The Sail System

This second iteration of the sail has a symmetrical shape around central tensioning.  The boom controls the sail, acts as an end plate to stop any thrust bleeding away. It also collects the skin and battens when reefed and completes the structure of the mast.. The whole assembly swivels on the central point. Again the mast has no significant effect on sail apart from holding it up.

Battens help to give shape to the sail material which has been cut and joined to give an aero-foil shape. The design of this batten comes directly from studying the nine section tail of a dragonfly. It is to be integrated inside the sail in pockets. The semi circular shapes are interlocking hinges, held together under compression. Between each section there are angled shoulders when all are closed on one side to form an E347 profile. On reflection I thought this made the sail too clumsy to build and may well suffer from too much wear to the pockets. Start again.

Here is a schematic of a batten on the boom to give me an idea of size of structure. This is a great break-through drawing because I saw the central tension should be applied not at the centre of the area (1/2 chord) of the sail but as close to the point of aero thrust generation. As the sail moves from one side (tack) to the other the central section barely moves. At this point I envisioned radiused tracks to maintain tension on the leading and tailing edges of the sail, (Leach and Luff) I had not got very far resolving the fixtures.

The breakthrough resulted in this wide, low aspect ratio, rig with a fore /aft mast. The boom is now a jointed trellis structure. Fairings can be added to reduce windage drag. The major advantage to this being the effort from the thrust of the sail can be accurately placed for the best characteristics of the hull shape. The red, central panel represents car seat belt webbing with a breaking point of 2500 kg. Handily it comes compatible with seat belt latches which are made from high grade stainless steal.

The big drawback is it’s ugly, clumsy, difficult to work around and ugly. Start again.

The batten was simplified from nine sections to seven. This image shows on the underside the shoulders have closed and the gaps on the top side show the angles that create a simplified E347 in the tensioned webbing. The whole sail surface is moulded together with the battens.

I decided to reduce the sail area, reduce the forces, make the battens all the same length and extend the tension system to the leading and trailing edges. The sail area is relatively low compared to racing boats, but by having two sets they can be balanced around the Centre of Lateral Resistance (CLR)

The boom is jointed and takes up the shape of the sail. The sail material is bonded to the webbing and batten sections.

To reef the sail the tension is released from below, through the mast, and sail lowered. The central latch is released and the one from above attached then either end are done, one at a time. At no time is the sail  released from the boom and is always held by two connections. The tension is re-applied.

Tension is maintained in the moulding of the sail, especially to give the offset nature of the top webbings. The first is to the leading edge, the second to the centre, the third to the sail at the same angle (45 deg.) as the first, and the fourth to the trailing edge. Reducing tension very slightly from below, through the boom, on the centre web will increase the arc in the battens.