Whether racing or cruising, tides and currents matter. But how accurate is the tidal information available and how can you best use routing apps and software to navigate tidal waters?
Why navigating the tides is so important
Wherever we have current (tide or otherwise), it will always influence the sailing wind and the course of the boat on the bottom. At anchor, we can accurately measure the speed and direction of the wind, which we call ground wind. However, if we are just drifting, the effect of the current will change the speed and direction of the wind we are measuring. This is what we generally call the sailing wind or the apparent wind. It means understanding and navigating the tides all the more essential.
This relationship between onshore wind and sailing wind is not only important for racing around canisters, but also for 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 we exit the Solent on a rising tide, we will usually choose the north shore, but the routing solution must also take into account the angle of the sailing wind. The simplest example is that of tacking and gybing angles with a favorable or unfavorable tide. We all watched our course and were disappointed with the angle of the counter-current tack – even to the point that we made little or no progress.
As boats become lighter and faster, with the ability to plane or even foil, wind angle becomes extremely important. A few degrees in either direction can make a significant difference in boat speed. We see this when balancing speed and angle in an inshore race where competitors are close together. However, in long distance races, we have to consider the tide, not only our course, but also the angle of the wind. The predicted wind direction will be the ground wind, which can be quite a different wind angle from the sailing wind.
A side current will change the true wind angle by about 1.5° for every tenth of a knot. It didn’t look like much – until we fell into a current greater than 0.6 knots when we witnessed a 10° shift from onshore wind to sailing wind; more than enough to change the sail settings or the difference between a good layline, or making a tidal wave or not.
Now we can get race routing solutions on our mobile phones or tablets. The latest generation of apps includes cloud routing where, by setting your heading and boat polars, a shore computer will run a number of different patterns, giving you a series of solutions and routes. By comparing the different model solutions, we can choose the one that best suits the real conditions. Routing solutions also include tidal and ocean currents. The best known of the companies offering this is PredictWind which (for a fee) provides GRIB file forecasts for weather and currents.
However, most racing yachts run on-board weather routing programs and sailors work tirelessly to perfect their boats’ polar charts. These are used to predict boat speed for all wind speeds and wind angles.
Most computer-generated polars are for flat water, and while a top helmsman may be able to navigate polars, most sailors will struggle once the waves and the added weight of gear on board will have been added. It is therefore useful to develop our own polars for the boat and the sails, and most navigation software has data logging facilities to help. This information is also used to develop crossover charts for different sails.
Weather routing depends on accurate information; there is no point trying to develop polars if your instruments are not properly calibrated. As true wind direction and speed are calculated from boat speed, heading and apparent wind, all must be accurate. If not, we better use apparent wind and generic polars for the boat and run them at less than 100%. But calibrating our instruments and generating polars in an area of moderate to strong tidal current is difficult and incorrect polars will give us a bad routing solution.
Real information to navigate the tides?
When entering information into cloud routing or in-vehicle navigation software, we will get a weather routing solution that takes all the variables into account. However, high water times will vary between different data sources. There are also other variables to consider; atmospheric pressure will change the height of the tides – not as much as a change in pressure of 1 mb corresponds to 1 cm. We rarely get changes of more than about 30cm due to atmospheric pressure changes, but 30cm can be a lot if you try to stay away from the tide!
Additionally, the wind direction will add or subtract the water depth as well as the timing of high and low tides. An onshore wind will increase water depth and tend to induce a stand at high tide. This will delay the onset of the outgoing tide, which is then likely to be stronger. When the wind blows parallel to the coast, the wind tends to create long waves and storm surges. These waves have a period of several hours and a wavelength of several hundred kilometers. All of these factors become increasingly important when navigating the tides.
While it’s hard to take all of these variables into account when navigating, it helps explain why the tidal information you have may not be the same as what you’re experiencing, either in time or amplitude. . On land, it is always worth checking the tide rate when passing navigation or racing marks.
Paper offshore tidal atlases have been used for many years, but we now tend to use electronic versions. These tend to be a mixture of theoretical and observational data and can vary between sources in accuracy for tidal navigation. With well calibrated instruments we will see a difference between COG and SOG in relation to boat speed and heading. By continuously monitoring the real effect and comparing it with the theory, we will have a very good idea of the accuracy of the data.
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