Chapter - 15

Sailing-In Theory

Sails have been used for thousands of years to drive boats through the water—even on the simplest rafts and dugout canoes of primitive tribes. The earliest forms were square-rigged types in which the sail set thwartships across the mast. However, like the small boy's raft with a blanket sail, they had one serious limitation; they could sail only before the wind. To go to windward they were forced to drift with a favorable current—or row. The art of using the wind's own power to propel a vessel against that wind had not as yet been discovered.

Centuries rolled by before a sailing rig was invented which enabled a boat to make any progress at all against the wind. The Arabs contributed an important development to the science of sailing when they worked out the lateen rig, a triangular sail supported by a yard at the head but no boom at the foot. The sail could be set in a fore-and-aft direction. Resembling the sails used on canoes today, these lateen sails were undoubtedly the fore-runner of our modern fore-and-aft rigs.

For years square sails drove our windjammers, like the famous clipper ships of the mid-nineteenth century. Under clouds of canvas spread from lofty spars stayed by a maze of rigging, they made phenomenal passages in the China tea trade. Displaced by steam, the big clippers have disappeared and with them the square rig, except in a few isolated cases. When the fore-and-aft sail came into vogue, the gaff rig was most popular, the principal sails being spread from a gaff at the head of a quadrilateral sail and a boom at the foot, the luff (forward edge of the sail) being secured to the mast. The after edge, the leech, is not attached to a spar.

While the gaff rig was at the peak of its popularity, the principles underlying the theory of sailing were not too well understood. But when the airplane made its appearance, necessitating intensive studies of the action of wind on wing surfaces, the analogy between wing and sail was recognized—the sail being a wing set vertically on the hull to drive the boat by giving a "lift" ahead instead of a lift upward as in the plane. Sailmakers then strove to shape their sails into a section resembling a bird's wing.

Airplane Theories Have Effect

No longer did sailmakers think in terms of pressure applied to the after side of the sail as the major factor in producing the drive needed to propel a hull. It became apparent that reduction of pressure along the luff on the forward side of the sail was the big factor. As soon as that principle was generally understood, gaffs and long booms on racing sailboats began to disappear, giving way to the tall '"Marconi" mast and triangular jib-headed sail. Proportion of luff to foot was radically changed so that now the luff may be roughly 22 times the length of the foot in a well cut sail.

The first thing to realize in studying the principle by which a sail is made to drive a boat is that the wind blowing on a sailboat has several different effects. First, it will heel her over as the wind hits the sails. Then it tends to blow her sidewise off her course. The sidewise slipping is called leeway. The wind also acts to drive the boat forward on her course. And finally there is a pivoting action as a result of which the bow may tend to come up into the wind or to fall off before it.

The architect studies all these tendencies and so designs his craft as to minimize the features which do not contribute to propulsion of the hull and to develop and utilize the factors which do tend to drive the boat along the desired course.

Now this first factor, heeling, while it does not help in driving the hull forward, does have a considerable effect upon the way the boat handles in winds of varying velocity. Realizing that the hull must of necessity heel to some extent, the architect designs the underwater lines so as to offer a minimum of resistance to passage through the water along the course at a certain angle of heel.

Just as an overpowered motor boat creates a lot of disturbance and uses up power in wave-making when driven too hard, so a sailboat pressed down under canvas beyond her normal sailing lines makes a lot of fuss without getting a proportionate increase in speed. The mere fact that her lee rail is under doesn't necessarily mean that she is travelling at top speed.

Stability Is Important

Furthermore this heeling is something that must be kept under control. Stability is needed to prevent the boat from capsizing under heavy wind pressures. To picture an extreme, a light unballasted narrow hull, sitting on top of the water, is easily knocked down. Witness the canoe under sail. Stability can be obtained in a light hull of little draft by increasing the beam. Our broad-beamed catboats are a good example. But once a broad, shallow draft hull has been pushed down beyond a certain safe angle, she too must capsize, like toy sailboats made of a flat board and sail.

On the other hand stability can also be obtained in a relatively narrow deep hull. This is accomplished by keeping weight low down in the hull, lowering the center of gravity. If the boat has no outside ballast she will carry ballast deep down inside the hull. If she has a deep keel, she will carry a casting of iron or lead bolted to the lower extremity of the keel, sometimes in the form of a bulb. Such a boat, hit by a puff of wind, heels over, easily at first. But the further she heels, the greater the tendency that weight on the bottom of the keel is exerting to right her. Knocked down on her beam ends, she will right herself when pressure on the sails is eased.

Landsmen often marvel when they see a yacht sailing "on her ear," masts inclined at a 45-degree angle and water boiling on deck around the rail. They feel an extra puff must certainly send her under. However, if she is properly designed, there are powerful forces acting to right her all the time—the further she heels, the stronger they become.

Now the second effect of the wind on the hull is to drive it off sidewise, to leeward. Here's something that must be definitely counteracted; otherwise our sailboat will take on the characteristics of a raft driven before the wind. Our shallow draft hull, like a rowboat, presents no vertical surface in the plane of the keel to resist being driven off by lateral pressure. So we fit a pivoted centerboard (see Figure 2) which can be lowered from its trunk when we go to windward.

Centerboards And Dagger Boards

Other expedients are the dagger board (Figure 3), which does not pivot but is raised or lowered vertically, and leeboards carried at each side of sailing canoes and many Dutch sailing craft. In each case we are presenting a vertical surface to the water in a fore-and-aft direction. This moves easily through the water in a direct line but resists any tendency to be driven sidewise due to water pressure on the leeward side of the board.

A deep-keeled vessel has this same expanse of surface below the water against which the water can act to minimize leeway. At the same time, to improve her sailing qualities, the surface which the hull above water presents to the wind is kept at a minimum. The sides of the hull above water are relatively low, compared to the deep draft. Technically, a boat with "low sides" is said to have little freeboard.

The third factor we are dealing with is that tendency of the wind which drives the hull forward. To guarantee that a maximum of propelling effect will be gained from every square foot of sail, underwater lines of the hull are made as clean as possible, of a shape that will allow it to slip through the water with a minimum of resistance, wave-making and eddy-making. Lines that taper off aft to a point conform to one of the principles of streamlining, allowing the displaced water to close in smoothly about the stern without suction, drag or needless fuss.

The fourth factor, the pivoting action of the hull, we shall shelve for the time being until we understand just how the wind pressure acts to drive the boat ahead. Referring to Figure 1, consider a sailboat on a northerly heading with the wind coming generally from the northwest as shown, sail trimmed in as close as possible. She is "close-hauled" and on the port tack (wind coming over the port side).

A Good Sail Draws The Boat Forward

Study of the effect of wind pressure on a properly designed wing surface has shown that there is an area of negative pressure on the convex side. So it is with a sail. In Figure 1, this acts to draw or pull the sail in the direction indicated by the arrow CW through the wind actually comes from the direction W. Now if we show this force graphically, allowing CW' to represent the force and direction of the wind, it can be shown according to physical laws that CW’ can be resolved into two forces. The parallelogram FCLW’ (dotted lines) shows how this is done. FC then represents the relative strength and direction of the forward component propelling the boat on her course, LC the strength and direction of the lateral component tending to drive her to leeward.
 
The question that the sailboat skipper must decide for himself is just how close to the wind he should sail to gain the most ground to windward. If he "pinches" her, (sails too close to the wind) he will find FC proportionately reduced as the sail loses its pulling power. If he eases the sheets and takes a course further away from the wind, (doesn't "point" so high) the sail draws better and FC is increased, although he has a longer course to sail to reach his objective to windward.

Fig. 1. With no sail set, boat tends to drift off to leeward toward X. With sail set, close-hauled, wind from direction W strikes sail and acts on it much the same as lift is given to an airplane wing, so that wind force is exerted in direction CW’ This is resolved into component forces—FC, the forward component which drives the boat ahead; and LC, the lateral component which drives her sidewise off her course

Tacking

The relative ability of sailboat racing skippers to judge from experience just how close to the wind they dare sail will often be the critical consideration on which will hang the outcome of their race.

While a sailboat cannot sail directly into the "eye of the wind" as a motor boat can, she is able to get to an objective dead to windward by "tacking." See Figure 4. She wants to sail from A to Z but the highest she can point toward the wind puts her on a course AB. So she sails the course AB (starboard tack), goes about on the port tack from B to C and repeats this, sailing the course ABCDE, etc., represented by the solid line, but making good on the average the course AZ dead to windward (dotted line). At CD she is on the starboard tack again as shown, close-hauled; at DE close-hauled on the port tack, etc.
 
If the course is not dead to windward, the length of the various "legs" or "boards" will not be the same as shown in Figure 4. For example every starboard tack may be a long board, every port tack a short board, or vice versa, depending upon the relative direction of the course to be made good and the direction of the wind.

Fig. 4. How a sailboat goes to windward by tacking

Now we can return to the fourth factor involved in the theory of sailing—the tendency of the wind to pivot the boat if the sails are not properly trimmed, if the hull is out of trim, or if the balance of sails has not been properly matched to the underwater design of the hull.

Fig. 5. Relationship of wind force which acts on sails to drive the boat to underwater part of hull which resists sidewise motion. Wind acts on mainsail as if its total effect were centered at A, on jib at B. With both sails set, combined effect is centered at CE (center of effort). Water presses from opposite side on under-body of the hull to resist sidewise motion (leeway). Center of this pressure is at CLP (center of lateral plane). Note CE and CLP are not quite in vertical line. Distance CE is located ahead of CLP is called lead

Referring to Figure 5, we have a Marconi knockabout showing the sail plan consisting of jib and mainsail and the underwater shape in profile. Now if we draw lines from the tack of a sail to the mid-point of the leech (disregarding curvature or "roach"); from the head to the mid-point along the foot; and from the clew to the mid-point of the luff, the lines intersect at a point. At this point all the force of the wind may be said to act. It is called the center of effort, and is located at A for the mainsail, B for the jib and at CE for the combined sail area of jib and mainsail together.

While it is not as easy to locate as the center of effort, a center can also be determined for the underwater portion of the hull (disregarding the rudder) against which the pressure of water may be said to act in resisting leeway. In Figure 5 this is at CLP (center of lateral plane, sometimes called center of lateral resistance). If you were to anchor the boat sketched in Figure 5 in a tideway, broadside to the current, you would find that the anchor line would have to be made fast at a point just a little abaft the after port hole in order to hold the keel at right angles to the current and the anchor line. This is a rough and ready way to determine the CLP.

Fig. 6. What happens when sails are not properly balanced. Wind is acting in direction of arrow at center of effort (CE). In sketch A, center of lateral plane (CLP) is too far aft. Therefore boat pivots, her bow falling off away from the wind. At B, CLP is too far forward. Again, boat pivots, this time bringing bow up into wind. To offset tendency sketched at A, rudder would have to be set to port (lee helm). At B, rudder would have to be set to starboard (weather helm)

If the boat were not heeled, it is evident that CE should be in the same vertical plane as CLP if we wish to prevent any pivoting action. If we anchor in the tideway as before, with the anchor line in the plane of CLP, but move CE toward the bow, the bow would swing one way when wind pressure was exerted on the sails. If we moved it toward the stern, the bow would swing the other way.

A better example is shown in Figure 6. Here the boat is sailing, with wind abeam. At A, the wind is acting on the boat at CE. If CLP is too far aft, the boat's bow falls off to leeward. To correct that the tiller would be pushed down, setting the rudder to port, and the boat would then be said to carry a lee helm.

At B, Figure 6, you have the opposite condition. Here CLP is too far forward, and the forces combine to turn the bow into the wind. Weather helm (tiller toward the wind, rudder to starboard, in this case) would be necessary to hold her on a straight course. Bear in mind that these examples are highly exaggerated to illustrate the principles clearly.

Form Of Underbody Changes With Heel

Under sail, the boat actually heels and does not stand vertically in the water as in our hypothetical illustration above. As she heels, the shape of the surfaces of the underbody exposed to the water changes and a wave is piled up under the lee bow, tending to push the bow to windward. As she heels, the CLP moves forward. Furthermore CE moves forward also when sheets are eased off instead of standing squarely amidships as in the profile drawing showing the sail plan. However, CE doesn't move ahead as far as CLP.

To take these factors into consideration the designer locates his CE ahead of CLP a short distance, varying with the characteristics of the boat—whether she be shallow or deep drafted, etc. This distance between centers is called the lead and may vary anywhere from about 2^ per cent of the waterline length in shallow draft centerboard boats up to roughly 8 per cent in deep draft boats with a long fore-and-aft underwater surface.

Another factor has a tendency to drive the bow into the wind as the boat heels. With a tall rig, and the boat heeled far over, that part of the wind force which is driving the boat ahead is exerted not in a plane vertically through the centerline of the boat, but far outboard over the water, on the leeward side of the boat. The hull resists being driven through the water, while the wind pressure acts almost as though you were to grasp the mast and push it ahead while holding the hull back. This is just another one of those points that the designer must consider, drawing on a good deal of experience in his calculations, if the boat is to perform well on all points of sailing at different heeling angles and in winds of different velocity.

Now with all of the above interacting factors to consider, it is easy to understand why a boat must be properly balanced as to her rig and hull design if she is to sail well. If her sails are too far forward, or her keel or centerboard too far aft, the bow will fall off from the wind and the boat carries a lee helm. The helmsman must keep his tiller to leeward (rudder to windward) all the time to keep her on her course.

On the other hand if the sails are too far aft, or the keel or centerboard too far forward, the reverse is true. Then she carries a weather helm and tends always to throw her bow up into the wind.

Weather Helm

A little weather helm is a good thing in a sailboat. If the tiller is let go, she will then swing into the wind and lose her headway. Or if a puff of wind hits her when the boat has so little way on that the rudder has practically no effect in steering, she will luff into the wind, automatically easing the pressure on her sails.

The complete theory behind the design of a sailboat and the principles that govern her action on different points of sailing are not easy to follow through scientifically, much less to describe in non-technical terms. But a little attention to the fundamentals discussed above may help the novice to understand how a sailboat, properly designed and handled, is able to turn the wind's force to her own advantage.

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