The predicted routes in both the 2D and 3D viewer and the 1 day, 3 day and 5 day positions on the predicted leaderboard are all calculated using a technique called ‘Yacht Route Optimization’.
‘Weather Routing’, which is the process of assessing tactical and strategic options uses this kind of process. In fact, the race uses a leading racing navigation software package to produce the website predictions.
The first input of the route optimization is the data model of the yacht’s performance which tells us how fast a yacht can go at any wind speed and at any angle to the wind.
When you combine this with the weather forecast, it is possible to predict all the places that the yacht could sail to in a specific time period -- for instance one hour. All these places are connected by a line called an isochrone (just as an isobar is a line connecting all the places with an equal barometric pressure, so an isochrone is a line connecting all the places a yacht can reach from its starting point in an equal period of time).
At the end of the first hour, the routing calculation checks the weather forecast to see what the wind conditions are all along the one hour isochrone. Then, once again combining this with the yacht’s performance model, it works out all the places the yacht could sail to in the next hour, to get a second isochrone.
This process is repeated over and over, with the two hour isochrone followed by the three hour isochrone and so on, expanding outwards from the yacht’s starting point like a wave rippling out from a stone thrown into a pond - only not quite so neat.
At some point one an isochrone will reach the yacht’s destination i.e. the finish line.
This defines the fastest time that the yacht could reach the finish and everything else in our predictions can then be calculated. The quickest route to the finish can be traced back through the expanding wave of isochrones, the arrival time can be predicted and compared to the rest of the fleet to get the finish order, and so on.
But weather routing is much more than just a software process. The process described above is just a simulation of which will be the fastest route to a destination point based on the data model of the yacht’s performance and the weather forecast.
There are many factors that can make what we see in this simulation very different to reality:
- Forecast degradation: the prediction ability of a weather model is degraded with time, which is why a 10 day simulation is not something you can believe with any certainty.
- Poor modeling of local phenomena: there are many small scale weather events like squalls, frontal lines, sea breezes, geographical wind shifts, light wind areas and so on, that the forecasts cannot take into account and which obviously impact the short term strategy.
- Tactics: under many situations a navigator or skipper can tactically decide not to follow the optimum course. This happens very often when the forecast confidence is low and it is better to stay with the fleet rather than going off on your own. This reinforces the fact that a route optimization tool is only one of many inputs in the strategical decision making process.
- Unexpected yacht performance: Especially in rough weather situations, the skippers can decide not to push the boat 100 per cent to preserve the gear and crew.
All this means that we shouldn’t be surprised to see the fleet depart from the predicted routes on the website -- but don’t worry, the crews