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FOOTY Class R/C Sailboat
Moonshadow is a narrow beam, lighter displacement derivative of Dingo, entered in last year’s
competition.
In principle, she is an extremely simple boat. It must, however admitted that achieving the target weights of 335 g displacement and 215 g ballast requires extremely precise building. These figures give a ballast ratio more or less the same as an IOM.
The hull form is virtually that of Akela with a rather less heavily snubbed bow and different treatment of the transom. The two boats share our current view of Footy hulls with deeply immersed transoms giving very high prismatic coefficients and centres of flotation and buoyancy well aft. The result is a high ‘hull speed’ hull and a very ‘long’ bow, which helps minimise submarining and has good wave penetration – improved by snubbing what is actually quite a fine bow.
The fin profile and section are derived from those of model sailplanes. The sections are not conventional NACA. They are relatively thick for reasons that should be obvious.
The rig is a ‘conventional’ McCormack rig with a 3mm leading edge spar, a 1.6 mm S/S torsion bar and a 1 mm carbon rod boom. Experience is beginning to suggest that the boat would stand a stiffer torsion bar.
Construction is basically very simple. The hull was built on a male mould without a transom. It is
very fine glass cloth with epoxide resin and is constructed integrally with the side decks. The bridge
deck for the mast and the transom are both balsa with limited carbon tow reinforcement. Steering is
by pushrod and the sheets have a 2:1 purchase.
The rudder is epoxide saturated balsa with a carbon stock, while the fin is epoxide saturated western red cedar. Many builders of model yachts seem to build structures out of wood and then put on a ‘protective layer’ or some sort of resin. Although in individual cases the practical result may be very much the same, the intellectual approach here is very different. We are building what is essentially a true sandwich structure where the (strong) resin impregnation of the outer layer provides the real strength and the unimpregnated core acts as the distance piece.
However you intellectualise it, the fin is incredibly stiff without any use of carbon or anything else exotic.
The bulb is based on ‘slender body theory’. This is the thinking used for commercial aircraft
fuselages, wing-tip and weapons pods on many military aircraft, etc. Low lift and low drag are
preserved over a wide range of angles of attack by a body with a long cylindrical central section and
relatively short ‘pointy’ ends. This has a number of advantages. First, since it is very difficult to say
at any given moment what direction the lift of the bulb is acting in (for good or bad), it would seem
a good idea to eliminate as much bulb lift (and drag) as possible and leave the job of preventing
leeway to the fin. Second, the shape of the ends is moderately critical, but the proportions are not. It
is therefore possible to set up to manufacture such bulbs to a wide range of exact weights from three
standard lead castings – a standard nose, a standard tail and a variable length centre section.
