Lower center of effort allows larger sail area.
Smaller dual masts are easier to raise and lower than a single larger mast.
Compression loads are removed from the main beam and no forebeam is needed.
Wishbones reduce sheet loads and allow reversal or flow over sails.
A problem with the biplane rig is that some configurations can cause one of the sails to be blanketed on reaches, Millennium Skeeter uses a wishbone rig with rotating mast to allow the leeward sail to be reversed so that it can get into clear air on reaches, see diagram
Aluminium is a very consistant engineering material with excellent mechanical properties and a long service life. It is an economical material which costs about $1.50 per pound. Its is easily anodized in a variety of colours such as grey, gold, red, green, blue and black and such a finish holds up well in a marine environment. Aluminium is available in economical custom extrusions and can be readily stretch formed into complex shapes.
The primary structure is designed for low labour input manufacturing. Custom extrusions are used as appropriate to reduce costs and integrate function. The beam section for instance has an integrated lip to support the bridgedeck and a groove for the bimini top, see example. Similarly the beam hinges are formed from a custom extrusion to minimize fabrication costs. Similar extrusions are used for the daggerboards and trunks and in the hull joint to provide both hard points for trailering and to facilitate weldup.
The basic construction of the hulls and interior consists of joining the hulls along the centreline from just two pieces. Each hull/deck side is stretch formed in one piece from 6061T6 aluminium. The internal structural supports are welded into the half hull and the primary interior structure is installed. At this time the hullsides are then joined together with the custom joint extrusion. This completed structure is then anodized in the required colour. Windows and hatches are then installed and final interior trim in way of the welded joints is completed.
The layout drawing shows the full width and the folded configuration. The beams are pivoted at the hull juncture and at the centerline. The bridgedecks lock the beams in place in normal use. When reducing beam the bridgedecks which are hinged alongside the hull are folded up towards the hulls from the boat centerline. With the bridge deck no longer stabilizing the beams the centers of the beams are pulled towards each other until the hulls are alongside each other. Because the masts are unstayed there are not mast support issues in this configuration. The main issue with this arrangement is that the now vertical bridgedecks limit visibility and mobility around the deck and access to the interior.