by Eric Coleman

This article will be much concerned with achieving maximum performance under sail, particularly against the wind, which occupies most of our sailing time. After all, we sail for pleasure, so if our boat has a satisfying windward performance, we are better off than those who have to use an engine — we have the choice. Throughout the design, we shall need to know how any particular feature may effect the performance, so some knowledge of how a boat behaves is desirable. The flow of air over sails and water past a hull is a complex subject which is only partially understood. It is not unusual to find more than one theory to explain a particular effect. For our purposes, it is the effect which matters more than the theory and, indeed, most of yacht design and much of ship and aircraft design is based on empirical results. Even today, the power required to drive a ship is calculated from the results of tests with models.

Theory, however, can point the way to progress in design. For instance, I have a theory that if I open up the toilet in the bows of my Rebell when she is travelling fast, air will be sucked down and run aft under the hull in the form of bubbles. These will greatly reduce the friction and enormous speeds will be achieved. A singular lack of results so far has delayed the publication of my paper on ‘Toilet Boost’, but if I increase the suction at the base of the toilet there is the possibility... On the other hand, when I modified the centreboard edges to give an aerofoil shape, there was a major improvement in performance. It was the theory which persuaded me to make the change. Another use of theory is that it helps us to tune up our boat, to get the best out of her. It is a substitute for years of practical experience. Within the text I have included theory which has a close application to practical considerations.

Vector Forces

Sailing to windward

Although anyone can understand how a yacht sails downwind, the ability to sail against the wind is more obscure. A yacht has a high resistance to sideways motion, due to the keel which grips the water, and a low forward resistance. We can roughly represent these conditions thus:

fig 1. How a yacht sails to windward

By placing a wedge shaped piece of soap on a table then pressing a finger on to it in the direction shown, the piece of soap, like the yacht, will move against the force producing its motion. It will be apparent that sails cannot be trimmed to enable a boat to sail directly into the wind.

The forces generated by sails can be illustrated by vectors which are simply arrows representing their direction and strength. Fig 2a shows lift and drag combined to give the resultant force R.

fig 2. Conversion of wind force by the sails

R can now be resolved into two components (fig. 2b) — F driving the boat forward and S a sideways or lateral force. S is comparatively large so, in spite of the lateral resistance of the keel, some sideways motion will result. The boat will sail crabwise and, although her heading will not change, the direction of motion or the course ‘made good’ will be along the line marked COURSE. θL is the angle of leeway. The effect is to reduce the windward performance because she is not sailing as close to the wind as her heading would indicate. Fig 2b can now be re-drawn as fig 2c where the forward and sideways forces are related to the course sailed. Fig 3 shows the boat tacking to windward. If, with no leeway, she could reach position A, then the effect of leeway would be that she could only reach B.

fig 3. The effect of leeway on windward performance

fig 4. Apparent wind direction

Fig 4a shows the relationship between the true wind WT (i.e. the wind which would be experienced by a stationary observer), the speed of the boat VC and the apparent wind WA which is the wind experienced by the boat. If the boat moves faster, the apparent wind draws ahead and strengthens (fig 4b). A relative increase in true wind speed, as in a squall for instance, brings the apparent wind aft, so the boat can be pointed higher (fig 4c).

fig 5. The pattern of forces

Fig 5 shows all the individual forces acting on the boat. LS is the lift of the sails, as in fig 2a, giving a forward component FS and a sideways component SS. DT now includes the total windage of rigging, hull and crew and has a component DS in line with S and an aft component DA. DH is the drag of the hull through the water. LK is the lift, at right angles to the water flow, generated by the keel and hull. At a steady speed and course, D = FS and LK = S so all the forces are in equilibrium.

I often see a pocket cruiser sailing along with the outboard propeller trailing in the water. The drag could be as much as 10% of the total hull resistance. If the outboard is tilted, DH will be reduced and D will be less than FS. This means that the speed of the boat will increase, but now LK will be greater than S so θL becomes less to compensate because this will reduce the lift. Thus, the boat is not only travelling faster but the leeway is also reduced, a point which fig 5 makes obvious. The other vectors will readjust themselves as the apparent wind draws ahead and strengthens and the higher speed will increase the drag of the hull itself. All these changes act in such a way as to restore the balance of forces.

Reaching and Running

When a boat is sailing on a reach, at right angles to the true wind, the apparent wind is still ahead, so the forces are approximately the same as for beating. On a broad reach (fig 4d) the apparent wind is at right angles to the course and this makes a significant difference to the pattern of vectors in fig 5. LS is in line with S and only contributes indirectly to D by its effect on hull resistance as a result of heeling the boat. The overall effect is more drive, less drag and less leeway. In moderate winds, the highest speeds are obtained on a broad reach.

As course is changed to bring the apparent wind aft, sails lose their lift because of stalling and become drag devices so LS disappears and is replaced by DT. All windage now gives forward thrust. With the wind directly aft, its strength is WT - VC which is why a fresh breeze is so deceptively gentle when one is running.