Appendix 12 Volume Three 5. Assessment of speed through the water 5.1 The following method is an accepted methodology to determine Ship Speed where an appropriate instrumental approach is not available or where the hull-mounted instruments are faulty. The speed through the water, V W , is calculated from the speed over the ground, V G , according to: (1) V= V− λxU T W G in which λ is an index that describes the direction of travel of the ship (λ = 1 for ships travelling towards Picton, and λ = -1 for ships travelling away from Picton), and U is the T speed of the tidal stream. The convention for the speed of the tidal stream is that it is positive for the flooding phase (i.e. tide level is rising) and negative for the ebbing phase (i.e. tide level is falling). For the purposes of calculating the speed of the tidal stream, the Marlborough Sounds is deemed to consist of three zones. Each zone is named after the tidal stream data point marked B, C and D that is located within each zone as shown on Marine Chart NZ 6153. The speed of the tidal stream in each zone is calculated from: 2π (2) U T =Acos 12 (t−t +t) HT L in which A is the amplitude of tidal stream speed (see Table 2 below), is the time in t hours since midnight, t is the time of the nearest high-tide in Wellington, and t is the HT lag between the tidal stream and the high-tide at Wellington (see Table 2 below). The times use for t and t must be to a consistent standard, either New Zealand Daylight HT Saving time (NZDT) or New Zealand Standard time (NZST) depending on the time of year. Equation 2 is a cosine fit to the tidal stream with a semi-diurnal period of 12 hours. The time of high tide is based on the figures published in the New Zealand Almanac (or an equivalent source). The tidal zones, their definitions, and values for the coefficients in equation 2, are given in Table 2 below. Table 2: Tidal zones and tidal speed parameters that might be adopted to assess speed through the water A t L Zone Description of Limits of Zone (knots) (Hour) Zone anywhere in Queen Charlotte Sound, and separated B from Tory Channel along latitude 41°14′ S at Dieffenbach 0.3 -0.4 Poin t. Zone in Tory Channel between latitude 41°14′ S at C Dieffenbach Point at its western end and longitude 1.8 -0.4 174°18′ E in Tory Channel at its eastern e nd. D Zone near the Heads in Tory Channel located east of 6.7 0.0 longitude 174°18 ′E. App 12 – 4