Understanding Marine Propeller Shaft and Stern Tube Systems

A technical guide for shipowners, naval architects, marine engineers, and vessel procurement teams

Introduction

The anchoring system is one of the oldest and most fundamental safety systems on any vessel — yet it remains one of the most technically demanding to specify correctly. An anchor that drags in deteriorating weather, a windlass that fails under load, or a chain that parts under shock loading can transform a routine anchorage into a maritime emergency within minutes.

For commercial vessels, offshore installations, and working craft of all types, the anchoring system must be engineered to hold the vessel securely across a wide range of seabed conditions, water depths, and environmental loads — and it must do so reliably, year after year, with minimal maintenance intervention.

This guide provides a technically grounded reference for everyone involved in specifying, procuring, operating, or maintaining marine anchoring systems, covering:

• How anchoring systems work and why correct specification matters

• The principal anchor types and their performance characteristics

• Windlass systems — design, operation, and selection

• Anchor chain grades, standards, and inspection requirements

• Holding power calculation methodology

• Classification society requirements

• Maintenance and inspection best practices

• Emerging trends in anchoring technology

How a Marine Anchoring System Works

A marine anchoring system consists of three interconnected elements that must function together as a system:

The Anchor — penetrates and engages the seabed to generate holding force

The Chain (or Cable) — connects the anchor to the vessel; its weight and catenary shape are critical to anchoring performance

The Windlass — the deck machinery that deploys and recovers the anchor and chain

Understanding how these three elements interact is essential for correct system specification.

The Role of the Catenary

When a vessel anchors, the chain does not run in a straight line from the hawsepipe to the anchor. Under normal conditions, the chain hangs in a curved shape — the catenary — with a horizontal section lying on the seabed near the anchor.

This catenary geometry is critical to anchoring performance for two reasons:

1. It ensures the anchor is pulled horizontally. Anchors generate holding force by being pulled along the seabed, not lifted vertically. If the chain angle at the anchor becomes too steep — because insufficient chain has been deployed or because the vessel is in very deep water — the anchor will be lifted rather than dragged, and holding power will be dramatically reduced.

2. It provides a shock-absorbing function. When the vessel surges in waves or wind gusts, the chain catenary stretches and straightens before the load reaches the anchor, absorbing dynamic energy and preventing shock loads from being transmitted directly to the anchor.

The practical implication is that scope — the ratio of chain length deployed to water depth — is a critical operational parameter. A minimum scope of 5:1 is generally recommended for moderate conditions; 7:1 or more may be required in severe weather.

Why Correct Anchor System Specification Matters

Anchor system failures are a documented cause of vessel casualties, groundings, and collisions. The consequences of anchor drag or windlass failure range from vessel damage and cargo loss to environmental incidents and loss of life.

Common causes of anchoring system failure include:

Each of these failures is preventable through correct specification, quality procurement, and disciplined maintenance.

Principal Anchor Types: Performance and Applications

1. Stockless Anchor (Patent Anchor)

The stockless anchor is the standard bower anchor on the vast majority of commercial vessels worldwide. Its defining characteristic is the absence of a stock (the crossbar found on older anchor designs), which allows the anchor to be hauled directly into the hawsepipe for stowage — a critical practical advantage on commercial vessels where deck space is limited and rapid anchor handling is required.

How It Works:

The stockless anchor has a shank connected to a crown piece that carries two flukes. When the anchor lands on the seabed and the chain is tensioned, the crown piece pivots relative to the shank, driving the flukes into the seabed. The holding force is generated by the resistance of the seabed material to the flukes being dragged through it.

Performance Characteristics:

Typical Applications:

• Cargo ships, bulk carriers, and tankers — primary bower anchors

• Container ships

• General commercial shipping

Classification Society Requirement: Stockless anchors on classified vessels must meet the weight requirements specified in the classification society’s equipment number (EN) calculation. The EN is a composite number derived from vessel displacement, windage area, and other factors that determines the required anchor weight and chain size.

2. High-Holding-Power (HHP) Anchor

High-holding-power anchors are designed to generate significantly greater holding force per unit weight than standard stockless anchors. Classification societies define HHP anchors as those achieving at least twice the holding power of a standard stockless anchor of the same weight in standard test conditions.

Design Principle:

HHP anchors achieve their superior holding power through optimized fluke geometry — larger fluke area, steeper fluke angle, and improved penetration characteristics — that allows them to dig deeper into the seabed and resist extraction more effectively.

Common HHP Anchor Designs:

AC-14 Anchor
The AC-14 is the most widely used HHP anchor in commercial shipping. Developed through systematic hydrodynamic and geotechnical research, it features a large fluke area and a fluke angle optimized for penetration in a wide range of seabed types.

• Holding power: approximately 2–3 times standard stockless

• Compatible with standard hawsepipe stowage

• Approved by all major classification societies as HHP

Spek Anchor
The Spek anchor features a distinctive design with a movable crown piece and large flukes. It performs well in soft seabeds and is widely used on vessels operating in areas with soft mud or sand bottoms.

Pool Anchor
The Pool anchor is a high-holding-power design commonly used on offshore vessels and in applications where superior holding performance is required.

Advantages Over Standard Stockless:

• Higher holding power allows use of a lighter anchor for equivalent holding force

• Better performance in soft seabeds where standard stockless anchors may drag

• Reduced anchor weight can improve vessel stability and reduce windlass load

Typical Applications:

• Vessels operating in areas with soft or difficult seabeds

• Offshore support vessels

• Vessels where anchor weight reduction is beneficial

• Any application where superior holding performance is required

3. Super High-Holding-Power (SHHP) Anchor

Super high-holding-power anchors represent the highest performance category, achieving four or more times the holding power of a standard stockless anchor of equivalent weight. They are used in specialized applications where maximum holding force is required.

Typical Applications:

• Offshore mooring systems

• Vessels operating in extreme weather regions

• Specialized anchoring applications where standard HHP performance is insufficient

4. Admiralty Pattern Anchor (Stocked Anchor)

The Admiralty pattern anchor — the classic anchor shape familiar from maritime iconography — features a long stock perpendicular to the flukes that ensures the anchor lands in the correct orientation for the flukes to engage the seabed.

Performance Characteristics:

• Excellent holding power in rocky and hard seabeds

• Reliable engagement across diverse seabed types

• Cannot be stowed in a hawsepipe — requires deck stowage

Limitations:

• Deck stowage requirement makes it impractical for most commercial vessels

• Handling is more complex than stockless designs

Typical Applications:

• Small craft and yachts

• Vessels where deck stowage is acceptable

• Historical and traditional vessel applications

5. Mushroom Anchor

The mushroom anchor has a distinctive inverted dome shape that embeds progressively into soft seabed material over time, developing very high holding force through suction and burial.

Performance Characteristics:

• Excellent long-term holding in soft mud and silt

• Holding force increases with time as anchor embeds deeper

• Not suitable for rocky or hard seabeds

• Cannot be quickly reset if dragged

Typical Applications:

• Permanent or semi-permanent moorings

• Buoy moorings

• Lightship and navigation buoy anchoring

• Applications where the anchor will remain in place for extended periods

6. Drag Embedment Anchor (DEA)

Drag embedment anchors are specialized high-performance anchors used in offshore mooring systems. They are designed to be dragged along the seabed under high tension, embedding progressively deeper until they reach their design holding capacity.

Common Types:

• Vryhof Stevpris

• Bruce anchor

• Flipper Delta

Performance Characteristics:

• Very high holding power to weight ratio

• Designed for permanent or semi-permanent offshore mooring

• Requires significant drag distance to reach full holding capacity

• Not suitable for conventional vessel anchoring operations

Typical Applications:

• Offshore platform mooring systems

• FPSO mooring

• Semi-submersible mooring

• Anchor leg mooring (ALM) systems

7. Suction Pile Anchor

Suction pile anchors (also called suction caissons) are large-diameter steel cylinders that are installed by pumping water out of the interior, creating a pressure differential that drives the pile into the seabed.

Performance Characteristics:

• Extremely high holding capacity — suitable for the largest offshore structures

• Precise installation positioning

• Can be retrieved and reused in some applications

• Requires specialized installation equipment

Typical Applications:

• Deepwater offshore platform mooring

• Subsea pipeline anchoring

• Offshore wind foundation systems

Anchor Type Selection Summary

Anchor Chain: Grades, Standards, and Specifications

The anchor chain is as critical to anchoring system performance as the anchor itself. An undersized, incorrect grade, or poorly maintained chain is a single point of failure in the anchoring system.

Chain Grades

Anchor chain is classified by grade, which reflects the steel’s tensile strength and the chain’s minimum breaking load (MBL) relative to its nominal diameter.

Most modern commercial vessels use Grade 3 (Q3) chain, which provides the best balance of strength, weight, and cost for standard anchoring applications. Grade 4 chain is used where maximum strength with minimum weight is required.

Chain Construction Types

Stud Link Chain
Stud link chain has a stud (crossbar) welded or cast across the interior of each link. The stud prevents the link from deforming under load and reduces the risk of kinking.

• Standard for commercial vessel bower anchors

• Required by classification societies for vessels above a certain size

• Provides consistent geometry for windlass wildcat engagement

Open Link Chain (Kenter Chain)
Open link chain without studs is lighter and more flexible but has lower strength for a given diameter. Used in some mooring applications and on smaller vessels.

Chain Certificates and Traceability

Classification society rules require that anchor chain be supplied with a certificate of conformity documenting:

• Steel grade and heat treatment

• Mechanical test results (tensile strength, elongation, impact toughness)

• Proof load test results

• Dimensional inspection results

• Manufacturer identification

Chain without valid certificates should not be accepted for use on classified vessels. Traceability of chain material is essential for maintaining classification society approval.

Chain Markings

Each shot (length) of anchor chain must be marked to identify its grade and position in the chain locker. Standard marking systems use paint, wire, or swivel links at defined intervals to indicate the amount of chain deployed during anchoring operations.

Holding Power Calculation

Determining the required anchor holding power is an engineering calculation, not a rule of thumb. Undersizing the anchoring system creates unacceptable risk; oversizing increases cost and weight unnecessarily.

Factors Affecting Required Holding Power

Environmental Loads:

• Wind force on the vessel’s hull and superstructure (function of wind speed and windage area)

• Current drag on the hull and underwater appendages

• Wave-induced surge forces

Vessel Characteristics:

• Displacement

• Windage area (projected lateral area above waterline)

• Underwater hull form

Anchoring Arrangement:

• Single anchor vs. multiple anchor arrangement

• Chain scope

• Water depth

Simplified Holding Power Estimation

For preliminary design purposes, the required anchor holding force can be estimated from:

Wind Force:
F_wind = 0.5 × ρ_air × C_d × A × V²

Where:

• ρ_air = air density (approximately 1.225 kg/m³)

• C_d = drag coefficient (typically 1.0–1.3 for ship hulls)

• A = projected windage area (m²)

• V = design wind speed (m/s)

Current Force:
F_current = 0.5 × ρ_water × C_d × A_underwater × V_current²

The total required holding force is the vector sum of wind and current forces, accounting for their relative directions.

Classification Society Equipment Number

For classified vessels, the required anchor weight and chain size are determined by the Equipment Number (EN), calculated according to the classification society’s rules:

EN = Δ^(2/3) + 2 × B × d + A/10

Where:

• Δ = vessel displacement (tonnes)

• B = vessel breadth (m)

• d = vessel depth to freeboard deck (m)

• A = projected lateral area of hull and superstructure above waterline (m²)

The EN is used to enter the classification society’s equipment table, which specifies the required anchor weight, chain diameter, chain grade, and chain length.

Windlass Systems: Design, Selection, and Operation

The windlass is the deck machinery responsible for deploying and recovering the anchor and chain. It is a safety-critical system — a windlass that fails to hold the chain under load, or that cannot recover the anchor in deteriorating weather, creates an immediate operational emergency.

Windlass Types

Horizontal Windlass

In a horizontal windlass, the wildcat (the sprocket wheel that engages the chain) rotates on a horizontal axis. The chain runs horizontally across the wildcat before dropping into the chain locker.

Advantages:

• Lower profile — suitable for vessels with limited freeboard

• Good visibility of chain during operation

• Widely used on smaller commercial vessels and yachts

Limitations:

• Chain routing requires careful design to avoid sharp bends

• Less suitable for very large chain sizes

Vertical Windlass (Windlass with Vertical Wildcat)

In a vertical windlass, the wildcat rotates on a vertical axis. The chain drops vertically from the wildcat into the chain locker below deck.

Advantages:

• Compact deck footprint

• Efficient chain routing — chain drops directly into locker

• Preferred for large commercial vessels with heavy chain

Limitations:

• Higher profile than horizontal designs

• Motor and gearbox typically below deck — requires watertight arrangement

Combined Windlass-Mooring Winch

Many commercial vessels use combined units that incorporate both the anchor windlass and mooring winches in a single deck machinery arrangement, reducing the number of separate units and simplifying the deck layout.

Windlass Drive Systems

Electric Drive

Electric windlasses use an electric motor (AC or DC) driving the wildcat through a gearbox. They are the most common drive system on modern commercial vessels.

Advantages:

• Clean, controllable power

• Easy speed control

• Compatible with vessel’s electrical system

• Low maintenance compared to hydraulic systems

Limitations:

• High starting current demand — requires adequate electrical supply

• Motor protection required against overload and water ingress

Hydraulic Drive

Hydraulic windlasses use a hydraulic motor driven by the vessel’s hydraulic power unit (HPU) or a dedicated hydraulic pump.

Advantages:

• High torque at low speed — ideal for heavy chain recovery

• Smooth, stepless speed control

• Compact motor size for given torque output

• Overload protection through pressure relief valve

Limitations:

• Requires hydraulic power unit

• Hydraulic oil leakage risk

• More complex maintenance than electric systems

Electro-Hydraulic Drive

Electro-hydraulic systems combine an electric motor driving a dedicated hydraulic pump, which in turn drives a hydraulic windlass motor. This arrangement provides the control advantages of electric drive with the torque characteristics of hydraulic operation.

Critical Windlass Performance Parameters

Rated Pull (Nominal Pull)
The continuous pulling force the windlass can exert at rated speed. This must be sufficient to recover the anchor and chain under the worst expected conditions — typically defined as recovering the full chain length at maximum scope in the design current and wind conditions.

Maximum Pull (Stall Pull)
The maximum force the windlass can exert at zero speed. This is typically 1.5–2.0 times the rated pull and represents the windlass’s ability to break out an anchor that has become deeply embedded.

Chain Speed
The rate at which chain is recovered, typically expressed in meters per minute. Classification society rules specify minimum chain speeds for commercial vessels.

Brake Holding Capacity
The maximum load the windlass brake can hold without slipping. This is a critical safety parameter — the brake must be capable of holding the full static load of the deployed chain plus the dynamic loads imposed by vessel motion in the design sea state.

Classification society rules typically require the brake to hold a load of at least 80% of the chain’s minimum breaking load (MBL).

Wildcat Design
The wildcat must be correctly designed for the chain grade and diameter being used. Incorrect wildcat geometry results in poor chain engagement, accelerated wear, and risk of chain jumping off the wildcat under load.

Windlass Sizing Methodology

Windlass sizing should be based on a systematic calculation of the loads the system must handle:

Step 1: Determine Chain Weight
Calculate the total weight of chain to be deployed (chain length × weight per meter for the specified grade and diameter).

Step 2: Calculate Catenary Load
Determine the tension in the chain at the hawsepipe for the design anchoring condition (water depth, scope, environmental loads).

Step 3: Add Dynamic Loads
Apply a dynamic load factor to account for vessel motion and wave-induced surge.

Step 4: Select Windlass Rated Pull
The windlass rated pull must exceed the calculated chain tension with an appropriate safety margin.

Step 5: Verify Brake Capacity
Confirm that the brake holding capacity meets or exceeds the classification society requirement (typically 80% of chain MBL).

Classification Society Requirements for Anchoring Systems

Classification society rules for anchoring systems are comprehensive and mandatory for all classed vessels. Key requirements cover:

Equipment Number Calculation

As described above, the EN determines the required anchor weight, chain diameter, chain grade, and chain length. Vessels must carry anchors and chain that meet or exceed the EN requirements.

Windlass Requirements

Classification rules specify:

• Minimum rated pull based on chain size and vessel type

• Minimum brake holding capacity (typically 80% of chain MBL)

• Minimum chain recovery speed

• Motor protection requirements

• Control system requirements

• Emergency stop provisions

Chain Locker Requirements

• Minimum volume to accommodate the required chain length

• Drainage arrangements

• Access for inspection and maintenance

• Chain stopper requirements

Inspection and Survey Requirements

Classification societies require periodic surveys of anchoring system components:

• Annual survey: Visual inspection of windlass, chain, and anchor; verification of brake function

• Special survey (every 5 years): Detailed inspection including chain measurement, windlass overhaul, and anchor inspection

Anchor and Chain Maintenance

Anchor Inspection

Visual Inspection (Each Use / Annually):

• Inspect all surfaces for cracks, particularly at the shank-crown junction and fluke roots

• Check fluke pivot pins and securing arrangements

• Inspect anchor shackle — verify pin is correctly moused

• Check coating condition — repair bare metal areas promptly

• Verify anchor weight marking is legible

Dry-Dock Inspection:

• Full dimensional survey — compare against original specifications

• Non-destructive testing (NDT) of high-stress areas

• Fluke pivot mechanism inspection and lubrication

• Full recoating if required

Chain Inspection and Measurement

Chain wear is the primary cause of chain degradation. As chain links wear at their contact points, the link diameter reduces and the chain’s strength decreases. Classification society rules specify the maximum permissible wear before chain must be replaced.

Typical Wear Limit: Chain must be replaced when link diameter has worn to 87% of the nominal diameter (i.e., maximum wear of 13% of original diameter).

Chain Inspection Procedures:

Visual Inspection (Annual):

• Inspect for cracked, bent, or deformed links

• Check stud condition — loose or missing studs indicate overloading or fatigue

• Inspect joining shackles (Kenter shackles) — verify lead plugs are intact

• Check chain markings — verify position markings are legible

Dimensional Measurement (Special Survey):

• Measure link diameter at wear points — compare against wear limit

• Measure link length — elongation indicates overloading

• Document measurements for trend analysis

Chain Cleaning:

• Remove marine growth and corrosion products

• Apply corrosion-inhibiting treatment to cleaned chain

• Inspect coating condition in chain locker

Windlass Maintenance

Routine Maintenance (Monthly):

• Lubricate all grease points per manufacturer’s schedule

• Check brake lining condition and adjustment

• Test brake holding capacity — verify chain does not slip under load

• Inspect wildcat for wear — check chain engagement

• Check hydraulic oil level and condition (hydraulic systems)

• Test emergency stop function

• Inspect electrical connections and motor protection (electric systems)

Annual Maintenance:

• Full inspection of gearbox — check oil level and condition; inspect for leaks

• Brake lining measurement — replace if worn beyond manufacturer’s limit

• Wildcat wear measurement — replace if worn beyond classification tolerance

• Motor insulation resistance test (electric systems)

• Hydraulic system pressure test (hydraulic systems)

• Full operational test under load

Common Windlass Failure Modes:

Environmental and Regulatory Considerations

Anchor Damage to Seabed Ecosystems

Anchoring in sensitive marine environments — coral reefs, seagrass beds, and protected marine areas — can cause significant ecological damage. Many port authorities and marine protected area (MPA) managers now prohibit anchoring in sensitive areas or require the use of designated anchoring zones.

Vessel operators should:

• Consult nautical charts and port authority notices for anchoring restrictions

• Use mooring buoys where provided in sensitive areas

• Avoid anchoring in areas with visible coral or seagrass

Anti-Fouling and Corrosion Management

Anchor chains stored in chain lockers are subject to accelerated corrosion from trapped seawater and biological activity. Regular chain locker cleaning, drainage maintenance, and application of corrosion-inhibiting treatments significantly extend chain service life.

Future Trends in Marine Anchoring Technology

Smart Anchor Drag Detection Systems

GPS-based anchor watch systems that alert the bridge team when the vessel moves beyond a defined radius are now standard on most commercial vessels. More sophisticated systems integrate GPS position data with wind, current, and chain tension data to provide early warning of anchor drag before the vessel has moved significantly.

Remote Windlass Monitoring

IoT-enabled windlass monitoring systems that track motor current, brake temperature, hydraulic pressure, and chain tension in real time are enabling predictive maintenance approaches that reduce unplanned failures and extend equipment service life.

High-Strength Low-Weight Chain

Development of Grade 4 and experimental Grade 5 chain materials is enabling anchoring systems with significantly reduced chain weight for equivalent holding capacity — an important consideration for vessels where topside weight is a constraint.

Autonomous Anchoring Systems

Research into autonomous anchoring systems — where the vessel’s navigation system automatically selects the optimal anchoring position, deploys the correct scope, and monitors holding performance — is progressing, driven by the broader trend toward autonomous vessel operations.

Conclusion

The marine anchoring system — anchor, chain, and windlass — is a safety-critical assembly that must be correctly specified, properly installed, and diligently maintained to perform reliably when it is needed most.

The key principles for ensuring anchoring system reliability are:

1. Select the correct anchor type for the seabed conditions — stockless and HHP anchors for standard commercial operations; specialized designs for offshore and permanent mooring applications

2. Size the system correctly — use the classification society’s Equipment Number calculation, not rules of thumb

3. Specify the correct chain grade — Grade 3 for most commercial applications; Grade 4 where weight reduction is critical

4. Size the windlass for the actual loads — rated pull, brake capacity, and chain speed must all be verified against calculated requirements

5. Implement a structured inspection program — chain wear measurement, brake lining inspection, and windlass maintenance are not optional

6. Comply with classification society requirements — annual and special survey requirements exist because anchoring system failures have real consequences

For shipowners, naval architects, and marine engineers, the anchoring system deserves the same rigorous engineering attention as the propulsion system or the mooring arrangement. A vessel that cannot anchor safely is a vessel that cannot operate safely.

Frequently Asked Questions

What is the difference between a stockless anchor and a high-holding-power anchor?

A stockless anchor is the standard bower anchor used on most commercial vessels. It provides reliable holding performance and can be stowed directly in the hawsepipe. A high-holding-power (HHP) anchor is designed to generate at least twice the holding force of a standard stockless anchor of the same weight, through optimized fluke geometry that allows deeper seabed penetration. HHP anchors are used where superior holding performance is required or where anchor weight reduction is beneficial.

How is the required anchor weight determined for a commercial vessel?

Required anchor weight is determined by the classification society’s Equipment Number (EN) calculation, which accounts for vessel displacement, breadth, depth, and windage area. The EN is used to enter the classification society’s equipment table, which specifies the minimum anchor weight, chain diameter, chain grade, and chain length. This calculation is mandatory for all classed vessels.

What is the minimum brake holding capacity required for a ship's windlass?

Classification society rules typically require the windlass brake to be capable of holding a load of at least 80% of the anchor chain’s minimum breaking load (MBL). This ensures the brake can hold the chain under the maximum expected static and dynamic loads without slipping.

How often should anchor chain be inspected and measured?

Visual inspection should be conducted annually and after any incident involving heavy anchoring loads or suspected overloading. Dimensional measurement of link diameter — to check against the wear limit of 87% of nominal diameter — should be performed at each special survey (every 5 years) or more frequently if the chain is operating in demanding conditions.

What causes anchor drag and how can it be prevented?

Anchor drag is most commonly caused by insufficient scope (chain length relative to water depth), incorrect anchor type for the seabed conditions, anchor weight below the required minimum, or sudden increases in environmental loads beyond the design condition. Prevention involves deploying adequate scope (minimum 5:1 in moderate conditions; 7:1 or more in severe weather), selecting the correct anchor type for the seabed, and ensuring the anchor system meets classification society requirements for the vessel.

What is the difference between an electric and a hydraulic windlass?

An electric windlass uses an electric motor driving the wildcat through a gearbox. It is clean, controllable, and compatible with the vessel’s electrical system, but requires adequate electrical supply capacity. A hydraulic windlass uses a hydraulic motor driven by a hydraulic power unit, providing high torque at low speed and smooth stepless control, but requires a hydraulic system and carries oil leakage risk. Electro-hydraulic systems combine both technologies, using an electric motor to drive a dedicated hydraulic pump.

Normanship supplies marine deck machinery, anchoring equipment, and mooring systems for commercial shipping, offshore operations, and shipbuilding projects. Contact our technical team for guidance on anchor and windlass specifications, classification society requirements, and certified marine deck equipment.