The Channel Factor, the First Hurdle

The first hurdle for the 44 boats in the Transat Jacques Vabre is the English Channel with its unique characteristics,  as it is a Channel that is not found anywhere else in the world considering the huge amount of shipping that passes through it (representing 20% of the world's movements), its tidal currents, among the strongest on the planet and the way it funnels the winds moving in from the Atlantic. The duos will have to overcome this hurdle, which involves 200 miles of sailing and will have a huge influence over the rest of the transatlantic race.

Le Havre lies pretty much in the middle of the Bay of the Seine and is about  200 miles from Ushant out to the west and 100 miles east to the Pas du Calais which delimit the Channel. But it is the Western Channel extending from the Atlantic Ocean to the Cotentin peninsula that creates the most pertinent effect,  a "wall" that causes a significant increase in the amplitude of the tidal ‘wave’ approaching from the west.

The tidal differential is surprising

What is striking about the configuration of the Channel is the tidal range between Aber Wrac'h at the west entrance and Granville in the corner of the Cotentin peninsula, particularly at the likes of St. Vaast -la- Hogue at the bottom of the Bay of the Seine and at Boulogne / mer on the edge of the Pas-de- Calais. On Monday November 4 the tidal coefficient reaches 101 in the afternoon at a range of 6.25 meters in Brest (HW 1722hrs) but reaches 12 metres in Cancale in the east with HW at 1934hrs. In fact there are nearly six meters of height differential between the corner of the Cotentin Peninsula and the opposite extremities of the Channel

As the tidal wave comes from the west to pass up the Channel to the North Sea it hits this " Norman Wall ," meaning the range is one of the biggest in the world at 13metres at Mont Saint Michel (Canada’s Bay of Fundy . This big movement of the Channel waters causes means strong tidal currents over the night of Sunday 3rd to Monday 4th  indeed more than 7kts are anticipated in the Raz Blanchard between Guernsey and Cherbourg.

At start time on Sunday, November 3 at 1302h the tide will have been dropping for three hours so the IMOCA Open 60’s are unlikely to be as affected by the tide off Cherbourg but for the Class 40’s they will likely see the middle of the rising tide and so that will be difficult for them. So it might be expected to see some head more directly for the English coast before tacking back after the Channel Islands if it is a beat.

As well as the tidal complications there is the large volume of sea traffic. 20% of the global maritime transportation passes through this  area. This requires traffic separation zones (DST’s), or ‘lanes’ to be established which are closed, no go zones for the Transat Jacques Vabre fleet.

The first DST area lies north of the Channel Islands  so the racers need hug the Normandy coast and the Raz Blanchard by Alderney or to head towards the NW by Torquay on the English coast. And in the W’ly wind when the fleet are tacking upwind this often means the stakes are higher in terms of early strategic choices.
 

The other DST is at Ushant where it again can again drive a wedge into the fleet if they are tacking upwind,  choices include going through the channel of Fromveur off Conquet where tidal currents are usually too strong (more than six knots) or going north away from the rumb line course to Brazil ...

And the wind

The usual trajectory of Atlantic depressions follows the west-east axis with a north-easterly component trajectory as they reach Europe. These moisture laden systems strengthen as they collect moisture passing over the oceans, but slow and degrade when they reach the European continent. They can cross the Atlantic at more than 40kts and create winds of more than 60kts. But the Channel creates a reinforce initially by addressing the ocean, then slowed down and degraded arriving on the European continent : they can cross the Atlantic more than 40 knots (70 km / h) and generate winds greater than 60 knots (110 km / h) as was the case Sunday, October 2013 27 ...

But what particularly interesting in the Channel is the Venturi created. Nornallly these low pressure centres pass between Iceland and Scotland, but sometimes further south between Ireland and England but even between Cornwall and NW Brittany as will be the case on Sunday.

The SW’ly winds of the low become very unstable over N Brittany and on the Cotentin peninsula when the wind is slowed and deflected by the terrain such as at the Raz Blanchard !

The escape from the Channel is often the most delicate section of the Transatlantic crossing when it is into a low pressure system, not least because there are often chaotic seas and difficult winds. Ideally you need to be in the leading pack to position yourself well for the rest of the course, but especially to get to the NW’ly shift and cross the Bay of Biscay before getting to the second big hurdle Cape Finisterre.