World Ocean Currents - Gulf Stream, Agulhas & Antarctic Circumpolar

Prerequisites

This lesson covers ocean currents for the RYA/MCA Yachtmaster Ocean. Before starting you should:

• Hold, or be working towards, Yachtmaster Offshore (or MCA OOW Yachts).
• Be fully competent in Day Skipper and Coastal Skipper chartwork: tidal stream vectors, course to steer (CTS), estimated position (EP), leeway, set and drift, course and speed over ground.
• Have studied the Global Meteorology lesson (trade winds, westerlies, Coriolis effect, monsoons), because ocean currents are mostly driven by the wind systems described there.
• Know how to fix your position by astro (Celestial Navigation lesson), because comparing fixes with DR is how you measure the current at sea.

Learning Objectives

By the end of this lesson you will be able to:

• Explain how ocean currents are generated: wind stress, the Coriolis effect, Ekman transport, gyres, western intensification and thermohaline circulation.
• Describe the main surface currents of each ocean and their typical rates.
• Recognise the hazards of strong currents, especially wind against current in the Gulf Stream, Agulhas and Kuroshio.
• Use routeing charts, current atlases and satellite current data in planning.
• Apply current vectors in DR, EP and course-to-steer calculations, and derive set and drift from successive fixes.
• Use currents to gain advantage on an ocean route.
• Allow for oceanic tidal streams and tide rips near islands, atolls and shelf edges, and recognise how El Nino and La Nina disturb the normal pattern.

RYA Yachtmaster Ocean syllabus items covered: ocean currents and their effect on passage planning (worldwide current systems, routeing charts and Ocean Passages for the World); the allowance for current in dead reckoning, estimated position and course to steer on long passages; finding set and drift from fixes; the hazards of wind against current; and the use of weather and ocean data (satellite current analyses, GRIB files) at sea. The lesson builds on the Coastal Skipper and Yachtmaster Offshore tidal stream chartwork and applies it on an oceanic scale.

What the examiner expects: a Yachtmaster Ocean candidate must be able to say, without notes, which way the main currents flow in the area of the passage, roughly how fast, what the hazards are, and how that affects the choice of route and timing. Expect questions such as "What current will you meet between the Canaries and the Caribbean?", "How do you know what current you are experiencing?" and "What would make you delay leaving Durban?". Expect also a vector problem: current plus boat velocity to give a ground track, or a course to steer across a stream.

Why Currents Matter

Ocean currents are generally weaker than coastal tidal streams but they run in the same direction for days or weeks. A 1 knot current is 24 miles a day: a 20-day passage with a favourable current gains a day or more, and an unallowed-for current can put you tens of miles from your DR. Close to land, or approaching low islands and reefs, that error can be fatal; many historical wrecks on the low atolls of the Pacific and the reefs of the Caribbean and Red Sea were caused by unexpected current set.

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Strong currents also change the sea state dramatically when the wind opposes them, and can turn a moderate gale into a dangerous one.

How Ocean Currents Are Generated

Wind stress and Ekman transport

The main driver of surface currents is the wind dragging on the sea. Because of the Earth's rotation, the surface water does not move straight downwind: it is deflected to the right in the Northern Hemisphere (left in the Southern) by about 45 degrees. Each deeper layer is dragged by the one above and deflected further, forming a spiral. The net transport of the whole wind-driven layer (the Ekman layer, roughly the top 100 m) is 90 degrees to the right of the wind in the Northern Hemisphere and 90 degrees to the left in the Southern.

Ekman transport has two very practical effects:

• Upwelling: where wind blows parallel to a coast with the coast on the left (NH), surface water is pushed offshore and cold, nutrient-rich water rises. Examples: off Portugal, North-West Africa, California, Peru and South-West Africa in their summer. Result: cold coastal water, rich fishing grounds, and frequent advection fog.
• Gyres: across an ocean, the trade winds to the south and the westerlies to the north push water towards the middle, piling it into a gentle dome under the subtropical high. Water flows round this dome, clockwise in the Northern Hemisphere and anticlockwise in the Southern. These are the great ocean gyres.

In practice, the surface current felt by a yacht also includes a direct wind drift close to the surface: after a few days of strong wind, expect a current of roughly 2 to 3 percent of the wind speed, setting somewhat to the right of downwind (NH).

Western intensification

The gyres are not symmetrical. Because the Coriolis effect increases with latitude, the return flow on the western side of each ocean basin is squeezed into a narrow, deep, fast current, while the flow on the eastern side is broad, shallow and slow. This is why the Gulf Stream, Kuroshio, Agulhas, Brazil and East Australian currents are all on the western sides of their oceans.

Thermohaline circulation

Below the surface, a much slower circulation is driven by differences in water density (temperature and salinity). Cold, salty water sinks in the North Atlantic (Greenland and Labrador Seas) and around Antarctica, spreads through the deep oceans and slowly rises elsewhere. This "global conveyor" takes centuries for a complete circuit and has no direct effect on a yacht's navigation, but its surface branches (such as the North Atlantic Drift) help keep north-west Europe mild, and the sharp temperature boundaries it creates are visible in satellite sea temperature charts.

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The Main Surface Currents

The Admiralty chart reproduced below, drawn for the book Ocean Passages for the World, shows the general surface circulation of all the oceans on one sheet. Pick out the great gyres (clockwise in the Northern Hemisphere, anticlockwise in the Southern), the westward Equatorial Currents with the Counter Current between them, the unbroken Southern Ocean current and the monsoon reversal in the northern Indian Ocean.

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Image: United Kingdom Hydrographic Office, public domain, via Wikimedia Commons (downscaled)

Use the Admiralty Routeing Charts (or US Pilot Charts) for the month: they show prevailing current direction, mean rate and constancy (how often the current flows in that direction). Rates in the table are typical; local and seasonal variations are large.

Equatorial currents

The trade winds drive the North and South Equatorial Currents westward across each ocean, typically 0.5 to 1.5 knots: a bonus for trade wind crossings from east to west. Between them, roughly under the ITCZ, the Equatorial Counter Current flows east. It is strongest in the Pacific (where it can help yachts heading east, for example from Asia towards Central America) and is weaker and seasonal in the Atlantic.

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Eastern boundary currents

The Canary, Benguela, California and Humboldt currents flow equatorward along the western coasts of the continents. They are broad, slow (about 0.5 to 1 knot) and cold, with upwelling near the coast. Effects for the yacht: a useful push south from Portugal to the Canaries, cool water and frequent fog where warm moist air blows over it (the summer fogs off California and Namibia), and a steady set that accumulates in the DR over a long passage.

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Western boundary currents

These are the strongest, warmest and most hazardous.

The Gulf Stream flows out of the Gulf of Mexico through the Florida Strait (over 4 knots in places), north along the US coast to Cape Hatteras, then leaves the coast and meanders north-east and east across the Atlantic, broadening and weakening into the North Atlantic Drift. Its north wall is a sharp boundary with the cold water of the Labrador Current: sea temperature can change by 10 C or more in a few miles. The Stream throws off meanders and warm-core and cold-core eddies (rings) that may have currents of 1 to 3 knots running in unexpected directions. The NASA visualisation below, built from satellite and model data, shows how far the real picture is from the neat arrow on a routeing chart: a fast orange core leaving the American coast, then a meandering jet surrounded by swirling eddies.

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Image: Greg Shirah, NASA Scientific Visualization Studio, public domain, via Wikimedia Commons (downscaled)

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The Kuroshio is the Pacific equivalent, flowing north-east past Taiwan and along the south coast of Japan at 2 to 4 knots, then east as the Kuroshio Extension. It interacts with typhoons and with winter monsoon north-easterlies to produce very steep seas.

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The Agulhas Current runs south-west along the east coast of South Africa at up to 4 to 5 knots, roughly along the edge of the continental shelf (the 200 m line). South of Africa it turns back on itself (the retroflection) and flows back east into the Indian Ocean, shedding huge warm eddies, the Agulhas Rings, which drift into the South Atlantic.

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The Agulhas is notorious. When a south-westerly gale (the "south-west buster" behind a cold front) blows against the current, enormous, steep, breaking waves and occasional freak waves develop, which have damaged large ships. Yacht tactics between Durban and Cape Town: wait in port for a favourable weather window between fronts; when a south-westerly is forecast, get either inshore of the 200 m line (where the current is much weaker) or well offshore beyond the main stream, or into port, before it arrives.

The Antarctic Circumpolar Current

The largest current in the world flows east all the way round Antarctica, unblocked by land, typically at 0.5 to 1 knot. Eastbound Southern Ocean passages gain 12 to 24 miles a day; westbound passages in these latitudes are, combined with the westerly gales, extremely difficult.

Wind Against Current

When wind blows against a current, the waves become shorter and steeper because the current shortens their wavelength while the wind continues to build their height. In a strong current (2 knots or more) the effect is dramatic: a Force 6 against the Gulf Stream can produce seas more typical of a Force 8 elsewhere, steep enough to break and knock down a yacht. The same happens in the Agulhas, the Kuroshio, and anywhere a strong tidal stream meets an opposing wind.

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Rules for the Gulf Stream (and similar currents):

1. Never enter the Stream with a forecast of strong wind with a northerly component (north-west through north-east), which opposes it.
2. Choose a window with light winds or winds from the south-west quadrant, flowing with the Stream.
3. Plan to cross quickly and roughly at right angles, allowing for the set in your course to steer. Use the latest satellite sea temperature and current analysis to find the axis, meanders and eddies, and to use a favourable eddy or avoid a contrary one.
4. Expect squalls and lightning over the warm water, especially near the north wall in summer.
5. Monitor sea temperature: a sudden rise tells you that you have entered the Stream.

Using Current Information

Sources

Climatological currents are averages. On any given day the actual current may differ greatly in rate and even direction, especially near strong currents and eddies. Use real-time data when you can, and measure the current yourself.

Measuring the current at sea

Keep an accurate DR based on the water track (log distance and compass course, corrected for leeway). Whenever you get a fix (astro, or GPS when available), compare it with the DR for the same time. The vector from DR to fix is the set and drift experienced since the last fix (including any errors in the log, compass and leeway estimate). Divide by the elapsed time to get the rate.

Worked Examples

Example 1: The effect of an unallowed current

A yacht heads 270 T at 6.0 knots through the water. There is a current setting 180 T at 2.0 knots. What is the ground track?

Draw the water track 270 T, 6 miles (one hour). From its end draw the current 180 T, 2 miles. The resultant is the ground track: tan angle = 2/6, angle 18.4 degrees, so course over ground 252 T, speed over ground 6.3 knots. If the navigator ignored this current for 24 hours, the yacht would be 48 miles south of the DR.

The error grows in direct proportion to the time it is ignored. That is why DR must be checked against fixes regularly and current estimates updated every watch.

Example 2: Finding set and drift

At noon on day 1 the yacht fixes by sun-run-meridian. The DR for noon on day 2, worked up from log and course, is 14 10 N, 045 20 W. The noon fix on day 2 is 13 58 N, 045 40 W.

• d.lat = 12 minutes south = 12 miles S.
• d.long = 20 minutes west; departure = 20 x cos 14 = 19.4 miles W.
• Distance = square root of (12 squared + 19.4 squared) = 22.8 miles.
• Direction: tan = 19.4/12, angle 58 degrees west of south: set 238 T.
• Drift over 24 hours = 22.8 miles = 0.95 knots.

This is a typical North Equatorial Current set. Apply it to the DR for the next day (as an EP), and compare again.

Example 3: Course to steer across a current

A yacht wants to make good 135 T across the Gulf Stream, which is setting 045 T at 2.5 knots. Boat speed through the water is 6.0 knots. Leeway is negligible.

The current is exactly at right angles to the desired track, pushing the yacht to the left of it. To cancel it, the yacht must aim into the current by an angle whose sine is 2.5/6.0 = 0.417: 24.6 degrees. Steer 135 + 25 = 160 T. The speed made good along the track is 6.0 x cos 24.6 = 5.5 knots.

On a chart you would solve this with the usual CTS construction: draw the required track from the start, lay off one hour of current from the start, then from the end of the current vector swing an arc of radius 6 miles (boat speed) to cut the track. The line from the end of the current vector to that point is the course to steer.

Because the Stream's rate varies across its width (fastest in the axis), many navigators simply steer to counter the average set over the whole crossing and accept being carried a little in the middle, rather than altering continually.

Example 4: Choosing a route to use the current

A yacht is leaving Grenada for Panama in January. The routeing chart shows the Caribbean Current setting west at about 1 knot along the route, strengthening off Colombia. With a 1,200 mile passage at 6 knots (about 8 days through the water), the current adds about 190 miles of help: a day and a half saved. However, the same area off Colombia combines strong trade winds and current with steep seas in winter: the plan keeps 80 to 100 miles off the Colombian coast to avoid the worst of the sea state near Cabo de la Vela.

Other Currents the Ocean Skipper Meets

• Labrador Current: cold, south-going, brings icebergs and sea ice to the Grand Banks from spring to mid-summer, and fog where it meets the Gulf Stream. Ice information is essential for any passage north of about 40 N in the western Atlantic.
• Caribbean Current and the Loop Current: the westward flow in the Caribbean at 1 to 2 knots, strongest off Colombia and through the Yucatan Channel, where it feeds the Gulf of Mexico and then the Florida Current.
• Mozambique Current and eddies: variable flow in the Mozambique Channel, with large eddies that can run against the general south-going direction. Passage-planning guides advise using the latest satellite analysis.
• Leeuwin Current: unusual warm, poleward flow down the west coast of Australia, strongest in the southern winter.
• Alaska and Aleutian currents and the Oyashio (cold, south-going off Japan, meeting the Kuroshio): relevant to north Pacific passages.
• Mediterranean: a surface inflow of Atlantic water through the Strait of Gibraltar (up to about 2 knots) and a deep outflow, so that yachts bound east make a little free progress, while those leaving westward face a contrary set, strongest in the Strait itself.
• Sargasso Sea: the calm centre of the North Atlantic gyre, with weak and variable current but also light winds (the Horse Latitudes) and floating Sargassum weed.
• Monsoon currents: in the north Indian Ocean the whole circulation reverses with the monsoon; the Somali Current in the south-west monsoon is one of the fastest in the world.

El Nino, La Nina and the variability of currents

The currents on the routeing chart are long-term averages. In an El Nino year the trade winds weaken in the Pacific, warm water sloshes east, the westward South Equatorial Current weakens and the Equatorial Counter Current strengthens, and the cold Humboldt Current is suppressed off Peru. A La Nina year does the opposite. Passage times across the Pacific can therefore differ from the guide book, and weather patterns (cyclone tracks, trade wind strength) change with them. Check the current ENSO status when planning a Pacific crossing.

Tidal Streams in Oceanic Waters

An ocean passage is mostly in deep water, where tidal streams are weak, usually well under half a knot. But the tide still matters in four places.

• Landfalls and departures: on continental shelves, in channels and around islands, tidal streams of 2 to 5 knots are common. Use the tidal atlas, the pilot book or the tide tables for the area.
• Atoll passes (for example in the Tuamotus): the whole tidal exchange of the lagoon flows in and out through a few narrow passes. The stream can reach 6 to 8 knots, the outflow against an onshore swell produces breaking seas, and slack water is often not at high or low water at the standard port. Local sailing directions give the offsets: arrive off the pass early, watch the water, and enter only at slack or with a fair stream and good light.
• Island wakes and shelf edges: the tidal stream or ocean current accelerates around headlands, and tide rips form where it meets a shoal or opposes the swell.
• Tidal range: the pilot book will give the range and datum for ports on remote coasts. Do not assume the 2 to 3 m of the Caribbean applies in, say, north-west Australia, where springs reach 10 m or more.

The principles are those you already know: pick the correct tidal diamonds or tidal atlas, correct for springs and neaps, and always work the vector triangle. The only difference is that on the open ocean the vector you apply is the mean current for the day, not an hourly tidal stream.

Working Current Into the Passage Plan

Gaining from a favourable current

A current is free speed, but only where you can reach it. On a Northern Hemisphere gyre the best route for an east-to-west crossing runs south of the centre of the high, in the trades and the North Equatorial Current; for a west-to-east crossing it runs north of the high, in the westerlies and the North Atlantic Drift, ideally using the Gulf Stream for the first part. The wind and the current agree because the wind drives the current.

When the current is in the opposite direction to your route, look for counter-currents and eddies or a route outside the main flow. In the Atlantic, yachts bound from Bermuda to the Azores often pass north of the Stream to avoid its adverse north wall.

The cost of an adverse current

A 1 knot adverse current on a boat making 5 knots through the water cuts the speed over the ground to 4 knots, so a 600 mile passage takes 150 hours instead of 120: a loss of 30 hours, or a quarter of the time. Plan fuel, water and food reserves accordingly, with the 25 percent margin the Ocean Passage Planning lesson recommends.

Estimated position with current

On a long passage keep a DR from the log and compass. Each noon (or each watch) work up the EP by applying the best estimate of current for the period: take the mean rate and direction from the routeing chart, adjusted by what your last fixes told you. Plot the DR, then at the end of the period apply the current vector to the DR position to get the EP. When a fix comes in, the vector from the EP to the fix is the error in your estimate of current. Update the estimate before the next period.

More Worked Examples

Example 5: Passage time with a favourable current

Distance 2,800 miles, boat speed through the water 5.5 knots average, favourable North Equatorial Current of 0.8 knot on average along the route. Speed over the ground = 6.3 knots; time = 2,800 / 6.3 = 444 hours = 18.5 days. Without the current: 2,800 / 5.5 = 509 hours = 21.2 days. The current saves about 2.7 days, but only if the route holds the yacht in it. A route that wanders 5 degrees of latitude out of the main flow loses most of the gain.

Example 6: A day's EP with current

At 1200 the yacht is at a noon fix of 24 30 N, 040 00 W. In the next 24 hours she steers 245 T and logs 138 miles, with leeway 5 degrees to leeward (wind on the starboard side, so leeway moves the track to port, that is the course made good is 245 minus 5 = 240 T). The routeing chart gives a current setting 270 T at 0.6 knot.

1. Water track 240 T, 138 miles. d.lat = 138 x cos 60 = 69.0 S. Departure = 138 x sin 60 = 119.5 W.
2. Current: 270 T, 0.6 x 24 = 14.4 miles: d.lat 0, departure 14.4 W.
3. Total d.lat = 69.0 S = 1 09.0 S. Total departure = 133.9 W.
4. New latitude = 24 30.0 N minus 1 09.0 = 23 21.0 N. Mean latitude = 23 55 N, so d.long = 133.9 / cos 23.9 = 146.5 minutes = 2 26.5 W.
5. EP: 23 21 N, 042 26.5 W. Without the current the d.long would have been only 130.8 minutes: the unallowed current moves you 16 minutes of longitude (about 15 miles) from the true position.

Example 7: Judging the current from the log and a fix

Over 3 days the log reads 420 miles, steering a mean course of 255 T with a total leeway of 4 degrees. The DR from the previous fix, worked from the log and course made good 251 T, ends at 19 07 N, 051 12 W. The new astro fix is 18 48 N, 051 51 W. Departure of the difference: d.lat 19 S; d.long 39 W; departure = 39 x cos 19 = 36.9 miles W. Distance = square root (19 squared + 36.9 squared) = 41.5 miles; direction: atan (36.9 / 19) = 62.8 degrees west of south, set 243 T; drift = 41.5 / 72 = 0.58 knot. This agrees with the North Equatorial Current of the pilot chart, so the navigator updates the daily current estimate to about 0.6 knot, 243 T.

Example 8: Wind against current in the Agulhas

A yacht in Durban plans to sail to Port Elizabeth, 400 miles to the south-west along a coast with the Agulhas current setting 2 to 3 knots south-west. At 6 knots through the water she makes about 8.5 knots over the ground in the stream, so the passage takes about 47 hours. The forecast gives a cold front with a strong south-westerly wind arriving in 36 hours. Even if she is making good progress, the passage will end in the front. The decision: delay departure until the front has passed and a north-easterly or light wind returns, or use the 3 or 4 intermediate ports (East London, Port St Francis) as bolt holes and move from one to the next in the weather windows. The Agulhas current is never crossed or followed in a south-westerly gale.

Practical Rules of Thumb

• 24 miles per day for each knot of unrecognised current.
• Expect 0.5 to 1 knot of current in the open trade wind belts, 2 to 4 knots in the main western boundary currents.
• Wind against current is dangerous when the current is 2 knots or more and the wind is above Force 5.
• Sea temperature is the indicator: a change of 3 C in a few miles means a current boundary. Record sea temperature every watch in the Gulf Stream and Agulhas.
• Birds, weed and colour change (blue Stream water, green coastal water) and lines of rips, foam or debris show convergence zones and current edges.
• Compare GPS speed over ground with log speed through the water: the difference along the track is the current component. This is the quickest way to use the instruments you have, but never rely on GPS alone for the exam: you must be able to work from log, compass and astro.

Exam Tips

• Know the vector triangle cold: water track plus current gives ground track; for a course to steer, draw the current first, then swing the boat speed arc to the track.
• Always quote set as the direction towards which the current flows, in degrees True. Do not confuse with wind.
• When asked "what would you do?", combine the answer: source of information (routeing chart, satellite analysis), the hazard (wind against current), and the action (delay, change route, enter at right angles).
• Show that you measure the current at sea rather than only reading it from a chart.

Common Mistakes

• Using climatological current figures as if they were exact; they are averages with large variations.
• Forgetting to allow for current in the DR over a long period, then being surprised by a large discrepancy at landfall.
• Confusing set (the direction the current flows towards) with wind direction (the direction it blows from).
• Attributing all the DR/fix difference to current when part is log error, compass error or leeway; check the log calibration and compass deviation.
• Entering the Gulf Stream, Agulhas or Kuroshio with a strong opposing wind forecast.
• Approaching low islands or reefs at night relying on a DR that has an unknown current error.
• Ignoring eddies, which can give a contrary current right next to the main stream.
• Applying the leeway correction the wrong way round, or forgetting that leeway and current are two separate effects.
• Entering an atoll pass at the wrong state of tide because the tide tables for the standard port were applied without the local offset.
• Taking the current from a pilot chart that is for the wrong month.

Summary

• Surface currents are driven by the wind, deflected by the Coriolis effect: net Ekman transport is 90 degrees to the right of the wind (NH), left (SH).
• Each ocean has gyres: clockwise in the NH, anticlockwise in the SH, with fast, warm, narrow western boundary currents and slow, cold, broad eastern boundary currents.
• Equatorial currents flow west in the trades (0.5 to 1.5 knots); the Equatorial Counter Current flows east under the ITCZ.
• Gulf Stream, Kuroshio and Agulhas run at 2 to 5 knots; wind against these currents creates dangerous seas. Never enter with a strong opposing wind forecast.
• The Antarctic Circumpolar Current flows east at 0.5 to 1 knot round Antarctica.
• Plan with routeing charts and real-time satellite current data; measure the current at sea by comparing fixes with DR.
• Apply current with vector triangles for EP and course to steer; an ignored 1 knot current is 24 miles of error per day.

Check Your Understanding

1. In which direction is the net Ekman transport for a north wind in the Northern Hemisphere, and what happens off a coast running north-south on the east side of the ocean when such a wind blows?

Answer: 90 degrees to the right of the wind's direction of travel: the wind blows towards the south, so the net transport is towards the west, i.e. offshore. Surface water is pushed away from the coast and cold water upwells near it (as off Portugal and North-West Africa).

1. Why are the strongest ocean currents found on the western sides of the oceans?

Answer: Western intensification: because the Coriolis effect increases with latitude, the return flow of each gyre is squeezed into a narrow, deep, fast current on the western side of the basin, while the eastern side has broad, slow flow.

1. Name the current that would help a yacht crossing the Atlantic from the Canaries to the Caribbean, and its typical rate.

Answer: The Canary Current near Africa, then the North Equatorial Current, setting west at about 0.5 to 1 knot.

1. What is the Agulhas Current's main hazard, and how do yachts avoid it?

Answer: A south-westerly gale blowing against the south-west-going current creates extremely steep, breaking seas and freak waves. Yachts wait for a weather window between fronts and, if a south-westerly is coming, get inshore of the 200 m line (where the current is weak), well offshore beyond it, or into port.

1. Why should you never enter the Gulf Stream with a forecast of strong northerly winds?

Answer: Northerly winds oppose the north-east-flowing current, shortening and steepening the waves dramatically and producing dangerous breaking seas far worse than the wind strength alone would suggest.

1. A yacht's DR at 0800 is 20 00 N, 060 00 W. Its fix at the same time is 19 52 N, 060 15 W, 16 hours after the last fix. What were the set and drift?

Answer: d.lat 8 miles S; d.long 15 minutes W, departure 15 x cos 20 = 14.1 miles W. Distance 16.2 miles; direction tan = 14.1/8, about 60 degrees west of south: set about 240 T. Drift 16.2 / 16 hours, about 1.0 knot (assuming log, compass and leeway are correct).

1. You want to make good 090 T at 5 knots through the water across a current setting 180 T at 1.5 knots. What course should you steer?

Answer: The current is at right angles to the track, setting south, so aim north of the track by arcsin (1.5/5) = 17.5 degrees: steer about 072 T. Speed made good along the track is 5 x cos 17.5, about 4.8 knots.

1. What is the Equatorial Counter Current and when might a yacht use it?

Answer: An eastward current lying between the westward North and South Equatorial Currents, roughly under the ITCZ. It is strongest in the Pacific and can help a yacht making easting, for example from Asia or the western Pacific towards Central America.

1. Give three sources of current information for ocean passage planning and one way to measure current at sea.

Answer: Routeing or pilot charts (monthly climatology), Ocean Passages for the World and Sailing Directions, satellite-derived current and sea temperature analyses (as GRIB files). At sea, compare a fix with the DR for the same time: the vector from DR to fix gives set and drift.

1. A yacht is bound from Las Palmas to St Lucia, about 2,800 miles. Her average speed through the water is 5.5 knots and the average favourable current is 0.8 knot. How long does the passage take, and how much does the current save?

Answer: Speed over ground 6.3 knots: 2,800 / 6.3 = 444 hours, about 18.5 days. Without the current: 2,800 / 5.5 = 509 hours, 21.2 days. The current saves about 2.7 days.

1. What are the indicators at sea that you have entered the Gulf Stream, and what does wind against current do to the sea state?

Answer: Sea temperature rises sharply (often several degrees in a few miles), the water changes to a deep blue, there may be rips, debris and Sargassum weed lines, and the log and GPS speeds diverge. Wind against the current shortens and steepens the waves; at 2 knots or more with Force 6, seas can be more like Force 8, steep enough to break and endanger the yacht.

1. Why should you not rely on the tide tables for the nearest standard port when entering an atoll pass?

Answer: In an atoll the lagoon fills and empties through narrow passes, so the times of slack water and the strength of the stream depend on the lagoon size, the pass width, swell and wind, and may be hours different from the standard port tide times. Use the local sailing directions, observe the pass, and enter at slack water or with a fair stream and good light.