Section 5 Definitions

5.1.1 The following definitions apply. All principal particulars not explicitly defined in the Trimaran Rules are to be as defined in the Complementary Rules.

5.2.1 Rule length, L _R, is the distance, in metres, on a waterline of the main hull, at the design draught from the forward side of the stem to the after side of the rudder post or to the centre of the rudder stock if there is no rudder post. L _R is to be not less than 96 per cent, and need not be greater than 97 per cent, of the extreme length on a waterline of the main hull at the design draught. In vessels without rudders, the Rule length, L _R, is to be taken as 97 per cent of the extreme length on a waterline at the design draught. In vessels with unusual stem or stern arrangements, the Rule length will be specially considered.

5.2.2 Waterline length, L _wl, is the distance, in metres, measured on a waterline at the design draught, from the fore side of the stem to the after side of the stern or transom of the main hull.

5.2.3 Side hull length, L _sh, is the distance, in metres, measured on a waterline at the design draught, from the fore side of the stem to the after side of the stern or transom of the side hull. For stepped side hulls above the waterline, and where the side hull extends significantly forward of the stem, or aft of the stern or transom, the side hull length will be specially considered.

5.2.4 Side hull waterline length, L _shwl, is the distance, in metres, measured on a waterline at the design draught, from the fore side of the stem to the after side of the stern or transom of the side hull.

5.2.5 Breadth, B, is the greatest moulded breadth including the main hull, side hulls and cross-deck structure.

5.2.6 Waterline breadth, B _wl, is the total moulded breadth of the three hulls on a waterline at the design draught, excluding tunnels.

5.2.7 Total waterline breadth, B _wltot, is the total moulded breadth of the three hulls on a waterline at the design draught, including tunnels.

5.2.8 Side hull breadth, B _sh, is the greatest moulded breadth of a side hull measured from the lower point of intersection of the side hull and cross-deck structure as shown in Figure 1.5.1 Definitions to the outermost point of the side hull. If the connection between the side hull and the cross deck structure is radiused, this point is to be taken as the point where the tangent line is at 45° from the horizontal.

Figure 1.5.1 Definitions

5.2.9  Main hull breadth, B _mh, is the greatest moulded breadth of the main hull measured from the lower point of intersection of the main hull and cross-deck structure as shown in Figure 1.5.1 Definitions. If the connection between the main hull and the cross deck structure is radiused, this point is to be taken as the point where the tangent line is at 45° from the horizontal.

5.2.10 Main hull waterline breadth, B, _mhwl is the greatest moulded breadth of the main hull on a waterline at the design draught.

5.2.11 Side hull waterline breadth, B _shwl, is the greatest moulded breadth of a side hull on a waterline at the design draught.

5.2.12 Depth, D, is measured, in metres, at amidships, from the top of the main hull keel plate to the moulded deck line of the uppermost continuous deck, at the side of the side hull. This uppermost deck is to be a deck shared with the side hulls and cross-deck structure. See Figure 1.5.1 Definitions.

5.2.13 Side hull depth, D _sh, is measured, in metres, at amidships, from the top of the side hull keel plate to the moulded deck line at the side of the uppermost side hull continuous deck. This uppermost deck is to be a deck shared with the side hulls and cross-deck structure. See Figure 1.5.1 Definitions.

5.2.14  LCG is the longitudinal centre of gravity of the appropriate hull, in metres, measured from the aft end of the L _wl for the loading condition.

5.2.15 Draught, T, is the design draught, in metres, measured from the moulded baseline of the main hull.

5.2.16 Side hull draught, T_sh , is the design draught, in metres, measured from the moulded baseline of the side hull.

5.2.17 Displacement, Δ, is the total displacement of the side hull and the main hull in tonnes.

5.2.18 Displacement, Δ_mh, is the total displacement of the main hull in tonnes.

5.2.19 Displacement, Δ_sh, is the total displacement of one side hull in tonnes.

5.2.20 Block coefficient, C_b , is the block coefficient at draught T corresponding to the waterline at the design draught, based on Rule length L_R as follows:

5.3.1 Unless otherwise stated, the variables used in the Trimaran Rules are expressed in the following units. For calculations performed using the Complementary Rules, those units given in the Complementary Rules are to be used:

5.4.1 Unless otherwise stated, the co-ordinate system is as shown in Figure 1.5.2 Co-ordinate system. It is a right-hand co-ordinate system with the X axis positive forward, the Y axis positive to port and the Z axis positive upwards. Angular motions are considered positive in a clockwise direction about the X, Y or Z axes.

Figure 1.5.2 Co-ordinate system