lightweight structure for trimarans and pod cats

Discussion in 'Boat Design' started by david ingles, Jan 26, 2015.

  1. david ingles
    Joined: Sep 2012
    Posts: 7
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    Location: AUSTRALIA

    david ingles Junior Member

    tris and cats put a lot of bending load on the crossbeams. Its possible to decompose these loads into compression (column) and tension loads by lifting the inboard end of a tri crossbeam quite high and running water stays to the outer end from low on the hull. The cross beams then become simple tubular sections which can be relatively light.
    The idea is illustrated in the concept model shown. The lateral loads are absorbed by the X-stays (double diagonal) at deck level. Rig stresses are minimised by making the height difference between the beams and the water stays as large as possible; for this reason this approach works particularly well with a centre-pod cat.
    Because the beams are fixed by stays there are relatively light stresses at the inboard and outboard ends so that the fixing can be light, the main load being compression on the joint. So the concept has applications for folding (towable) tris. In testing this i used a flat plate with four high corners; any rigid rectangular shape would suffice and this can be thought of as being the central hull for a tri or the pod for a cat. My model pictured uses a central plate with a double-bottom; the horizontal tubes (which replace the water stays) slide in and out of the double bottom and are then fixed in place by the angled tubes which can have a spinnaker pole -type end fitting. The resultant structure was quite strong and rigid (see photos).
    I then thought we could make the towable structure even wider by having folding lower beams allied with click-on upper beams. Again the resulting structure was quite rigid. However the folding process becomes more difficult.
    My current thinking is to stick to the original concept by having a single strong beam at each hull point, with folding to be achieved by having the beams slide in upward-sloping short tubes at each of the hull attachment points.

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