Come fly with me
Given how rarely a new International Moth design appears perhaps it is little surprise that this one has been long in the gestation… However, the Thinnair is up and flying at last, a joint effort between sailmaker Mike Lennon and world champion International 14 designer Dave Hollom
The chance to design a Moth, the Thinnair, the whole caboodle other than the rig, came as a welcome challenge. It stemmed from my work on International 14s and on glider wing sections.
The design of an International Moth can conveniently be divided into three parts: the hull, the foils and the rig. Neces- sarily each has a bearing on the rest and so each cannot be considered in complete iso- lation, but for the purposes of this article we will look at the two parts that involved myself, the foils and the hull, individually.
The foils
At the end of 2009 Seahorse published an article by me entitled ‘Enigma’ (issues 358 and 359) which described the dynamics of Moth sailing. A quick revisit will bring the reader up to speed but, to summarise, the study found and gave a physical explana- tion of what every Moth sailor knows: that it pays to sail heeled to windward and that this heel angle is critical to maximising speed. The reasons are many, but in large part it is because surface-piercing foils are
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very inefficient at producing the required hydrodynamic forces, lift or, in this case, side force. However, by heeling to the correct angle, the required hydrodynamic forces – lift and side force – can both be pro- duced on the horizontal (when upright) foil. To digress for a moment, Team New Zealand’s success in the last America’s Cup owed much to this principle. All of the syndicates used an L-shaped foil layout – in other words, the ‘dagger- board’ was approximately vertical and the lifting board was approximately hori - zontal so that side force, to resist the horizontal rig force, was produced by the vertical board, and lift force, to support the weight of the boat, was produced by the horizontal board. ETNZ used this concept early in the cycle but in the latter stages they graduated to a foil that had about half the vertical and half the hori- zontal foil cut away and the two parts were then joined by a diagonal section. This diagonal section, when at the opti- mum angle, is analogous to the dynami- cally angled lift foil of a Moth and forms
the hypotenuse of a shallow triangle, of which the cut-away parts of the horizontal and vertical boards form the opposite and adjacent sides. Because the hypotenuse is always shorter than the sum of the other two sides of a triangle and as the forces on the hypotenuse, resolved into a vertical and horizontal component, are (operating at the same lift coefficient Cl) the same as those produced by the vertical and hori- zontal parts of the same triangle, the required forces are produced by a shorter foil of less area.
If this concept is taken to its ultimate for those particular boats the wetted area of the foil is reduced by about 25 per cent. ETNZ, mainly for class-specific reasons, only used this concept for about half of the span of the vertical and horizontal boards and so saved about half that (12.5 per cent) in wetted area. Nevertheless, any competitive designer would give his eyeteeth to make such a saving with, apparently, no downside. Once foiling viscous drag, at the same Cl, is pretty well proportional to area and, allowing that the rudders and elevators
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