The examiner uses construction terms without explaining them, and expects you to do the same. This is the working vocabulary:
| Term | Meaning |
|---|---|
| Keel | The backbone — the principal fore-and-aft strength member along the bottom centreline |
| Frames | Transverse ribs rising from the keel, giving the hull its shape and transverse strength |
| Stringers / girders | Longitudinal members tying the frames together — longitudinal strength |
| Beams | Transverse members supporting the decks, tying the two sides of the hull together |
| Bulkheads | Vertical partitions — structural, fire (class divisions), or watertight subdivision |
| Stem / stern post | The forward and after extensions of the keel structure |
| Garboard strake | The plating strake next to the keel |
| Sheer strake | The uppermost side strake at deck edge — highly stressed; why cuts and welds there matter |
| Sheer | The fore-and-aft curve of the deck line, rising toward the ends |
| Camber | The transverse curve of the deck, shedding water to the sides |
| Bilge keel | External longitudinal fins at the turn of the bilge — roll damping |
| Double bottom | Inner and outer bottom plating forming tanks — grounding protection, ballast/fuel stowage, and LOW weight |
| Material | Strengths | The operational catch |
|---|---|---|
| Steel | Strong, ductile, fatigue-tolerant, cheap to repair anywhere | Corrodes — coatings and anodes are a maintenance regime, not decoration |
| Aluminium | Light — speed and top-weight savings; no rust | Loses strength FAR below visible red heat (~200–300°C) — an aluminium superstructure fire is a structural race, which is why boundary cooling matters so much; galvanic corrosion with dissimilar metals; specialist welding |
| GRP / composite | No corrosion, mouldable, light | Burns — and feeds the fire; osmosis in older laminates; poor radar reflectivity (your own tender barely paints); repairs are laminating jobs |
Bulkheads divide the hull so that flooding is confined and the vessel survives damage — the collision bulkhead forward being the first and most important. The design assumption is that the boundaries are INTACT: watertight doors closed at sea (opened briefly on authority, logged), cable and pipe penetrations properly glanded, and closing appliances maintained and drilled.
Related vocabulary: margin line (the line below which the deck at side must not be immersed after assumed damage), downflooding points (the openings that set the angle at which water enters — vents, doors, hatches), and freeing ports (which shed boarded water and must never be blocked).
The disc and lines amidships both sides showing the minimum permitted freeboard by zone and season — S (summer), W (winter), WNA (winter North Atlantic), T (tropical), F/TF (fresh, tropical fresh). Loading beyond the applicable line is an offence and an insurance problem. The letters beside the disc identify the assigning authority.
Numerals at stem, stern and often amidships both sides, read against the waterline for the actual draughts — the numbers your UKC calculation, squat estimate and stability condition all start from. Reading them (and knowing your marks' units) is basic officer competence.
| Term | What it measures | Used for |
|---|---|---|
| Displacement | The actual WEIGHT of the ship — the water she displaces (tonnes) | Stability calculations, squat, handling |
| Gross tonnage (GT) | A dimensionless index of total enclosed VOLUME | Regulation thresholds — manning, certification, "under 3000gt" itself |
| Net tonnage | Volume index of the earning spaces | Dues and fees |
| Deadweight (DWT) | Carrying capacity — the weight of everything the ship can lift (cargo, fuel, stores) | Commercial description; loaded minus light displacement |
| Item | Variants | Operational meaning |
|---|---|---|
| Rudder | Balanced / semi-balanced; spade (yachts) vs skeg-hung | Effectiveness depends on flow — prop wash across it is why the kick ahead steers; no flow, no rudder |
| Propeller | Fixed pitch (FPP) vs controllable pitch (CPP); handed left/right | FPP handedness gives transverse thrust (stern walk); CPP keeps shaft direction constant — astern thrust by pitch reversal, different low-speed behaviour, and zero-pitch instead of stopped |
| Stern/bow thrusters | Tunnel, azimuthing | Effective at rest to ~2–3kn; thermal and electrical limits |
| Stern gland / shaft seal | Traditional packed gland vs mechanical/water-lubricated seal | A routine bilge-round item — a failing seal is a slow flooding you find early or late |
Every hole in the hull is a potential flooding source with a valve on it: main and auxiliary sea suctions (with strainers and often high/low inlets), overboard discharges, log and sounder transducers, the stern gland. The OOW should know where the vessel's hull openings plan is, how the valves are operated (and which close automatically), and the emergency response: a failed fitting is isolated at its valve — which is only possible if someone knows where it is in the dark, with water rising.
Corrosion control is a system: coatings (the barrier), sacrificial anodes (zinc/aluminium blocks that corrode preferentially, protecting the metal they're bonded to), and on many vessels impressed-current systems. Galvanic corrosion attacks where dissimilar metals meet in seawater — the aluminium hull with a bronze fitting, the stainless bolt in the aluminium mast. Anodes are inspected and renewed at haul-out; a vanished anode has been doing its job and its replacement is protecting your stern gear.
The OOW's fabric rounds — bilges dry and clean, limber holes clear, coating breakdown noted, anodes' state at surveys — feed the planned maintenance system and the class/flag survey cycle (annual, intermediate, renewal/special) that keeps the certificates in Section 5 alive.
What is the difference between gross tonnage and displacement?
Your vessel has an aluminium superstructure. What does that change about fighting a fire in it?
You find a bilge slowly making water in the lazarette. Where does your construction knowledge take you?