Hull Outfitting

Every anchor, length of cable and windlass fitted to a ship is sized from a single derived number, the equipment numeral, not from a designer's estimate of how big an anchor "looks right" for the tonnage. The reasoning behind the numeral is that a ship at anchor is really being held against wind and current acting on everything above the waterline, plus the inertia of the hull below it. Displacement alone captures the second part. It says nothing about a ship that is short and low with a small deckhouse compared with one of the same displacement that is long, high-sided and carries a large accommodation block well above the waterline — the second ship presents far more windage and needs more holding power for the same anchor to work.

The displacement term Δ^(2/3) grows more slowly than Δ itself, which is deliberate: holding power needed against current and inertia scales roughly with a characteristic area of the hull, not its full volume. The 2hB term uses an effective height h, measured from the summer load waterline to the top of the highest enclosed house, together with the moulded breadth B, as a simple stand-in for the side area exposed to wind. The A/10 term then adds the actual measured profile area of hull, erections and houses above the summer waterline, which corrects for ships whose superstructure does not fill the h×B envelope, or overflows it. None of these terms is optional in the sum, and an exam question that quietly gives you a boxy, high-sided vessel is testing whether you remember all three.

Once EN is known, it is looked up in the classification society's equipment table, which returns — as a fixed entry, not a calculation — the number and mass of bower anchors, the diameter and grade of stud-link chain, the total length of cable to be carried, and the windlass duty. The designer's job stops at computing EN correctly and reading the table; it is not a design exercise in the same sense as scantlings or stability.

Cable length is measured in shackles, an old but still universal unit equal to fifteen fathoms.

Each shackle joint in the chain is marked so the officer paying out or heaving in can read, by eye or by counting turns on the gypsy, exactly how much cable is out. This matters for two separate reasons: enough scope has to be paid out for the catenary to hold the anchor flat against the seabed rather than dragging it, and the crew must know when to stop heaving before the anchor is pulled hard up into the hawse pipe and damages the shell plating.

The windlass itself is specified by a duty condition, not just a peak capability: a rated speed at which it must be able to heave in continuously, sustained for a stated period, plus a braking capacity to hold the ship once the cable is secured. The two requirements are different mechanically. The motor and gearing set the heaving speed and the continuous pull available while actively hoisting; the brake, usually a friction band bearing directly on the cable lifter, is what holds the suspended weight once the clutch is disengaged and the motor is not driving. A windlass can be entirely adequate for hoisting and still be let down by an undersized or poorly maintained brake, which is why the two are checked separately rather than assumed to track each other.

Two mooring lines and their equipment follow a broadly similar logic: minimum breaking load and the number of mooring lines and winches are also read from a table keyed to the equipment numeral, on the reasoning that a ship large enough to need heavier anchors is also large enough to need stronger, more numerous mooring lines to hold it alongside against wind and current.

These two words are used carelessly in conversation and precisely in design, and mixing them up is one of the most common ways to lose marks. A watertight closure is designed to resist a static head of water pressing on it from either side — the condition that exists, for example, at a bulkhead door below the waterline if the compartment on one side floods. A weathertight closure only has to resist water finding its way in from above or from the side under wind and wave action — spray, green water breaking over a weather deck, or heavy rain — and is not expected to hold back a standing head of water pressing from underneath or from one flooded side.

The construction reflects the duty. A watertight door is a heavy, gasketed steel door with multiple dogs or a hydraulic sliding mechanism, mounted in a reinforced frame, and it is tested with an actual or equivalent hydrostatic head to confirm it holds under pressure. A weathertight hatch or door instead relies on a compression gasket closed by a smaller number of cleats or dogs, sufficient to exclude water thrown at it but never proven against a standing head, and it is tested with a hose playing a jet of water at the closed seam rather than with pressure from behind. Fitting a weathertight door where the compartment arrangement actually calls for a watertight one is a real design error, not just a vocabulary slip — it leaves a boundary that will let water through under exactly the flooding condition the ship's subdivision was meant to survive.

Access openings — manholes, sidescuttles, ventilator coamings — are classified the same way and dogged, gasketed or glazed accordingly, and their closing arrangement is shown on the ship's damage-control and closure plans so the crew knows, compartment by compartment, which doors must be shut before going to sea and which can be left for weather protection alone.

Steel in seawater corrodes electrochemically, and a ship's corrosion protection is really two complementary systems working together rather than either one on its own. The coating is a barrier: an intact paint film simply keeps the electrolyte away from the steel, and where it stays intact almost no corrosion current can flow at all. No coating stays perfectly intact in service — it is abraded by cargo, chafed by mooring lines, holed by welding spatter during repairs, or simply breaks down with age — so a second, independent system is needed to protect the steel that the coating has stopped covering.

Cathodic protection works by forcing the steel to be the cathode of the corrosion cell rather than the anode, so the current that would otherwise dissolve iron flows into the steel instead of out of it. Sacrificial anodes — aluminium or zinc alloys, chosen to be more electrochemically active than the hull steel — are bolted to the hull and simply corrode themselves in place of the structure, driven by the natural potential difference between the two metals; they need no external power and are replaced as they waste away. Impressed current cathodic protection (ICCP) instead drives current from an external DC power supply through permanent, largely inert anodes, controlled by a reference electrode that continuously adjusts the output to hold the hull at the target protective potential. ICCP needs power and control equipment but avoids the periodic anode replacement and drag penalty of a large sacrificial anode array, which is why it tends to appear on larger, faster ships where that trade-off favours it.

Because of that link, the coating specification is dominated by surface preparation rather than by the paint product itself. A near-white metal blast to grade Sa 2½ removes mill scale, rust and old coating residue down to a sound, anchored profile that the new coating can key into; skipping or skimping on this step leaves contamination under the film that causes early blistering and disbondment however good the paint chosen is. In practice, surface preparation determines most of a coating's service life, and product selection accounts for comparatively little of the variation seen between a coating that lasts its intended interval and one that fails early.

A ship is subdivided into fire zones by boundaries — bulkheads and decks — that are classified by letter according to what they are made of and how long they hold back fire and hot gas while still limiting the temperature rise on the unexposed face. The classification is structural fire protection: it is about buying time for escape, firefighting and containment, and it is a completely different requirement from thermal or acoustic insulation fitted for comfort, even though both can be lagging wrapped around the same steel.

An A-class division is steel or equivalent, insulated with approved non-combustible insulation to a tested standard, and its suffix denotes the number of minutes it has been proven, by standard fire test, to resist the passage of flame and to keep the temperature rise on the unexposed side within limits — an A-60 division is materially different fire protection from an A-0 division of the same steel, purely because of the insulation and testing behind the number. B-class divisions use lighter non-combustible construction rated for a shorter proven period, typically used for internal partitions such as cabin bulkheads where less fire load and less time are expected. C-class divisions are simply required to be built of non-combustible material, with no fire-integrity time attached at all.

The arrangement of all these boundaries — which bulkheads are A-class, which decks, where the B-class cabin partitions sit, and how the divisions tie together at doors, ducts and cable penetrations — is recorded on the ship's fire control plan, which is also the document the crew and shore firefighters use to locate boundaries, extinguishing systems and escape routes during an actual incident.

Lifesaving appliances are arranged as a chain of linked spaces rather than as isolated pieces of kit, and the exam expects the arrangement logic as much as the equipment list. Crew and passengers first move to a muster station, where they are accounted for, briefed and issued lifejackets; from the muster station they proceed to an embarkation station, positioned so the route between the two is short, protected from weather so far as practicable, and does not cross machinery casings or other hazards; and from the embarkation station the launching appliance — davit, free-fall arrangement, or marine evacuation system — actually gets the survival craft into the water. A lifeboat stowed in a perfectly good davit is of no use if the muster route to reach it is blocked or unlit, so the arrangement is judged as a whole sequence, not station by station.

Launching appliances themselves are sized and tested for the loaded mass of the craft plus its full complement, with the davit, falls and winch brake all rated to lower that load under the worst realistic condition of list and trim the ship might have at the time of an emergency, not just when upright and even keel. This is the same reasoning that sizes a windlass brake for the actual suspended load rather than an idealised at-rest condition — lifesaving equipment is specified for the casualty condition it exists to serve, not the calm condition it is normally seen in during drills.

Access equipment sits alongside lifesaving appliances in the outfit list because it shares the same purpose of moving people safely on and off the ship: accommodation ladders and gangways for use alongside, and pilot ladders for boarding at sea, each with their own rigging, lighting and rescue-arrangement requirements so that the means of getting aboard does not itself become the hazard.

The three examples below carry the equipment numeral, windlass duty and cathodic protection reasoning from the sections above through to full numerical answers, in the layered, multi-step form the exam actually sets.