Section 1 Introduction

The sailing vessel is a complex artefact. It comprises a hull to keep it afloat, a sail or sailing rig to drive it along using the power in the wind, some means to stop it moving sideways in response to the transverse force produced by the sails and some way to keep it more or less upright. These several parts all have to work together to produce a practical design and the way in which they work together is special to the wind-driven vessel. Each part can be understood separately but their separate roles can only be understood in the light of the behaviour of the sailing rig whether it is a single sail or several sails working together. After all that is what sets it apart from every other boat. I have seen all sorts of explanations of the action of sails and the fact that there are so many means that it is not easy to find a single explanation that satisfies everyone.


When I was reading a thread on a boat design forum a statement was made that the commercial use of sails as a means of propulsion of surface vessels is not seen to be sufficiently viable to attract funding for any scientific investigation and that, in fact, sails are used exclusively for leisure activity. This is a cogent statement and it appears to explain why there appears to be no agreement on how sails actually work, no structured test data, and, at the same time, a significant number of people keen to design and build their own boats. However I am not wholly persuaded that it is so simple because there is no guarantee that scientific investigation is possible nor that, if such investigation is possible, the outcome would be of much use to ordinary designers. A case in point is the experience of those who attempt to use the data on aerofoils that was painstakingly collected by the NACA.


In the early 20th century there was a dearth of information on the performance of aerofoils and wings. It was difficult to predict the area required for a wing to lift a given weight at a known speed and to know how the wing would perform in different attitudes. The USA sought to provide such data. They set up the NACA. Whatever the NACA was charged to do it actually adopted a clear strategy. It was to provide a bench mark for what was physically possible for an aerofoil by testing large, prismatic, models made to the highest standards of accuracy of profile and surface finish in large wind tunnels and then leave designers to make what use of the data that they could. Designers quickly pointed out that they had to design wings that could be made by normal production methods with control surfaces and high lift devices all built in and the performance of such wings was a far cry from the performance of the test models. NACA made some response but there was no hope that they might provide the data that the designers sought because no one could say with any confidence what constituted a typical practical wing. What was needed was benchmark data on a wide range of evolving designs and the NACA strategy could never give data that could be of direct use to designers except perhaps those who design high performance gliders. My impression of the work of the NACA is that they gave us a considerable range of aerofoil profiles and their characteristics from which to choose potentially practical sections and showed us how important it is to get the first 10% of the profile accurately made and free from any form of roughness. These are very important contributions to aviation but not what they set out to do.


By comparison scientists in the 19th century when they sought to predict the flow in natural and man-made channels swiftly concluded that the character of the wetted surface was very important but they did not try to get rid of the problem by only testing channels that were straight, prismatic and smooth. They tested all sorts of real channels with wetted surfaces that were described in a few words such as "planed timber", "slightly tuberculated iron" or "well-laid brickwork" so that anyone could visualise what was meant. Empirical expressions were devised and coefficients worked out and the outcome is still in use. It was a totally different strategy to that of the NACA.


It seems to be certain that the same strategy, in another guise, has been adopted for real aeroplane wings and that the major manufacturers of aeroplanes have accumulated much practical data about the wings that they have used. That data must be stored along with a description in words of what was actually tested. They must by now be homing in on the ultimate wing configuration.


One must recognise that the design of aeroplanes is not just about the design of wings because the aeroplane must be capable of controlled flight in three dimensions and to fly as economically as possible. Then one must first understand how an aeroplane flies and then design engines and stabilising surfaces as required and a streamlined fuselage to hold it all together and accommodate people or goods as the payload.


There are obvious similarities between aeroplanes and sailing boats. The hull is comparable with the fuselage but now does the job of the wings in keeping everything afloat, the control surfaces that work in only two dimensions are the keel and rudder but the significant difference is that there are no engines and fabric sails are used to drive the boat. There are those who think that sails are directly comparable with wings and that they operate in the same way but this is not the case as we shall see.


If these sails drive the boat then it is essential to find an explanation that is consistent with established science. There is no doubt that if we knew the magnitude and direction of the forces produced by interaction of the sails with the wind we can explain how they drive the boat; the problems arise when we try to quantify these forces for a given sail or a given combination of sails. We have a further need that is not easily satisfied and that is how to design combinations of sails to work in the best possible way.


If this is the case it seems to be an obvious thing to seek to gather experimental data for fabric sails along the lines of the work of the NACA. But the prospect of testing sails of innumerable shapes and sizes with all sorts of surface finish attached to all manner of masts and spars and used in a natural wind at low speed is too daunting to even contemplate. It is clearly not possible to devise a testing programme that will lead to anything that can be stored in a systematic way for use in design and throwing money at the problem will not alter this position.


Nevertheless wind-driven vessels have been built that proved to be very suitable for the task for which they evolved. Ships like the windjammers and clipper ships, sailing barges, sailing trawlers and pilot cutters came through their periods of design evolution long before aeroplanes came through the same process. Each had a well defined role that can be compared to airliners, cargo aeroplanes, feeder airliners and mail-carrying aeroplanes. The sailing vessels were reliable and one might note that the fighting ships that were used by the Europeans to pursue their wars could stay on station during a blockade for months on end requiring only food and water for the crew and fuel for cooking. In this respect they have only been equalled by nuclear-powered submarines. Both depend on reliability of equipment.


So far as I know little of the reasoning that went into the evolution processes for these sailing vessels have come down to us. We have only the designs to study and sometimes the artefacts. I suppose that the nearest thing to documentation that we have has come from naval records that were kept because naval vessels were built in naval dockyards and not by private companies. Even so the records seem to be more historical than scientific. Those days are over and now wind driven-vessels are built for leisure even if that leisure is derived from being aboard a commercial sailing ship for fare paying passengers.


So what do people actually do with their sailing or power boats?


The most common thing is nothing. The boat spends most of its existence stationary in marinas, at moorings or out of the water. It seems to me that most people who fancy owning a boat find that it involves considerable physical effort, travelling and all with nowhere special to go when the boat is actually under way. Very often the boat turns out to be an expensive alternative to a caravan. However for sailing boats, as distinct from power boats, there is the element of interest that stems from using sails to drive the boat.


I watch leisure sailing boats and it is often evident that not much effort is going into sail setting. Some people appear to set the rig to get the boat on the move and then just steer. They can do this because the soft sail is not particularly responsive to the swinging of the wind and the continual variation in its speed. The sail behaves in this way because the flow never follows the leeward surface of the sail and, in aeronautical terms, is always detached and, if it is set at a high angle to the mean direction of the relative wind, changes in the speed and direction of the wind just increase or decrease the force on the rig without any sudden changes in the way that the rig is working. (See the section of wing sailing to see what happens with a rigid wing.) Then the hull with its very non-linear resistance to motion does not speed up as much as one might expect when changes in the wind increase the force on the rig and conversely it does not slow down as much as one might expect when the speed drops and the angle of attack increases. So idling along is what a sailing rig does best.


The sailing activity that is most rewarding is racing and a large number of people race. However racing must be organised and the way that this has worked out is that most racing is in classes and it is usual for people to buy their boats or yachts simply because there is no permitted alternative to buying a particular design so that everyone sails a boat that is measurably identical to all the other boats in the class. Learning to race a yacht takes a long time and a knowledge of how the yacht actually works is only a small fraction of what must be learnt.


I think that the number of people who design either their rigs or their boats is very small when compared with the total number who sail. Some of those people will design in the total absence of any theory or data and others will make every effort to understand the way in which sailing rigs actually work and find any data that is going. Most will lie somewhere between these extremes. They are all doing what they want to do in their own way.


This led me to think that I should introduce my contribution to the way that sailing vessels actually work by looking at sailing in the round. In reading this remember that I have never sailed so I am dependent on observation of others sailing and on information from those who do sail. What I can do is look at the applied physics of sailing and try to fit this and the practical information together to understand it a little better. In my experience every contribution contains something that can be useful.