By ShipCalculators.com · Published 26 April 2026 · last updated 28 May 2026

Marine Anchor and Anchor Handling Equipment

Anchors and anchor handling equipment are among the oldest pieces of ship’s equipment, with the basic function (holding a ship in position by gripping the seabed) traceable to antiquity. Modern ships rarely anchor in open ocean as routine, but anchoring stays essential: waiting for berth at congested ports, sheltering from heavy weather, lying off cargo terminals, supporting offshore work, and providing emergency holding when propulsion or steering fails. The anchoring outfit on every classed merchant ship is sized by the IACS Equipment Number, $EN = \Delta^{2/3} + 2.0(hB + S_{fun}) + A/10$ , which maps a ship’s displacement and windage to a required anchor mass, chain diameter, and chain length. This article covers anchor types, stud-link chain grades and their proof and breaking loads, the windlass and chain stopper, the hawse pipe and chain locker, and the holding-power basis that the whole outfit rests on.

Contents

Background

The IACS (International Association of Classification Societies) Equipment Number, or EN, sizes the anchor and chain outfit for every classed merchant ship. It’s a single dimensionless figure built from displacement and windage, and it indexes a table that returns the number of anchors, the mass per anchor, the chain diameter, and the chain length. IACS UR A1 states the formula directly:

EN = \Delta^{2/3} + 2.0(hB + S_{fun}) + \frac{A}{10}

Each term answers a different load. The displacement term $\Delta^{2/3}$ uses moulded displacement in tonnes to the Summer Load waterline; raising it to the two-thirds power gives a quantity that scales with the ship’s wetted area rather than its volume, which is what the current load acts on. The middle term $2.0(hB + S_{fun})$ is the windage on the side: $h$ is the effective height in metres from the Summer Load waterline to the top of the uppermost deckhouse (the freeboard $a$ plus each tier $h_i$ wider than $B/4$ ), $B$ is moulded breadth, and $S_{fun}$ is the effective front projected area of the funnel. The last term $A/10$ adds the side projected area $A$ of the hull, superstructures, houses and funnels above the waterline that lie within the equipment length and exceed $B/4$ in breadth. So a high-windage ship of modest displacement, a car carrier or a large container ship with stacks of boxes on deck, lands on a higher EN than a low-profile tanker of the same tonnage, and gets a heavier anchor outfit for it.

The relation has been refined over decades of casualty analysis. Rev.7 (2020, Corr.1 2021) rewrote the windage terms to handle very large ships and tall funnel-deckhouse arrangements explicitly, and Rev.8 followed in June 2023. The point worth keeping in view is what the EN does not do: it sizes a temporary harbour outfit for sheltered water, not a holding system for any sea state. UR A1 says so in its own scope. The section on the harbour basis below returns to that limit, because it governs how the equipment can honestly be used.

How the Equipment Number maps to the outfit

Table 1 of UR A1 is a step table, not a continuous function. The computed EN drops into a band, and the band returns the outfit. The bands run from EN 205 to 240 at the bottom up past EN 16000 at the top, and every band specifies the same four things: number of bower anchors, mass per anchor, total chain length, and chain diameter for each of the three steel grades.

Read across one band to see the logic. The 205 to 240 band calls for two bower anchors of 660 kg each, 302.5 metres of chain in total, and a stud-link diameter of 26 mm in Grade 1, 22 mm in Grade 2, or 20.5 mm in Grade 3. Move up to the 1300 to 1390 band and the figures climb to two anchors of 4050 kg, 522.5 metres of chain, and 64 mm Grade 1 (50 mm in Grade 3). The chain length in column 4 is the total for both anchors; A1.2.2 requires it to be split in roughly equal parts between the two. So 302.5 metres total is 11 shackles of 27.5 metres each, divided as a bit over 5 shackles per anchor. The mass tolerance is loose by design: A1.4.1.1 lets an individual anchor vary by 7 percent from the table mass as long as the pair still totals what two equal anchors would weigh.

Two anchors is the standard answer across the whole table, which is why ships carry a port and a starboard bower at the bow. The chain diameter changes with grade because the table fixes the required breaking strength, not the bar size: a higher-grade steel hits the same strength in a thinner, lighter bar, which is the entire reason the grade ladder exists. The companion IACS Equipment Number calculator works the formula and the lookup in one step.

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 carry a precise meaning in UR A1, not a marketing one. A1.4.1.2 defines an HHP anchor as one with a holding power of at least twice that of an ordinary stockless anchor of the same mass, and it must work without prior adjustment or special placement on the sea bottom. The payoff is mass: when an approved HHP design is fitted as bower anchor, A1.4.1.2(b) allows each anchor to be 75 percent of the Table 1 mass for an ordinary stockless anchor. Lighter anchors then permit lighter chain, so the whole outfit drops in weight. Approval requires full-scale comparative holding tests under class supervision per A1.4.2, and the proof test in A1.4.4 loads the HHP anchor to the Table 2 value for an anchor mass 1.33 times its actual mass. Common HHP designs include the AC-14 (a development of the Spek pattern), the Pool TW, and proprietary anchors from makers such as Vryhof and SHM. HHP anchors are now standard on large commercial ships for that weight saving.

Super High Holding Power (SHHP) anchors push the definition further. A1.4.1.3 sets the bar at a holding power of at least four times an ordinary stockless anchor of the same mass, or at least twice an approved HHP anchor of the same mass. The allowed mass reduction is correspondingly larger: A1.4.1.3(d) permits an SHHP bower anchor to be reduced to 50 percent of the Table 1 ordinary-stockless mass. The catch is that SHHP use is limited to restricted-service ships, and the individual anchor mass is generally not to exceed 1500 kg. SHHP proof testing in A1.4.6 uses the Table 2 load for an anchor mass twice the actual mass, followed by dye-penetrant surface inspection. The fourfold and twofold ratios are class-defined absolutes, not approximate ranges, which is why an anchor either holds its designation or loses it on test.

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. Its weight and the friction of chain lying along the seabed damp the snub of a yawing ship and flatten the pull on the shank, which raises holding capability well beyond what the anchor alone returns.

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 is where UR A1 gets quantitative. The three anchoring grades are Grade 1 (mild steel), Grade 2 (special quality), and Grade 3 (extra special quality), often written U1, U2, U3 on certificates. Table 4 of UR A1 gives the breaking load BL and proof load PL in kN as functions of chain diameter $d$ in mm. The Grade 1 breaking load is the base:

BL_1 = 9.80665 \times 10^{-3}\,[d^2(44 - 0.08d)]

The higher grades scale off it directly. Grade 2 has $BL_2 = 1.4\,BL_1$ and Grade 3 has $BL_3 = 2\,BL_1$ , so Grade 3 chain is exactly twice as strong as Grade 1 of the same diameter. The proof loads run $PL_1 = 0.7\,BL_1$ , $PL_2 = BL_1$ , and $PL_3 = 1.4\,BL_1$ . Notice the consequence: Grade 2’s proof load equals Grade 1’s breaking load, and the proof load is the figure each link is actually tested to in the chain-testing machine before acceptance. Working a sample through the formula, a 50 mm Grade 1 chain gives $BL_1 = 9.80665 \times 10^{-3} \times 2500 \times (44 - 4) = 981$ kN, the Grade 2 chain of the same diameter breaks at about 1373 kN, and Grade 3 at about 1961 kN. Those figures match the UR A1 Table 5 acceptance loads. Use the IACS chain breaking load calculator to run any diameter and grade.

The grade ladder is why the equipment table lists three diameters per EN band. Because the table fixes a required strength, a stronger grade reaches it in a thinner, lighter bar. A ship that would need 64 mm Grade 1 can meet the same outfit requirement with 50 mm Grade 3, cutting chain weight by roughly a third and easing the load on the windlass and the bow structure. Most ocean-going merchant ships now carry Grade 3 for that reason; Grade 1 survives on smaller and older vessels. Common diameters span 26 mm at the bottom of the table to over 120 mm on the largest bulk carriers and crude tankers. Offshore mooring uses a separate ladder, R3, R3S, R4, R4S, R5, with higher minimum yield and breaking strengths, but those grades belong to long-term station-keeping rather than the ship’s own anchoring outfit.

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 set by IACS UR A2, which defines three performance figures verified by test during construction: the continuous duty pull the machine sustains at nominal speed, an overload pull held for a short interval, and a hoisting speed. The continuous duty pull is keyed to the chain proof load and the anchor mass, so the windlass is matched to the chain the equipment table assigned to that EN band, not specified in isolation. The Lloyd’s Register summary of the 2018 UR A1, A2 and Rec 10 package describes the same coupling: windlass design follows from the chain the ship is required to carry. The machine must raise the anchor from a stated depth at the nominal speed without stalling, and the overload pull covers the extra demand when the anchor breaks out of the seabed.

The brake, not the motor, is the load-holding element once the anchor is set. UR A2 requires the brake to hold a defined fraction of the chain breaking load with the windlass not powered, which is the figure that actually carries the ship’s drift load at anchor. Continuous motor torque is for hauling at speed; the static holding job belongs to the band brake and, where fitted, the chain stopper described below.

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.

Chain Stoppers and Supporting Structure

The chain stopper is the device that takes the anchor load off the windlass and transmits it directly into the deck structure. On most ocean-going ships it’s a bar or pawl that drops between two chain links forward of the windlass, so that when the anchor is set the standing load runs stopper to deck rather than through the cable lifter and brake. A windlass band brake can hold a great deal, but it isn’t meant to carry full drift load indefinitely; the stopper is.

UR A1 A1.7.1 fixes the design loads for the supporting hull structure, and the figures are explicit. The chain stopper structure is designed to 80 percent of the chain cable breaking load. A windlass with no stopper, or with a stopper attached to the windlass itself, is also designed to 80 percent of chain breaking load, because the windlass then carries the standing load. Where a separate stopper is fitted and is not attached to the windlass, the windlass structure drops to 45 percent of chain breaking load, since the stopper now takes the larger share. The loads are applied in the direction of the chain cable, and sea loads from green water are added per UR S27. The permissible stresses in A1.7.3 are checked on net thickness after deducting the corrosion addition, so the structure is sized for end-of-life scantlings, not as-built.

The practical reading of these numbers: a 78 mm Grade 3 chain breaks at roughly 5390 kN, so a stopper foundation for that chain is designed to about 4310 kN, more than 430 tonnes of pull into the forecastle deck. That’s why the stopper, the windlass bedplate, and the surrounding deck plating are heavily insert-plated and bracketed. A stopper that pulls out of the deck under load is a documented casualty mode, and the 80 percent and 45 percent splits exist so the load path is unambiguous regardless of how the builder arranges the machinery.

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.

Holding power and the harbour standard

The number that matters at anchor is holding power: the horizontal force the seabed resists before the anchor drags. For a stockless anchor it scales with anchor weight and a holding-power coefficient that depends on seabed type, fluke angle, and how deeply the flukes set. Firm sand or stiff clay returns a coefficient several times the anchor weight; soft mud or hard rock returns much less. The chain adds to this in two ways the anchor alone can’t. Its weight pulls the shank down toward horizontal at the seabed, which is the angle at which flukes bite rather than skip, and the catenary of chain lying along the bottom absorbs surge as the ship yaws and snubs, keeping peak tension off the anchor. This is why scope, the ratio of chain paid out to water depth, matters: more chain on the bottom means a flatter pull and more damping. The anchor holding power and anchor chain catenary calculators quantify both effects, and the anchor scope tool works the scope-versus-depth relation.

Here’s the limit the IACS outfit carries, stated in UR A1’s own scope: the anchoring equipment is intended to hold a ship in good holding ground in conditions such as to avoid dragging, and where the holding ground is poor the holding power of the anchors is reduced. The standard assumes a sheltered or moderate anchorage, a defined current, moderate wind, and no appreciable sea or swell. It is a temporary, harbour-water standard. It is not designed to hold a ship against a storm in an exposed anchorage, and it never has been. A master who reads the EN outfit as a foul-weather holding guarantee has misread the requirement.

That distinction separates ship anchoring from deep-water mooring outright. A floating production unit or a drilling rig moored for months uses purpose-designed drag-embedment anchors, R-grade chain or wire, and an analysed mooring pattern with calculated holding margins for the design storm. The ship’s bower anchor and its Grade 3 chain are sized to a band in a step table for sheltered holding; the offshore mooring is engineered to a site-specific environmental load case. Confusing the two has grounded ships that trusted a harbour outfit in conditions it was never sized for.

Ship motion at anchor connects back to hull form. A high windage profile, the same one that drove the EN windage terms up, also gives the ship a larger yaw and a wider swinging circle, which loads the anchor in surges rather than steadily. Underwater, the current load follows the wetted area that the displacement term stands in for, and the way a hull sits and swings relates to its block coefficient and its stability, the same GZ curve and righting arm that governs how it heels and rolls. The anchoring outfit is one corner of a connected naval-architecture problem, not a standalone fitting.

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 rated at 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.

Limitations

The IACS Equipment Number sizes a harbour outfit, not a foul-weather one. UR A1 states the equipment is for holding in good ground in conditions that avoid dragging, and the figures assume a sheltered or moderate anchorage with a defined current and no appreciable sea. Treating the EN outfit as adequate for an exposed storm anchorage is the single most common misuse, and it has grounded ships. Anchoring in heavy weather calls for the master’s judgment on scope, ground, and lee, not a table lookup.

The EN formula and Table 1 are step functions. A ship one tonne over a band boundary jumps to the next outfit, and the bands are coarse, so two near-identical ships can carry different chain diameters. The mass tolerance of 7 percent per anchor and the rounding in the Table 5 test loads mean the as-fitted outfit is never an exact match to the computed EN. Use the figures as the certified minimum, not a precise design point.

Holding-power coefficients are not in UR A1. The requirement sizes mass, diameter, and length; it does not return a holding force, because that depends on seabed type, fluke set, and the dynamic loading of a yawing ship, none of which the table knows. Any holding-power number quoted here or in the calculators is an estimate from a coefficient that must be chosen for the actual ground. Soft mud, weed over hard bottom, and shelving rock all defeat a correctly sized anchor.

The chain load formulas give design and test loads for new, undamaged chain. In service, link wear, corrosion, and prior overload reduce strength below the Table 4 value. UR A1 requires replacement when the mean diameter of a worn link falls below a defined limit, and the proof and breaking loads quoted apply only while the chain stays within that limit. Surveyed wear and certificate history, not the original rating, govern a chain’s actual capacity.

Class rules differ in detail. DNV, ABS, and Lloyd’s Register all implement UR A1, but their hawse-pipe, chain-locker, and survey provisions carry society-specific requirements, and restricted-service notations change what’s required. Always work from the governing society’s current rules and the ship’s class record, not a generic summary.

Conclusion

Marine anchor and anchor handling equipment stays essential despite dynamic positioning, mooring buoys, and other station-keeping options. A correctly sized anchor, certified chain, capable windlass, sound chain stopper, and the hawse pipe and chain locker that carry the gear give a ship its sheltered-water holding. The people responsible for these systems need the design basis (the IACS Equipment Number), the regulatory references (UR A1 and A2), the operational practice, and the maintenance regime that together keep the outfit fit. The hardware is evolving through monitoring and partial automation, but the physics holds: weight on the bottom, a flat pull on a set fluke, and chain catenary to absorb the snub.

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