PLANS AND DRAWINGS

62. To persons not familiar with the readings of plans and drawings representing the worked-out design of motor boats and other craft, the following brief explanations will prove useful, especially in cases where a prospective yacht owner must judge the qualities of his boat from drawings submitted by the designer.

63. Plans.-In general, three views are necessary to project the shape of a boat. These views, with the necessary lines and curves, are shown in Fig. 60. The sheer plan is a side view of the proposed boat. The half-breadth plan is a view looking from above the boat; one side only, being drawn, because the two sides are alike. The third view is known as the body, or section, plan, because it represents the form of the body of the boat in sections.

64. In the sheer plan is shown the load water-line, L. W. L., which is extended across the body plan. It will be noticed that this water-line is divided by vertical lines at equal distances apart, and that these lines are numbered consecutively from bow to stern. These lines are projected to the half-breadth plan below, dividing it into an equal number of parts. To find the shape of the body of the boat at any one of these lines, the different intersections of the line in the half-breadth plan are plotted and projected in the body plan. Usually, in the plotting of sections, all forward of midship line, in this case 7, are drawn on one side of the center line, while sections aft of the midship line are plotted on the other side. Thus, in the body plan of Fig. 60, the forward sections are to the left and the aft sections to the right of the center line.

The sheer plan is further divided by horizontal lines running parallel with the load water-line, and which, like the latter, are extended across the body plan. Any number of such water-lines, as they are called, may be used, but for the sake of

clearness, only two are here shown. By measuring the distance of the water-line from the center to each section in the body plan and then plotting them in the half-breadth plan, the area of the boat enclosed by each line is shown.

65. In the body plan are shown two dotted vertical lines; and, running from the center, intersecting the plan at different angles, full lines. The former are known as buttocks and the latter as diagonals. The buttocks are drawn at a fixed distance from the center of the boat in both the body plan and the half-breadth plan. From the latter plan, they are projected to the sheer plan, as shown in Fig. 60, corresponding letters x, x1, x2, and y, y1, y2 indicating the same points of intersection in both plans.

The diagonals are very useful for the purpose of fairing up and adjusting the lines of the boat so that frames and planking of body will fit together smoothly. In Fig. 60, diagonals 1 and 2 are plotted on the opposite side of the half-breadth plan by measuring and transferring the distances of each point of intersection in the body plan. Thus, the distance of ʼn on diagonal 1 from the center line in the body plan, is equal to n 2 in the halfbreadth plan. Likewise, the distance of the points z, u, and e of diagonal 2 are equal to the distance from the center line to the corresponding letters along diagonal 2 in the half-breadth plan. The same applies to corresponding letters b, c, and d and p and q of diagonals intersecting the aft sections of the body plan.

66. It is evident that before the plans of a vessel are drawn, the general characteristics of the boat must be decided upon. This may be done either by a preliminary set of drawings in which the general arrangement and principal features are embodied, or plans may be drawn from a worked-out wooden model of the proposed boat. In the first method, the actual plans and lines are prepared from the preliminary set of sketches, and, based on them, calculations are made of the weights, stability, trim, strength, speed, etc. If the shape of the hull has been determined by model, the plans are drawn according to results ascertained by the experiments of the model.

When working out his design, the naval architect uses special tools and instruments. Among them are the splines and weights for drawing the various curves, such as buttocks, water-lines and diagonals. The spline a ƒ, Fig. 60, is a flexible batten, generally made of celluloid, or hard rubber, though the best designers use lancewood battens altogether, as rubber or celluloid battens are liable to kink out of shape. When properly adjusted, these splines produce a fair curve passing through the desired points of intersection. To hold the spline in place, weights of the shapes shown in the figures are ranged along it at suitable intervals. The spline shown is adjusted to draw the aft portion of the diagonal 1 after the intersections taken from body plan are plotted.

67. Curve of Areas. A curve sometimes shown on the plan of a boat is the so-called curve of areas, which is obtained as follows: After the area of each section below the load waterline, in square feet, has been calculated, these areas are laid out in some linear scale. Thus, if the midship section contains 20 square feet, a distance of 2 feet is laid off vertically from the center line on the half-breadth plan on, say, an inch scale, assuming each foot on that scale to represent 10 square feet; the areas of all the other sections are projected in the same way on their respective lines. If no error has been made in calculating the areas, a batten or spline, bent through the extremity of each line, should make a fair curve, as shown in Fig. 61. By means of this curve the area of the midship section up to any mean draft may be determined.

68. Center of Buoyancy.-Designers using the symbol of dead flat, I, to indicate the midship section of the boat, sometimes employ the same symbol to indicate the center of buoyancy, which in most motor boats is very close to the middle of the water-line; if anything, it is a trifle aft of it, as in Fig. 61. This center of buoyancy is identical to the center of gravity of the area enclosed by the curve of areas just referred to. It is the point at which the boat is supposed to pivot or balance, theoretically, if made to float exactly even with the load waterline as drawn. In case a great deal more weight of wood enters