Freeboard is the vertical distance from the waterline to the freeboard (upper) deck at side. It is required because it provides:

• Reserve buoyancy: the volume of the hull above the waterline that keeps the ship afloat when waves or damage raise the external waterline. Adequate reserve buoyancy is essential for floating headroom and survivability.
• Reduced shipping of water and deck wetness: a higher freeboard gives better watertight integrity against seas sweeping over the deck, protecting cargo, stability and the crew.
• Improved stability and reduced angle of heel by keeping the deck edge and openings (doors, hatches, air pipes, vents) well above water, and by providing a larger intact range of stability (area under the GZ curve).
• Comfort and safety in a seaway: better freeboard limits immersion in following/quartering seas and reduces green seas on deck, lowering risk of damage and of loss of stability.

The minimum freeboard is fixed by the International Load Line Convention, giving the load-line mark which must never be submerged.

The condition of assignment is the series of requirements that a ship must satisfy to receive its load line certificate, ensuring the hull strength, watertight integrity and the protection of openings are commensurate with the assigned freeboard. It covers the scantlings and weathertightness of the freeboard deck, the strength of the superstructure and hatch covers, the provision and closing of watertight and weathertight openings (doors, scuttles, vents, air pipes, sounding pipes, sill heights, freeing ports), and the availability of bulwark freeing arrangements. These are maintained by:

• keeping the hull structcuy sound and free from corrosion/defects;
• maintaining all weathertight and watertight closures, gaskets, coamings and securing arrangements in good order;
• ensuring proposed and deck fittings (gangways, cranes, lashings) do not open ill-hidden ports above the assigned deckline;
• carrying out planned and classification surveys (annual/periodical) and maintaining the load line certificate, the certificate being renewed at the load line survey;
• reporting any damage or alteration to the hull or openings so the surveyor can confirm the condition still satisfies the rules; unauthorised alterations that reduce the assigned freeboard are prohibited.

(Provide a diagram: a ship's side at midship with the freeboard deck line, and four waterlines at different heights.)

• Type A ships are designed to carry only liquid cargoes in bulk, with a closely guarded deck and high integrity (e.g. tankers). They get the lowest (smallest) freeboard because the spillover risk is small, giving maximum carrying capacity.
• Type B ships are general cargo ships not covered by type A; they require a greater freeboard.
• Type B60 and B100 are refined type B ships to which a reduced freeboard is permitted provided the ship satisfies additional strength and stability criteria - B60 allows 60% of the tabular difference between B and A freeboards to be taken off, B100 allows the full 100% reduction (i.e. the B freeboard may be reduced to the type A level) providing the prescribed additional conditions are met. Hence the freeboards order: Type A smallest, B100 next, B60 next, and plain Type B the largest freeboard.