‘What’s the best course to steer?’
No, Nick has got this completely wrong. He is asking Sarah to keep the boat on a heading that will result in a course made good of north. This means that she will have to set off on a course steered that is so far west of north that she will actually have a speed over the ground of a little over 2 knots. By 1600 the tide will be slack and he will have to steer north and they will actually be making good three knots. Then as the stream builds in a westerly direction she will need to steer a progressively more easterly course and the speed made good will start to drop again. They will probably arrive at their destination at about 1900, an hour too late for safety.
The east- and west-going tides will, in this particular case, more or less balance out so their fastest course to steer will be north. Their track will be a curving line off to the east, then returning to the west but throughout the four-hour passage their speed made good in a northerly direction will be three knots. In general the quickest way to make a cross-tide passage is to work out what the total set will be for the whole voyage and then work out just one course to steer. If it is a long passage it might be worth heading for a destination a few miles up-tide (at your estimated time of arrival), then if you do not quite maintain your expected speed at least you shouldn’t finish up having to fight your way up-tide in a dying wind and if you sail a little faster than expected you will at least have the speed to make a late course alteration.
Speed is seldom steady in a boat under sail so it makes sense to adjust your course to steer whenever the speed changes significantly and to check that you are on the right course every time you halve the distance to your destination.
Nick and Sarah needed to be back in their home port by two hours after High Water at the very latest and they had time in hand, thanks to the engine. Then a floating line got in the way.
Nick didn’t see it until there was a bang and the engine stopped dead. He immediately suspected a fouled prop and, looking over the stern, his suspicion was confirmed, a length of blue, buoyant polypropylene rope was trailing in the wake. As the boat lost way he managed to pick up the end of the offending rope with the boathook and bring it on board but no amount of heaving would free it.
Half an hour later, after much prodding with the boathook, attempts to pull the rope sideways with the engine out of gear, and to cut it off with a bread knife lashed to the end of the boathook had failed, they realised that the only way they had any hope of clearing it, short of drying the boat out, would be to dive over the side, don a face mask and attack it at close quarters. But with a short, steep swell left over from yesterday’s strong breeze that seemed an impractical proposition and they resigned themselves to setting sail and making the best of the light following wind.
By the time they had the main and genoa set, the time was 1400. High Water was at 1600. The GPS plotter showed that the bar at the entrance to their home river was 12 miles due north.
A tidal diamond on the chart, a couple of miles off the entrance, showed the directions and rates of stream.
The stream at the river entrance was no more than a knot until a couple of hours after high water, when it increased rapidly and a dangerous breaking sea built up on the bar if there was a significant onshore sea or swell. With the genoa boomed out, the log showed a speed over the ground of 3 knots. An easy time-speeddistance sum, thought Nick. They should just make the river entrance in time, as long as the wind held.
‘What course would you like me to steer?’ asked Sarah, now on the helm.
‘Never mind about the compass,’ Nick answered, ‘Just steer by the plotter, so that our course over the ground is the same as the bearing of the waypoint. That means we should make good a straight line to our destination and a straight line is always the shortest distance between two points.’
Easier said than done, thought Sarah, but she did her best.
Has Nick given her a sensible course to steer?