Tuesday, 17 February 2009

VOR: Burning Down the Black Tide

Ericsson 3 full on. Image copyright Gustav Morin/Ericsson 3/Volvo Ocean Race.

by Mark Chisnell

Torben Grael and Ericsson 4 led the fleet past Japan overnight, burned a hole in the Black Tide and headed east into the vastness of the Pacific Ocean. China, South Korea, Japan... the countries are flying by faster than on a European bus tour - and if it hadn’t been done already I’d be thinking Around the World in Eighty Days...

Meanwhile, both Telefonica Blue and Ericsson 3 are setting up to make one of the all time great comebacks. Ericsson 3 have made mincemeat of their seven hour deficit at the start line, to get right back in the game – trading places with Green Dragon overnight. While Telefonica Blue are just 8 hours behind the leader, after starting 19 hours in arrears.

It’s been fast, wet and uncomfortable – but at least it’s got a lot, lot warmer. And by 10:00 ZULU this morning, the wind had eased as well, and life was edging back from the extreme. Ericsson 4 led the front quartet east-southeast in a 20 knot (TWS) northerly (TWD). Ken Read and PUMA had crept a little closer, with Ericsson 3 and Green Dragon edging north and making losses as a result.

We covered the reasons for the resurgence of the late-starters in yesterday’s TEN ZULU, and when we left the fleet yesterday morning, they were all powering down the Yellow Sea in the same strong northerly wind direction. Everyone was reaching on port gybe, headed south-east for the southern tip of Japan. It was not long after the TEN ZULU was posted that opinions started to diverge on the fastest route out of the Yellow Sea. Ericsson 4 was edging towards the low road, while Green Dragon and PUMA were holding further north.

The balancing act that the skippers and navigators had to perform was between the medium term strategy – to get as far east as they can in the northerly breeze (more on that later) – and the short-term tactical need, which was to avoid getting slowed by the lighter wind swirling off Japan to their north.

So... the closer they sailed to the southern tip of Japan, the more quickly they gained ground to the east. But the more likely they were to get slowed by the land’s malign influence on the breeze. And the wind direction allowed them to do either, simply by sailing a wider or narrower True Wind Angle (TWA).

The Dragon's skipper took it as a warning

Green Dragon’s northern strategy didn’t last long – you can see an interesting little kink in their track, just south of South Korea. It was due to a patch of lighter wind from a more westerly direction caused, in this case, by South Korea messing up the breeze.

The Dragon’s skipper, Ian Walker, took it as a warning, and as soon as they were clear of the bad air, they started steering a more southerly course – eventually converging with the track taken by both the Ericsson boats. That same light patch was still there when Telefonica Blue came through about eight hours later – you can see an identical blip in their track. The boat that didn’t hit it, but should have, was PUMA – and she held north all the way in towards Japan.

You can see the kink in everyone’s tracks as they rounded the southern tip of Japan – worst affected was PUMA, who were closest, while least affected were Green Dragon, the furthest away. The lesson of South Korea was obviously taken on board...

But if we’re going to get a fair appraisal of how these different strategies worked, we need to look at how the mileage changed from where they started to diverge back in the Yellow Sea (about 10:00 ZULU yesterday), to where they all converged again after passing Japan (about 01:00 ZULU today).

If we look at today’s Distance to Leader (DTL) graph (I’ve dropped Telefonica Blue out so the scale is better), we can see that in that period Ericsson 4 almost doubled her lead on PUMA and Green Dragon, but took just three or four miles out of Ericsson 3 – who went more or less the same way. So you can either score one to Team Ericsson navigation - or you could wonder about the relative boat speeds.

There’s a medium term gain to be made by staying in the Kuroshio

Once the top four all got clear of Japan, there seems to have been a pretty broad consensus on the need to get east, with every one sailing a narrower wind angle (TWA). Green Dragon and Ericsson 3 initially went for an even more easterly route than the other two – and lost miles because of it – before assuming the same course.

So why do they want to go east, when the next waypoint and scoring gate are to the south-east? There are two reasons - the first is the long-term need to exit the Doldrums as far east as possible, to get a fast sailing angle in the south-east trade winds.

There’s also a medium term gain to be made by staying in the Kuroshio, or Black Tide, which caused so many of the problems on Leg 4. It’s a warm water current, and it’s flowing north-east – boosting them in a direction that they want to go. The spectacular rise in Sea Temperature charts their transition into the current.

Fortunately, this time the wind is blowing more across the current, than against it, and the impact on the sea state is lessened. Nevertheless, the waves have built, the Maximum Wave Height rising from 2.5 metres to five metres in the past 24 hours, matching the rise in Sea Temperature.

So I think we can expect everyone to continue to hold a course north of the rhumb line for a while yet. The breeze has already started to ease, as anticipated in the forecast, and it will eventually veer (rotate clockwise) from the broadly northerly flow they’ve ridden out of the Yellow Sea, to the east-northeasterly breeze of the trade winds.

Today’s Predicted Route image suggests that the fleet will turn south as they enter the trade winds and start to point at where they want to go. It’s hard to imagine a better forecast, and it’s a dream start to this marathon leg.

Volvo Ocean Race

No comments: