8 – Tidal streams and currents

Definition and origins of currents

Currents reflect the horizontal movement of water whereas tides, in the strictest sense, only reflect vertical movements. These currents influence the ship's position and are therefore important to understand.

The horizontal movement is primarily caused by the gravitational pull of celestial bodies, see Chapter 6 on the astronomical origins of tidesChapter 6 on the astronomical origins of tides. But also other factors are in play:

• Differences in water temperatures caused by heating and cooling due to the Earth's atmosphere.
• Differences in salinity caused by rain, evaporation and estuaries.
• Wind induced friction.
• The Coriolis force which is a consequence of the Earth's rotation.

Ocean surface currents

Prominent features in the map, see Figure 8.1 of the major ocean surface currents include the subtropical gyres centered on 30 degrees latitude in each of the major ocean basins.
The Earth's rotation – the source of the Coriolis force as mentioned above – and the change in wind direction with latitude (from the east in the tropics and from the west at mid-latitudes) cause the circulation of the gyres to be clockwise ↻ in the northern Hemisphere and counterclockwise ↺ in the southern Hemisphere.

The well-known Gulf Stream in the Atlantic and its counterpart in the Pacific, the Kuroshio Current, are strong surface currents (e.g. 30 NM wide, 500 m deep, and with a velocity over 4 knots) that carry heat northward from the tropics.

The deep ocean currents (not shown) are caused primarily by water density differences and in general return the (now colder) water back towards the tropics.

To predict the behavior of major ocean currents several references are available. The Sailing Directions (also see this list of Planning guides) contain some information on normal locations and strengths of ocean currents.
Nevertheless, the Pilot Charts are by far the best reference for predicting the direction and speed of these currents. On these charts, arrows indicate the direction of the prevailing current; a number printed above the arrow indicates the average speed.
Since this information is based upon historical averages, it won't predict the actual ocean current encountered with 100% accuracy.

Coastal currents

Ocean surface currents need not be considered in coastal areas.
When close to the continental shelf, the horizontal movement of water is defined mainly by two terms:

• Tidal stream or tidal current: gravitational
• Current: gravitational, rivers, wind, thermohaline (temperature / salt) imbalances, but mind the conflicting definitions between UK, USA and other English speaking nations…

Tidal streams are described by drift / rate for its speed and set for its direction or bearing.

In order to address tidal streams you will need to use tide tables in conjunction with a tidal atlas, or the tidal diamonds found on the nautical chart.

Tidal atlases

An atlas is a collection of maps or charts, and is named after the Ancient Greek mythological figure Ἄτλας “Bearer (of the Heavens)”, and a tidal atlas presents tidal stream information in the form of 12 or 13 chartlets, one for each hour of the tidal cycle.

The arrows indicate the direction (set) of the tidal stream, and their length and thickness give an impression of its rate.
More accuracy is given by the numbers, arranged in pairs, that show the rate in tenths of knots at neap and spring tides.
For instance 37 , 52 gives a neap tide of 3.7 knots and a spring tide of 5.2 knots. The period (or comma) indicates the geographical position to which the figures refer.
To interpolate for tides that are between neap and spring either the Rule of sevenRule of seven or an interpolation chart enclosed in the atlas is used.

Furthermore, dangerous areas are shown:

• Larger areas affected by overfalls, races or tide rips are highlighted by colour or contour lines.
• Specific positions by .

Though several layouts can be used, usually the direction of the tidal stream is shown by arrows, which are heavier where the tidal streams are stronger. Figures against the arrows give the mean neap and spring drift or rate in tenths of knots.
For example, indicates a mean neap drift of 2.1 knots and a mean spring drift of 4.6 knots.
A short thin arrow shows a weak tidal stream; a fatter and usually longer one shows a faster one.

Tidal atlases provide not only a graphical version but also show a wider area, and are therefore a very useful addition to tidal stream diamonds.

Tidal apps are to be used alongside the paper publications as well as the tidal layers of the chart plotter. The online and electronic versions will often allow animations and selecting a specific arrow will usually show set and rate, with the precision of a tidal diamond.

An alternative to a tidal atlas is a tidal diamond, which are often less accurate and less easy to use, yet more readily available.
Tidal diamonds are symbols on the nautical chart that indicate the direction (bearing) and speed of tidal streams, consisting of a capital letter inside a rhombus , both magenta coloured. The diamond symbol is placed on the chart for the location where tidal stream data is available, and tabulated elsewhere in the chart.
On any particular chart each tidal diamond will have a unique capital letter starting from “ A ” and continuing alphabetically. This in contrasts to the lower case letters in squares used for the vertical movement of tides, see Tidal levels in the chartTidal levels in the chart.

Somewhere on the same chart – generally on land – will be a tidal diamond table such as the one shown below:

Columns of the diamond table

The three columns show the bearing / direction / set of the tidal stream, and its speed / drift / rate, in knots, at both spring tide and neap tide.

• Dir : Direction of streams (degrees).
• Sp   : Rate at spring tides (knots).
• Np : Rate at neap tides (knots).

Note, that “Dir” is direction towards as opposed to wind directions. 

Rows of the diamond table

The thirteen rows are the hours of the tidal cycle showing the 6 hours before high water, high water itself and the 6 hours after high water.

The times on the table are related to the moment of high water of the standard port or reference port. In this case Eastbourne harbour in England.

There can be a LW slack water before flooding and there can also be a HW slack water before ebbing: the tidal flow turns and changes direction, indicated by S l a c k.
Conversely, during ebbing and flooding the rates are strongest.

Tidal atlases vs tidal diamonds

Tidal diamonds give the tidal streams at that location only, an adjacent diamond may yield very different sets and rates; sensible interpretation and comparison is integral to estimate the streams at intermediate positions.

Where tidal diamonds have the advantage of precision, particularly in terms of direction – which can be difficult to measure on the chartlets of a tidal stream atlas – the latter has the advantage of creating a good overall view of the changing patterns of the tidal streams, especially for positions in between tidal diamonds.

Mind that both types of presentation use relative times with respect to HW (or sometimes LW) at a particular port, instead of clocktime.

Three ways to deal with tidal streams

Finally, to get our Course to Steer (CTS), we must adjust for leewayleeway (in case of cross wind), and for variation & deviationvariation & deviation resulting in a compass course to tell our helmsperson. We can use the equation below; remember that CTW = Course through the Water = Water Track is the conclusive line drawn in the construction, which is still a true course.

CTW  ±  leeway  ±  var  ±  dev  =  CTS

• Leeway: assuming a strong westerly wind we estimate the leeway to be −8° (with wind from port we must substract, hence the “minus”). The Water Track adjusted for leeway is 338° − 8° = 330°T.
• Variation: the inner compass rose tells us that in 2024 the variation is 2° 15' W = −2° 15'. Adjusting also for variation gives 338° − 8° − 2° = 328°M.
• Deviation: for a compass course that the helmsperson can steer by, we need to adjust for deviation as well.
  Our deviation for sailing ~330° (the leeway is large enough to take into account) is about +3°, see the deviation tabledeviation table.
  CTS = 338° − 8° − 2° + 3° = 331°.

Finding time of passage

The equation for working out how long it takes us to reach our destination B is straightforward, since speed equals distance divided by time, and rewriting this gives:

Less straightforward is deciding on the vessel's speed. This is explicitly not the 5 kn that we estimated, since this is the Speed through Water (STW) which takes no account of the tidal effect. What we actually need is the Speed over the Ground (SOG).

To find the SOG we go back to our finished construction in Figure 8.16 and measure the distance from A to the point where the Water Track meets the Ground Track. The measured distance is 5.43 NM and if we plotted lengths for one hour, the vessel's SOG will be 5.43 kn.
If the construction had been based on e.g. a half hour passage the SOG would be 10.86 kn because the vessel would cover 5.43 NM in half an hour.

The distance A – B likewise is measured on the chart and is 6 NM.

$\text{T}=\frac{{\text{6 NM}}_{}}{{\text{5.43 kn}}^{}}=\text{1.10 hours}$

If we multiply this by 60 minutes we have our time of passage in minutes.

$\text{T}=\text{1.10}\times \text{60}=\text{66 minutes}$

Allowing for several tidal streams

With opposing tidal streams

Crossing a narrow channel and sailing from C to  D, as shown in Figure 8.17, is a special case that permits mental arithmetic, since all tidal streams are neatly aligned in two opposite directions, i.e. rectilinear. So, instead of having to draw each tidal stream construction we can resort to a single one.

• Measure the distance across (32 NM).
• Estimate boatspeed (4 knots).
• Work out time of crossing (8 hrs).
• Add up all east and west going tidal streams for those hours. It isn't necessary to plot all the tidal vectors, and simply listing the speeds for each hours will do.

• Plot the balance (0.7W), which is the resultant tidal vector, see Figure 8.18.
  Perhaps surprisingly , the (resultant) tidal vector may cross hazards or land, or finish at a hazard or on land.
• Strike off distance travelled in time (4 knots × 32 NM) to give course.
• Apply leeway, variation and deviation.

With opposing tidal streams – crossing a wider channel

If tidal streams change direction every hour, i.e. rotary tidal streams, it is necessary to plot tidal vectors for each hour, as shown in Figure 8.19 for six hours, which is the estimated time it takes us to sail from E down to F.

Figure 8.20 shows the full construction; the first tidal vector starts at E and the other tidal vectors each start where the previous tidal vector finishes. From the end of this resultant, or tidal vector chain, strike off the distance travelled (speed × time) to find the course.

But as everything can vary on a long passage, plot an EP every hour, and work out a new course to steer each time you half the distance to your destination.

It is also good practice to aim off so you arrive up-tide of the harbour (or rather down-tide if the waypoint is a small hazard or buoy).

Course shaping

Applying Course to Steer, i.e. Shaping a course, is also useful to work out when to change course and where to place a waypoint. In the previous examples the destination was a given, now let's look at Figure 8.21.
We are currently at A and want to know when to tack so we can clear the headland by half a nautical mile.

• Draw an arc with a ½  NM radius around the cape.
• Mark in a Water Track at 045° to the wind.
• Measure down 5 knots for the boatspeed.
• Draw in 1 NM at 180°T for tide. (the “T” is optional)
• Mark in the Ground Track,
• …and extend it.

• Work out the Ground Track from point A,
• …and extend it.
• Where the two Ground Tracks cross B is the place to tack.
• The length of the Ground Tracks show that we are only making 4.25 knots over the ground.
• Measure the distance A – B, which is is 7 NM.
• Answer: we need to tack in about 1 hr 40 min.

Tacking to windward

Without any tidal streams: if your destination is directly to windward, it is best to stay within 10° of the down-wind line (rhumb line), as illustrated by Figure 8.22, to take advantage of any wind shifts that might occur.
Always try to set off on the tack which is more directly towards the destination.

However, with a tidal stream the best windstrategy is lee bowing, i.e. keeping the tidal stream on the ship's lee bow, to sail on a lifted tack. To demystify this exploit we much first discuss apparent wind.

Apparent wind

The wind perceived on a yacht is affected by three separate factors.

• The first is the true wind experienced at that place, irrespective of yacht's movement.
• The second is the wind generated by the passage of the yacht through the air, which is an opposite direction to the yacht's course and at the same speed.

These two are combined to form a resultant which is usually known as the apparent wind, and is without flow of water.

• The third factor occurs when the motion of the yacht over the sea is modified by a tidal stream (or other flow). The apparent wind perceived onboard will be the resultant of waterflow, yacht motion through the water, and true wind.

These three factors contributing to the apparent wind are usually represented by the vector system, as detailed in Figure 8.23.

Two worthwhile scenarios to reflect on:

• A yacht at anchor will only experienced true wind, e.g. 10 kn from the north, even in tidal waters. Motion induced wind is zero, and tide induced wind is zero since the yacht cannot be moving with any tidal stream.
• A motor boat travelling at 10 kn on a windless day without any water motion (tidal or otherwise) will only experience the induced wind of 10 kn directly on the bow caused by its own motion. True wind is zero, and tide induced wind is zero.

Lee bowing

But, if we have to allow for a change in tidal stream, an advantage is to be gained by letting the tide push us up to windward – just like yacht A is doing in Figure 8.24. Notice that this yacht is keeping the tide on the lee bow, hence the name lee bowing.

The tide and true wind act together to form a resultant wind or tide induced wind which is lifting yacht A, and putting it in a good position when the tide turns.
But the same resultant wind will be heading yacht B on his chosen tack and placing it in a poor position when the tide turns.

Passage planning in the real world it’s not so simple. The apparent wind direction can alter, so it then makes even more sense to keep close to the rhumb line and play the wind shifts.

Two types of EP

1 – Non-tidal estimated position

We already explored the first type of EP in chapter 4chapter 4, combining a single LOP with with dead reckoning (DR)dead reckoning (DR), plotted on the chart with a square  .

This version of EP is in essence non-tidal, since dead reckoning is based on course steered and distance travelled alone; it takes no account of leeway or tidal stream.
Combining the single LOP with a DR position optimises the certainty of our position.

2 – Tidal estimated position

The second type is the tidal estimated position, which is plotted on the chart with a triangle  , and although both course and distance are plotted, this is distinct from dead reckoning, because tide and leeway are considered.

This estimated position also differs from Course to Steer, since leeway and tide are allowed to happen, and a CTS is meant to cancel out these forces. Moreover, EP means plotting what already has happened, whereas CTS means planning in the future.

There are five steps involved in estimating an hourly position on a chart. Figure 8.25 illustrates how to estimate your position while allowing for tide and leeway.

• Make allowance for compass deviation and variation in order to get an accurate true course steered, dotted line A – B. This will amount to a few degrees only but becomes significant over a large distance.
• Make allowance for leeway to give a true Water Track.
• Draw a line in pencil from the last known position A on the chart and mark off the distance run in the previous hour in nautical miles, as recorded by the log.
• From this mark C then plot the tidal set and rate in knots from here.
• The final mark D is the vessel’s new estimated position (EP) and a line can then be drawn between it and the last known position A.
  This line A – D is known as the Course made Good (CMG) or Ground Track and its length is the actual distance covered over the ground.
  This distance, divided by the time (in this example one hour) also gives the actual Speed over the Ground (SOG).

Tidal streams and sea state

The wave conditions or sea state can be influenced by either tidal streams or winds, or by a combination of winds and tidal streams.

Notorious are the overfalls, races or tide rips that arise where strong tidal streams are deflected upwards to the sea surface or are broken up into turbulence by obstructions on the sea bed. These localized phenomena result in steep and irregular waves close downtide of the obstruction.

Though the severity of the overfalls is affected by certain wind conditions, they originate from the tidal streams interacting with the seabed and are not created by winds, hence they can be predicted and recurring and are marked on charts and tidal atlases as a potential hazard.
And hazardous these can be: have a look at the Corryvreckan whirlpool to get an insight into such ferocious tidal flows.

Symbols in the nautical chart for currents and tidal streams

	Flood tidal stream with rate
	Ebb tidal stream with rate, no feathers
	Relatively constant current (in restricted waters), non-tidal Variable current (in restricted waters), non-tidal
	Overfalls Tide rips Races
	Eddies
	Chart Diamond Position of tabulated tidal stream data with designation Magenta rhombus and upper case lettering
	Breakers
	Breakers near shoal water or the coastline
	Breakers over off-lying shoals are charted by to avoid obscuring the shoal soundings or feature.
	Ocean current. Details of current strength and seasonal variations may be shown:

Tide races will occur wherever the speed of the tide is accelerated by a constriction, such as a headland, see Figure 8.27, or forced through a narrow channel, Figure 8.28. This increase may produce currents of 10 or more knots in localised areas, probably far more than the official information indicates. In these conditions great care must be taken with the navigation as the vessel's position may be changing too fast for conventional navigation techniques.

The safest times to pass through a tide race are when tide is slack and when the wind is with the tide.
In contrast, avoid tide races or overfalls:

• In strong winds
• With the wind against the tide
• When the tide is at its strongest

Visibility

The turbulence in tide races or overfalls can bring cold water in upwellings to the sea surface. If the conditions for fog forming are on the threshold – such as when fog banks are forecast – the drop in sea surface temperature can cause fog banks, reducing crucial visibility at precisely these dangerous spots. 
In general, fog will happen when the air temperature drops to within 2° C (5° F) of the dew point, and the air becomes saturated.

Signs of tidal flow

In tidal waters, it is important to understand what the tide is doing. With experience, it is possible to perceive tidal flow, perhaps by the ripples that a flow creates, or by the interaction of the tide with wind and swell.
In an area of variable tide, the wind will tend to pick out areas where the tide is flowing against it with small waves, whereas regions of opposite flow can appear flat.

Areas of flat water without ripples indicate where the flow is slow or forming eddies, in contrast with areas with notably steeper waves or ripples.

Also look for a debris line with visible seaweed, plastic etc, as a layer between the immiscible ebb and flood waters.

You can use AtoNs to assess tidal stream – look for the “cushion” waves ahead of a buoy or beacon and the flattened area behind indicate flow; more on waves, wind and tides.

Coastal tidal effects overview

In coastal navigation it is crucial to understand that the shape of the seabed and surrounding land can greatly affect the speed and direction of tidal streams and local wave conditions, see Figure 8.30.

• A – In deep water offshore there is a strong tidal stream.
• B – Inshore in the shallower water the stream is less.
• C – Headlands and ledges can increase the rate of flow and cause overfalls as the water rushes and tumbles through the narrowed gap.
• D – Headlands and bays can also cause back eddies where the tidal stream can actually turn back on itself and swirl in the opposite direction.

Both the reduced rate of flow in the shallows and the back eddies along the coast can be used to great advantage to sneak up over the tide.
Local knowledge, pilot books, or a close look at the tidal stream atlases will show where these areas are.
At the same time the confused seas caused at headlands should be avoided by small craft by either going inshore or going further out to deeper waters.
Again, pilot books will advise on the best route to take.

Wind over tide effect

Aside from the effects of coastal geography on tide, we should consider the influence of wind, which either creates smoother seas when blowing with, or creates rougher seas when blowing against the tidal stream.
This “wind over tide effect” results from waves being modified by tidal streams by increasing (Figure 8.31) or shortening (Figure 8.32) the wavelength relative to the height of the waves.

Although not specifically related to tide, it is useful to recognise convergence of waves at headlands and divergence in front of the bays, see Figure 8.33, which can intensify or weaken the effects that coastal geography has on tidal flow.

Wind driven currents

Aside from gravitational (i.e. tidal streams), rivers, and the thermohaline imbalances which all influence horizontal movement of water, we should consider the remaining factor of wind.
Friction between the wind and water at the ocean surface results in a surface current flow at between 20 and 45 degrees to the direction to which the wind is blowing.
This effect will vary according to the depth of water, with smaller angles in shallow water and larger angles in deep water.

The speed of wind driven current is typically about 3% of the surface wind speed, so in 20 knots wind you would expect a rate of 0.6 knots and in 30 knots of wind a rate of 0.9 knots.

Tidal gate

A final word should be reserved for tidal gates: the areas of coastal water where, at certain times, the flow of the tide prevents safe or timely progress in a given direction.

Above, we already discussed wind-over-tide, overfalls and tide races; rough situations where a vessel travelling at e.g. 4 knots will come to a standstill when faced with an extra 4 knots of contrary tide. Also, the faster tide might result in more treacherous overfalls.

The slower progress means that you will be in those adverse conditions for longer, causing wear and tear on both the crew and the vessel’s gear.
So, even though the tidal gate might not be literally shut, it will be prudent to wait until it is “open”.
Once the tide gate is “open” the tide will be with you, progress will be faster, safer and more comfortable and any time lost waiting might easily be made up.

Overview

• Flood: when the tide is coming in and filling the bay, fjord, firth or estuary.
• Ebb: when the tide is going out and draining the bay, fjord, firth or estuary.
• Slack: when the tidal flow turns and changes direction, a LW slack before flooding and a HW slack before ebbing: S l a c k.
• Springs: when the Moon and Sun are in alignment to produce the maximum gravitational pull and hence the biggest and fastest tides.
• Neaps: when the Moon and the Sun are at their weakest alignment so the gravitational pull is less and the tides are at their smallest and slowest.
• Range: the height difference between high and low water, which varies every day.
• Current: is a horizontal movement of water. Currents may be classified as tidal and nontidal.
  Tidal currents are caused by gravitational interactions between the sun, moon, and earth and are a part of the same general movement of the sea that is manifested in the vertical rise and fall, called “tide”.
  Nontidal currents include the permanent currents in the general circulatory systems of the sea as well as temporary currents arising from more pronounced meteorological variability.
  In British usage, “tidal current” is called tidal stream, and “nontidal current” is called current.
  The terms tidal current and tidal stream can be used interchangeably, but the polysemantic current always needs context.
• Eddy: area of recircling water caused by main tidal stream flowing past a headland or obstruction. Eddies are larger and flatter than wirlpools / maelstroms.
• Set: the direction or bearing of a tidal stream.
• Drift: the speed or rate of a tidal stream.
• Gyre: a large system of circular ocean currents formed by global wind patterns and forces created by Earth's rotation.
• Ocean surface current: A continuous and regular movement of ocean water flowing along a definable path, maintaining the earth's thermodynamic balance. Horizontal pressure gradients, winds and the Coriolis force are three prominent causes of these currents.
• Intended track: usually refers to the line connecting two waypoints of a route, i.e. a leg of a route we intent to sail aka the desired track. When we slip off of that track we can display how far off of the track we are, which is called the “cross-track error” (XTE).
• Tidal diamonds: are shown on nautical charts, such as the example on the right for location “D”, at locations where tidal stream information has been measured: set and drift. This information is included in the same nautical chart and tabulated in a tidal stream table.
• Tidal stream atlas: these show the tidal streams for each hour of the tidal cycle. These comprise a total of 13 (or 12) tidal charts ranging from 6 hr before HW till 6 hr after HW (high water in a certain standard port). So, these charts are relative to the time of HW and to use them we must know the absolute time of HW (or LW as in the example below).

• Tidal atlas / stream arrow: have often associated numbers (for example 11.20) that give the average speed of the current at neap and spring tide. In this example, the rate at neap tide is 1.1 knots and at spring tide is 2.0 knots.
  • The thicker the arrow, the faster the water it represents is flowing
  • Two figures relate to springs (bigger) and neaps (smaller)
  • The decimal place is not included: e.g. 40 is not forty but 4.0
  • Speeds are given in knots
  Instead of numbers, modern publications will also use colours to indicate rate, as illustrated in the Canadian atlas example above.
• Fetch: the distance over open sea which the wind can blow. A bigger fetch will lead to bigger waves, and a smaller fetch will only produce smaller waves.
• Tide race: happens when the tidal flow is constricted or squeezed through a horizontal gap. it flows faster.
• Overfalls: where the tide is constricted by the sudden decrease in depth; a vertical gap. When the volume of water is sufficient and the speed of water is sufficiently fast this will result in surface turbulence.
• Apparent wind : the speed and direction from which the wind appears to blow with reference to a moving vessel, movement due to tide and / or movement due to the vessel's motion through the water.
• Resultant (vector): is the sum of two or more individual vectors, resulting in a single vector that produces the same effect as is produced by the individual vectors collectively.
• Vector: in physics, is a quantity that has both magnitude and direction, represented by an arrow whose length is proportional to the quantity’s magnitude. Although a vector has magnitude and direction, it does not have position. That is, as long as its length is not changed, a vector is not altered if it is displaced parallel to itself.
  In contrast to vectors, ordinary quantities that have a magnitude but not a direction are called “scalars”. To qualify as a vector, a quantity having magnitude and direction must also obey certain rules of combination.
• Vector system: a vector parallelogram for addition and subtraction.
• Course to Steer (CTS): involves predicting the course that will take the yacht to a chosen position. It takes into account:
  • The tidal stream that will be experienced
  • The estimated speed of the yacht
  • Any leeway
  CTS plotting is a very important competency and is one of the most frequently applied techniques in navigation.
• Estimated position (dead reckoning): is the combination of a single LOP with a DR position without allowances for tidal streams, currents and leeway, plotted on the chart with a square. This EP is very common when no tidal information is available and is an improved position based on a DR position.
• Estimated position (tidal): is derived from the last know position (Fix, Rfix, but also DR) with allowance made for leeway and the effects of currents and tidal streams. From the last known position a current vector with a length for the chosen time span is drawn to plot the EP.
  This EP is plotted on the chart with a triangle.
  The line between the last known position and the EP is the Ground Track, at least the estimation of it…
  The tidal EP is a “go with the flow” sailed track, and therefore the opposite of an CTS when you plan to counter leeway and tidal streams by planning to steer upwind and upstream.
• TSS aka a traffic separation scheme is where you should avoid sailing a CTS since a yacht should cross a shipping lane at a right angle to avoid endangering ship traffic, and to minimize the duration of the crossing.

  

• Pilotage: the art of finding your way around within sight of land, using visual references: piloting. It is considered distinct from “navigation”, which often refers to finding ones way away from land, or without good visual references: navigating.
• Course shaping: Also known as “shaping a course” or “Course to Steer”, is the estimated course that a vessel should steer in order to arrive at a specific waypoint, taking into consideration the effects that wind and tide will have on it.
• Lee bowing: The exploit of keeping the (tidal) stream or current on a sailing ship's lee bow to be lifted towards a windward destination, i.e. sailing a shorter distance.
• True wind: the wind relative to a fixed point on the earth.
• TWA (True Wind Angle): the angle of the wind, relative to the yacht if it was stationary.
• TWD (True Wind Direction): the true direction of the wind relative to north.
• Motion wind (induced wind): the wind a moving vessel would experience in still air, without any flow of water.
• Tide induced wind: the wind a non-propelled vessel would experience in still air, due to flow of water.
• Wind driven current: also known as "wind induced current” or “wind generated current” is a flow in a body of water that is generated by wind friction on its surface. A well known oceanic wind driven current is the Gulf Stream.
• Wind over tide effect: when wind, or waves are going the opposite direction to a tidal stream, the waves become shorter, taller and often break. This can make for challenging conditions off headlands and in tide races. When wind, or waves aline with a tidal stream the seastate and sailing becomes smoother.

  

  

• Tide gate: (tidal gate), is an area of coastal water that needs to be negotiated at certain times to avoid undue delays or dangerous situations such as “wind over tide” conditions, tide races or overfalls.