requires the navigator to evaluate the chosen anchorage position considering water depth, seabed type, surrounding traffic, weather forecast, and shoreline relationship. The chosen position should provide adequate swinging circle (the area the ship sweeps as wind and current rotate it around the anchor), clear of other anchored vessels and shore obstructions.
Speed reduction approaching the anchorage allows the ship to be at minimal headway when the anchor is dropped. Drift sideways or astern provides the chain tension needed to lay the chain along the seabed rather than piling on top of the anchor, which would prevent anchor setting.
Anchor deployment involves walking the anchor down (lowering at controlled speed using the windlass) until the anchor reaches the seabed, then paying out additional chain in a controlled manner as the ship’s drift creates chain tension. Length of chain deployed (the scope) is typically 5 to 10 times water depth in normal conditions, increased to 10 to 15 times depth in heavier weather or worse seabed.
Brake-and-lock procedure secures the chain at the desired scope, with the band brake applied tight and the cable lifter mechanically locked to the windlass frame. Once braked and locked, the chain holds the ship through the friction of brake and the structural strength of the locking mechanism, removing reliance on continuous windlass operation.
Anchored watchkeeping monitors the ship’s position relative to the dropped anchor, watches for dragging (the anchor losing grip and the ship moving in the direction of wind/current), monitors weather changes, and maintains regular bridge watch with appropriate communications and emergency procedures.
Position monitoring uses GPS or DGPS systems with anchor watch alarm functionality, alerting watchkeepers if the ship moves outside a defined circle around its expected position. Bearings to fixed shore objects, sounding-line depth observations, and visual observation supplement electronic monitoring.
Anchor recovery begins with windlass operation hauling chain steadily aboard at the rated lifting speed. As the chain shortens, the ship’s drift relative to the anchor brings them closer together. The “anchor aweigh” condition occurs when the chain is short enough that the anchor lifts free from the seabed, after which the anchor is hauled to the hawse pipe.
Foul anchor situations occur when the anchor’s chain wraps around the anchor or fluke, or when the anchor catches on seabed debris, cable, or pipeline. Recovery may require backing the chain (paying out additional length) to allow the anchor to free itself, deploying the second anchor for stability, or in extreme cases buoying the chain and slipping the foul anchor with shore assistance.
Slipping the cable is the emergency procedure of releasing the chain at the bitter end shackle, leaving the entire chain and anchor on the seabed. This is done when the ship must depart immediately (fire, weather, collision risk) and recovery is impractical. The chain and anchor are typically marked with a recovery buoy for later recovery.
Anchor and Chain Maintenance
Anchor and chain maintenance is essential to safe operation, with regular inspection, cleaning, and overhaul scheduled around major surveys.
Routine cleaning during use includes washing chain with seawater as it comes aboard during anchor recovery, removing seabed mud, salt deposits, and marine growth. Wash water connections at the hawse pipe area direct fire main pressure water onto the chain as it passes through.
Annual inspection examines anchor condition (corrosion, fluke wear, shank straightness), shackle condition at the anchor connection, swivel function, chain link condition (excess wear, distortion, cracks), shackle condition at chain joints, and bitter end attachment integrity. Class surveyors typically witness annual inspection of anchor equipment.
Periodic overhaul, scheduled at 5-year intervals coinciding with major surveys, involves complete chain ranging on shore, with each link visually examined and high-stress areas non-destructively tested. Worn or damaged links are replaced individually or as complete shackle sections. Anchors are dimensionally checked against original drawings, with worn flukes built up by welding and machining as required.
Chain weight and dimension checks during overhaul verify that wear has not reduced chain strength below acceptable limits. Chain link diameter at high-wear points should not be reduced more than 10 percent from original dimension, with greater wear requiring replacement of affected sections.
Galvanising restoration on anchor and chain involves stripping original galvanising, surface preparation, and re-galvanising for further service life. Galvanising substantially extends chain and anchor service life by preventing corrosion in the harsh marine environment.
Hawse pipe and chain locker inspection during dry-docking includes structural examination, coating renewal, drainage system testing, and bitter end attachment verification.
Windlass maintenance includes regular lubrication of the cable lifter and gearbox bearings, inspection and adjustment of brake systems, hydraulic system checks (for hydraulic windlasses) or electrical system inspection (for electric windlasses), and periodic load testing to verify continued capability.
Specific Applications
Different ship types use anchor and anchor handling equipment in characteristic configurations.
Tankers and bulk carriers typically have two stockless or HHP anchors at the bow with separate windlasses and chain lockers, with chain lengths of 11 to 13 shackles per anchor. The anchor sizes range from 7000 to 17000 kilograms depending on ship size, with the largest ULCCs requiring anchors of 21000 kilograms or more.
Container ships have similar bow anchor arrangements but often with reduced chain lengths reflecting their typical operation in well-equipped ports with shore mooring assistance. The high windage of large container ships (with stacks of containers above deck) drives Equipment Number to higher values, requiring larger anchors and chains than other ship types of equivalent displacement.
Passenger ships and cruise ships have anchor arrangements similar to tankers and bulk carriers, with attention to the visual presentation of the bow area where anchors and chains are visible to passengers. Anchor recess arrangements (fully or partially flush with hull) are common on cruise ships for hydrodynamic efficiency.
Offshore supply vessels (OSVs), platform supply vessels (PSVs), and anchor handling tug supply vessels (AHTS) have specialised anchor handling equipment for offshore mooring operations, in addition to the vessel’s own anchors. AHTS vessels carry winches with massive pulling capability (50 to 750 tonnes brake load) for handling rig anchors, plus stern rollers, towing pins, and chain compressors that allow controlled chain handling on the open deck.
Naval auxiliaries and military ships have anchor arrangements similar to commercial ships but with attention to structural reinforcement of the bow area to withstand combat damage and operational stress. Some naval ships have additional aft anchors for specialised deployment.
Inland and coastal vessels may use simplified anchor arrangements with smaller anchors and shorter chains reflecting their typical operation in protected waters.
Future Developments
Anchor and anchor handling equipment continues to evolve in response to operational requirements and design improvements.
Active anchor monitoring systems use load cells, position sensors, and accelerometers to provide real-time data on chain tension, anchor position, and ship drift, alerting watchkeepers to incipient anchor dragging before significant position change occurs. Modern systems integrate anchor monitoring with bridge displays and shore-based fleet monitoring centres.
Dynamic positioning (DP) systems on offshore vessels and increasing numbers of commercial ships provide alternative or complementary station-keeping capability, with DP-only operation feasible for short-duration station-keeping in conditions where anchors might be impractical. DP combined with anchoring (DPA) provides redundancy for long-duration offshore operations.
Improved anchor designs continue to be developed with computer modelling and at-sea testing, with progressive improvements in holding power per weight pushing the boundaries of what is achievable. The development of SHHP and beyond-SHHP anchor designs is ongoing.
Lighter-weight chain alternatives including high-strength steel (R5 grade) and composite synthetic mooring lines (for offshore mooring rather than ship anchoring) provide reduced weight per unit strength.
Autonomous and remote anchor operation using cameras, sensors, and remote control systems is being developed, with the ultimate goal of reducing crew exposure to the dangerous foredeck environment during anchor handling. Some merchant ships now feature semi-autonomous windlass operation with operator supervision rather than direct control.
Conclusion
Marine anchor and anchor handling equipment remains essential ship’s equipment despite the increasing prevalence of dynamic positioning, mooring buoys, and other alternative station-keeping technologies. The combination of properly sized anchor, certified chain, capable windlass, and robust hawse pipe and chain locker arrangements provides reliable holding capability across the wide range of conditions ships encounter at anchorage. Crew members responsible for these systems must understand the design principles, regulatory framework (particularly IACS Equipment Number), operational practices, and maintenance requirements that together ensure safe effective anchoring. As the maritime industry evolves through automation, monitoring, and alternative technologies, anchor equipment is evolving with it, but the fundamental principles of holding power, chain catenary, and proper anchor handling technique remain unchanged from generations of seafaring practice.
Related Calculators
- Anchor Holding Power Calculator
- Anchor Chain Catenary Calculator
- Anchor Scope Calculator
- IACS Chain Breaking Load Calculator
- Windlass Pull Rule Calculator
- Anchor Windlass Hydraulic or Electric System Calculator
- Tanker Mooring Drift Anchor Calculator
Related Wiki Articles
- Marine Mooring Equipment and Winches
- Marine Hydraulic Systems
- SOLAS Chapter V: Safety of Navigation
- SOLAS Chapter II-1: Construction, Subdivision and Stability
References
- IACS UR A1 - Equipment
- ISO 1704 - Ships and marine technology - Stockless anchors
- ISO 7892 - Chain and chain components for sea-going service
- DNV Rules for Classification of Ships - Pt 3 Ch 11 Hull Equipment
- Lloyd’s Register Rules and Regulations for the Classification of Ships - Pt 3 Ch 13 Anchoring and Mooring Equipment