Far left: revealed in all its glory… Katie Nurton’s Departure, upon which she won the 2016 POW Cup racing with Nigel Ash. Note those veed forward sections… again. Left: Hollom’s Hijack National 12 design which with its even deeper V-sections forward but more curved aft shape takes account of the much higher proportion of displacement sailing in this non-trapeze and non-spinnaker class. The current National 12 champion is a Hijack design
Of course in the real world thickness is more often than not determined by strength and stiffness requirements, so the only way of reducing T/C ratio while retaining these is to have a longer junction. As interference drag varies linearly with the length of the junction, and also as the square of the T/C ratio, the longer the junction for a given thickness the lower the junction drag. Thus the longer chords selected for the vertical foils, besides reduc- ing wave drag, also reduce interference drag. The bulb that joins the centreboard to the main lifting foil also helps in this respect. Essentially, it is speeding the flow up before the flow reaches the foils so that, when the flow reaches the foils and is about to accelerate around them, the flow around the bulb is slowing down. This has a moderating effect on any flow accelera- tion around the foils.
This is handled differently on the rudder. Because of the broader chord on the vertical foil, adopted to reduce wave drag, we had enough length to be able to attach the horizontal tail foil aft of the maximum thickness of the rudder so that the flow around the tail foil only starts accelerating when the flow around the rudder is start- ing to decelerate. In that way maximum velocities in the junction are reduced and the adverse pressure gradient is less steep.
The hull
The Thinnair hull is essentially a piece of flotation material upon which to mount the foils and rig. It is also an office for the helmsman. That said, to expedite take-off it must have low drag in the range of speeds up to and including take-off speed (about 8kt) and, once airborne, it must have as low an aerodynamic drag as possible. It must also allow as soft a touchdown as possible with a minimum increase in drag for the occasions when speed is not enough to keep the craft permanently foil-borne. Preferably it must also have the highest resistance possible to nosediving. Finally, and perhaps as importantly as any other requirement, it must be a workable office. Most of the lift prior to take-off will be provided by the foils, but any hull lift can only help and any negative lift or suction would, of course, be counterproductive. My thoughts on hull shape are well docu-
mented in my 2010 article ‘Departure’ (issue 364). But, to briefly recap, contrary to popular opinion, V-sections plane better than U-sections because they produce more lift; when combined with a U-section bow a high-prismatic hull is produced that never- theless has fine waterlines and a narrow waterline beam with low wetted area and low wave drag. Just what you want to promote early take-off. Additionally, any convex hull curvature in the direction of the local flow will produce low pressure and thus suction, tending to pull the boat back into the water, which is the last thing you want if you wish to escape the watery bonds. The hullform is very similar to that of my International 14 but is narrower and pro- portionally deeper so that the sectional shape is very much deep-V. This shape, according to both my logic and my VPP, gives very low drag and as the boat rises the waterline quickly becomes narrower. As waterline beam is one of the main drivers of wave drag, this drag is minimised just when it is becoming dominant. Addi- tionally, when the reverse happens and the boat sinks back down to the water, rather than the sudden increase in buoyancy and drag that occurs with the more traditional flat-bottomed or U-section Moths, the buoy- ancy on this boat will increase in a more gradual manner. So the progressive increase in buoyancy provided by this hull shape will not only result in a smaller, more gradual increase in drag as the boat settles down, but on occasion only a small increase in buoy- ancy may be necessary to keep the boat pretty much on its foils – and this will be achieved with only a minor increase in drag. In my opinion, one of the most ridicu- lous concepts to be foisted on sailors is that of the so-called wave-piercing bow. Its supporters claim that it reduces any increase in drag when the bow depresses and is thus less likely to cause a nosedive. However, I would argue the reverse, that drag with such a bow rises more as it is depressed and that a nosedive is more likely than with a conventional bow and that in reality a bow overhang, the reverse of a wave-piercing bow, is better at pre- venting or minimising nosedive behaviour. A nosedive is caused by the nose-down pitching moment generated by the driving force being some distance above the boat
and the equal and opposite drag force being below the boat. As these forces are equal and opposite, to remain in equilib- rium, the reduction of one will result in the reduction of the other, and vice versa. As we are trying to sail as fast as we can we do not want to reduce the driving force so we must reduce, if possible, the drag force or, at the very least, reduce the rate at which it rises. On a wave-piercing bow the waterline gets shorter the more the bow is depressed and shorter waterlines increase drag rather than decreasing it, so drag rises more than with a more conventional vertical stem. Also, the primary restoring moment is provided by the increase in buoyancy as the bow depresses and a wave-piercing bow increases the buoyancy far more slowly and by a smaller amount than a conventional bow with more reserve buoy- ancy. And even more slowly than a bow with an overhang.
Additionally, in the Moth and probably most other boats, a reverse bow puts the forestay closer to the mast which increases mast compression; now the hull either has to be stronger and thus heavier or it bends more. Also the bowsprit, upon which the wand is mounted, has to be longer which adds more weight. For all these reasons we decided to stick to the plumb bow. The wing racks that support the Thin- nair trampoline have a higher dihedral angle than is normal at the moment. The effect of this is to increase the helmsman’s contribution to righting moment with heel. Once airborne the most important requirement is that the hull has minimum aerodynamic drag. The rules allow a fair- ing for the gantry that supports the rudder and as this tidies up the flow in this area the opportunity was taken, and one was designed by Cam Stewart and Mike Lennon, as was the neat and low turtle deck that lowers the rig and thus reduces heeling moment. They also designed a sexy-looking detachable fairing that joins the turtle deck to the bottom of the cockpit with a slot through which the control lines pass. The end result is an aerodynamically clean hull… but when all is said and done the helmsman represents by far the great- est aerodynamic drag. However, every little bit really does help.
* From a poem written by Pilot Officer John Gillespie Magee, an American air- man who joined the Royal Air Force at the outbreak of the Second World War. He was killed descending through cloud when his Spitfire collided with another aircraft. Dave Hollom, West Yorkshire
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