Left: among many standout features on Team New Zealand’s raceboat, Te Rehutai, for the purposes of this article a couple of less obvious details are among the most significant – the foil arms on the Kiwi design exit the hull beneath the chine, much lower than rival designs; also look closely and you will discern the vortex strip on the bottom of the very fair full-length skeg. So nothing like the very boxy skeg on Britannia, however (above) ,which looks as if it was created with some hydroplane-style ‘bounce-off’ in mind
provide a bit more buoyancy if the hull should come into contact with the water. I would also suspect the skeg will now incorporate a vortex generator (a longitu- dinal sharp-edged strip) to provide the beneficial vortex which helps to further seal any gap between hull and sea. I would question whether the additional
buoyancy provided by this, apparently more voluminous, skeg will outweigh the suction, or at least the reduced lift, that it will provide compared to a concave V-section. Other than that a very nice boat. Her clean lines and low wetted area mean that she should reach take-off speed easily and quickly and so should be good in marginal foiling conditions. To me she is still the boat that makes the most technical sense and that goes for the foils, which we will discuss later. Terry Hutchinson’s Patriot is another
pretty boat that looks right and I’m sure will be fast both semi-foiling and in lift-off. Whether she will be fast once on the foils will obviously depend on the foils them- selves. My only slight question regarding the hull is whether that skeg is deep enough or runs as far aft as it could. I know she sails in quite a pronounced bow- down attitude and in that attitude her skeg gives a good seal with the sea surface but the resulting tip vortex, which helps seal any gap between bottom and sea, is released earlier than it might be if the skeg were carried further aft as on Luna Rossa. Research has shown that lift-induced
drag in aircraft reduces as the wing tip is moved aft. That is why swallows and swifts and other birds that have to travel vast distances at minimum drag, as well as fast swimming fish, have crescent-shaped wings or fins with the tip well aft of the
root trailing edge of the wing or fin. This research (originally by Van Dam)
did show that moving the wing tip aft reduced induced drag, but it didn’t go on to show if this tendency reduced after it had been moved a certain percentage of the root chord aft or conjecture as to the physical reason for this finding. Perhaps it is because, as the downwash, which is the source of lift-induced drag and itself is the result of the tip vortex, is moved further aft, the downwash doesn’t have to be as steep and thus its effect on the wing is reduced. Remember, at small downwash angles induced drag is proportional to
of how it turns out – Václav Havel
Hope is not the convic- tion that something will turn out well but the certainty that something makes sense, regardless
downwash angle so that any reduction in induced drag must be because the down- wash angle has reduced. Anyway, it does seem that the further
aft the tip vortex is moved the lower the induced drag and one would expect this to be true at both ends of the foil. I know that the gap underneath the boat is small and the tip vortex is helping to seal any gap there is, but that seal is not perfect, so I would imagine that the further aft the vortex is released the lower any induced drag at that end of the foil will be. Thus carrying the skeg all the way to the stern seems to make sense. There is a downside, of course, that when it gets wet the wetted
area and drag increase. However, Patriot’s canoe body is very
smooth and flat in her aft body in a longitudinal direction, as is Luna Rossa. This is important because convex curva- ture, as mentioned previously, speeds up the flow and, according to mathematician Bernoulli, produces low pressure. Low pressure on the bottom is the last
thing you want on a boat that is trying to lift out of the water. It is a force that is working against the foils, making them work harder and thus producing more drag. This slows acceleration and, as lift varies as velocity squared, reduces the build-up of lift from the foils and thus delays the time it will take for the craft to reach take-off speed. That’s why planing dinghies have or should have very flat runs in the aft part of the boat. I predict that both Patriot and Luna
Rossa will be good in marginal conditions and getting onto the foils quickly when needs be. Perhaps, due to her shorter skeg with less wetted area, Patriot will be the quickest in these conditions? One further observation about Patriot is
that if you take the skeg away her hull has a spoon bow. Without the skeg her bow waterline ending would be well back from the bow giving her, effectively, a shorter static waterline. Bearing in mind that when in displace-
ment mode length is speed, this restricts her maximum speed and thus may mean that the drag rises more rapidly than it should as the boat approaches take-off. In this respect the skeg in the forward part of the boat doesn’t effectively extend her waterline length. It doesn’t have the volume necessary to do that. Looked at in another way her prismatic coefficient (the
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GILLES MARTIN-RAGET
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