Left: Mike Lennon sails the Thinnair design displaying the veed hull sections forward flowing back into the more curved aft shape. The approach follows the theme used in Hollom’s successful International 14 and National 12 dinghy designs. Rather than treat the board/foil connection as an engineering challenge in pursuit of minimal frontal area the Thinnair ‘bulbs’ (above) were approached from the start as performance enhancers. Also note the dihedral on the rudder foil in the background. Having started out (right) with curved wings it proved impossible to maintain a smooth trampoline – the wrinkles looking ‘unbearably draggy!’ No aero stone left unturned… new twin-skin tramps (left,with airbag inside) hang off an elliptical front beam and an aft beam with a pintail trailing edge


remain the same area, it probably resulted in a saving of around 5-6 per cent in total viscous drag. That is a very nice margin, thank you, and so it turned out to be. So simple and yet only Team NZ committed fully to this configuration. Strange? Even stranger if you consider that the America’s Cup is won, more often than not, by a technical innovation so that the best chance of winning the event is by innovating. And yet, in the last two edi- tions of the Cup, it has only been New Zealand who have come up with the goods. Perhaps the sailors need to remem- ber that the Cup is as much if not more a technical competition as it is a sailing com- petition and it will require more than their undoubted skills to win; thus priority should surely be given to setting up a team structure that encourages free thinking? It is perhaps ironic that the order implicit in a strong management structure is the enemy of invention. Innovation requires lateral thinking and order implies vertical thinking. Perhaps what is required is a department tasked with nothing else but the generation of ideas, with a man- agement system loose enough not to stifle free thinking but strong enough to ensure the objectives are met. Critically, though, the whole organisation needs to embrace a risk-taking culture, because without risk there is no gain. That is how Team New Zealand won the 2017 America’s Cup. ‘It is better to be had for a sheep than a lamb’. ‘Better to have tried and failed than not to have tried at all’. ‘Fortune favours the bold’. ‘Who dares wins’. There are many more but all implore us to take risks if we want to succeed. And remember, ‘If you don’t lead you follow and if you fol- low you can’t win.’


Reverting to our Moth, if all the lift is produced on the angled horizontal foil this leaves the vertical foil as no more than a pylon on which to attach the horizontal


foil, and it has no need to produce any side force. Now the only design aim for the ver- tical foil is to reduce drag to a minimum. A surface-piercing foil is subject to basically two drags, viscous and wave, and here, as in most design situations, there is a compromise. Logically, for minimum wave drag a large chord and a small thick- ness/chord ratio (T/C) are required. A large chord means a longer effective water- line length and thus a lower Froude number, and generally long waterlines mean lower wave drag. Likewise, small T/C ratios mean slimness and slim bodies have less wave drag than fat bodies. However, a large chord means a larger wetted area and viscous drag is greatly governed by wetted area which gives rise to the compromise. Without tunnel or tank, for us the decision has to be, to an extent, intuitive. There are, however, other factors. To produce minimum wave drag on a foil not required to produce a lift force requires a section that has the smoothest velocity (pressure) distribution with the minimum of humps or peaks at zero lift and a rea- sonably high prismatic. To produce mini- mum viscous drag requires a section that produces the maximum laminar flow at the correct range of Reynolds numbers (Res) without undue separation at zero lift. Fortunately, at the relevant Res both requirements are, to an extent, comple- mentary so that there is not too much of a


‘Oh! I have slipped the surly bonds of Earth And danced the skies on laughter-silvered wings’


—Pilot Officer John Gillespie Magee, 1922-1941*


compromise on designing a section to achieve both aims. In the circumstances we managed to design a section that pro- duced, theoretically, 25 per cent less vis- cous drag than other sections currently in use, and so we felt safe to have a some- what broader chord to reduce wave drag. Bearing in mind that, for strength con- siderations there has to be a minimum thickness, by making the chord greater the T/C reduces for that minimum thickness, making a slimmer foil, which, as men- tioned, again reduces wave drag. The same section is used for both the vertical foil and the rudder but the T/C ratio varies. The design philosophy behind the hori- zontal lifting foil is a little different from current thinking and is a result of collabo- ration between myself and Denis Oglesby, a longtime collaborator on glider and other foil designs who was the co-author of a series of Seahorse articles in 2004 about foil design entitled ‘Go with the flow’ (issues 288-292).


Essentially, flapped foils, where the flap is used to change the lift coefficient to achieve a near-constant ride height, only operate with the flap in neutral on rare occasions. Most of the time the flap is to some extent up or down. It seemed logical, therefore, that the section should be designed to be at its most efficient with the flap at some angle, either up or down, other than neutral.


The same logic applies to competition gliders, which are either thermalling, to gain height (flying very slowly at minimum sink at high Cls so with some down flap), or penetrating (flying fast to the next thermal with minimum height loss and thus with neutral or, depending on wind direction, up flap). We designed, with success, a number of such glider sections and decided to use the same logic to design a Moth lift foil. The resulting profile was no more draggy than existing foil sections with


SEAHORSE 51


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