Though very different on initial inspection the first AC75s of American Magic (far left) and Ineos Team UK have a lot in common below the waterline with their flat scow shapes. But that is where most similarities end, with the American Magic design team taking a much more holistic approach to reducing windage with this gentle overall scow profile, while the designers of Ben Ainslie’s Britannia appear to have dealt with this fundamental issue in a rather more granular fashion. Britannia is seen with its retrofit skeg and bustle. Where the deep, rounded ‘bulges’ running along the underside of the first boats of Team New Zealand and Prada both add considerable buoyancy, the mini keelson on Britannia is an aerodynamic add-on to help close the gap between the hull and water with the potential side benefit of straightening the waterflow beneath the bow prior to lift-off
THE MEAT Induced drag force is given by the formula: Di = L2
/0.5*q*V2 *Pi*B2 will be around 180m2 but to that must *k
where: Di = induced drag force, L = lift force, q = density, V = apparent wind speed, Pi = 3.142, B = span and k = efficiency factor. We have already calculated the lift force (30,000n), air density is taken as 1.225, we will assume an apparent wind speed of 50kt or 25.73m/s and we already have the span at 26m and we will take k as 1.2. Using these figures induced drag works out at about 871 newtons. To work out the profile or viscous drag, as it alters with lift coefficient (Cl), we first need to know the (Cl) of the sails (S = area): Cl = Lift/0.5qV2
S so, at this windspeed,
Cl = 0.322. At this Cl profile drag coefficient will be around 0.008. Converting that to drag force via the formula Drag = Cd*0.5*q*V2
*S gives
746 newtons. However, we must also add the parasitic drag of the rest of the vessel swept by the air. The surface area of the hull and deck
which both viscous and induced drag are equal will be around 43kt (17kt true wind) at a rig Cl of about 0.5. Above this true windspeed it will pay to reduce viscous or base drag, below that windspeed it will pay to reduce induced drag. The latest rules for the 2021 Match
allow racing between 6.5 and 23kt TWS (current adjusted) and 17kt TWS sits a little above midway between those extremes, which creates a dilemma. Do teams con- centrate on reducing induced or base drag? As already mentioned, just about every-
thing that reduces induced drag increases base or viscous drag and vice versa so there is a decision to be made. That decision only became a little easier when the wind limits for the Match were published in February. You can perhaps see why Team New Zealand and the COR did not rush to disclose that information – leaving their competitors having to wait before making vital design decisions. Nasty business the America’s Cup but all’s fair in love and war and the America’s Cup is war.
be added the area of the two arms and one lift foil which will be out of the water so we can add another 22m2
of surface area making a total of, say, 202m2. We
can also safely assume, due to cockpit cut-outs, fittings, rigging and crew and so on, that it will have a higher drag coefficient than the relatively smooth sails so if we guess it at a little more than double that of the sails we might use a figure of 0.020. However, profile drag of a wing or, in this case, a sail is worked out on the planform area of the wing or sail, which is roughly half the total surface area of the wing or sail. The area for the hull etc is the actual
physical area so we must halve the drag coefficient to 0.010 which would give a drag force of 820 newtons. Adding this to the profile drag of the sails gives 1,566 newtons of viscous or base drag, which is almost twice as much as the induced drag so that at an apparent wind speed of 50kt (true wind 20kt) the most important drag to reduce is viscous.
Bearing all this in mind we can perhaps
see in their boats the thinking of the design teams. With a complex deck shape, which is in essence a complicated end plate to an end plate, intended to increase the effective span and thus the k number the British team are concentrating on reducing induced drag. Likewise, Prada with their spine running
down the centreline – designed to produce a vortex running the length of the boat which, as these boats run close to the water surface, tends to seal the gap between hull bottom and water surface preventing the flow of air from the high to the low pressure side and thus increasing the effective span. This appears to be a more elegant solution than the retrofit spine of Team Ineos. I am a great believer in the principle that
if it looks right it will usually perform right. True, when some new shapes and innovations appear they might seem a little strange but, with some thought, you will usually see that there is purpose in their shape and if you can see that purpose and imagine the thought process of the
designer it will start to look right even to the non-technical observer. However, no matter how much I look at
the first British boat it doesn’t look right. I can quite imagine Bob Fisher, with his great knowledge of boats over a very long period of time, looking at what was before him and slowly scratching his chin. It looks clumsy and I really cannot see that those ‘walls’ will act as efficient end plates. The deck is an endplate in its own right.
What this boat is doing is adding an end- plate to an endplate and every time you do that the gain diminishes. It just seems to me that the swept area is going to increase viscous drag by far more than any reduc- tion in induced drag. By comparison, Prada’s solution to this
problem is much more elegant. It looks right and, while it will increase viscous drag a little, that increase will be a lot less than the British solution. However, I am very patriotic so I do hope I am wrong. Britannia is also Ineos’s boat 1 so it may
simply be a brave stab into left field. Britannia 2will be particularly interesting. Moving onto the other two boats, Team
NZ and American Magic both seem to make no attempt to reduce induced drag other than the obvious one of sealing the gap between sail and deck which costs nothing in viscous drag, so perhaps they expect the wind, over the course of the competition, to be above that break- even point. Alternatively, with all their resources they will have a much better handle than I on the relative magnitude of the two drags and perhaps they put that break-even point at a lower wind speed than I have estimated. Whatever rig design path you follow you
need a hull that will reach take-off speed with the minimum of drag and that take- off speed will determine the sort of hull you need. The equation for finding take-off speed is: V=(L/0.5qSCl)0.5
[where: V=veloc-
ity, L=lift, q=density, S=lift foil area and Cl is of course lift coefficient]. Lift in this case is lift normal to the lift
foil which we have already decided will be angled at about 22° so as to produce a vertical lift force of 74,556n and a side force of 30,000n. These forces become the
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