Question 1

RISQ Q10.5 recommends mooring winch brakes be set to render at 60% of the ship's design Minimum Breaking Load (MBL), with the winch's stall load never exceeding 50% of design MBL. Why are these two figures both kept well below 100% of MBL, rather than setting the brake to hold right up to the line's actual breaking strength?

Accepted Answer

A brake set close to the line's full breaking strength would allow tension to build to a level where the line itself becomes the failure point under shock loading — by rendering (slipping) at a safe margin below MBL, the winch brake fails in a controlled, less violent way (paying out line) rather than allowing the mooring line to part suddenly, which is a genuinely more dangerous failure mode (snap-back) for personnel nearby A rendering brake is a deliberate, engineered failure mode: it is far safer for the winch to controllably slip and pay out line under excessive load than for the mooring line itself to part suddenly under a higher tension. Setting the brake to render well below the line's MBL ensures the brake — not the line — is what gives way first, converting a sudden violent parting (with snap-back risk) into a controlled release.

Question 2

RISQ Q10.4's guidance states that mooring tails should have a Tail Design Break Force (TDBF) of 125-130% of the ship's design MBL — higher than the mooring line itself — and explicitly warns that increasing TDBF further "will not necessarily increase tail fatigue life and may undermine the integrity of the mooring system by reducing system compliance." Why would making the tail even stronger actually reduce overall mooring system safety?

Accepted Answer

Mooring tails are designed to be the more elastic, compliant component of the system, absorbing shock loading through stretch — making the tail progressively stiffer/stronger reduces this elasticity ("compliance"), meaning shock loads that the tail would otherwise absorb get transmitted more directly to the rest of the mooring system instead, which can increase rather than decrease the risk of failure elsewhere This is a genuinely counter-intuitive but important engineering principle worth understanding clearly: "stronger is always better" is not true for a component whose functional role depends partly on its elasticity. The tail's job includes absorbing shock through stretch — over-engineering it for raw strength at the expense of compliance can shift risk elsewhere in the system rather than eliminating it.

Question 3

RISQ Q10.13's guidance explicitly recommends AGAINST painting snap-back zones on the mooring deck, on the basis that doing so "may give a false sense of security" since "the whole mooring deck may be considered a danger zone." Why would marking only specific snap-back zones create a safety problem rather than improving awareness?

Accepted Answer

Snap-back is governed by the geometry of the line under tension at the moment of failure, which can change as the line angle, lead, and tension point shift during mooring operations — a fixed painted zone reflects one assumed geometry and can give crew false confidence that standing just outside the painted area is safe, when in fact the actual dangerous zone has moved This is a clear example of a well-intentioned safety measure creating a worse outcome than no measure at all — a static painted zone implies a static hazard, but the actual snap-back danger zone shifts with line geometry throughout a mooring operation. Treating the entire mooring deck as hazardous, rather than relying on a painted boundary, avoids the false confidence that "I'm standing outside the marked zone, so I'm safe" would create.

Vetting — RISQ Mooring Practice Questions