A Climatological Overview of the Race
The Sydney-Hobart yacht race can be strategically quite difficult, mainly due to route laying very close to land. The only section of the race not sailed in coastal waters is the crossing of Bass St; an often trecherous waterway where large amounts of wave and wind energy are funnelled through from the Southern Ocean to the west. The wind and current in coastal waters are more difficult to anticipate and may alter significantly over just a few hours.
During December and January, the weather pattern across southern Australia is typically in a 4 to 6 day cycle. During that cycle, cells of high pressure migrate eastwards; their 'typical' path running from the Indian Ocean, under WA, across the Bight, through Bass St and then out across the Tasman Sea towards the North Island of NZ. These migrating high pressure cells are separated by one or several cold fronts from the Southern Ocean that regularly interact with the summertime heat trough over the Australian mainland. In their west to east migration, the high pressure systems usually cross the Sydney-Hobart rhumb line at Bass St, or around latitude 40S. As a consequence, the race crosses the mean ridge of high pressure and it is common for the fleet to experience a near complete compass swing (left or backing) of winds as they sail south. North of Bass St, the winds are predominately onshore; from S-SE to N-NE, depending upon the phase of the cycle. South of Bass St, the winds are more varied. The isobars indicate a majority of NW to SW flow, but these gradient winds can easily shift into a SE or NE onshore flow near the Tasmanian east coast, either as a twist to the flow or afternoon sea breeze.
FORMULATION OF THE RACE PLAN
The route south should be divided into a number of sections and there are a several factors that should contribute to these divisions and the overall game plan:
a. Dividing the race route into three is the most obvious and common race strategy; the 3 sections being the NSW coast, Bass Strait and the Tasmanian coast. Climatologically and navigationally, these are logical divisions as the weather, winds and sea conditions are generally different across these regions. As well, the NSW and TAS coastlines navigationally constrain the sailing tactics. If the standard 3 divisions is chosen, then care is required in choosing the two boundaries; entering at Gabo Island and leaving at Banks St or Swan Island. It is common to find that these boundaries are not positioned exactly at the state borders; ie the boundaries are not fixed, but vary according to the prevailing wind.
b. A better strategy is to ignore the obvious coastal-ocean divisions and concentrate more on the where the expected change in the wind, wave and current is expected. Coastline geography is important, particularly at night.
c. The day - night - day cycle (diurnal phasing) has a dramatic impact on the surface winds, particularly near the coast. At various stages thoughout the day, limits should be set where the proposed course/route is not permitted to be closer to the coast and/or farther offshore. Another method of setting limits is to set a boundary to the spatial distance away from the rhumb line that you are prepared to travel. However, it should be remembered that it is the influence of land and changing atmospheric stability from day to night that controls the the low level wind structure.
d. The anticipated shift in the wind as the weather system(s) move and develop - the all important forecast! The anticipated change (and rate of change) of wind direction and wind speed might easily save an extra tack or gybe.
e. The changing current pattern - where the strongest favourable currents are located and how the favourable current is expected to vary, both spatially and temporally as well as the possibilty of unfavourable counter, or return, current very close to the coast.
All these factors should be considered prior to the start of the race. On leaving Sydney Harbour, it is essential to have an overall race strategy well planned - remember than an electronic failure could leave you without access to any of the updated weather information available during the race.
Divide the race into 6 hourly sections for 30 hours ahead and then update this 30 hour forecast updated every 6 hours. The 6 hourly increment forecasts are probably best based on the main hours; eg 1200, 1800, 0000, 0600 etc in local time. This division places 1200 and 1800 as 'daytime' conditions and 0000 and 0600 as 'nighttime' conditions.
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