Chapter 2        The model monohull racing yacht

 

Yachts are designed with a specific task in mind. A yacht designed to sail across the Atlantic in the shortest time will not carry any comforts for the crew if these in any way limit the speed of the yacht. By comparison a yacht designed for holiday cruising will place creature comfort high up on the list of requirements and speed will not be important. Study of these designs may be interesting and it may be profitable to try to follow the designer’s reasoning. Some yachts will be designed to conform to the rules of some class of yacht so that it can be raced. History shows that such rules have often led to yachts that are “distorted” by the rules. A study of the history of class rules, such as that given in the Shell Encyclopaedia of Sailing, will show how difficult it is to decide whether a feature of a sailing yacht was designed to increase the speed or to get round a constraint of the rules. For example the Genoa foresail will be fitted where the rules require only the area of the fore triangle to be measured. Would it be used if the rules required the whole area of the foresail to be measured? Hulls designed for rules that limit the overall length will be quite different to those where the waterline length is measured. The shapes of keel weights depend on whether there is a restriction on draught and so on.

 

This means that we must be careful not to draw the wrong conclusions from some feature on a full sized yacht or on a model in the absence of a proper explanation of its origin. It is also wise to take into account the differences between a model racing yacht and a full sized yacht. For example, on a full sized yacht the Genoa is loose footed because it can be set by the crew and be taken down and replaced by a spinnaker for running before the wind whereas, on a model, the fore sail must be mounted on a boom and cannot be changed.

 

This text will be limited to designs that have been developed for model yacht racing.

 

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Figure 2-2 Marblehead

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Figure 2-1 “A” boat

There is really just one basic design. It comprises a hull with, a fin or a keel, (which in some way incorporates a streamlined lead weight), a rudder and one of anything up to 5 suits of sails. All the suits of sails have the same arrangement of a roughly triangular mainsail and a triangular fore sail with suitable mast and rigging. The main sail is attached to the mast and to a boom that swings on the mast. The fore sail is attached to a fore stay and to a boom that is pivoted on the deck. This sail arrangement is called a Bermuda rig and is shown in Figure 2-1 and there is a derivative of this rig called a swing rig shown in Figure 2-2 where both sails swing on the mast.

 

The model racing yacht has evolved to tack upwind at about 40+° to the wind at a reasonable speed, to reach across the wind, and to run downwind and also to be capable of racing in all but the most severe wind conditions. For this to be possible the sails must produce a forwards force and usually this also produces an unwanted transverse force. The transverse force must first be resisted by the combined effect of the keel weight and the stability of the hull to limit the heel and to stop the yacht overturning and then by the hull, fin (or keel) and rudder in providing the sideways force. It must be seen as fortuitous that our boats are heavy enough and can go fast enough when pointing into the wind to permit them to tack head to wind. If things had been otherwise racing with boats gybing from tack to tack would not have been the same.

 

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Figure 2-3
This is an appropriate point to outline the functions of the several systems of a yacht in preparation for a more detailed treatment later.

 

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Figure 2-4

The obvious starting point is to consider the sailing rig, which provides the motive power. In model boats the sails must, according to the class rules, be capable of being rolled. This precludes the use of rigid sails although a few sail battens are permitted to allow some extension of sail area outside the basic triangle. The sails are mounted on a mast and booms and the whole arrangement above the deck is called the rig. The sails are curved to deflect the wind and so produce a force. Figure 2-3 shows the forces on the two sails on a plan view of a yacht that is heeled in response to the forces on the sails. The forces are not normally horizontal nor are they likely to be both in the same plane. Wherever they act they can be combined into a single force and then that force can be represented by two other components[1], one acting fore and aft and the other across the boat. The forward component drives the boat and, because it acts off centre and above the deck, tends to make the boat dip its bows and turn into wind. The transverse component also tends to overturn the boat and to combine with other forces on the boat to make it change its course. We need to understand how the forces on the sails are generated, how they change with different points of sailing and how sails can be designed to improve yacht performance.

 

Figure 2-4 shows the main forces that act on a yacht. The sails could not work without the resisting force generated by the fin. So let us look at the function of the fin and rudder. With perhaps the exception of running before the wind the sails always produce a transverse force which will make the boat move sideways if it is not resisted. The main resistance comes from the action of the fin and rudder. The fin works like an aeroplane wing. It makes a small angle to the course of the yacht and acts like an aerofoil to produce lift in the opposite direction to the transverse force produced by the sails. This is what is needed. If we are to design a fin or understand its mode of action we shall need to know something about aerodynamics.

 

The fin, which is symmetrical, because it has to work either way cannot function by itself any more than an untwisted wing can fly by itself. Another surface is needed to stabilise it like the stabiliser of an aeroplane. In the case of a yacht the stabiliser is the rudder. This is another aerofoil that can be turned at will by remote control. Its function is more complicated than one might think and we shall have to apply aerodynamic principles to understand it. There are many forces acting on the yacht and they all change with different sailing configurations. Not surprisingly, even when the sails are properly set for best speed, these forces usually combine to make the yacht change course and the rudder must be used to generate a force to produce a balance. As any rudder action leads to unwanted resistance to motion the fastest yachts are “well balanced”, that is, they need little rudder action to hold a course.

 

We have seen how the fin can resist the transverse force but the sails and the fin both tend to overturn the yacht and this also has to be resisted. It is done partly by the inherent stability of the boat and partly by the lead weight fitted either as part of the fin or as a separate bulb attached to the fin. The effect of the weight is to lower the centre of gravity of the boat. As the boat heels the hull produces a force tending to restore the boat to an even keel and at the same time the centre of gravity swings sideways also tending to restore the boat to an even keel.

 

Our hull has plenty to do. Its most obvious task is to keep everything afloat but it must also house the radio equipment and support the rig and the fin and rudder. Then it is required to contribute to the stability and provide an upward force at the bow when the sails are driving the bow down. It has to be strong enough to withstand all the rigging forces, the water forces and the knocks that are inevitable in racing and during handling in a rescue boat. Given all these functions the hull and all its underwater parts must be designed and made to permit the greatest maximum speed through the water and to be easy to drive by small sail forces when winds are light and when the yacht is pointing into the wind.

 

Clearly the rig and its interaction with the other parts of the yacht constitute a very complicated system.

 

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Figure 2-5

However there is a further effect to be considered. It is the effect of the motion of the yacht on the water near to the surface in producing a wave pattern. On the face of it one might expect the bow waves to be asymmetrical because the hull is making a small angle to its course. However yachts seldom move without also heeling, even when running. The heeling makes the two sides of the bow take different angles and produce bow waves of different height that give an asymmetrical wave pattern. See Figure 2-5. This wave pattern tends to produce forces which act to right the yacht. This all serves to complicate an already complicated motion and we shall have to pay some attention to wave patterns.

 

When the way in which all the systems function is understood there is still the problem of actually putting detail to the practical parts of a racing yacht. Then there is the further problem of setting up a rig. This involves using all the rigging adjusters on the mast and booms so that the sails have the right shape for all sail positions although the “right shape” is not something that can be written down very easily. Once set, the sails must be moved continually by the skipper to make the best possible progress round the course. The first is done before the yacht is raced, the second during racing.



[1] Strictly there will be three components but we are only interested in two of them.