B U L L E VA R D
WINNING FORMULA
WAVE RIDER
How can boats sail faster than the wind? A skipper and a chart-flipper explain what’s with the yacht
FOR ALL TO SEE “It is true that sailboats can go faster than the wind,” says Dr Martin Apolin, of the physics faculty at the University of Vienna, “but it only applies to boats with minimal water resistance. An Extreme 40 catamaran can reach a speed of 40 knots (74kph) in a wind of 28kn (52kph). “However, intuition suggests that a boat would sail fastest with a tailwind: the boat is pushed by the wind and can, at best, sail as fast as the wind itself. To understand faster-than-wind sailing requires three observations and a few vectors. “Let’s stick with the speed mentioned earlier and assume that the catamaran is sailing at 90° to the wind, on a ‘beam reach’. “Step 1 (Fig. 1): Calculating the ‘apparent wind’. The wind you would perceive if the boat were at anchor is known as ‘true wind’ (in our example here, it has a speed of vtW = 28kn). Because the catamaran moves through the air as it sails, it gives rise to an air stream (vAS = 40kn). The two wind velocities are added together to form the apparent wind, which can be calculated with a Pythagorean formula: vAW = √ v²tW + v²AS ≈ 49kn. The sailor feels this overall wind and determines the course and pitch of the sails. “Step 2 (Fig. 2): Calculating the total force of the sail. Because the sail curves, it functions like a vertical wing. This creates a drag force (FD) and lift (FL). At 90° to the wind, the latter is allocated to the apparent wind parallel to the water surface. Step 3 (Fig. 3): Calculating propulsion. The total force that the wind creates upon the sail operates on the mast and the catamaran. It can be broken down into two components: transverse force, which is intercepted by the rudder and helm and which has no impact on forward momentum (but can produce spectacular listing), and another component in the direction of travel, propulsion. It’s this latter component, smaller in relative terms, which can propel the boat faster than wind velocity, as counterintuitive as that may sound. That’s because this propulsion is greater than if the catamaran were sailing with a tailwind (assuming that water resistance is low).” www.extremesailingseries.com
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WORDS: MARTIN APOLIN. PHOTOGRAPHY: ROB TRINGALI/RED BULL CONTENT POOL. ILLUSTRATION: MANDY FISCHER
OUT TO SEA “Extreme 40 catamarans go fastest at beam reach [a course steered at right angles to the wind], with minimal water resistance,” says two-time Olympic sailing champion Roman Hagara. “That means calm water and the least possible friction coefficient from the two hulls. We achieve the latter by ‘nanocoating’ the hulls with a particle-thin layer of a material that reduces friction. “Recently, people have been working on specially bowed helms, which can create huge uplift. Soon boats could be floating on the surface with only the rudder and the helm maintaining contact with the water.”