General comments on freeboard and some fundamental concepts

The ship needs an additional safety margin over that required for static equilibrium in calm seas to maintain buoyancy and stability while operating at sea. This safety margin is provided by the reserve of buoyancy of the hull components located above the waterline and by the closed superstructure. In addition, the freeboard is fixed and prescribed by statute. The freeboard regulations define the freeboard and specify structural requirements for its application and calculation.

The freeboard F is the height of the freeboard deck above the load line measured at the deck edge at the mid-length between the perpendiculars (Fig. 1.6). The load line is normally identical with the CWL. If there is no deck covering, the deck line is situated at the upper edge of the deck plating. If there is deck covering, the position of the deck line is raised by the thickness of the covering or a part of this.

Figure 1.6 Freeboard F

The freeboard deck is usually the uppermost continuous deck, although, depending on structural requirements, requests are sometimes granted for a lower deck to be made the freeboard deck. The difference in height between the construction waterline and the uppermost continuous deck is still important in design, even if this deck is not made the freeboard deck.

Superstructures and sheer can make the freeboard in places greater than amidships. Sheer is taken into account in the freeboard regulations. The local freeboard at the forward perpendicular is particularly important (Fig. 1.7). The regulation refers to this as 'minimum bow height'. For fast ships, it is often

Figure 1.6 Freeboard F

advisable to make the bow higher than required in the regulations. A high bow with a small outward flare has a favourable effect on resistance in a seaway.

A 'ship with freeboard' is a ship with greater freeboard than that required by the freeboard regulation. The smaller draught resulting from the greater freeboard can be used to reduce the scantlings of the structure. For strength reasons, therefore, a 'ship with freeboard' should not be loaded to the limit of the normal permissible freeboard, but only to its own specially stipulated increased freeboard.

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