Early planing shapes. The hollow forward sections of the Catalina flying boat hull and fast launch (far left) were the default solution for high-speed hulls in the first half of the 20th century. The step on the Catalina hull is a feature that extended to a small number of powerboats but more typically the hollow sections washed out into almost flat stern shapes. Raymond Hunt (next month) changed everything when he perfected the deep-V hull (left) which handled better across a range of sea states. And man plus bubble (top) – American Magic arrives in New Zealand
opposite and adjacent sides of a triangle and the force normal to the lift foil becomes the hypotenuse of the same triangle which, according to Pythagoras, gives us a total force on the lift foil of 80,365n. Assuming a foil area of around 3m2
, this
lift force of 80,365n and an achievable Cl of, say, 1.7, then the speed to achieve take- off becomes 10.78kt which is just about hull speed for a boat of this length. But the lift from the foil does not suddenly come in with a bang, it is progressive. Remember lift force varies as velocity squared so if the full 80,365n of lift required is provided at between 10 and 11kt then at around 5kt it will be producing about a quarter of the required lift force. Thus as speed increases the hull lifts progressively out of the water and viscous and wave drag diminish. Looking at the first four boats, two paths
have been followed. Magic and Ineos have gone for a scow hull with a rounded bow and rounded sections. By comparison, Prada and ETNZ have gone for a more conventional sharpish bow. Prada has more of a V-section and ETNZ a more multi- faceted solution – a bit like a conventional V-section with the apex cut off to form a flat bottom and then something akin to a dustbin strapped underneath as a ‘bulb’. I can follow the thinking behind the
bulge on the bottom but question its exe- cution. If it is meant to provide a little buoyancy to keep more of the boat out of the water when the wind pressure eases, it is surely the wrong shape? According to Bernoulli’s well-known theorem, when a fluid flows round a convex curved surface it speeds up and produces low pressure or suction, in this case on the bottom of the boat. Not exactly what you want if you
58 SEAHORSE
are trying to lift your boat out of the water; instead it is going to tend to suck the boat in before take-off, reducing the amount of natural lift, rather than lift it and help the foils to get the boat flying. To digress again for a moment. Early
fast powerboat designers used concave V-sections, as did flying boats, because they gave the greatest lift. The problem, however, was that they only really worked in smooth water. Once they encountered waves, because there was so much lift from these sections, they pounded very badly and thus could not maintain, in a seaway, the high speeds they were designed to achieve – or, in the case of the flying boat, what they needed to achieve for release. The solution, used by designers such as
Commander Peter Du Cane, was to incor- porate convex V-sections in the bow where the pounding occurred. Because the convex shape sped the flow around these sections and produced a lower pressure they greatly reduced the pounding prob- lem and enabled higher speeds to be main- tained in a seaway. This shape persisted until Raymond Hunt perfected the deep V-section which, because of the acuteness of the V, produced less lift, which cured the problem in a seaway but at some cost to speed in calmer waters. Moving back to our AC boats, pound-
ing should not be a problem. Pounding becomes a problem when, normally at high speed, the whole weight of the boat is stopped as it falls back into the water typi- cally from a wave. When one of these AC boats falls back into the water, the foils will still be taking most of the weight. Remember, force equals mass times acceleration. If the mass, because most of
that mass is still supported by the foils, is small the pounding force will be small. So, to supply as much hull lift as possible so that the foils don’t have to work as hard and thus improve take-off, surely a V or, better still, even a concave V-section should be used. Also, as speed rises during take-off, a V-section, besides producing more lift, progressively narrows the water- line beam as the boat rises, reducing wetted surface area and thus viscous drag and, as beam is a big driver of wave drag, reducing wave drag as well. When the boat then comes off the foils
and starts to settle in the water additional buoyancy is added progressively and the increase in drag is also progressive. By contrast a flat U-sectioned boat nar-
rows more slowly as it rises, at least to start with, and so carries more drag during most of this transitional phase and when it settles back into the water it does so with a rapid rise in buoyancy and drag and some suction. The boat that comes nearest to this, in
my opinion, ideal concept is Prada which has a concave V-section that not only pro- duces more lift but, as already mentioned, also produces a beneficial vortex to seal the gap between hull and sea and so make the rig more efficient. My only slight criticism is that it does not seem to have a chine where bottom meets side. A chine increases the lift produced by the hull by better maintaining the pressure on the hull bottom right out to the edge. By contrast, a rounded bilge tends to lose pressure as it turns into the topsides. Of course, the biggest factor affecting
speed, once on the foils, will be the design of the foils themselves. But that’s another story.
q
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