Navigation: Staying on course despite wind and currents
Wind and currents alter a yacht’s actual course. By accurately assessing drift and current offset, you can plan your course more precisely and reach your desired position even in difficult conditions.
Although the course setting explained in the previous episode aligns the bow precisely with the destination, wind and current can cause the yacht to drift. As a result, the actual course travelled over ground differs from the intended course.
In order to identify where the lateral drift is taking the boat and to counteract it by steering accordingly, the course correction is adjusted to take account of wind drift and tidal drift. Wind drift can be realistically estimated primarily through experience. Nautical charts, tidal atlases and other nautical documents provide the necessary guidance regarding the influence of tidal currents.
Navigation series
Assessing wind drift correctly
Wind-induced drift can often only be estimated. After all, it depends on a wide range of factors, including the type of boat, wind strength, current sail configuration, lateral plan, heel and correct trim.
Added to this is the course relative to the wind: whilst when sailing dead downwind, naturally, there is hardly any lateral drift to speak of, it becomes all the more noticeable when sailing close-hauled.
Here, the sail’s profile generates aerodynamic lift that acts at right angles to the wind direction and laterally to the keel line. The fact that the boat nevertheless achieves forward propulsion is down to the underwater hull. The lateral plan counteracts lateral drift, whilst its design minimises drag. Nevertheless, a certain transverse force is always at work, which, in addition to heel, manifests itself as a drift through the water towards the leeward side.
In short: the boat doesn’t just travel in a straight line, but also cuts through the water at an angle – in the old days, people used to say ‘across’ – to the keel line.
Long-standing owners know from experience under what conditions they can expect their yacht to drift, and to what extent. New owners and charterers are advised to constantly compare their intended course with their actual course in order to assess and take account of any drift.
Detailed tables on leeward drift or corresponding data sets are sometimes also produced for racing yachts.
In order to take wind-induced drift into account when navigating, the course-change scheme (see Part 2) must be expanded to include an additional factor: the wind allowance (BW). This describes the angle between the boat’s true heading and its movement or course through the water (KdW):
MgK (magnetic compass bearing) =
Dev (deviation) =
mwK (magnetic course) = MW (magnetic variation) =
rwK (true course) = BW (bearing for wind) =
course through the water (KdW) =
If we are sailing in open waters and want to know where the drift will take us, the bearing on the compass (MgK) would need to be converted into a KdW, from top to bottom.
To obtain a bearing that will keep our yacht heading straight through the water towards the destination, the course taken from the chart is set as the KdW and the steering course (MgK) is calculated from bottom to top (with the sign reversed).
Whether the degree value assigned to the BW represents an empirical value or is taken from a loading table, the following generally applies: the BW always has a positive sign when the wind is from the port side (when the yacht is shifted to starboard) and always a negative sign when the wind is from the starboard side.
Effect of the flow
If there is also a current, the water itself moves relative to the seabed. This causes anything floating in the water to be carried along with it. Consequently, the yacht’s movement through the water no longer corresponds to its course over the seabed, meaning that a supplementary current correction (BS) is required. The BS describes the angular difference between the true heading (KdW) and the course over ground (KüG), which corresponds to the chart course (KaK):
MgK (magnetic compass bearing) = Abl (deviation) =
mwK (magnetic bearing) = MW (magnetic deviation) =
rwK (true bearing) = BW (bearing for wind) =
Course through the water (KdW) = BS (course for current) =
KüG (course over ground) =
In the case of a surface current caused by wind, it might be possible, under certain circumstances, to use a single input for the offset – after all, BS and BW would then have the same cause (though not necessarily always the same effect).
The situation is different with tidal currents, whose direction and strength change continuously as the tide ebbs and flows. This requires a coordinated flow profile, which in turn necessitates reliable data on the prevailing tidal current.
Electricity diamonds
Information on tidal currents can be found in a variety of sources – including nautical charts of tidal areas. These often show so-called current diamonds, which are labelled with identification letters. A tidal current table printed on the chart provides detailed information on the tidal current at the relevant position for each diamond. The current data is listed in hourly intervals – from six hours before to six hours after high tide at a reference point. The reference point is indicated above the table.
To find out when high tide occurs at your chosen location, take a look at the tide calendar or the more detailed tide tables for the area (there are, of course, smartphone apps for this too). Once the high-water time at the reference point is known, we can look at the relevant row in the table – for example, two hours before high water, if we are at the river junction at 2 pm and high water is due to occur at the reference point at 4 pm (Please note: you must take daylight saving time into account).
Spring, Nipp and Mittzeit
In addition to an angle indicating the direction of the current, there are two figures for the current speed in knots (kn) – one for spring tides and one for neap tides. Background: At spring tides, the Earth, Moon and Sun are aligned (full and new moon), which further amplifies the forces that generate the tides. This results in a greater tidal range, which manifests itself in stronger tidal currents.
At neap tides, the Sun is at a right angle to the Earth–Moon axis, resulting in a low tidal range and relatively weak tidal currents. Between these phases lies the intermediate tide – in which case one would need to interpolate between the values for spring and neap tides.
This so-called ‘tide age’ is also indicated in the tide table. Please note: anyone relying solely on the phases of the moon may misjudge the situation, as in some areas the topography causes the aforementioned effects to occur with a delay.
In the German Bight, for example, the so-called spring tide delay can amount to several days. The official German tide calendar for the area contains a table showing the age of the tide, which already takes the spring tide delay into account, whilst in some other tide tables it is specified separately for each reference location.
The Electricity Atlas as a resource
Another source of information on tidal currents is tidal atlases. These contain charts with current arrows showing the direction of the current in a specific sea area at different times. Here, too, there are usually map sheets covering the period from six hours before to six hours after high tide at a reference point (in exceptional cases, these may also be based on astronomical times). In addition, there is a longitude and latitude scale along the edge of the map to help locate the relevant position.
Using a tidal current atlas, it is possible to determine the tidal current prevailing at almost any location within the area covered, whereas the tidal current symbols on nautical charts are often spaced much further apart.
Leafing through a tidal atlas also gives you a clear picture of the tidal patterns in the area (compared with the bare figures in a tidal table). This allows you, for example, to gauge at a glance when the best time would be to complete a particular section of the route.
After all, in tidal waters, the aim is to sail as far as possible with the current in your favour. Particularly in narrow stretches, such as at Seegatten, an unfavourably flowing current can prove extremely treacherous, especially as it is amplified here. Conditions also become unpleasant when the current runs against the prevailing wind direction, resulting in heavy swells.
The direction of the current is determined by referring to a current triangle and the nearest current arrow. The current is always indicated in the direction in which it flows, which corresponds to the direction of the arrow. Depending on the tidal station, the current speed can be identified by the length, shape and/or colour of the arrows.
In addition, there are figures where attention must be paid to the unit of measurement. In the ‘Atlas of Tidal Currents for the North Sea’ published by the Federal Maritime and Hydrographic Agency (BSH), the figures are given in knots – whilst in older editions of the BSH publication ‘Coastal Tidal Currents in the German Bight’ in centimetres per second (cm/s; the current edition also uses knots).
The British ‘Admiralty Tidal Stream Atlases’ feature values separated by commas. Here, one value applies to neap tides and the other to spring tides (both given in tenths of a knot). ‘04.08’ means 0.4 knots at neap tide and 0.8 knots at spring tide.
Some tidal current charts, however, only show average tidal current conditions, so corresponding adjustments must be made during spring tides (see the relevant instructions for use).
Nautical charts for the German North Sea and Baltic Sea coasts can also be viewed online on the BSH website (www.bsh.de, under the ‘Data’ section, then ‘Forecasts’ and ‘Currents’).
This is how the current triangle is formed
To determine a course taking into account the calculated current offset, vector addition by drawing a current triangle has proved effective. A current triangle consists of the following legs: the course through the water (KdW), the course over ground (KüG) and the direction of the current (StR).
The length of the legs corresponds to the respective speed: the speed through the water (FdW) for the KdW, the speed over the seabed (FüG) for the KüG, and the current speed (StG) for the StR.
The specific procedure depends on which side of the triangle is being calculated: the KüG or the KdW. StR and StG can be found in the tidal current table or the current atlas. The FdW is either read from the log or estimated based on the wind conditions expected along the route (wind force, bearing to the wind). The FüG, on the other hand, must always be calculated, as it is influenced by the current, which still needs to be factored in.
First electricity assignment
Let us first assume that we have read the current heading (MgK) from the compass and now want to know where the prevailing current will carry our yacht over the seabed.
Once the MgK has been converted into a KdW, a vertical meridian is first plotted on a chart as a north reference. The KdW is then transferred from this meridian using the bearing triangle. Its length corresponds to the FdW read from the log.
When scaling the vectors, a standard scale must be defined, for example, one node per centimetre. The current vector is now marked at the end of the KdW vector – in the direction of the StR and with a length equal to the StG.
Finally, the triangle is completed by drawing a further side from the KdW’s starting point on the meridian to the end of the current vector. This third vector corresponds to the KüG, and its length to the FüG.
Second electricity assignment
If a bearing is to be determined, the course bearing taken from the chart is first plotted on the meridian. This cannot yet be adjusted, as the direction bearing is still unknown. The current vector is therefore plotted from the starting point of the course bearing.
All that is missing is the KdW leg, the length of which is, after all, known in the form of the FdW. This length is placed in the compass, and an arc is drawn round the end of the current vector that intersects the KüG.
Finally, the tail end of the current vector is connected to this point of intersection. This third leg corresponds to the KdW we are looking for. At the same time, the point of intersection on the KüG marks the length of the FüG, which could be used to calculate, for example, the estimated time of arrival (ETA).
Third electricity assignment
Ultimately, careful tidal current data input can also help to identify wind drift and wind-induced surface currents. After all, if – despite the current data input – there is a discrepancy between the calibration point and the observed location, the wind must be the only cause. Either by pushing the boat sideways through the water, and/or in the form of an unexpected wind-induced current.
Anyone who keeps a close eye on the connections (see Episode 2) can actually determine this drift quite accurately using what is known as the third current component – because the so-called cutlery displacement (BV) describes its direction and magnitude.
However, the distance of the offset only corresponds to the offset speed in knots if the BV is based on one hour. If the coupling interval is longer, OK and OB must, so to speak, be ‘coupled back’ to their positions after one hour.
However, you can also simply construct an OK for the relevant time on the coupling curve and then shift the BV in parallel to this point. The feedback OB for the same time is then located at the point where it intersects with the KüG.
In practice, however, when assessing current conditions on board, one should not rely solely on the sources mentioned, but should always keep a watchful eye out for any signs that might indicate a deviation in the current.
For example, at bends and narrows, as well as in bays and near harbour entrances, eddies and counter-currents sometimes occur which flow in a completely different direction to the predicted tidal current. The wild ripples on the water and the dancing buoys bear witness to this.
Flow data on a plotter, smartphone and tablet
Some navigation programmes and electronic nautical chart sets for chartplotters already include data on tidal currents in the relevant area. This data can then be taken into account during electronic navigation. In addition, there are various apps that allow you to access current data provided you have an internet connection. Furthermore, the Federal Maritime and Hydrographic Agency (BSH) offers data packages on surface currents in German coastal areas for download.
Book recommendations
Sailing in tidal waters provides a practical introduction to planning and undertaking sailing trips in tidal waters. The book explains the physical principles behind the ebb and flow of the tides, methods for determining tidal times, and typical problems encountered in coastal waters.
Exercises and tasks is aimed at readers who not only wish to understand course, drift, current and tides, but also want to practise these concepts through specific exercises. The brief introductions to each topic are supplemented by exercises and detailed solutions.
How do you determine wind drift and current drift whilst on board – do you rely on charts and a plotter, or do you regularly check your course using your own observations?
Sven M. Rutter
Freier Mitarbeiter