Routing in tide

Professional meteorologist and expert sailor Chris Tibbs explains how to navigate more efficiently through tidal waters

- Chris Tibbs

When cruising, tides and current are important. But how accurate is the tidal information available, and how can you best use routing apps and software to navigate in tidal waters?

Wherever we have current (tidal or other) this will always influence the sailing wind and the boat's course over the ground. At anchor, we can accurately measure the wind speed and direction, which we call the ground wind. However, if we are just drifting the effect of the current will alter the wind speed and direction that we are measuring. This is what is usually called the sailing wind or apparent wind.

This relationship between the ground wind and sailing wind is not just important for racing around the cans but also when venturing offshore. We can look at the direction of the tide and decide in a general sense where we want to go; for example, when beating out of the Solent in a flood tide we will usually choose the north shore, but the routing solution should also take into account sailing wind angle. The simplest example is the tacking and gybing angles when with a favourable or adverse tide. I'm sure we've all looked at our track and been disappointed with the tacking angle when against the tide-even to the point that we've made little or no progress.

As boats get lighter and faster, with the ability to plane or even foil, wind angle becomes incredibly important. A few degrees one way or another can make a significant difference in boat speed.

However, for offshore we need to take the tide into account, not just on our heading but also for wind angle. The forecast wind direction will be the ground wind, which can be quite a different wind angle to the sailing wind.