The B&G Report - Volvo Ocean Race - Mark Chisnell - Leg 4 Week 2

by Mark Chisnell on 20 Feb 2015 Top Volvo Ocean Race correspondent, navigator and sailing analyst, Mark Chisnell writes a regular report for B&G on the current race and trends he sees developing. This week he Looks at the first few days of Leg 4.

Mark Chisnell – Leg 4 Week 2 – The Big Split

Caps off to Capey
First up – caps off to Team Brunel’s Andrew ‘Capey’ Cape and Team SCA’s Libby Greenhalgh. Together they pulled off probably the most significant move of the race so far, creating a 150nm split from which Team Brunel at one point took an 80nm lead, with the fleet in line astern behind them… and I didn’t see it coming. Sorry about that, but that’s yacht racing, full of the potential for being wrong.

A week ago this blog thought the strategic option was to clear the Philippines and turn south-east, go fast and low to stay west and play the islands to try to avoid the huge hole in the wind developing off Papua New Guinea. No one took that option, for good or bad. So what did they do instead? They went north, and then east.

A Problem of Scale
I think this moment of rather indifferent performance is the right place to point out one of the tricks of weather routing. The calculations done by the Deckman’s predicted routing software are based on a mathematical model of the boat’s performance (called a Polar Table), and the weather forecast. And most of the respected weather forecasting programmes provide information for the next 7 to 10 days.

So if your destination is less than ten days away, then the Deckman can have a pretty good stab at the optimum route all the way there. If it’s more than ten days away then you’ve got a problem. Where do you tell the routing software to aim for when you know it’s going to fall short of the destination? In the case of a 20-day leg, the Deckman’s 10-day route prediction might take you to a position where you are unable to take advantage of a very predictable weather pattern for the second half of the leg.

The answer usually lies in a thorough analysis of the historical data. This is a simplification – since there are many subtleties that we don’t have space or time for today – but essentially, the navigator’s pre-race preparation will involve calculating the optimum route for the ‘average’ weather that they are likely to see on each leg.

(The definition of average is the tricky bit, there may be several ‘averages’ each one based on a particular, commonly-occurring weather pattern.)

This ‘average’ optimum route is then used to guide the short-term optimum route predictions that are calculated using the 7-10 day weather forecast. So you tell the Deckman to aim for the point on the 'average optimum route' that happens to be at the furthest extent of the currently available weather forecast. Or, in other words, use the lessons of history to guide the short-term predictions from the Deckman.

So what do the lessons of history teach us about this section from the Luzon Strait out into the Pacific?

A History Lesson
In the last race, Puma took a route to the north and then east before they finally turned south. The long-cut took them to the bottom of the leaderboard, but when they did finally turn south they did it in better breeze. Breeze that stayed better for a long while, eventually taking them past the fleet – who were miles to their west – to end up in second place. In the previous race it was Ian Walker that took Green Dragon on an eastern adventure that – despite a slow boat – worked out just fine for him too.

It’s a little bit like the Doldrums crossing in Leg 1 where the optimal route is to get west to the narrowest crossing point. In Leg 4 in the North Pacific, you need to get east before you can go south.

So last week, instead of aiming the Deckman straight down the great circle route to Auckland, perhaps we should have aimed it far east out into the Pacific. Aimed it at a point seven days away along the route that Puma had taken last time. If we had, we would probably have got the same answer as Andrew Cape and Libby Greenhalgh.

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