b) when changing tack, a rotation has to occur so that the parts of the mast below the boom move across towards the leeward side. This enables the sail to take on the correct camber.

c) the best way to visualise this pronation/supination movement is to think of the upper part of the mast, the front middle segment, and the boom forming a single unit (labelled 1). The back middle segment, and the lower part of the mast form another unit (2). Joints at the bottom of 1 and the top of 2 allow the two units to spin around a vertical axis in relation to each other (the axis for pronation/supination). When the top of the mast and the end of the boom move away from us, the middle part of 2 moves towards us. This internal rotation of the mast is different from the rotation of the mast base when the rig is being sheeted in or out when sailing.

a) implementation in a dinghy rig:

Here is a short video showing how the pronation/supination movement was originally implemented on a Mirror dinghy. This was the earliest free-standing version of the Transition Rig, and the mast uses biology-inspired joints. It was made in 2000. More recent dinghy rigs have a simplified structure.

b) implementation in windsurfing rigs:

The first Transition Rig prototypes were made for windsurfers, and my initial assumption was that if I incorporated the necessary joints, then the correct pronation/supination rotations would occur automatically when tacking and gybing. (For a description of the joints that allow pronation and supination to occur, please click here.) I visualised that the positive air pressure on the windward side of the sail would be enough to swing the lower joints across to the leeward side. Out on the water, I soon discovered that the opposite was true - as soon as the sail powered up on a given tack, the lower joints flipped across to the windward side, that is to say, the wrong way. This gave the lower third of the sail a very poor aerofoil shape. Pushing the joints to leeward took some effort and they would soon flip back to windward. On reflection, I realised that the force for this contra-rotation was being generated by the upper part of the sail, which has approximately twice the area of the sail below the boom. The top segment of the mast was being pushed away to leeward, and this resulted in a force on the lower joints to windward. I then had to find ways of overcoming this problem. I was looking for a method that caused the least distraction for the sailor, who when windsurfing already has their hands full, quite literally - it is not good to be trying to force an unwilling rig to adopt the right form when also trying to harmonise all the other forces at work.

Here are three of the solutions I tried for windsurfing rigs:

side view, showing cross-tube behind middle
mast struts

view from behind, showing roller attached to the
back of one of the middle mast tubes.
Two interconnected locks are in the open position

the lower mast joints can swing from side to side,
with the cross-tube running on the roller

mast strut rotated to left and locked

cross-tube with locks (1999)

A curving tube is attached across the boom, passing behind the rear strut in the middle segment of the mast. A roller is attached to the rear strut and the cross-tube can move from side to side, supported on the roller, as pronation and supination occur. Two locks are fitted, one on each side of the cross-tube, and they are interlinked so that if one is opened, the other opens at the same time. They can also be closed at the same time. When sailing, the downwards pull of the sailor holding on to the boom causes that side of the boom to tilt downwards slightly and the increased pressure of the curving cross-tube on the roller forces the rear strut away from the sailor towards the other side of the boom. When it reaches the stop, the sailor can operate the lock to hold the mast in the correct configuration until the next tack or gybe.

In practice, this arrangement works quite well. It does however require manual operation of the locks by the sailor who must remove one hand from the boom to operate the lock.

There are, however, some disadvantages. The cross-tube, locks, and roller all add weight and complexity to the rig. If the boom is adjusted to allow for sailors of different heights, then the roller support must be adjusted too. The presence of the cross-tube means that there needs to be a hole in the corresponding part of the sail to allow the tube to pass through. This is not an ideal situation, because the hole will allow equalisation of pressure between the windward and leeward sides of the sail, reducing efficiency.

The next example eliminates the need for a cross-tube.

the left side lever has been rotated down and locked,
tightening the rope attached to it and rotating the
rear middle segment to the opposite side

viewed from below - the ropes and pulleys

levers and rope (1999)

In this arrangement, there is a lever attached to each side of the boom. A rope from each passes a pulley and travels through the inside of the boom, around the front of the mast and to the other side of the boom. Here it passes another pulley and is attached to the rear middle segment of the mast. When the lever is pulled down to the boom, it tensions the rope and pulls the middle segment to the other side. Because the rope is attached below the level of the lever's pivot, the lever is stable in this closed position.

When beginning a tack or gybe, both levers are in the raised position to release the rear middle segment. After completing the manoeuvre, the lever on the sailor's side is pulled down to pronate the mast to the opposite side.

This system worked, but like the previous solution requires a hole in the sail for the ropes attaching to the mast. Care also had to be taken to avoid trapping the fingers under the levers. Once again, this is not an automatic system, and therefore not ideal.

The next example produces the required movement automatically.

angled plates are attached inside the front end
of the boom

the attachment of the boom to the mast
allows the boom to tilt from side to side

the right side of the boom has tilted down,
and the plate is pushing the mast segment
to the left

the left side of the boom is tilted down,
and the mast segment is pushed to the right

angled plates attached to boom (2001)

In this arrangement, pronation and supination are carried out by tilting the boom and having angled plates push the rear middle mast strut in the right direction. Tilting of the boom is achieved by having a suitable attachment of the front of the boom to the mast by way of a universal joint, and by the sailor putting weight on the boom while sailing.

This set-up has three advantages over the preceding one - there is no need for locks operated by the sailor, there is no need for a hole in the sail, and the movements occur automatically. However, the sail wears more quickly where the angled plates press it against the mast segment during sailing. It also requires modification of a standard boom.

a rotating sleeve (green) is held in position
on the mast by two stops (above and below).
A standard boom is clamped to the sleeve
and the boom is free to rotate around the mast

the entire mast has been rotated to the right
in relation to the boom

c) compromise solution

It was always my intention to make the Transition Rig as easy to use as possible, and I was not happy with the complications being added by the need to move the lower part of the mast to leeward when changing tack, particularly when windsurfing. I decided to compromise, and experimented with rigs in which the pronation/supination rotation within the mast was eliminated, and instead the boom was able to rotate around the mast. This was achieved by fitting a composite sleeve around the mast that was free to rotate, and then clamping the boom to the sleeve.

With this arrangement, when the sail powers up on the new tack, the forces in the sail swing the top of the rig to leeward in relation to the boom, and the lower joints have to follow suit. So the correct movement becomes automatic and natural. However, a small price is paid in terms of aerodynamic efficiency since the rig no longer "cups" to windward.

This compromise approach is to be recommended if you want the simplest solution to the pronation/supination issue - it is simple in structure and simple in use. I am using this solution on my current windsurfing and free-standing rigs.

d) conclusion

To get the greatest aerodynamic benefit from the Transition Rig, it is worth going to the trouble of incorporating the pronation/supination rotation within the mast. This produces a "cupping" of the rig to windward and improves its performance - as the lower joints move to leeward, the tip of the mast moves to windward. However, this rotation adds complexity, and requires the sailor to operate an additional control - it is not automatic.

The alternative is to eliminate the pronation/supination movement within the mast, and instead allow the front of the boom to rotate in relation to the mast. Then the movement of the lower joints to leeward becomes automatic, and an additional control is not required. The cost of this simplification is a slight loss of aerodynamic efficiency. The "cupping" effect is absent as the top segment of the mast is now deflected slightly to leeward.