Nav in a nutshell: Curved track or straight?

On passage it’s sometimes quicker to be swept along a curved track rather than fight the tide to stick to a straight one, says Dick Everitt

TAGS: boating Nav in a Nutshell navigation practical

GPS will keep us on the straight and narrow by constantly showing how much we need to adjust our heading for changes in tide, wind and boat speed.

We can then follow a straight track between two waypoints, which is fine when the tide’s flowing with us, or for short trips when it’s running weakly across our track. And it’s ideal for fast powerboats that can almost ignore tidal streams that are only a small percentage of their boat speed.

For instance, if we’re doing 25 knots and there’s a 2-knot cross tide, we’d only have to allow about 5°. But in a sailing boat doing 5 knots, we’d have to allow 23° (see diagram A).

It’s for this reason that if we’re in a slow boat crossing two or more tidal streams, we can actually save time staying on one heading and let ourselves be swept off along a curved track rather than fighting the tide to stay on the straight line the GPS wants us to follow.

It sounds odd that it is quicker going further along a curved track rather than staying on the straight track – but hopefully by the end of this article you’ll see why.

Course to steer

To construct a ‘course to steer’ for a passage lasting a few hours and crossing complicated tidal streams, we need to identify which tidal vector will influence us for each hour of the passage. To do that we decide when to start, and mark up the following pages of the tidal atlas with the correct times.

Then we can lay a scrap of paper along the passage in the tidal atlas and divide the distance up into the number of hours the passage is likely to take.

As we turn each page of the atlas for each hour, we place the paper on the passage line and see which is the nearest tidal arrow that will affect us.

With a Breton plotter, transfer the strength and bearing of each (explained in a previous Nav in a Nutshell) to the chart and link them all together head to tail in a wiggly line.

Then strike off the same number of hours’ worth of predicted boat speed. (See diagram B).

Some people prefer to draw this diagram on tracing paper, moving the trace from page to page and then measuring off the course to steer.

This will roughly average out the tidal streams and give us a constant heading to steer. Once adjusted for leeway we’ll be swept off in a curve and then hopefully swept back again.

The great thing with GPS is we can keep an eye on our cross track error (how far we’re off the straight line) and plot our position every hour – either by monitoring it on the on-screen chart or via the highway page, if available.

Comparing that with a log reading and EP every hour is a good check on progress and makes sure we’re not being swept into any danger areas.

As things change during the passage we might have to adjust our ‘course’ at the halfway stage and again as we halve each subsequent bit of the passage.

We can do the same thing for longer passages with opposing tidal streams, such as crossing the English Channel. Luckily if we are crossing roughly at right angles, we can just add all the east-going tidal streams and subtract all the west-going ones, because they run backwards and forwards at about the same angle.

But first we must…

■ Estimate an average boat speed for the predicted conditions

■ Work out roughly how long the passage should take.

■ Decide on a departure time – working back from an arrival time which could be defined by tidal height, lock times, slack water, daylight hours and crew endurance etc.

■ Mark each page of the tidal atlas in pencil with the tide times.

■ Identify the nearest tidal stream arrow that will affect us on the correct page for each hour of the passage.

■ Add them all up to see how much offset to allow for tide.

■ Estimate your leeway and make an allowance for it.

Again, it’s quicker to stay on a constant heading and allow ourselves to be swept in a gentle curve rather than fighting the tide trying to stick to the straight GPS track.

For example, leaving at 0500 at a boat speed of 5 knots, crossing the 65 miles from Weymouth to Cherbourg, we could chop an hour or so off the passage time of sticking to the straight GPS line (see diagram C).

You can check this by plotting a series of EPs to stay on the straight line and another set of EPs following the curved track.

Motor-sailing is a useful option to keep the speed up, dodge shipping or placate the ‘Are we nearly there yet?’ crewmembers.

This Nav in a Nutshell article by Dick Everitt was published in the March 2012 issue of PBO. For more useful archive articles explore the PBO copy shop.