3. BASIC CONSIDERATIONS

Co-ordinate system

The co-ordinate system and hull parameters are shown in Figure 3.1; x is positive astern, y is positive to starboard, and z is positive upwards. The undisturbed water surface is the plane z = 0. Hull centreplanes are parallel to the x-axis. The shapes of the hulls can be different from each other, however, they are assumed to be symmetric with respect to their own centreplanes. The overall vessel need not be laterally symmetric.

Figure 3.1: Co-ordinate system for general multihulls

Each hull has a nominal centre point (x = si, y = wi, z = 0). The centre of the main hull, hull 1, is always located at the origin. The centres of all other hulls are then measured relative to the main hull, i.e. to hull 1. si is the longitudinal separation distance of the i'th hull measured relative to the first hull and is defined as positive astern. Hence by definition, s1 = 0. wi is the lateral separation distance of the i'th hull relative to the first hull (positive to starboard). Hence by definition, w1 = 0. For example, in the arrangement shown in Figure 3.1, s2 has a negative value, and w2 is positive; for hull 3, s3 is negative and w3 is positive.


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