波からの衝撃を吸収するために係留尾翼が必要なのはなぜですか?

2026-07-22 14:39:34

Mooring tails are flexible synthetic extensions fitted to the end of a vessel’s primary mooring lines. They are specifically engineered to stretch and absorb sudden shock loads from waves, functioning much like a car’s shock absorber to protect the whole mooring system. This article explains why these components are necessary for wave shock absorption, based on maritime practices and technical behavior.

The Problem of Wave-Induced Dynamic Loads

At sea, wave action creates continuous vessel motions such as heave, sway, and surge. When a swell hits a moored ship, the primary lines—often wire or high-modulus synthetic ropes with minimal stretch—transmit force quickly and directly. Without adequate cushioning, such a sudden jolt could easily snap lines or place undue strain on hull fittings and deck equipment. Primary lines made of steel chain or stiff fiber have limited ability to dissipate rapid load spikes, magnifying the effect of sudden movements.

Elastic Deformation as a Natural Shock Absorber

Mooring tails introduce a compliant section into the system. Constructed from highly elastic materials like nylon (20–35% elongation) or polyester (~12%), they undergo elastic deformation under load. When a wave attempts to pull the vessel, the tail stretches progressively, absorbing a portion of the kinetic energy that would otherwise become instant tension in the main line. This slows the rate of force build-up, spreading energy absorption over time and distance. Nylon tails can stretch up to 20–30% before breaking, significantly reducing peak loads on hardware.

Energy Dissipation Through Hysteresis

Certain tail materials show hysteretic behavior: not all absorbed energy returns upon contraction, as some is lost as heat via internal friction. This is valuable under repetitive wave cycles, lowering force oscillation amplitude seen by primary lines and preventing cumulative stress amplification. By dissipating vibrational energy, tails keep tensions within safer bounds.

Prevention of Snap-Back and Line Failure

Wave shocks can cause main lines to break suddenly, creating dangerous snap-back. Mooring tails bear the brunt of wave force, reducing wear and stress on primary ropes and decreasing snap-back risk for crew. Their elasticity allows the ship to absorb surges without becoming dislodged, even in crosswinds.

Even Load Distribution and System Protection

Tails create a buffer zone that smooths peak tensions, distributing forces evenly across the mooring arrangement. This prevents overload on individual connection points and protects bollards, winches, and vessel structure. They also act as sacrificial elements, extending main line service life.

Material and Length Considerations for Wave Absorption

For high-energy wave environments, nylon is preferred for maximum shock absorption, while mixed polyester-polypropylene offers balanced durability. The 22-meter tail rule is recommended for exposed berths to ensure enough elongation against surges. OCIMF MEG4 requires tail strength 25–30% above main line MBL to handle transferred wave loads.

In summary, mooring tails are needed to absorb wave shock because they convert sudden wave energy into controlled stretch, dissipate heat via hysteresis, prevent snap-back, and distribute load, safeguarding vessels and port safety in dynamic conditions.