Do You Know How Sails Work? Find Out More Here
August 26, 2009 by Chris Wethered
Filed under Sails
Daniel Bernoulli
The question how sails work was answered by a Swiss scientist in the eighteenth century called Daniel Bernoulli. Before elaborating, it is important to remember that the wind has the same characteristics as a liquid: Wind flows.
When wind reaches an obstruction it is splits and goes around the obstruction, then joins up again. The speed of the wind as it negotiates the obstacle varies.
Concave And Convex
This is what happens with yachts moving through the water. So lets look at the physics in tangible language! As you are aware, the different parts of a sail or foil, play a specific part in the process.
How sails work: As the wind moves around the sail shape, the speed differential moves the boat through the water
What is happening?
When the wind reaches the foil:
- The wind must find a way around the foil
- As the wind flows around the foil, the windward side of the foil makes a concave shape and the leeward a convex one
- The draft of the foil is measured in the middle
- The draft is the point at which the wind on the leeward side is moving fastest
- More draft equals more speed
Push And Pull
The push and pull between the windward and leeward develops due to pressure differential. High pressure on windward repulses low pressure on leeward. As air reaches the foil it splits up. The air that flows to leeward speeds up and that to windward slows.
An area of low pressure develops to leeward and high pressure to windward. The “push” and “pull” combine together to move the boat through the water.
the result - lovely efficient shapes!
It’s all about pressure change:
- To leeward a channel develops between the accelerated air and the unaffected air
- The draft of the foil narrows the channel
- The channel accelerates the wind towards the unaffected air
- The faster air leads to decreased pressure and a chain reaction begins
- As air heads towards the foil, more is drawn to the low pressure area
- This increases the difference in pressure
A different approach is used for flying the spinnaker
Point of sail
The result is that the boat accelerates to it’s maximum speed for that point of sail. The angle of wind to the direction of travel has sparked the whole industry. It’s impossible to sail straight in to the wind. The reason is that air reaches the foil evenly on both sides. There is no pressure differential, the sails just flaps uselessly.
Moving at angles to the wind
Moving at angles
Harnessing wind, therefore requires the yacht to move at angle to it. This is why yachts tack to change direction. Back in 1783 Daniel Bernoulli published the now famous Bernoulli Effect. How wind works, has fascinated sailors ever since!
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