Predictwind - Sydney Hobart record still under threat 48hrs from start

by Richard Gladwell, Sail-World.com NZ on 23 Dec 2016 The race record in the Rolex Sydney Hobart would still appear to be under threat 48 hours from the start of the 630nm offshore classic.

Using the routing function of wind and weather prediction application, www.predictwind.com, the latest model run by Sail-World, shows that in an extreme situation over 7 hours could be slashed off the record of 1day 18 hrs 23 mins set by Wild Oats XI four years ago.

The more conservative route shows a finish time of six hours under the record, with one of the weather feeds producing an optimum route for a supermaxi that will take within a few minutes of the current record.

Average speed for a supermaxi under the extreme situation gives an average speed for the supermaxi of 19-20kts for the race, with the more conservative approach dropping that average speed back to 19kts, and 16.6kts for the route which shows the race winner missing the record by five hours.

In previous years it is common for predictions of the race record to fall, but as navigators and tacticians make their course decisions the focus comes on winning the race, rather then slashing the record.

The reason for running the extreme and conservative routes is that the extreme approach will show which side of the direct route is favoured, and at this time out from the race start generally the extreme version points to an easterly approach with the trade-off of sailing extra distance at a faster speed.

In the Extreme scenarios is only 34-50nm further than the actual 630nm race distance. But interestingly with the current models the conservative route is around 50nm longer than the minimum distance, but of course it does allow for better boat positioning options as the weather information is updated and route options change.

In reality, the lead boats tend to stay very close to the most direct route but will move to the east if that option is shown as being favoured under the extreme model.

Obviously, as the time left to the race start reduces the weather feeds (there is a choice of four feeds available in the Predictwind application), will tend to align more closely and the routing options start to fall into the same area of weather.

The extreme scenario run through Predictwind's routing function gave the application complete freedom to determine the optimum route from Sydney Heads to the finish line in Hobart. As with previous years when this approach has been used a series of extreme easterly course options were chosen to take advantage of the expected northerly wind to drive the yachts down the NSW coast and then set the boats up to miss the worst of the southerly front, expected to hit at midnight, which usually lasts for about four hours.

However the current models show (since confirmed by other sources) the usual front being replaced with area of light winds around dawn before a moderate ESE breeze comes into play. There is an area of strong southerly winds lurking around Bass Strait, however the routing is able to skirt these winds and they dissipate later in the day. Of course that scenario can change closer to the race start.

Earlier predictions showed the usual southerly front making a visit on the first night.

The interesting point with the extreme model is that three of the four feeds are in broad agreement as to the optimum route recommending a course out to the east before coming back to the Tasmanian coast. Both the extreme and conservative approaches to routing show the race record falling, the question is just by how much?

Under current models the front is not shown as hitting the fleet at all, and at worst just a light wind period before swinging back through the east. The slowest prediction again shows no front at all, but instead a period of light winds in the early hours of December 27. All models are agreed about the wind strengthening from the north on December 27 giving the lead boats a fast run down the Tasmanian coast.

To look at the race options in a more conservative way, a series of way points are inserted into the routing model to pulls the yachts into a more direct course (but still standing off to the favoured easterly side).

The Predictwind sees these waypoints as a series of rounding marks dotted down the route between Sydney and Hobart and treats these as a series of marks that must be rounded, and the optimised route will pull the boat's course back into line at these points.

That produces a distortion in itself, which will almost certainly mean that the course is more conservative than would otherwise be taken - but that is then reflected in the time taken, and even under that constraint the record, at this stage would definitely seem to be on the line.

The real issue with the extreme model is that as the race progresses the weather feeds and their optimum course predictions vary quite markedly and 'banging the corner' is never a good option in yacht racing - particularly offshore, and will usually leave a boat well away from the direct route, with longer distance to sail and with less favourable course angles and options to sail to reach the finish.

Simply a boat which takes the extreme option can rarely recover to reach a new optimum position when the forecast feeds change their information, and that is why most navigators and tacticians opt to stay close to the most direct route and then position then accordingly as weather unfolds as the race progresses.

Keys to look for in the early stages of the race will be which boats opt to move offshore (sometimes in quite an extreme way) and then follow the progress of that option compared to the boats who have tended to stick to the more coastal route. In the past races , the key time has been around dawn on December 27, after the passage of the usual front, and as the boats set themselves up to cross Bass Strait.

If the wind does turn back into the north and freshen on December 27, then there will be a very fast race, however, if the front does arrive, lingers, or winds turn light, then the odds of the race record falling will reduce markedly.