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Captain Bob Wallace hangs onto the handrail as the 5 meter swell pours onto the deck. The strong trade winds and high speed caused the boat to heal to starboard at a severe angle.
Photo : Genevieve Johnson

October 27, 2003
The Coriolis Effect
Real Audio Report

Log Transcript

This is Genevieve Johnson speaking to you from the Odyssey, seventeen degrees south of the equator in the Indian Ocean.

Since leaving the Maldives, we witnessed spectacular sunsets, drenching squalls and a glaring tropical sun. Currently, the crew is taking advantage of full-scale trade wind conditions as we sail a course to Mauritius. With 25-30 knot winds and a 4-5 meter swell at our port quarter, the Odyssey is practically flying.

Even though the winds aren't particularly strong, when they blow consistently over time, the seas slowly build into enormous swells.

Trade winds are generally restricted to tropical and sub-tropical latitudes and are dependable all year round, although they may vary in strength, dropping off a little in the summer months.

Mariners learned to rely on the trades centuries ago. The Spanish realized the value in the regularity of the trade winds early in the 16th century. They sailed from Spain to obtain riches from the colonies of the new world, utilizing the tropical trades to get there and the concurrent westerly winds at higher latitudes to return to Europe.

Trade winds are caused as a result of the earth's rotation. Winds run parallel in the northern and southern hemispheres along either side of the equator, blowing west simultaneously. The rotational velocity of the earth varies with latitude, increasing from the poles toward the equator and matter not rigidly attached to the earth's surface, such as air and water, undergoes an apparent deflection known as the Coriolis effect. Consequently, objects that move freely relative to the planet appear to drift to the right of their intended path, or clockwise in the Northern Hemisphere and to the left of their intended path, or counter clockwise in the Southern Hemisphere.

Trade winds are caused as a result of the earth's rotation. As the planet spins, matter not fixed to the surface of the earth, such as air and water, undergoes an apparent deflection known as the Coriolis effect. Therefore, equatorial winds rotate clockwise in the Northern Hemisphere and anti-clockwise in the Southern Hemisphere. The Pacific, Atlantic and Indian Oceans all experience, year-round trade wind conditions. An exception to the rule is the land-locked Northern Indian Ocean which is governed by the northeast and southwest monsoon seasons.

Trade winds diverge to the north and south upon reaching continental landmasses, turn back at higher latitudes and blow back east as westerly winds. After crossing an ocean, they will once again hit land and run down the coast before returning to their westerly course along the equator.

The Coriolis effect is named after its French discoverer, engineer and mathematician Gaspard Gustave de Coriolis (1792 - 1843)

You can test the Coriolis effect yourself by observing the way water spirals clockwise down a kitchen sink or toilet bowl in the northern hemisphere, and counter clockwise in the southern hemisphere.

For the past week, the trade winds caused Odyssey to sail healed over to starboard at a severe angle. Water frequently pours over the handrails and gushes up through the freeing ports, washing across the deck like a flash flood. Movement around the boat is challenging and each maneuver must be carefully planned, requiring excessive concentration. Sleep comes easily to those bedding down against the starboard hull, but is illusive for those of us on port. If a crewmembers hat were to fly off in the wind while on observation watch, it would be all but impossible to turn the boat around against the trades to attempt retrieval. Of course, if a crewmember went over, we would make an effort!

By midday today, we traveled 203 miles in 24 hours. We are averaging 8.5 knots and are sometimes exceeding 9.5 knots. Captain Bob Wallace has been onboard Odyssey since 1986. In 14 years, he does not recall ever having seen the boat sail so far and so fast.

The Odyssey was built to sail and she is at her best and most spectacular when her engine lies dormant, the jib, main and mizzen are set, and only the sound of the wind and waves accompany her.

The trade winds are weaker closer to the equator. At the beginning of the passage, the seas were calm and the sunsets were brilliant.
Photo : Chris Johnson

We liken Odyssey under sail to a quote by Henry David Thoreau -

    "A creature of two elements, she is akin to a swift and shapely fish below the water line, above, she is a strong-winged, graceful bird."
    Henry David Thoreau from ďA Week on the Concord and Merrimack Rivers', 1849


  • "Oceanography - An Introduction to the Planet Oceanus"
    Paul R. Pinet. West Publishing Company 1982. pg 163-167


Written by Genevieve Johnson

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