It must not be supposed, if of two vessels of 75 tons each one is a schooner, and the latter makes a course of 41 points from the wind, and the cutter 4 points, that there will be, providing their general speed and area of canvas be equal, only seven minutes' difference between them at the end of a twenty miles' thrash to windward. The schooner will probably be the equal of the cutter with the wind abeam; but it is a very different matter when the wind and the sails make a very small angle, as in close-hauled sailing. For such sailing a great portion of the sails near the upper, lower, and after edges are ineffective ; con sequently, the greater the number of parts a vessel's sails are in, the more edges there will be, and the greater will be the loss of propelling power ; and, further, the eddied wind thrown off by the sails greatly interferes with the direct or impelling currents of wind. Beyond this, a schooner suffers in stability, inasmuch as she has to carry the weight of two masts and two sets of rigging, instead of one mast and one set of rigging, and for any given area the heeling moment of the sails of a schooner will be greater as the centre of effort will be higher; for this reason (though otherwise she might have been equal), she would not carry her canvas so effectively, as the effectiveness of the canvas is practically reduced in proportion to the sine of the angle of heel.
This leads us up to the point that sails are not really planes, but surfaces which are more or less concave. There is no doubt that the general pressure on a surface is equal whether that surface be a plane or a concave one, providing the areas are equal ; but, on surfaces like those of sails, it is not the direct pressure (such as it would be when sailing dead before the wind) that drives a vessel ahead ; but, as previously explained, a component of the wind force which strikes the sail at some angle. The exact value of this component for a concave surface is, so far as we know, undeterminable ; but experience has taught us that it is vastly larger for flat surfaces. The oft-quoted example of the America's sails as against the wind-bags of British yachts in 1851, and the practice in consequence of the last quarter of a century, are sufficient evidence of the truth of this.
When the wind force applied to a sail comes obliquely from ahead, as in close hauled sailing, there is a plus pressure on the fore part of the sail, and Mr. Osborne Reynolds' illustration has very completely proved centre of pressure is far ahead of the centre of area, as indicated by the letters a, b, c, d, Fig. 11, page 22. Let A (Fig. 12) be a projection of a plane moved obliquely through water or the air, in the direction of the arrow, by a line attached at p. So long as the line is kept attached at p the plane will keep in an oblique position, and the centre of pressure will be at p ; but if the line were attached to the centre of the figure at z, the plane would move square to the line of pull, as shown by B, and the centre of pressure would necessarily be at x. If when the line were attached at p, the centre of pressure remained at x, it is apparent that upon pulling the line the plane would be overbalanced, and would assume a horizontal position.
Mr. Wm. Fronde has given this subject a great deal of attention, and, in speaking of sails, says : A striking indication of a distribution of fluid pressure on a curved surface is supplied by the prim& facie paradoxical curvatures into which sails often arrange themselves under the effect of wind, as is specially noticeable in jibs. In these sails especially many sail makers, for reasons which it would be out of place to enter on here, cut the canvas with an extravagant roundness or convexity of outline on the anterior edge of the triangle (or "lull of the sail" as it is called) before roping it ; and as the rope is made somewhat shorter than the rectilinear dimension of the side of the triangle, the prominent edge of the con vexity becomes gathered in, so as to form, immediately behind the rope, a narrow tapered belt of slack canvas, which becomes conspicuously bagged out by the pressure of the wind.
Now it is a most noticeable fact, familiar doubtless to all who have studied the " sit " of sails, that when the vessel which carries such a sail is " close-hauled," that is to say, when the wind strikes the sail obliquely from ahead, say at an angle of 45° with the line of the keel, the general wind pressure which the reaction of the rest of the sail produces swells out the " baggy " belt of canvas, not simply to leeward, but also so much forward that an observer viewing it in a direction at right angles to the vessel's course can see the convexity protruding itself ahead of the bolt rope, although, from the direction of the wind current as a whole, that part of the sail, when thus protruded by the internal pressure, must experience externally also a considerable direct pressure on its convex or (so to call it) leeward side. As the vessel is pressed closer to the wind, it is this part of the sail which will first begin to flap or " lift ; " but this will not happen until the greater part of the windward surface of the sail is brought so nearly edgways to the wind, that the flatter, or less " baggy," portions of its surface are nearly relieved of pressure.
There is no doubt that, if the fore part of sails did not go into the " bag " described by Mr. Fronde, they would be much more effective; and, within certain limitations, the heavier the canvas, or the more rigid and unstretchable it can be made by narrowness of cloth or other means, the more wind will the sails usefully resolve. Sailing masters generally well understand the importance of having the fore part of a sail flat and " unbaggable." Hence, during a match, we frequently see them wetting the lull of a main sail, to shrink the flax and so strain this part of the sail flatter. But in old-fashioned sails (and in some ill-cut modern ones), the after part of the sail also went into a bag, and the idea was that the wind should not be allowed to escape. But the real effect of a bag in the after part is to make a "back sail ; " and, of course, a back sail retards a vessel's progress, and, in the case of after sail, helps to turn the vessel's head towards the wind, by pressing her stern to leeward. This bagginess in the after part is more or less apparent in all sails, and in the case of one set on a gaff the mischief of the tendency to turn the vessel towards the wind is somewhat remedied as the peak goes off to leeward, so that actually only little more than half the sail remains at the angle the boom is trimmed to. The conclusion is that the general pressure on a baggy sail is the same as on a flat surface of equal area if that pressure be applied at right angles to the plane ; but if applied obliquely the component of the pressure (represented by F a, Fig. 10) which drives the vessel ahead is smaller, with a baggy sail, whilst the pressure that drives her to leeward, and assists in heeling her, is much larger. Before the wind this is a matter of no consequence ; but by the wind it is evidently of the utmost importance that the sails should be perfectly flat, and that they should be well cut, without folds or girts of any kind, and that they should never go into bags in consequence of the canvas being soft or elastic.