The third or vertical component of the pressure tends to add to the immersion of the vessel ; but unless she were heeled to a very considerable angle this part of the pressure would have little effect, and in no case could serious or important consequences arise through the extra immersion due to the vertical pressure of the wind.
If a vessel resembled a true hemisphere, with no keel or dead wood of any kind, it is certain that she would, under the influence of a wind pressure, proceed to leeward in the direction F b much faster than ahead in the direction F a ; but a yacht is so formed that she offers very great resistance to lateral or sideway motion, and very little to headway. In similarly formed vessels, the difference between the resistance to lateral motion and to forward motion is generally taken as proportional to the area of the midship section and the area of the longitudinal section, it being always understood that only the immersed portions of these sections are referred to. This proportion roughly is as 10 to 1 ; but in reality it does not show the actual relative value of the lateral resistance and resistance to forward motion which depends upon the form of the vessel and her area of immersed surface. For speeds of 6 knots this relative value will be found by simply comparing the resistance due to surface friction and the resistance to broadside motion. Thus, say a vessel has 1000 square feet area of immersed surface, and a plane of 450 square feet area for lateral resistance ; at six knots the frictional resistance on the immersed surface, consequent on forward motion will be equal (nearly) to +lb. per square foot or 2501b. in the aggregate. That is, the whole force required to move the vessel at a speed of six knots will be 2501b., that being the total resistance at such a speed.
The resistance on the plane of 450 square feet (see Fig. 8, page 14), to lateral motion will not be one of " friction," but of direct pressure, and at a speed of six knots would be 1121b. per square foot, or 50,400lb. in the aggregate. But the vessel would not have this enormous lateral motion, as the force that world drive the vessel ahead at the rate of six knots an hour would have very little influence in driving her side ways. In the diagram (Fig. 10) let F a represent a force of 2501b. which overcomes the resistance to forward motion, and F b a force which is three times 2501b. or 7501b. The force of 7501b. is exerted on a plane of 450 square feet area, which is equal to 1.661b. to the square foot, and the resistance that would balance a force of 1.661b. per square foot would be met with at a speed of 0.68 knot. Then, if the headway were six knots, and the leeway knot, the "angle of leeway" would be 6°. We are not aware if anyone has actually tested the leeway a yacht will make when close-hauled under a wind pressure that will give her headway at the rate of six knots an hour ; but it has been variously estimated from 8° to 11°, or from + point to 1 point of the compass. We are inclined to think, so far as our observation goes, that a cutter yacht when sailing a course 4 points from the wind will, including leeway, make a course of about 4i points ; it would, however, be impossible to ascertain this accurately, as the wind does not remain sufficiently constant in strength and direction even whilst a mile could be traversed.