Chapter 10

What Makes Her Go?

Snipe on starboard tack with crew hiking.

It may sound simple to explain why a sailboat goes. She is blown along with the wind just as your hat goes sailing down the alley. But the man who is to learn the art of sailing cannot stop there. He must be ushered through enough theory so he will be better able to understand why he must handle sails and rudder in certain ways to obtain the desired results. If we were to simply accept the analogy of the hat and the alley, we would be pretty well up against it to account for the fact that a boat can sail in a great many directions other than in the one toward which the wind is blowing.

Whether we like it or not, we have to get into engineering far enough to understand the law that where there is action there must be an equal reaction. Think of it this way. You are trying to push a wheelbarrow up a grade. You put into effect an exertion of, let us say, 100 pounds and the load moves ahead. Stop to think about it and you will agree that the barrow must also be pushing, but in the reverse direction. You are pushing her ahead and she, the one-wheeled obstinate, is trying to stay where she is. If you wanted to get technical about it and had the proper instruments, you would find that when you push ahead with 100 pounds of energy, the barrow would push back with exactly the same amount. One of the laws of Newton, the famous mathematician, is that action and reaction are equal. If the wind blows against a sail with a force of one pound per square foot, the sail, and the boat to which it is attached, pushes back with an exactly equal force.

At this point, we run into a subject we wish we could duck. It flies in the face of a lot of humbug, and some truths, that have been written in recent years about sailboat propulsion. For many centuries, men had a good idea of why the wind blowing on a sail sent the boat ahead. Briefly, it did push on the sail but because the canvas was set at varying angles to the wind, it was decided, in accordance with the laws of physics, that it was old Newton's law of reaction that moved the boat. That is surely the case when the boat is sailing directly before the wind. Like your hat, she is being blown along. But what if the wind is coming from one side? The boat still goes ahead at right angles to the force. How? It is easy if we follow the Newtonian law. The wind strikes the sail at an angle to its surface. There is a reaction to that force. Let's be unscientific for long enough to simplify the result by saying that the reaction "bounces" off the canvas. Did you ever play pool or billiards? A ball struck at an angle reacts to the force of the cue by going off at another angle to the direction of the cue. There you have an example of the reaction "bouncing" off at a direction different from that of the force.

Technically, we have what is called a parallelogram of forces. We'll be mercifully brief with this. This mouth-filling term looks like Fig. 1. Lines A-B and C-D represent the direction we want the boat to take. Lines A-C and B-D represent the side slip or leeway caused by the pressure of the wind on the sail. The diagonal line C-B is the component of the two forces— in other words, the theoretical path of the boat. The sides of the rectangle should be directly proportionate to the forces. In a heavy breeze, the proportions of the rectangle of forces would vary. As the sides change in length, the diagonal C-B will also change. Line C-B is what we are after as it represents both forward and sideways motion. By designing boats to exert the least resistance to motion ahead, and the greatest resistance to movement sideways, we change the proportions of the parallelogram so that C-B becomes longer and at less of an angle to the keel. In theory, we are attempting to shorten A-C and B-D and lengthen A-B and C-D so that our pet C-B line is going where we want—which is ahead and not sideways. Having gotten that off our chest, we can thankfully return to subjects with less gibberish.

All of this makes a good deal of sense and checks up with what we can do with billiard balls, roller skates, wheelbarrows, and all sorts of things that move due to some external force. This theory of sail propulsion is accepted by the majority of folks who stop to think. Unfortunately, the airplane had to get into the picture and gum the works up in the minds of those who are apt to jump to a conclusion should it sound sufficiently mystical.

Three views of a Raven sailing close-hauled on the port tack. Her large cockpit takes 10 people day-sailing comfortably and her beautifully built molded-mahogany hull is capable of high speeds.
 
Exploring the airplane theory we immediately agree with the statement that a major portion of the lift of an airplane is on the upper surface of the wing. The wing is partly pushed upward by the force of the rush of air, but it is also sucked upward with even more power by—forgive us—an area of negative pressure built up by currents deflected into turbulence by the leading edge of the wing. Theorists argue quite rightly that, in a hurricane, it is often the windows on the leeward side of a building that are broken and that they are broken outwards. Keeping it simple, we can say that the flow of air at very high speeds has created a suction on the side farthest away from the wind's force. Every word of which is the simple truth, so help us.

Somewhere around the time that aeronautical engineers discovered the leeside suction effect, a sailor got it into his head that a similar action might take place with a sail. If suction drags window frames out in hurricanes and lifts airplanes into the wild blue yonder, why not believe that suction draws boats ahead? Why not, indeed, until you stop for a few seconds to remember that this suction effect only occurs with appreciable power at very high wind velocities. When the airplane's speed drops below a mile a minute or so, the rescue teams dig the unfortunate crew out of holes in the ground. When the house is played upon by an ordinary breeze, there is not enough suction on the leeward side to do more than blow out a match, if even that much action is felt.

If you drive an open car, try this. At speeds of 15 miles an hour or less, light a match behind the windshield. The chances are that the flame will hold until you apply it to your Luckyfield coffin nail. Look for traffic cops and then speed her up to 50 miles an hour and try the same thing. The match will blow out toward the windshield. Wind currents rushing around the windshield are blowing forward in the same direction the car is moving. At the low speeds, the wind speed was not enough to create any appreciable suction. Right there is the kernel of the entire problem.

Sailing boats are not airplanes nor houses in hurricanes. A small boat sailing at four miles an hour is really moving. If she makes twice that speed, you are going like the proverbial bat out of hell. It will take a mighty strong breeze to create any such speed. In any average summer breeze, it is utterly impossible to set up enough leeside suction effect to draw the boat along. There may be a slight effect, which is why the tall, narrow, airplane-wing-like jib-headed sail is a few seconds per mile faster than the gaff-headed mainsail. But to allege that the leeside suction effect— usually called the aerodynamical theory of sail action—is what sends the boat along, is little more than an unthinking attempt to apply a theory that depends upon high wind velocities to a condition where low velocities are inevitable. If you want to prove the fallacy of the aerodynamical theory, you need nothing more complicated in the way of research paraphernalia than a pair of good eyes and a lighted cigarette. Some nice summer day, hold the butt up on the windward side of the sail and watch the direction of the smoke. Now crawl around forward of the sail and hold the cigarette up on the lee side of the mast where—according to the airplane boys— there will be so much suction pulling the boat over the bounding billows. By rights, the butt should be just about jerked out of your hands by the leeside effect. You will find, in all average airs, that there isn't enough suction to do more than make the smoke lazily flitter about. Ahead of a gaff-headed sail, you may find even less. And yet, to quote one writer, the aero dynamical effect "has been proven in thousands of cases." Unfortunately, he failed to state one specific instance or to offer any method of proving the assertion.

Snipe running before the wind with its jib extended by means of a whisker pole.

Getting away from controversial issues, let's see what happens when the wind strikes a sail. Unless the boat happens to be sailing directly before the wind—remember the hat-in-the-alley comparison —the wind will be blowing toward the side of the boat. In fact, it may even be coming at considerable of an angle to the side. With the proper adjustment of the sails, you will be able to sail very close to the direction from which the wind is coming. This is because you set the sails to "bounce" the reaction opposite to the direction you want to go.

Left: Another view of the Snipe running before the wind. Right: A Raven with the wind directly astern.

Some of the power of the wind will be exerted in an effort to drive the boat sideways. Look at Fig. 1 again. That effort is opposed by the underbody of the boat. The keel or the centerboard presents considerable area resisting the tendency toward side slipping. Some slipping, called leeway, does take place, but its amount is relatively little in a well-designed boat underway with the sails set at the proper angle. Compare it with ice skating. You kick out more or less sideways with the free foot. If the skates are not sharp, there will be a side slippage, but if the skate on the ice is sharp, it will dig in and, like the keel or center-board, prevent the sideslip so that you move ahead.

Perhaps it can best be explained by reference to Figs. 2, 3, 4, and 5, which show a boat under four main types of sailing. In Fig. 2, the wind is blowing from dead aft. Striking the sail, the reaction is "bounced" off the canvas and the boat goes ahead like our hat down the alley. You are sailing on what is called a run.

In Fig. 3, the wind has shifted so it is coming over the starboard side fairly well abaft amidships. You have shifted the sail to an angle about as shown until the reaction is again directed aft. You are getting more sideslip. Your course is a broad reach.

Fig. 4 shows the wind directly abeam, so you have brought the boom farther inboard; the reaction is still going aft and the boat is moving ahead. Leeway is getting pretty tough, but you still sail many feet ahead for each foot you slide down hill.

Fig. 5 shows the extreme forward position of the wind. Lots of boats will not go at all with the wind coming from so far forward. You are now beating to windward and also beating your brains out trying to keep her just so. Here, a good skipper can keep the sail filled and the boat moving. The lubber alternately luffs and falls off.

If you have any difficulty in understanding the direction of the wind and the reaction arrows, just assume that you are throwing a rubber ball against a brick wall. The direction of throw is the wind striking your sail and the bounce of the ball is the reaction. Even the best boat with ample lateral-plane area will sail sideways more than it should if the sails are improperly handled. Here is where the fundamentals of propulsion by the wind come in handy. If you base your sail handling on the false premise that the sail should be trimmed as close as possible at right angles to the direction of the breeze, you are never going to get very far. If, on the other hand, you work on the theory of the "bounce," and trim your sail so that the "bounce" will be directed aft, you are in a fair way to become a sailor.

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