The skeg on American Magic’s Patriot does not extend aft as far as their rivals’, instead offering flatter aft sections that – aside from the lack of skeg and rounded chine – closely mimic the second New Zealand boat. The foils debuted by American Magic are relatively small and indeed in early ‘not to be relied upon’ practice races the US team showed at its best in more breeze with bursts of real pace at times. Terry Hutchinson’s crew already have the most time on the water – as planned – and it shows with slick manoeuvres only rivalled by the Defender. With Airbus a prominent sponsor of the New York YC challenge commonsense dictates the US foils will be on the money… more so than the surprisingly agricultural foils that contributed to the USA losing the Cup in Bermuda back in 2017
relationship between the displacement of the boat and that of a prism of the same length and breadth and having a sectional shape identical to the largest section of the boat) is low and higher prismatics tend to give lower drag at higher Froude numbers. On the plus side this shape gives con-
cave waterlines and sections in the bow which slow the flow of water in that area and again, according to Bernoulli, pro- duces high pressure in the bow area which will help take-off. Safe to assume a very strong design team have decided that the greater lift in the bow trumps the possible increase in drag. Contrast this to Britannia. Yes, she has
a skeg that runs the full length of the boat and, yes, if you take the skeg away in the bow area, similarly to Patriot she has a spoon bow. But there the similarity ends. Rather than a hollow veed section forming the skeg with a strong ‘sealing’ vortex which, when it does meet the water, does so progressively and with little increase in drag, we have a voluminous skeg with a flat bottom that, because its edges are not as acute, will not produce as powerful a vortex and which, rather than adding vol- ume progressively, will add it all at once. I guess the designers of the striking
British raceboat reason that the flat bottom will generate some lift and also, because of its extra volume, will remain above the water surface with more certainty. They may be right but the flat bottom has a convex longitudinal curve and may there- fore produce suction which will reduce the buoyancy effect. Note that the V-sections of Patriot, because they are concave, will produce lift as they meet the water. Then there is the very angular sectional shape. Bearing in mind that a straight line
62 SEAHORSE
is the shortest distance between two points and that the hypotenuse of a triangle is always shorter than the sum of the other two sides, due to Britannia’s angular sectional shape the section perimeter length is large and the enclosed volume is small. To enclose the required volume, therefore, the wetted area must be very large and wetted area is drag. By contrast, the sections of Patriot and
Luna Rossa are conventional convex V- sections that contain more volume with a smaller wetted surface area than those of Britannia but with the V going concave near the centreline to form the skeg with, in the case of Patriot, a very sharp apex that will produce a powerful vortex. Also, when these boats come into contact with the sea they will do so with buoyancy building progressively with low drag and the concave V producing lift.
Finally, not only must the hull perform
as a low-drag hydrodynamic object to get on the foils quickly, it must also perform as a low-drag aerodynamic object to reduce parasitic drag when flying. Sailboat design is all about reducing
drag and increasing righting moment. If you are clever enough to do both you will sail faster and closer to the wind upwind and lower and faster downwind. Game over. However, the Lord giveth and the Lord taketh away. Most things that increase righting
moment also increase drag and vice versa, so it’s not quite that easy. However, once flying righting moment on these boats is fixed or nearly so (you could increase it a little by heeling to windward and also by flying low, or reduce it by flying too high) so your aim regarding the hull should be to minimise aero drag. For that you need a
SAILING ENERGY/AMERICAN MAGIC
GILLES MARTIN-RAGET
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