534. Q.—­In the case of a board moving in water in the manner of a paddle float, or in the case of moving water impinging on a stationary board, what will be the pressure produced by the impact?

A.—­The pressure produced upon a flat board, by striking water at right angles to the surface of the board, will be equal to the weight of a column of water having the surface struck as a base, and for its altitude twice the height due to the velocity with which the board moves through the water.  If the board strike the water obliquely, the resistance will be less, but no very reliable law has yet been discovered to determine its amount.

535. Q.—­Will not the resistance of a vessel in moving through the water be much less than that of a flat board of the area of the cross section?

A.—­It will be very much less, as is manifest from the comparatively small area of paddle board, and the small area of the circle described by the screw, relatively with the area of the immersed midship section of the vessel.  The absolute speed of a vessel, with any given amount of power, will depend very much upon her shape.

536. Q.—­In what way is it that the shape of a vessel influences her speed, since the vessels of the same sectional area must manifestly put in motion a column of water of the same magnitude, and with the same velocity?

A.—­A vessel will not strike the water with the same velocity when the bow lines are sharp as when they are otherwise; for a very sharp bow has the effect of enabling the vessel to move through a great distance, while the particles of water are moved aside but a small distance, or in other words, it causes the velocity with which the water is moved to be very small relatively with the velocity of the vessel; and as the resistance increases as the square of the velocity with which the water is moved, it is conceivable enough in what way a sharp bow may diminish the resistance.

537. Q.—­Is the whole power expended in the propulsion of a vessel consumed in moving aside the water to enable the vessel to pass?

A.—­By no means; only a portion, and in well-formed vessels only a small portion, of the power is thus consumed.  In the majority of cases, the greater part of the power is expended in overcoming the friction of the water upon the bottom of the vessel; and the problem chiefly claiming consideration is, in what way we may diminish the friction.

538. Q.—­Does the resistance produced by this friction increase with the velocity?

A.—­It increases nearly as the square of the velocity.  At two nautical miles per hour, the thrust necessary to overcome the friction varies as the 1.823 power of the velocity; and at eight nautical miles per hour, the thrust necessary to overcome the friction varies as the 1.713 power of the velocity.  It is hardly proper, perhaps, to call this resistance by the name of friction; it is partly, perhaps mainly, due to the viscidity or adhesion of the water.