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Reading Lines Plans

Lines plan

One of Rob's lines plans from the

Mystic Seaport watercraft catalogue

    In order to understand more fully how to make half-models, you will first need to understand how to read a lines plan.  The lines plan is a drawing which describes the size and shape of a boat hull by showing four different two-dimensional views. Each view represents the shapes made by slicing the boat one or more times in a specific plane. If the hull of the boat is thought of as a thin shell, then when a portion of the hull is sliced off in any plane, the edge of the shell will form a line typical of the plane which made the cut. The next four illustrations based on a lines plan show these planes. You can think of the lines plan as a topographic map of the hull; once you understand the positions of the planes you can visualize the shape of the boat.

    The planes are distinguished by whether they pass through the boat either longitudinally - that is, fore and aft - or transversely - that is, athwartships. The four sets of lines which make up the complete drawing are: the buttock lines, which appear in the profile view; the waterlines, which appear in the half-breadth view; the sections which appear in the body plan view; and the diagonals, which usually appear by themselves.

     A buttock line is made by passing a plane longitudinally and vertically through the hull (Figure 1). The number of buttock lines given in any drawing depends on the size and complexity of the hull shape, but there will usually be three to five buttock lines.

Fig. 1 buttocks

    A waterline is made by passing a plane longitudinally and horizontally through the hull (Figure 2). Usually you will find three or more waterlines in a drawing.

Fig. 2 horiz/longitud

    A section is made by passing a plane transversely and vertically through the boat (Figure 3). There are generally six to ten sections for the main part of the hull, sometimes with additional sections more closely spaced at the ends of the boat where the shape tends to be more difficult to define.

Fig. 3 vert/transv

    A diagonal is made by passing a plane longitudinally through the hull, but this time the plane is tilted so that it angles through the boat instead of cutting through horizontally like a waterline (Figure 4).

Fig. 4 diag.