How do boats sail into the wind ?

When I first started sailing it was relatively easy to understand how a boat sailed downwind and how your top speed was limited by the wind strength and the drag created as the hull pushed the water out of the way. Further to this it was also obvious enough to understand how a boat could not sail directly into the wind, it was however tricker to understand how the sails allowed you to beat to windward. 

To answer this question we have created the first in a series of videos on sail trim in an effort to answer the question, adding further detail in the remainder of this blog. Watch the video linked below or read on to learn.

What makes a boat sail to windward ?

What Factors limit the Top Speed of you boat ?

The top speed you can achieve under sail is influenced by three primary factors, the windspeed, the force generated from your sails and the energy required to displace the water as the boat moves through the water. The wind speed is out of your control and you can only make minor adjustments to the way the boat sits in the water by adjusting where you crew weight sits on the boat, this leaves the sails and how you set them up as the primary tool you can use to maximise your top speed. If you want to reach you top speed and stay their your crew must continually trim the sails to maintain maximum forward momentum. 

How do the sails generate forward force when sailing to windward ?

When a boat is sailing to windward the forward force is generated by the wind particles as they travel past the sails. The particles separate when they approach the luff of the sail, as they flow down either side of the sail the particles move at different speeds depending on the camber in the sails. The particles move faster over the leeward side of the sail, the speed differential between the particles creates a pressure difference, it is this pressure difference and the speed differential of the wind particles which propel the boat forward.

What is sail camber and how much should you have ?

Sail camber is the curvature which is built into the sails. When sailing there are multiple ways you can adjust the amount and position of the camber in your sail, matching the camber to the wind strength is key to generating the maximum forward motion. The position of your maximum camber which is point of sail with the deepest curve relative to the perpendicular between the luff and leach of the sail is key to getting the maximum top speed. A good starting point is to have your maximum camber 25% from the luff of the sail in light winds, moving it to the middle of the sail in medium condition and taking to 75% from the luff of the sail in the heavy airs. When it comes to amount of camber you should aim for to achieve your maximum top speed you want to have the greatest dept in the sail without having too much to cause the wind particles to separate from the leeward side of the sail. If you have too much dept in the sails, the particles on the leeward side of the sail will separate, this separation and uneven airflow results in excessive sideways force which intern result in the boat heeling over rather than moving forward.

What can you do to adjust the camber in your sails ?

Nearly all the control lines you have on your boat will have the ability to impact the camber of your sails, learning how these alter the camber and which one to use and when will  ensure you are always getting the most out of your sails. Our first video explores how sheet and forestay tension allow you to move the camber forward and aft in addition to altering the maximum camber of your headsail. 

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