Underneath everything
Higher and faster, lower and slower – in the end it all comes down to drag... Dave Hollom
It was in 1907 that Dr FW Lanchester FRS stated that ‘the problem of sailing yacht mechanics resolves itself into an aerofoil combination in which the aerofoil acting in the air and that acting underwater mutually supply each other’s reaction. The result of this supposition is evidently that the mini- mum angle at which the boat can shape its course relative to the wind is the sum of the under and above-water gliding angles.’ In other words, how close a boat can sail
to the wind or point is entirely due to the combined aerodynamic and hydrodynamic lift/drag ratios. And yet 118 years later the widespread and dogged belief of many sailors remains that how high a boat will point is primarily due to how much lift the keel can produce. Before, hopefully, explaining why Lan-
chester was right, a few words about one of the most underrated engineers of his age would not be out of place. Lanchester was a British physicist, engineer and aerody- namicist who is credited with the discovery of the circulation theory of lift, the origin of wing tip vortices and many other areas
56 SEAHORSE
of human discovery including such diverse things as battlefield tactics and the build- ing of the first British automobile, together with many inventions still used today in the automotive industry. He also founded and ran his own car
company, The Lanchester Motor Com- pany, which ran from 1899 until 1955. His contribution to science was probably underestimated because he was brilliant at developing the theory but not so good at communicating or putting numbers to those theories. For instance, it fell to Ludwig Prandtle to unravel Lanchester’s thinking and put his circulation theory of lift and his vortex theory of lift-induced drag into a completed theory. Moving back to just why one boat will
point higher than another, it is easy to see how one would think that it is lift that makes a boat point. Intuitively, if your boat lifts away from the boat to leeward and squeezes up to the boat to windward of you, it is easy to relate that to the perceived fact that your boat must be producing more lift than the other two boats. After all, lift is working in the direc- tion that will lift us above the boat to lee- ward and under the boat to windward, in other words ‘point higher’. However, nothing can be further from the truth.
Hydrodynamic lift is a reaction force
and is always equal and opposite to the aerodynamic force provided by the rig. One must always balance the other. The easiest way to appreciate that it is
drag and not lift that dictates the angle to the wind that we can sail is to look at the aerodynamic and hydrodynamic force vectors. Looking first at the aerodynamic forces, lift force is always considered to work normal or at right angles to the oncoming flow, in this case the apparent wind. However, the total aerodynamic force also contains a force that we call drag and that is always considered to work tangentially or parallel to the onset flow, in this case against the apparent wind. Now, looking at the angle that these
forces make with the direction of travel of the boat, if the apparent wind angle is at any angle to the boat track the lift force vector (excluding drag) will always have a component that is aligned with the boat track. If the apparent wind angle is the same as the track angle there will be no component acting along the track. However, as soon as the apparent wind
angle becomes wider than the track angle there will be a component that is working forward, and the greater the difference between the track and apparent wind
DAVID BRANIGAN
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