U2 vs. U3 Anchor Chains: How to Choose?
A Canadian client recently encountered issues with insufficient anchoring load capacity—wind and wave conditions in their operational area were harsher than anticipated, and the originally specified 50mm stud-link U2 anchor chain lacked the necessary tensile strength. They faced two options:
- Option A: Upgrade to a 52mm stud-link U2 chain (increase diameter, maintain grade).
- Option B: Switch to a 50mm stud-link U3 chain (maintain diameter, upgrade grade).
After comparing the data, the client ultimately selected Option B—the 50mm U3 chain. This article outlines the selection logic to assist procurement and technical personnel facing similar situations.
Key Data Comparison
| Parameter | 50mm U2 | 50mm U3 | 52mm U2 |
| Proof Load (PL) | 981 kN | 1,370 kN | 1,060 kN |
| Weight per Meter | 54.75 kg/m | 54.75 kg/m | 59.22 kg/m |
| Tensile Strength Increase (vs. 50 U2) | — | +40% | +8% |
| Weight Change | — | Unchanged | +8% |
| Cost Change | — | Approx. +2–3% | Approx. +8% |
What Are the Key Differences Between U3 and U2?
Tensile Strength Improvement
Moving from 50mm U2 to 50mm U3 increases the proof load from 981 kN to 1,370 kN—a 40% increase. In contrast, moving from 50mm U2 to 52mm U2 raises the proof load to only 1,060 kN—an 8% increase. The U3 grade achieves a significantly higher increase in tensile strength while maintaining the same diameter.
Weight and Cost
Chains of the same diameter in U3 and U2 grades have identical weights (54.75 kg/m). The density difference between CM490 and CM690 steel is less than 1%, making the weight difference negligible. Switching to U3 requires no changes to the chain locker design and adds no extra load to the vessel. Regarding costs, the processing fee for U3 is about 5% higher than for U2, yet the total cost increases by only about 2–3%. In contrast, switching to a larger-diameter U2 chain results in an approximately 8% increase in total price due to the 8% increase in weight per meter.
Windlass Compatibility
Shipboard windlass chain wheels, chain stoppers, and hawse pipes are designed for a 50mm chain diameter. Since U3 maintains the same diameter, it is fully compatible. Although the 52mm U2 chain is only 2mm larger, its compatibility with the existing windlass still needs to be verified.
Low-Temperature Performance and Fatigue
The low-temperature toughness of CM490 depends on whether it has undergone normalizing treatment. CM690, having undergone quenching and tempering, features a finer grain structure and a lower ductile-to-brittle transition temperature, making it safer for use in cold waters. Additionally, the quenched-and-tempered microstructure of CM690 offers superior fatigue resistance, resulting in a slower rate of diameter reduction due to fatigue and a longer service life before replacement is required.
Key Factors to Consider When Choosing
1. Mooring Load Requirements: What is the required increase in tensile capacity? U3 offers a 40% margin, whereas larger-diameter U2 chains typically provide only 8–15%.
2. Windlass Compatibility: Can the chain diameter be changed? Shipboard applications usually do not allow for this, making U3 the preferred choice; however, buoy or aquaculture projects are not constrained by windlasses, allowing for greater flexibility in selection.
3. Operating Temperature: Low-temperature impact toughness is critical in cold waters; the quenched-and-tempered treatment of U3 offers greater reliability in this regard.
4. Need for Additional Ballast: If conditions involve extreme wind and waves, a heavier chain may be required to provide catenary weight for system stability; in this case, a larger-diameter U2 chain is the appropriate choice. This is the primary scenario where a larger-diameter U2 chain is selected.
5. Certification Requirements: Marine anchor chains must comply with classification society standards. U3-grade anchor chains meet the certification requirements for U2-grade chains; essentially, a chain certified as U3 can be used in applications originally specifying U2. Conversely, a U2-grade chain cannot be used in applications requiring U3-grade specifications.
Points to Note
- U3 Supply Specifications: Lead times for U3 and U2 are essentially the same, and all specifications can be manufactured. It is a misconception that "U3 requires customization"; the difference simply lies in stock availability for standard specifications.
- Classification Society Certification: Regardless of whether you choose U3 or a larger-diameter U2, verify certification requirements. Changing the chain diameter may necessitate recertification.
- Total Cost Calculation: Look beyond the unit price of the chain. While U3 costs 2–3% more, it avoids the potential expense of replacing the windlass; conversely, a larger-diameter U2 chain is 8% more expensive upfront and incurs higher shipping and installation costs.
Decision Guide
| Your Situation | Recommended Solution |
| Shipboard application requiring higher tensile strength | Prioritize U3 (Same diameter, +40% tensile strength, +2–3% cost) |
| Buoy/Aquaculture project requiring higher tensile strength | Prioritize U3 (Same weight, lower cost, higher tensile strength) |
| Extreme wind/waves requiring added ballast weight | Choose larger-diameter U2 (+8% weight, increases catenary weight) |
| Cold operating environment | Prioritize U3 (Guaranteed low-temperature impact toughness) |
Contact Us
Unsure whether to choose U3 or a larger-diameter U2? Contact our engineers and provide the following details: current chain diameter, required proof load, windlass model, and operating region. We will help you determine the optimal solution.
SMEOCEAN — Marine and Aquaculture Mooring Solutions
More Links:
CM690 76mm Stud Link Anchor Chain – Manufacturing Process
27.5-Meter Anchor Chain for 10,000-Ton Ships
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