The Wind in Your Sails

One of the things that most annoys people new to sailing is the apparently impenetrable mystery that surrounds the business of being propelled along by the wind.

In order to enjoy being aboard a boat you’ve got to feel useful and involved, but this is almost impossible if events are hard to predict and incomprehensible as they happen. The nautical jargon floating about on deck ought to be precise and illuminating on the subject – instead, it tends to frustrate still further any efforts to understand what on earth is going on.

Through no fault of your own you are reduced to the status of cargo. Most people find this sudden demotion from capable adult to irresponsible child hard to bear, and as a result sailors are sometimes written off as elitist and deliberately arcane, long before the intricacies of their art and the subtlety of their relationship with the forces of the natural world can be appreciated.

Fortunately, however, learning one thing – and one thing only – will dissolve most of the mystery, and as you stand upon a deck gazing at all the complicated arrangements of cloth and string that have evolved to harness the Thing, your understanding will empower you to enjoy the present and anticipate the future. You’ll pull on the right bit of rope, duck your head at the correct moment, and respond not as an unreasoning animal to the roaring voice of the skipper, but to the situation itself. This Thing is the secret of the wind in the sail.

Everyone already knows, as they watch a plastic bag tumbling down the street or try to put up a tent in a gale, that the wind will blow anything it can away. Sailing downwind works on the same principle, and simply involves dangling out a bit of sailcloth, attached at all the corners, and sitting back to steer. It was certainly mankind’s first effort at sailing, and it worked wonders for the Phoenicians, the Vikings and for the rest of the early maritime civilisations.

Barnabas sailing downwind towards St Michael's Mount

Barnabas sailing downwind towards St Michael’s Mount

As well as being a bit boring though, downwind sailing does rather beg the question – asked with increasing desperation by many sailors – how the hell do I get back? There are pictures from ancient Egypt of vessels proceeding to windward along the Nile, and it’s probably true also that the Vikings and Phoenician vessels could also sail somewhat against the wind. Given the exhausting nature of continuous rowing, their only other option, how else could they have travelled so far?

So here is the crux of it: how do sailors achieve this paradoxical feat of using the power of the wind against itself? As so often happens, the answer lies somewhere else. We have to look into the sky for the solution.

Thinking about the wing, evolved several times by nature and of course latterly by man, is part of the answer. Imagine an aeroplane driven forward on the runway by its engines. Air is moved over its wings, and because of a combination of the angle of the wings and their shape, the airstream is caused to travel faster over the top of the wing than it does beneath it. In other words, the wing divides the airstream, and that on the top moves relatively faster than that below.

Strangely enough, the faster flow of air above exerts less pressure on the wing than the slower flow moving below. It’s to do with the fact that particles moving faster get further apart: fewer air particles = less pressure. This difference between higher and lower pressure areas causes lift, a force that pushes the wing into the sky. Works every time, you’ll be relieved to know.

Airflow over a wing creates lift

Airflow over a wing creates lift

If you have ever been on a boat and asked how the sails work, I bet someone will have told you about wings. And, probably, you already know about them anyway. But in itself, this explanation does not properly answer the question, because obviously there are major differences between the wing of an aeroplane and a boat’s sail. Firstly, an aeroplane requires an engine to push it forward and create the air movement over the wing. Secondly, a wing lifts you vertically upwards – where is the equivalent of that on a boat?

A boat’s version of a wing – the sail – points up into the air. So imagining the scene from above, looking down onto the vessel, helps with working out the effect. If you think about it this way, the lift generated by the sail is a horizontal force, not a vertical one. Imagine the boat’s sail, pulled in so that it catches the breeze and forms its curved wing shape. The lift generated by the air moving over the sail is roughly at right angles to the sail.

One important factor is that the lift really wants to move the boat sideways through the water. And so it does; so would a round coracle with a sail on it. But boatbuilders over time have evolved hull shapes that like to go forward – or back – but not sideways. The hull or keel of the boat resists the sideways force, and much of it is converted into forward motion because of the angle of the sails relative to the hull; the lifting force always points forward of the beam. So it is in fact the lift of the sail itself that moves the boat through the water. And as the vessel gathers speed into the wind, so the wind appears to grow stronger, and the lifting force increases.

An approximation of how the wind drives Barnabas

An approximation of how the wind drives Barnabas through the water

And that is it, really. All sailing theory is a variation on this one grand theme; all forms of hull shape and rig are merely methods that have evolved to harness it; and all the abstruse terminology simply labels its component parts.

So when you’re next on a sailing boat, imagine the forces at work and you’ll find meaning in the nautical madness.

Barnabas on a close reach

Barnabas on a close reach

Posted in: Articles

Written by Cornish Maritime Trust