Ship Anchor Weight Calculator
Determine the optimal anchor mass and chain size based on Equipment Number (EN)
Instructions: Enter your vessel's dimensions and displacement below. The calculator uses the Equipment Number (EN) formula to estimate required anchor weight.
Metric (Meters, Tonnes)
Imperial (Feet, Long Tons)
Total weight of the vessel in tonnes.
Please enter a valid positive displacement.
Widest point of the hull in meters.
Vertical distance exposed to wind in meters.
Side projected area above waterline in sq. meters.
Standard Stockless (Conventional)
High Holding Power (HHP)
Super High Holding Power (SHHP)
HHP anchors allow for ~25% weight reduction; SHHP ~50%.
Recommended Anchor Weight
0 kg
Equipment Number (EN)
0
Min. Chain Diameter (Grade 2)
0 mm
Est. Holding Power Required (30kts Wind)
0 kN
*Calculation based on simplified IACS Equipment Number formula: EN = Δ2/3 + 2Bh + 0.1A.
Wind Speed vs. Required Holding Force
Holding Power Requirements by Wind Speed
| Wind Speed (Knots) | Wind Pressure (Pa) | Required Holding Force (kN) | Safety Status |
|---|
What is Ship Anchor Weight Calculation?
Ship anchor weight calculation is the critical maritime engineering process used to determine the appropriate mass and size of an anchor and its associated chain cable (rode) for a specific vessel. This calculation ensures that a ship can maintain its position safely under various environmental conditions, such as high winds, strong currents, and wave action.
The primary metric used in professional ship anchor weight calculation is the Equipment Number (EN). This dimensionless number, standardized by classification societies like Lloyd's Register, DNV, and ABS, aggregates the vessel's physical characteristics—displacement, beam, and windage area—to assign a regulatory minimum for ground tackle. Correctly calculating the anchor weight is vital for the safety of the crew, the vessel, and the surrounding marine environment.
While small pleasure crafts might use simple length-based tables, commercial vessels and large yachts must rely on precise mathematical formulas to ensure compliance with international maritime safety standards.
Ship Anchor Weight Calculation Formula
The foundation of modern anchor sizing is the Equipment Number (EN) formula. Once the EN is derived, it is cross-referenced with standard tables to find the required mass.
The Equipment Number (EN) Formula
The standard formula adopted by the International Association of Classification Societies (IACS) is:
EN = Δ2/3 + 2 × B × h + 0.1 × A
| Variable | Meaning | Unit (Metric) | Typical Range |
|---|---|---|---|
| Δ | Molded Displacement | Tonnes | 10 – 200,000+ |
| B | Molded Breadth (Beam) | Meters | 3 – 60 |
| h | Effective Height (Waterline to top of house) | Meters | 2 – 40 |
| A | Profile Area (Windage Area) | Square Meters | 10 – 5,000+ |
Mathematical Explanation:
- Δ2/3: Represents the resistance due to the underwater hull form and current drag.
- 2 × B × h: Approximates the wind load on the hull and superstructure from the front.
- 0.1 × A: Accounts for the wind load on the side profile of the vessel.
Once the EN is calculated, the recommended ship anchor weight is typically derived. For a standard stockless anchor, the mass (kg) is roughly proportional to the EN, often approximated as Mass ≈ 2.5 × EN for smaller ranges, though strict lookup tables are used for certification.
Practical Examples of Anchor Sizing
Example 1: 50m Superyacht
Consider a luxury yacht with the following specifications:
- Displacement (Δ): 500 tonnes
- Beam (B): 9.5 meters
- Height above waterline (h): 8 meters
- Profile Area (A): 350 m²
Step 1: Calculate EN
EN = (500)0.667 + 2(9.5)(8) + 0.1(350)
EN ≈ 63 + 152 + 35 = 250
Step 2: Determine Weight
Using standard tables for EN 250, the required mass for a High Holding Power (HHP) anchor would be approximately 495 kg. If using a standard anchor, it would be heavier, around 660 kg.
Example 2: Small Commercial Trawler
- Displacement: 120 tonnes
- Beam: 6 meters
- Height: 5 meters
- Profile Area: 80 m²
Calculation:
EN = (120)0.667 + 2(6)(5) + 0.1(80)
EN ≈ 24.3 + 60 + 8 = 92.3
Result:
An EN of ~92 requires a standard stockless anchor of approximately 240 kg or an HHP anchor of roughly 180 kg.
How to Use This Ship Anchor Weight Calculator
- Select Unit System: Choose between Metric (Meters/Tonnes) or Imperial (Feet/Long Tons).
- Enter Displacement: Input the vessel's full load displacement. This is the most significant factor in the ship anchor weight calculation.
- Input Dimensions: Enter the Beam and Height. These determine the wind load profile.
- Profile Area: If known, enter the side windage area. If left blank, the calculator estimates it based on typical vessel proportions.
- Select Anchor Type: Choose "Standard" for conventional anchors or "HHP" (High Holding Power) if you are using modern designs like Delta, Rocna, or AC-14 types.
- Analyze Results: Review the recommended weight and chain diameter. Use the "Copy Results" button to save the data for your logs.
Key Factors That Affect Ship Anchor Weight Results
Several variables influence the final ship anchor weight calculation beyond simple geometry:
- Anchor Holding Power Efficiency: Not all anchors are equal. An HHP anchor has at least twice the holding power of a standard stockless anchor for the same weight. This allows for a 25% weight reduction in regulatory calculations.
- Bottom Type: The calculation assumes "good holding ground" (typically sand or stiff clay). In soft mud or rock, the theoretical weight may not provide sufficient holding, requiring heavier gear or specific anchor designs.
- Windage and Superstructure: Vessels with high superstructures (like cruise ships or car carriers) have a much higher EN relative to their displacement because wind force dominates the load equation.
- Current Drag: While the formula accounts for displacement, vessels operating in areas with extreme currents (4+ knots) may need to exceed the regulatory minimums calculated here.
- Chain Weight (Catenary): The weight of the chain itself contributes significantly to holding power by creating a catenary curve. A lighter anchor often requires a heavier or longer chain to maintain the pull angle at the seabed.
- Scope Ratio: The calculator assumes a standard scope (ratio of chain length to water depth). If you are forced to anchor with short scope (e.g., in a crowded anchorage), the required anchor weight effectively increases to compensate for the poor pull angle.
Frequently Asked Questions (FAQ)
Does this calculator apply to all types of ships?
This tool uses the IACS Equipment Number formula, which is the standard for commercial vessels and large yachts (>24m). For small recreational boats under 10m, simple length-based tables are often sufficient, though this calculation remains a safe conservative estimate.
What is the difference between HHP and SHHP anchors?
HHP (High Holding Power) anchors are certified to have at least 2x the holding power of standard anchors. SHHP (Super High Holding Power) anchors have at least 4x the holding power. This allows for weight reductions of 25% and 50% respectively in the ship anchor weight calculation.
Why is displacement more important than length?
Displacement represents the mass of the ship. When a ship surges at anchor due to waves, the energy that the anchor system must absorb is directly related to the vessel's mass (Kinetic Energy = 1/2 mv²).
How do I estimate Profile Area if I don't know it?
A rough approximation used in preliminary design is: Area = Length Overall × Height × Coefficient. The coefficient varies but 0.7 is a reasonable average for standard hull forms.
Does the calculator account for chain weight?
The calculator provides the minimum chain diameter based on the Equipment Number. The weight of the chain is implicit in the EN regulations, which assume a chain of standard weight is used to provide the necessary catenary effect.
Can I use a heavier anchor than recommended?
Yes, exceeding the calculated ship anchor weight is generally safe and provides extra security. However, ensure your windlass (winch) motor is powerful enough to lift the heavier anchor and chain.
What wind speed is the calculation based on?
Standard classification rules typically assume a wind speed of 25 m/s (approx 48 knots) and a current of 2.5 m/s (approx 5 knots) acting simultaneously.
How does water depth affect the calculation?
The EN formula does not directly include depth. However, depth dictates the length of chain required. A general rule is to carry chain length equal to at least 6 to 10 times the typical anchoring depth.
Related Tools and Internal Resources
Explore our other marine engineering tools to ensure your vessel is fully equipped for safety:
- Anchor Chain Length Calculator – Determine the optimal scope and rode length for various depths.
- Mooring Load Calculator – Calculate forces on dock lines and bollards during storms.
- Vessel Displacement Tool – Estimate your ship's displacement based on hull geometry.
- Wind Load Calculator – detailed analysis of wind pressure on superstructures.
- Marine Safety Equipment Checklist – Comprehensive guide to required safety gear.
- Buoyancy & Flotation Calculator – Understand hull stability and reserve buoyancy.