Question 1

RISQ Q10.21's guidance on anchoring equipment limitations notes that the class society's equipment number calculation assumes specific wind/current conditions and a scope (chain-length-to-depth ratio) of 6-10, but that "large ships at deep anchorages do not have sufficient chain on board to reach scopes of such magnitude," and that a vessel in ballast presents more exposed side area to wind than the calculation's waterline-based assumptions account for. What is the practical implication for a Master's anchoring decision in deep water or in ballast condition?

Accepted Answer

The vessel's actual holding capacity in these conditions can be meaningfully less than the class equipment number might suggest at face value — a Master who treats the equipment number as a guaranteed holding capacity regardless of actual scope achieved or actual wind exposure is relying on a calculation whose underlying assumptions may not hold for the specific conditions at hand This is precisely the kind of "the rule was derived under assumptions that may not match your actual situation" understanding that distinguishes genuine seamanship judgement from rote rule-following — the equipment number is a minimum-requirement calculation built on stated assumptions (specific wind/current, achievable scope), and a Master anchoring in deep water or in ballast needs to recognise when those assumptions are not actually being met by the real conditions.

Question 2

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 3

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.