QUESTION OF SEAMANSHIP
What’s the best course for a fast passage?
QRob is skipper of a 9.1m (30ft) cruiser racer in Cherbourg with crew Alan and Pete. They are preparing to sail back to the Solent via the Needles Channel which is 60 miles to the north. The weather is fine, the sea state is calm but the wind direction is northerly 10 knots, right on the nose and is forecast to stay that way. The Channel Light Vessel is also giving northerly 10 knots. The
AThe key to this question is the tide-induced wind. If there was no true wind and a 4-knot east-going tide, a yacht underway would experience a 4-knot easterly wind. Superimpose a 10-knot northerly wind to the 4-knot east-going tide and the trigonometry shows that the wind moves to 22° True and increases slightly to about 10.8 knots. When the tide turns towards the west and increases to 4 knots the wind yacht should make 5 knots in those conditions and it sails about 45° to the wind. It is a spring tide with maximum streams of over 4 knots at both the French and English sides of the Channel. Currently the tide is just beginning to turn towards the east. Alan thinks they should leave on a starboard tack, going about when the tide turns. He reasons that by lee-bowing the tide the yacht will stay closer to the rhumb line and get there quicker. Pete says that if you will be about 338° True and 10.8 knots. That’s a huge lift on both tacks if they start off on starboard and tack on the turn of the tide, and a huge header if they get it the wrong way round. Starting on a starboard tack will get them within about 6 miles of the Needles after 12 hours at 5 knots. If they start on a port tack in the same time they will end up over 30 miles south of the Needles. This of course assumes that the wind direction and speed stay constant, and that the tidal effect draw the triangles for the next 12 hours with equal tides both ways it makes no difference to the time of arrival, you could head off on a port tack, travel faster over the ground for a greater distance, tack on the turn of the tide and travel faster back. Rob has always thought that yachts going to windward should lee-bow the tide, but he’s puzzled because Pete’s calculations seem right. Are they? Why should lee-bowing make a difference? over 12 hours is the same to the east as it is to the west. In the real world the wind is never that constant and a big lift might change the plan. The navigation across the Channel is also complicated by the tidal stream running slightly NE/SW rather than at right angles to a south/north course. The leebow boat is not in a stronger wind. Boats close together on opposite tacks experience the same tide-induced wind, however strong or weak the tide.