Whether racing or 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?
Why navigating tides is so important
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 we usually call the sailing wind or apparent wind. This means understanding and navigating tides all the more essential.
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. We have 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. We see this as we balance speed and angle in a coastal race where competitors are close. However, in longer distance racing we need to take navigating 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.
A current from the side will change the true wind angle around 1.5° for every tenth of a knot. It doesn’t sound like much – until we get into a current in excess of 0.6 of a knot when we’re looking at a 10° shift in the wind from ground wind to sailing wind; more than enough to change sail settings or the difference between a good layline, or making a tidal gate or not.
We can now get racing routing solutions on our mobile phones or tablets. The latest generation of apps include cloud routing where, by setting your course and the boat’s polars, a computer ashore will run a number of different models, giving you a series of solutions and routes. By comparing the different model solutions, we can choose our best fit to the actual conditions. The routing solutions also include tidal and ocean currents. The best known of the companies offering this is PredictWind which (at a cost) provides GRIB file forecasts for weather conditions and current.
Most racing yachts, however, run onboard weather routing programs and navigators work tirelessly on perfecting their boats’ polar diagrams. These are used to predict the boat’s 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 sail to the polars, most sailors will find it difficult once waves and the extra weight of equipment on board has been added in. It’s therefore useful to develop our own polars for boat and sails, and most navigation software programmes have datalog facilities to help with this. This information is also used to develop crossover charts for different sails.
Weather routing depends on accurate information; it’s pointless trying to develop polars if your instruments are not properly calibrated. As true wind direction and speed is calculated from boat speed, heading and apparent wind, all need to be accurate. If they’re not, we’re better off using 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 poor routing solution.
True information for navigating tides?
When inputting information into cloud routing or onboard navigation software, we will get a weather routing solution that takes into account all the variables. However, the times of high water will vary between different sources of data. There are also other variables to take into account; atmospheric pressure will alter tide heights – not much as a 1mb change in pressure computes to 1cm. We rarely get changes of more than around 30cm due to changes in atmospheric pressure, but 30cm can be a lot if you’re trying to keep out of the tide!
Additionally, the direction of the wind will add or subtract to water depths and also the timing of high and low tides. An onshore wind will increase water depth and will tend to induce a stand at high water. This will delay the start of the ebb tide, which is then likely to be stronger. When the wind blows parallel to the coast the wind tends to set up long waves and storm surges. These waves have a period of hours and a wavelength of hundreds of kilometres. All of these factors become increasingly important when navigating tides.
While it’s difficult to take into account all these variables when sailing, it helps explain why the tide information that you have may not be the same as what you are experiencing, either in time or magnitude. Inshore, it’s always worth verifying the rate of tide when passing navigation or racing marks.
Offshore paper tide 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 for accuracy for navigating tides. With well calibrated instruments we’ll see a difference between COG and SOG compared with the boat speed and heading. By continually monitoring the actual effect and comparing it with the theoretical we will get a very good idea on the accuracy of the data.
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