It would not have been unreasonable to expect that 2024’s AC75s, which for teams like Luna Rossa and Ineos would be their third generation AC75 designs, would have converged more than proved the case. Ineos Britannia features this enormous deep, long, aero-driven skeg, a no-doubt heavily researched effort to minimise the average gap between skeg and water surface. The bulky and deep hull is probably the result of similar thinking, making for the greatest possible effective rig span. By contrast Luna Rossa looks almost like a sailing boat. Whether or not it was due to the better sealing effect of the straight skeg on Ineos, the British boat proved more forgiving in manoeuvres while the Italian AC75 was frequently visibly difficult to control up-range while also flying less steadily
offset sideways from the hull so that they do not carry the lift down to the water surface very satisfactorily and the air can escape very easily around and under the hull. It has the aerodynamic advantage over the Moth in that it is relatively easy to seal the bottom of the rig to the deck but from there down a seal needs devising. The closer the boat can fly to the water
the smaller the gap and the more nearly that doubling of span can be achieved. However, flying that close to the surface makes it far more likely that the hull will, on occasion, contact the water and with such a large area in contact this will result in a large increase of drag. With a very slim centreline structure
below the hull the boat can fly very close to the surface and if contact does occur drag will rise less and much more slowly. If this structure or ‘skeg’ is V-shaped along the centreline, the flow across the V will trip into a vortex that will partially seal any gap between hull and water and allow the boat to sail a little higher above the surface without losing too much of this endplate effect. If not V-shaped then an upside-down T-section vortex stimulator can be attached to the bottom of the
skeg, which will also allow a vortex to form. A quick word about vortices. If they
come into contact with a surface (an aero- dynamicist would call it a wall), such as a road or the water or the underside of a hull, the vortex will walk. Its direction of travel (see pg54) is in the same direction as the edge flow within the vortex that is in contact with the wall. So, looking from behind, a vortex that is rotating clockwise between hull and water surface will be dragged to the right by its contact with the hull bottom and left by its contact with the water. The result is it will stay attached to the
hull and the water surface and remain on the centreline, forming a seal between hull and water. Just how good that will be depends upon the power of the vortex and gap it is asked to bridge – but it will be better than the same gap without a sealing vortex. That is why boats with rounded skegs have flat plate/T-shaped vortex generators attached. Vortices can burst, though, and cease to
form a seal. However, by having a vortex generator near enough the length of the boat the vortex is continuously re-energised so that bursting should not be a problem. Many years ago American scientist Van Dam conducted a number of experiments
on wing planforms. He reasoned that nature is rarely wrong and birds, which had to travel large distances and wanted to have minimum drag to improve their lift/drag ratio (Cl/Cd) and thus the amount of energy they consume for covering a given distance, had wings that were either crescent shaped (swallows and swifts) or had a tip that was some way behind the root of their wings (albatross and seagull). He also noted the same tendency in fast swimming fishes and dolphins. At this stage it should be mentioned that
birds that soar, such as hawks, eagles and vultures, have a very different wing plan- form with splayed tip feathers that break the tip vortex up into a lot of smaller tip vortices having a lot less combined energy than one (or two) big vortices. This was the inspiration behind aerodynamicists such as Richard Whitcomb of NASA and John Spillman of Cranfield University pro- ducing winglets on modern transport air- craft, which was eventually used by Ben Lexcen on the keel of Australia II. Van Dam essentially utilised an early
fluid dynamic program to analyse different wing planforms. He used an elliptical spanwise area distribution, with no
SEAHORSE 49
ALAMY
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