Materials and framing
Vessels of this class are built in steel, aluminium alloy or fibre-reinforced plastic (FRP), each with its own framing system that carries loads into the structure: keel, floors, frames, stringers, girders, deep beams, bulkheads and pillars. Transverse framing (many closely spaced frames) suits smaller hulls; longitudinal framing (fewer, heavier longitudinals) resists hogging and sagging efficiently in larger ones; most ships use a combined system. Watertight bulkheads subdivide the hull for damage survival, and fire bulkheads (A-, B- and C-class divisions) contain fire. The midship-section drawing shows how these members are arranged.
Corrosion and its control
- Corrosion is chemical (ordinary rusting/oxidation) and galvanic (a current flowing between dissimilar metals immersed in seawater, which eats the less noble metal).
- Cathodic protection controls galvanic corrosion using sacrificial anodes of a less noble metal (zinc or aluminium) that corrode in place of the hull, or an impressed-current system that drives a protective current.
- Coatings, keeping bilges and voids clean and dry, and prompt attention to damaged paint all slow corrosion.
- Stray-current corrosion from poor electrical bonding can be aggressive and is a reason to keep the bonding system sound.
Classification and statutory oversight
A classification society — a member of IACS such as Lloyd's Register, DNV, ABS or Bureau Veritas — sets construction and survey standards, assigns a class notation, and surveys the vessel throughout her life to confirm she remains built and maintained to those standards. Class is separate from, but works alongside, flag-State statutory certification (safety, load line, pollution). The OOW should know which society classes the vessel and roughly what the survey cycle involves.
WARNING — Sacrificial anodes are consumed on purpose Anodes waste away by design to protect the hull, shafts and rudder. When they are gone, protection stops and the metal they were guarding starts to go instead — so their condition is checked and they are renewed on schedule.