Sailing with the current

Wind is half the story. A knot of current adds or takes away a mile every hour, and over a passage it adds up: setting off against the stream can cost hours, setting off with it can save them.

Routes now sail in the surface current of SMOC, from Copernicus Marine, tide included, and the boat feels the wind on the water rather than the forecast wind alone: the route seeks out a fair current and keeps away from a foul one. The summary tells what the current did: the miles it gained or lost towards the destination over the whole passage.

SMOC covers every sea of the world, hour by hour, more than a week ahead. NOAA's RTOFS, without the tide, is the other choice, a second opinion on the great ocean currents such as the Gulf Stream, and a route can also be computed with no current at all, to see what the current is worth.

Take Cherbourg to Guernsey through the Alderney Race, where the stream at springs runs faster than many boats sail. Leave with it, and the current as SMOC sees it gives nearly ten miles, but with the wind against the tide the Race raises a steep, dangerous sea; leave against it, and the route shows the hours it costs. Compute the passage for a few departure times and compare the miles the current gives or takes: you know which tide to catch.

SMOC sees the current every 9 km or so, enough offshore and along open coasts; inside a narrow tidal channel or close along the rocks, the stream escapes it, and the tidal stream atlas still has the last word on the hour. RTOFS has no tide at all: in tidal waters, keep to SMOC. And past the end of the current model's forecast, the route carries on in its previous run where that one reaches further, then without current.