Section 2 Definitions and symbols

2.1.1  Air gap. The air gap, G _A, is the minimum vertical distance, in metres, from the static waterline to the point considered in an operational condition. In no case is G _A to be taken greater then G _A(max) as indicated in Figure 2.2.1 Definition of air gap.

Figure 2.2.1 Definition of air gap

2.1.2  Allowable speed V. The allowable speed used in the computation of environmental loads is the design speed, in knots, associated with a nominated operational environment in which the craft is certified at corresponding operational displacement.

2.1.3  Beaufort Number. Beaufort Number is a measure of wind strength.

2.1.4  Bilge tangential point. For craft with partially submerged hull(s), the bilge tangential point is defined as the tangential point of the bilge with an oblique line sloped at 50^o to the horizontal at the LCG, see Figure 2.2.2 Definition of bilge tangential point and G _s for craft with partially submerged hulls . For craft with fully submerged hull(s), the bilge tangential point is defined as the intersection points between the hull and the design waterline.

Figure 2.2.2 Definition of bilge tangential point and G _s for craft with partially submerged hulls

2.1.5  Deadrise angle. For craft with no clearly defined deadrise angle at the LCG, the angle, in degrees, to the horizontal of the line at the LCG formed by joining the lowest point of the hull or underside of keel and the bilge tangential point is to be taken as the deadrise angle θ_D, see Figure 2.2.2 Definition of bilge tangential point and G _s for craft with partially submerged hulls . For craft with hulls of asymmetric section, where the inner and outer deadrise angles differ, the smaller of the two angles is to be used. For craft with fully submerged hull with circular sections, the deadrise angle is to be taken as 30^o.

2.1.6  Displacement mode. Displacement mode means the regime, whether at rest or in motion, where the weight of the craft is fully or predominantly supported by hydrostatic forces.

2.1.7  Froude Number F _n. The Froude Number is a non-dimensional speed parameter and is defined as:

2.1.8  LCG. The LCG is the longitudinal centre of gravity of the craft in the loading condition under consideration.

2.1.9  Maximum wave height. In general the maximum wave height, in metres, will be taken as 1,667 times the significant wave height. Where, for design purposes, a wave length is required this will be taken as the waterline length subject to any restriction resulting from limiting height to length ratio and wave profile angle.

2.1.10  Non-displacement mode. Non-displacement mode means the normal operational regime of a craft when non-hydrostatic forces substantially or predominantly support the weight of the craft.

2.1.11  Operating waterline is the waterline for the operating condition under consideration.

2.1.12  Period. The period is defined as the average time interval between upward crossings of the mean value.

2.1.13  Sea state. Sea state is an expression used to categorise wave conditions and is normally defined by sea spectrum, significant wave height and period distribution.

2.1.14  Significant wave height H _1/3. The wave height, in metres, used in the determination of craft motions and loads is a significant wave height, H _1/3, defined as the average of the one third highest waves in a short term wave measurement record.

2.1.15  Support girth. The support girth, G _s, is the girth distance, in metres, measured around the circumference of the shell plate between the tangential points or chines, as appropriate, of the hull for a mono-hull craft. For multi-hull craft it is to be taken between the inner and outer bilge tangential points or chines of the individual hulls. See Pt 5, Ch 2, 2.1 Parameters to be used for the determination of load and design criteria 2.1.4 and Figure 2.2.2 Definition of bilge tangential point and G _s for craft with partially submerged hulls .

2.1.16  Surviving wave height H ₀₃. The wave height, in metres, used in the determination of the structural integrity of a craft and is defined as the wave height with three per cent probability of exceedance. If this value is unknown, the following equation is to be used to determine H ₀₃:

2.1.17  Taylor Quotient . The Taylor Quotient is defined as

where V is defined in Pt 5, Ch 2, 2.1 Parameters to be used for the determination of load and design criteria 2.1.2 and L _WL is defined in Pt 5, Ch 2, 2.1 Parameters to be used for the determination of load and design criteria 2.1.19.

2.1.18  Volumetric speed number F _v. The Volumetric speed number is defined as:

where ∇ is the moulded displacement, in m³, of the craft corresponding to the design waterline.

2.1.19  Waterline length. Waterline length (L _WL) is as defined in Pt 3, Ch 1, 6.2 Principal particulars

2.1.20  Protected structure, see Figure 2.2.3 Definition of wet-deck protected and unprotected structure. A protected structure is one in which the wet-deck component under consideration is enclosed by port and starboard side inboard structure, where `side inboard' is as defined in Ch 4,1.5.6 of Parts Pt 6, Ch 4, 1.5 Symbols and definitions 1.5.6, Pt 7, Ch 4, 1.5 Symbols and definitions 1.5.6 and Pt 8, Ch 4, 1.5 Symbols and definitions 1.5.6 for craft of steel, aluminium alloy and composite construction respectively.

Figure 2.2.3 Definition of wet-deck protected and unprotected structure

2.1.21  Unprotected structure, see Figure 2.2.3 Definition of wet-deck protected and unprotected structure. An unprotected structure is one in which the wet-deck component under consideration is not enclosed by port and starboard side inboard structure, where `side inboard' is as defined in Ch 4,1.5.6 of Parts Pt 6, Ch 4, 1.5 Symbols and definitions 1.5.6, Pt 7, Ch 4, 1.5 Symbols and definitions 1.5.6 and Pt 8, Ch 4, 1.5 Symbols and definitions 1.5.6 for craft of steel, aluminium alloy and composite construction respectively.

2.2.1  L_R, B, D, C_b, L_WL and T are as defined in Pt 3, Ch 1, 6.2 Principal particulars.

Figure 2.2.4 Definition of Symbols

2.2.2 The displacement, Δ, in tonnes, used in this Part is the mass of the craft in the loading condition under consideration.

2.3.1 The minimum value of significant wave height, H _1/3, see Pt 5, Ch 2, 2.1 Parameters to be used for the determination of load and design criteria 2.1.14, in metres, used in the determination of accelerations and loads is, in general, not to be taken less than that given in Table 2.2.1 Minimum significant wave height, H _1/3 for the appropriate Service Groups defined in Pt 1, Ch 2, 3.5 Service area restriction notations.

2.3.2 The designer/Builder is to provide the value of significant wave height for use in the determination of the Rule loadings and, further, is to ensure that such a wave height is appropriate to the intended area of operation and/or service. In this respect the statistical wave data may be required to be submitted in support of the wave height nominated.

2.3.3 A reduction in the minimum value of significant wave height for a particular Service Group will be specially considered, provided that satisfactory statistical wave data for the intended service area are submitted for approval. See also Pt 5, Ch 2, 2.1 Parameters to be used for the determination of load and design criteria 2.1.14.