a breeze. There is also a slight leaning towards the lighter boats being a little faster but there is not that much in it. An insight into which way to go can
also be had by looking at what has hap- pened in similar classes. The trend in the A Class, which uses the same formula, has been for boats to become lighter, even to the extent of going into the penalty area of displacement – that results in sail area being lost at a greater rate with reductions of displacement than it is within the no penalty area. I believe also that the overall trend in IOR was for the boats to become ever lighter. Perhaps there is a clue there?
The concepts Four concepts appear to fit our rule: (1) An overhang at each end. (2) A plumb or almost plumb bow with a stern overhang. (3) A double-cranked short bow overhang plus a stern overhang, as pioneered by Laurie Davidson with his America’s Cup ACC design, NZL-60. (4) A plumb bow and a plumb transom with no overhangs at either end.
In conversation with the late Britton
Chance many years ago, he maintained that there was nothing to choose between concepts one and two and that boats to both concepts had won championships. Theoretically, the plumb bow has the
advantage that the static waterline length is greater than with a bow overhang, thus for any given displacement and prismatic the frontal area will be less. If the frontal area is less the water has to
accelerate less to pass the hull. Lower accel- erations mean lower drops in pressure and, bearing in mind that the wave system around a boat is mainly a consequence of the pressure distribution around that boat, lower drops in pressure mean a lower and less draggy wave system. Also, as viscous drag is approximately V2
dependent, lower
local velocities mean lower viscous drag. 54 SEAHORSE
Volume coefficient (Volume*1000/Length3 . Gener-
Technically, such a boat has a lower )
or, in old-fashioned terms, a lower Displ/ Length ratio (Displ/(0.01*Length)3
ally speaking boats with lower volume coef- ficients have less drag for the reasons given. The argument for concept one is that
once moving the static waterline extends, but of course the frontal area is still greater than for concept two. The old America’s Cup (ACC) Rule that
followed on from the 12 Metre Rule, although using a different formula to trade length, sail area and displacement used a very similar means to determine length, ie it was measured a certain distance above the waterline and combined with girths, so that similar concepts were possible under that rule as well. Concepts one and two were tried but
concept three, the Davidson cranked bow overhang, finally dominated. It is, in essence, a cross between concepts one and two. For a given measured length it has a longer waterline than concept one but a shorter waterline than concept two. Concept four gives the greatest water-
line length for any given measured length and thus the lowest volume coefficient and frontal area for any given displacement and length. It does, however, not increase sailing length with speed, as does a boat with overhangs.
The shape Throughout the history of yacht design sectional shapes have swung back and forth between V and U-sections. Some- times this has been influenced by the rating rule in operation at the time. For instance, at the turn of the century
the Dixon Kemp Length and Sail Area Rule favoured flat scow sections, because, as only waterline length was measured they better extended sailing length beyond the waterline endings with heel. Mean- while, the International Rule favoured deep V-sections.
However, there have been many classes
where there would appear to be no strong reasons, dictated by a rating rule, that would favour either type of section and yet, at various intervals, sections have switched from U to V and back again. Uffa Fox in his book Sailing Boats tells
the story of the early development of the International 14 dinghy. Before 1927 all boats in the class were designed with U-sections. However, in 1927 Uffa designed and built Avenger with V-sections – in her first season, 1928, out of 57 starts she won 52 times, came second twice and third three times. A convincing display of supe- riority and not entirely due to Uffa’s skill as a sailor for he had previously sailed his own U-section designs with nothing like the same success. Predictably, from then on all dinghies of
a similar type were of V-section until, in about the mid-1970s, a U-sectioned devel- opment class design again won convinc- ingly (it was, I believe, the National 12 March Hare, designed by Mike Jackson). From then until the present day U-sections have remained supreme but appear to be, as we speak, in the process of change. By the way, the story is much the same
in both model racing yacht design and full- sized keelboat design – with one or two exceptions, most model and full-sized keel- boats are now of U or similar section. Proponents of the U-section argue that
such a shape gives less wetted surface area, for a given volume, and thus viscous drag will be reduced. However, U-sections only definitely minimise wetted surface area if the beam/depth ratio (B/T) is the same for both the U and the V-section and this may or may not be true when stability is taken into account. Also, wave drag might be different for the two types of section so that it might not be all that obvious which is the favoured section type. Froude, in his famous experiments,
discovered that to minimise wave drag, volume should be placed as far away from
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