The Coriolis Force and decisions in yacht racing

The Coriolis Force, or effect, causes southern hemisphere sailors to prefer to sail on the opposite tack to their northern hemisphere counterparts. But which way is best, and why?

Readers may have seen the Coriolis effect demonstrated by throwing a ball from a moving platform (eg train window, rotating merry-go-round) to someone standing still nearby. It is not easy to throw accurately. The ball leaving the thrower’s hand has angular momentum. That is, it is travelling at an angle to the straight line between the thrower and the recipient. The ball appears to curve away from the target. The Coriolis Force, named after a French scientist who was first to explain how the effect worked, is not actually a force but rather an effect caused by the angular momentum.

When it comes to wind systems of the world, a wind blowing in the southern hemisphere towards the equator will shift to the left. This is because the surface of the earth is rotating more quickly in space towards the east at the equator than it is at latitudes further south. The angular momentum from the latitudes further south is insufficient to keep up with the faster travelling area to the north so they fall behind their target. Trade winds blow from around 30 degrees south towards the equator, but as they shift left, they become south east trades. Winds blowing away from the antarctic polar region become the polar easterlies. Winds blowing towards the polar area shift left because their angular momentum is more than sufficient and they overshoot the target to become the prevailing westerlies of the roaring forties and furious fifties.

In the northern hemisphere, the same principle applies but the shift direction is reversed. Winds shift to the right in the north. That is, winds blowing towards the equator shift right to become north east trades. The polar winds shift right to become easterlies and the westerlies come from winds blowing towards the pole but are then shifted right.

As winds blow away from high pressure towards low pressure zones, they circulate high pressure systems anti-clockwise in the south owing to the left shift and clockwise in the north owing to the right shift. Similarly, they circulate low pressure systems anti-clockwise in the north and clockwise in the south.

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Surface friction slows the wind so the lessens the shift. By virtue of being less shifted to the left, the slower wind appears to shift to the right in the SH. In the NH, instead of the normal tendency to shift right, slower wind appears to shift left.

Reminder: sailing on a port tack means the wind is coming from the port side of the boat, left when looking forward. A gust of wind is a stronger burst that has been ‘less slowed’ by surface friction. In the SH, that gust will therefore come from the left. That means, if you are sailing close hauled upwind on port tack, the gust will lift you higher since it comes from further left. You will get to the windward mark sooner. Whereas, if you were sailing on a starboard tack, that gust would knock you away from your upwind mark.

In the north, gusts will lift you on starboard tack and knock you on port tack.

There is a further consideration as to sail shape. The wind at the top of the mast will be less slowed than wind at deck level. On a port tack, more sail twist will be helpful in the SH. That would present a curve in the sail from less at the top to more at the bottom suited to the changing wind angle from top to bottom. On starboard tack, a twist in the sail is curving in the wrong direction. So flatten the sail, mainsheet on, traveller low, car forward for headsails.

If the weather is deteriorating with a deepening depression forming, you will know if it’s coming your way by of a falling barometric pressure. To avoid the centre of the depression, sail close hauled on port tack in the SH and sail close hauled on starboard tack in the NH. Once the barometric pressure starts to rise, you remain on that relevant tack but easing sheets to a broad reach. This way, you will claw away from the dangerous centre of a deep low.

So whether it is racing to pick up the advantages of the gusts, setting sail-shape correctly or racing to avoid a potentially dangerous depression, port tack is your friend in the south and starboard tack is your friend in the north. All due to the Coriolis force.