Structure and materials
Yachts of this size are built in steel, aluminium alloy or composite, with a framing system — keel, floors, frames, stringers, girders, deep beams, bulkheads and pillars — that carries the loads into the structure. Watertight bulkheads subdivide the hull for damage survival and fire-rated divisions contain fire. The Chief Mate should be able to read the general arrangement and the midship section, and know where the strong points, the tank boundaries and the watertight and fire boundaries lie.
Stresses on the hull
- Longitudinal: hogging (crest amidships, deck in tension) and sagging (trough amidships) as waves pass; and stresses from uneven loading.
- Local: panting of the bow plating, pounding/slamming of the forefoot, racking when rolling, and vibration.
- Discontinuities such as hatch corners concentrate stress and are where fatigue cracks start.
- Corrosion (chemical and galvanic) is controlled by coatings and cathodic protection, and by keeping bilges and voids clean and dry.
NOTE — Load within the hull's strength Uneven loading — too much weight in the ends or amidships — adds to the wave-induced hogging and sagging and can overstress the hull. The Chief Mate distributes weight so the ship is not strained by the loading itself.
WARNING — Report cracks and corrosion A crack at a hatch corner or heavy corrosion in a tank or void is a structural warning. The Chief Mate reports and records such findings on rounds rather than leaving them for the next survey.