The concept
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When a sailing rig is in use, considerable forces can be generated throughout the rig by the energy in the wind, and the forces can change rapidly in degree and direction. Therefore, if we are contemplating a jointed mast such as the Transition Rig, we have to be sure that the joints can withstand the forces being applied. Also, because the rig will be extending and folding, as well as rotating about its vertical axis when in use, we will not be able to attach stays or shrouds to support the mast - it has to be free-standing. (The windsurfing version is a little different in this regard, since the sailor provides support to the rig by holding on to the boom.) At the same time, the mast cannot be unduly heavy, since additional weight higher up would upset the stability of the craft. Therefore, there are a number of design issues to keep in mind while developing the Transition Rig mast. Here I shall review my experience with different types of joints.


A) Biology-inspired joints


When I began making jointed masts, I modelled the joints on biological joints. I used deep concavo-convex articular surfaces for the hinge joints because I knew from my experience as an anatomist that these were very stable and could resist twisting forces well. Straps made of aluminium or thin stainless steel were used to hold the joints together, rather like the collateral ligaments of a biological joint. To ensure that the joints were not too heavy, I made them from hollow composite materials such as glass or carbon reinforced epoxy resin.

Making the joints was very time consuming. First the exact shapes of the components had to be modelled in wood, and then moulds were made in which the composite components could be formed. (Notes on mould-making here.) It took me several months to create a new mast using these methods. If I changed any variables in the mast design as a result of testing I then had to redesign the joints and make new moulds.

As time went by, I came to the conclusion that I needed a more rapid way of making masts and joints so that I could try out ideas quickly and speed up development of the Transition Rig. I had found that very small changes in the geometry of the mast, for example the positioning of the joint pivots, could radically affect the way it worked. I made the conscious decision to simplify the design as much as possible, and began making the joints in a different way. This is summarised in the next section.

B) Simplified joints


I began to make joints from thin aluminium plate wrapped around the mast tubes and pop-riveted in place. In this way, I could quickly change the geometry of the mast by drilling the pivot holes in different places, and a new mast could be made in hours rather than months. The pivots were made from M6 stainless bolts and Nyloc nuts.

To simplify the bending of the joint plates I made formers to suit the different tube diameters I was using. First, the plate is cut to size and drilled - the image below shows an upper  joint plate for the 6 sq m rig:

The flat plate is clamped in position on top of the former and then hand pressure applied to each side of the plate to begin the bend. When the sides have dropped below about 45 degrees from the horizontal the whole unit can be placed upside-down in a vice to continue the bending in a controlled way until the cheeks of the joint plate are parallel. The plate can then be attached to the mast tube with pop-rivets.

The simplified aluminium joints work well and are very practical. I have to admit, though, that I like the look of the biology-inspired joints, and one day, when the design of the mast has been optimised, I would like to make myself a mast using them for purely aesthetic reasons.

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