Chapter 15 The mechanical arrangement of sailing rigs
We now have enough information to form a view about the design of sails. However there are some design constraints which come from the mechanics of the rigs in which the sails must work. There are two different sailing rigs for model yachts. The basic rig is the Bermuda rig with the fore sail attached to a boom that swings on a swivel attached to the deck. This rig is often adapted for use in A boats by the use of a radial jib fitting instead of the swivel. Then there is the swing rig which is really the basic rig fitted to a false deck that swings with the mast. In each case we can regard the rig as having two elements. They are the standing rigging, comprising the mast and booms and associated wire and cord, and the sails. The design of each is affected by the design of the other but they can only be considered separately.
The standing rigging used for the Bermuda rig
Figure 15-1 shows the commonly used standing rigging. The principal item is the mast. This may be stepped on the deck or mounted through the deck. A mast must be able to withstand sailing loads and must be light. Masts made from carbon fibre composites are strong enough and light enough for this purpose although they are not permitted in some classes. They could be used without other support but this is not likely because the mast will bend under the sail forces and, as the shape of the mast may be used to control the shape of the sails, most skippers will opt to retain the facility to control the shape of the sails.
So the mast is usually supported. This support turns out to be rather more than just holding the mast up so that sails can be attached to it. For a Bermuda rig the mast should be straight when viewed from the stern because the yacht has to sail on both tacks. However the bending of the mast fore and aft permits the changing of the camber of the main sail if this sail is suitably designed. Let me deal first with the sideways support and then with the fore and aft support.
The sideways support is provided by the shrouds and, where fitted, the spreaders. The function of the shrouds and spreaders is to restrict the sideways bending of the mast in an attempt to keep the mast straight. The mast, the shrouds, the spreaders and the deck form a structure but the fact that the mast can resist bending forces means that it is not a simple structure and therefore not amenable to routine calculation. (See Chapter 3 page 8).
The builders of model yachts use the arrangements shown on Figure 15-2a, b and c. The arrangement in Figure 15‑2a is the most simple and, if the two shrouds are suitably tensioned, the upper point of attachment of the shrouds is quite well fixed laterally. However it should be recognised that when the shrouds are subjected to a wind load from the sails a force that can be quite large is added to them. Most yachts are rigged with the shrouds lightly tensioned and this means that the added load is applied to the windward shroud and the leeward shroud goes slack. This means that a force that is nearly equal to the force on the shroud is exerted on the mast by the shroud and is exerted on the bottom of the mast tube and hence on the hull. The force is then transferred to the deck beam and the shroud fixing by the internal structure of the yacht. It is likely that the shrouds supporting the mast of a tall suit for a Marblehead would be attached at the upper measuring point of 68² above the deck. The deck fixings for the shrouds would be offset from the centre line by between 3² and 6² and, if we suppose that the sails exert a transverse force of say 1 pound at the upper point we can calculate the force in the shroud for any offset. The force in pounds = 68/O where O is the offset in inches. The result is given in Graph 15-3. Clearly large forces can be exerted on the shroud but they are not unmanageable. However for small offsets the extension of the shroud and the movement in the fixings coupled with the increasingly troublesome geometry means that the single shroud becomes less satisfactory.
Arrangement, b, is probably the most used. The obvious change is the use of spreaders. Each spreader is attached to the mast and, in a model, the shroud is either fixed to the outer end to make a frame or the shroud runs through a hole in the end of the spreader. One might ask whether it is better to let the shroud slide or to fix it at the spreader. This involves deciding what difference it makes. Either way the force in the shroud is reduced by the increased angle at the top. If the shroud is free to move in the spreader the spreader has to resist a bending load. If the shrouds are fixed to the top of the mast, the spreaders and the shrouds become a frame and this is quite rigid. However, if we accept that the primary reason for the use of these wires and struts is to prevent the undue bending of the mast so that we can control the shape of the sails, then all we really need is for the mast to remain reasonably straight under the sail forces. Masts on models are, generally, quite stiff and, if the obvious bending that would occur on an unsupported mast can be eliminated, the fact that there is a small “S” bend in the mast is not serious. The rig with shrouds that are free to slide in the spreaders produce a different “S” bend to those that are fixed.
In arrangement, c, the extra shrouds and the creation of a diamond with the extra wires complete the triangles but the mast is a member which can resist bending so it is not a simple frame. Nevertheless these extra shrouds do provide considerable extra stiffening if this proves to be necessary.
Now we must look at the way in which the mast is supported fore and aft. The standing rigging without sails is shown in Figure 15-4. The fore sail would be mounted in a triangle formed by the fore sail boom, the fore stay running from the luff end of the boom and up to an eye fixed to the mast, and the leech line which runs from the leech end of the boom to the same eye on the mast. Both lines can be adjusted for length. The fore sail boom is attached to the deck through a swivel of some sort and this swivel is much nearer to the luff than the leech. The mast is fitted with a mast-head crane to which the back stay is attached. The other end of the back-stay is attached to the aft end of the hull and a device to tension the back stay is fitted. The use of a crane makes space for a main sail with a battened roach that would otherwise foul the back-stay and increases the purchase on the top part of the mast.
In the diagram the mast is shown in a mast socket which is the most common arrangement. Clearly, if the fore stay is slack and the mast socket is properly fixed, the tightening of the back stay will bend the mast backwards, as shown in Picture 15-5a, and apply a bending load to the aft end of the hull.
In Chapter 12 we looked at the arrangement of the fore sail as in Diagram 12-5. In effect the two forces exerted through the fore stay and the leech line act at the eye on the mast to pull the mast forwards if the back-stay is slack. Tightening the adjuster on the fore stay and perhaps the adjuster on the leech line will transfer the force acting at the swivel to the mast and bend it as shown in Picture 15-5b. Then the forward section of the hull is under a bending load.
If now the back stay is tightened the top of the mast will bend backwards to put an “S” bend in the mast and put all the hull in bending and at the same time unload the mast socket. (See Picture 15-5c. Sail makers use this facility to shape the mast to let sailors adjust the set of the sails they make. Evidently the fore stay and the back stay can be used for more than just supporting the mast.
It seems to be unlikely that sails would be designed to work with a mast with an “S” bend in it. Yet, if the requirement is for the maximum practical tension in the fore stay, an “S” bend is the likely outcome. There is a compromise to be made here between having a light and flexible mast which will bend and a heavier more robust mast which will resist the bending. As most people buy sails for their yachts and these sails are not matched to the masts the usual decision is to use a stiff mast.
Now we can turn our attention to the arrangements used to attach the sails to the rig.
Diagram 15-6 shows the common arrangement of the main sail. The luff of the sail is attached to the mast using mast rings (or perhaps a bolt rope in a groove in the mast). The luff is located and kept free from wrinkles by the uphaul and downhaul. The clew is attached to the free end of the boom with an outhaul adjuster. The boom is free to swing from side to side on the goose neck and the up and down movement of the boom is controlled by the kicking strap. These very simple adjustments with, of course, mast shaping, are all we have to control the set of the sail once it has been designed and made.
Now we can consider the mounting of the fore sail. We can see where it has to go in Diagram 15-7 that shows the sail mounted. It is an obvious arrangement to mount the luff of the jib on the fore stay, use an uphaul and a downhaul to keep the luff of the sail free from wrinkles, and to make arrangements to attach the clew to the boom with an out haul adjuster. The fore sail cannot be adjusted in the same way as the main sail because we have no control over the shape of the fore stay (except for its tension of course). So we have to set it to the design shape and then make what alterations we can to the camber using the out haul and to the twist by adjusting the length of the leech line. Add to this the inherent tension problem caused by the use of a swivel and it is clear that the design of this, the more important of the two sails, is not going to be easy
Where class rules permit the inherent weaknesses of the above arrangement for the fore sail can be avoided by the use of a radial jib fitting. Such a fitting is shown in 15-8. The boom swings on a substantial ball bearing and its angle can be adjusted using the knurled screw on the left. This gives a much better control of the fore sail leech than can be obtained with a leech line and swivel. The axis of the bearing can be altered to either increase or decrease the twist in the sail as the boom is sheeted out. The fore stay can be mounted on the deck forwards of the fitting so that the camber increases as the boom swings out. The radial jib fitting gives much more scope for sail design than the simple swivel and less is left to chance. In addition we lose the unwanted drag of the leech line.
The swing rig as it is used is shown in Figure 15-9. Most of the parts are the same as they were for a conventional rig. The main change is the introduction of a mast block to which are attached a short boom forwards and a new curved main boom aft. (The curve is to clear the water when heeled.) Both of these have good fits in the mast block so that the mast can be given a considerable bend by tightening the back stay and the forward boom can stand the force exerted on the fore sail swivel by the fore stay and the leech line. The other major change is the use of adjustable outhaul on the main sail and an adjustment at the clew to control the twist, which, for an ordinary rig, would be set using the kicking strap.
There are of course no shrouds and it is probable that the swing rig has evolved as a direct result of the availability of suitable carbon fibre tubing that is strong enough and light enough to use without support. It is hard to see a reason for the considerable bend in the mast especially as there is another version of the swing rig that has no back-stay. It means that the luff of the main sail must be cut to suit the bent mast.
With these mechanical constraints we can look at the design of sails.
 One designer I know uses a very light and flexible mast and accepts the resulting S bend. I have been unable to find out whether he cuts his sails to suit.