Prada’s Luna Rossa is generally acknowledged as the prettiest of all the AC75s and for once with an Italian challenger not just in the colour scheme. Every aspect of this design presents smoothly to both the wind and water and in those ‘provisional’ early skirmishes the boat looked like the best all-rounder among the challengers. Foils were larger than on the US design but with a similar tip treatment using miniscule winglets. The UK alone were still trying complex cranked foils with dramatic anhedral change midway down the span
nice smooth shape with minimum frontal and wetted area and with the minimum of lumps and bumps above and below. Looked at in that light Patriot and Luna Rossa win hands down over Britannia which is angular with a large wetted area. Also, contrary to popular belief, you
should aim for minimum lift from the hull in any direction, even up. Lift comes with a drag penalty, which varies inversely with aspect ratio. The aspect ratio of the hull is appallingly low so any lift comes with a big drag penalty. Much better to allow the foils with their far higher aspect ratio to produce that lift. They will do it with less drag. Also, any lift from the hull will reduce the effective weight of the hull and that will reduce the righting moment. Better, perhaps, to have the hull producing downforce which will increase the righting moment. Is this one reason why they seem to fly with a bow-down attitude? Of course a bow-down attitude also
reduces the amount of vertical rudder in the water and thus reduces drag, but at the expense of living a little more dangerously. First impressions of the Cup Defender’s
second boat are that her design thinking is actually quite similar to Britannia but with all the edges rounded off and she looks so much better and more workmanlike for it. She also sports a chine between bottom and topsides which readers of my last article will know I suggested would be good as it maintains the pressure on the bottom of
64 SEAHORSE
the hull right out to the edge but per- versely, on this boat, that chine becomes a slightly more rounded affair towards the stern where I would have thought a very hard chine would have been most effective. Wetted surface wins out again perhaps? The Kiwi sections are a very concave V
which again, readers of my last article will understand, gives good hydrodynamic lift, and they have got rid of that dustbin strapped to the bottom of their first boat and replaced it with a sausage. We know from the design of powerboat
bow sections that the convex V pounds in a seaway while a concave V gives a softer ride. That is because the hollow V gives more lift and the convex V less lift. By now readers will realise that, in my
opinion, humps and bumps on the bottom of boats suck. Literally. The flow round them forms suction, which is not good for speed, or take-off. The way the crew are housed in pods out of the airflow is particu- larly elegant on the New Zealand raceboat. The wide, flat stern is also good as it
means that the buttock lines (the fore and aft lines that define the shape in a longitu- dinal direction) in the aft part of the boat are very flat with the minimum of convex curvature and therefore the minimum of suction. Yes, the wetted area will be large but the slightest heel will reduce this to a minimum. The only part of the New Zealand boat I would take issue with is the shape of the
skeg. It’s rounded in all directions and as such will produce suction, which will reduce some of the buoyant lift it will give when it meets the water surface. It’s also so rounded that to produce the beneficial vortex it has to have a sharp strake or vortex generator down the centreline. Under the rules of this competition the
teams are not allowed to use tanks or wind tunnels and so must rely on CFD (Compu- tational Fluid Dynamics) to test ideas about shape etc. CFD relies on solving the Navier- Stokes equations in order to model the fluid flow. The trouble is that these equations are so complex and lengthy that they are almost impossible to solve completely even with the world’s largest computer. To come to a solution within an acceptable time requires that some of those equations will not be solved and assumptions will be substituted for them. The remaining equations can then be solved and a useful answer delivered in a sensible timeframe. However, some of the answers are more
dependant upon those assumptions than others and so, in some cases, because those assumptions are not always totally correct odd things can happen. For instance, British Aerospace were once designing a tracked carriage to carry the Rapier guided missile and it incorporated a dome on top of the vehicle. CFD predicted that it would produce a downforce whereas Bernoulli and commonsense would predict that it produced lift which, in actuality, it did.
RICHARD GLADWELL
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88 |
Page 89 |
Page 90 |
Page 91 |
Page 92 |
Page 93 |
Page 94 |
Page 95 |
Page 96 |
Page 97 |
Page 98 |
Page 99 |
Page 100 |
Page 101 |
Page 102 |
Page 103 |
Page 104 |
Page 105 |
Page 106 |
Page 107 |
Page 108 |
Page 109 |
Page 110
