Reading the Lines

Shipwrights, Ancient and Modern. An Explanation of the Lines used to Delineate a Yacht. Designing to a Rating Rule

Apart from the cognoscenti there are many model yachtsmen to whom the art of designing a yacht savours of black magic, and the lines themselves are as meaningless as the signs of the Zodiac.

In the olden days, when a shipwright designed a vessel, he took a block of wood and carved out a halfmodel. When he had faired this to a shape that satisfied him, he proceeded to saw the block through and took off the sections he obtained. He then went to his mould loft and laid out the sections full size from his model. The section moulds were duly set up on the keel and faired up again with battens. He was then ready to build.

Many a beautiful vessel was evolved in this way, and there is no doubt that many of these old fellows had a wonderful eye for a ship. Another point in their favour was that in fairing by batten they were obliged to get a natural curve. Even today many boats are "designed" in this manner, and some of the men who do so, still style themselves "yacht and boat designers" This method is, however, too hit or miss for these modern times, and increasing knowledge has made it possible to do on paper all the old shipwright did with his halfmodel and a great deal more besides. Even today a good eye for a vessel is an invaluable possession to a designer, even as a good ear is to a musician, and just as a sheet of music is intelligible to a musician so are a yacht's lines to a designer.

So, nowadays, the old shipwright's procedure is exactly reversed, and instead of making the design from the model, the model is made from the design. The first step in designing is to understand what the lines represent. In Fig. 6 the deckline of a yacht is shown, but as the two sides of a vessel are alike, there is no need to duplicate the work, and only one side is usually shown. The craft is, therefore, considered as if it were longitudinally cut in half by a vertical line from deck to keel. This line appears on the deckline, and is known as the "centreline." What has to be considered is really the lines of a half model.  The same yacht is shown in elevation in Fig 7. This plan is usually known as the "sheer plan," and takes its name from the sheer, which is the side elevation of the deckline. A straight line will also be seen on this diagram, showing the line of the plane of flotation of the yacht. This is called the " load water line " (L.W.L.).

Treating the yacht as bilateral instead of as a halfmodel, and cutting her through horizontally on the L.W.L., a plane would be revealed shaped as Fig. 8. As, however, the yacht is being treated as a halfmodel, the part that is shown as a dotted line can be dispensed with. Comparing Figs 7 and 8, the L.W.L. appears on the sheer plan as a straight line, and in plan as a curve. In order to save space, the deckline and waterline are shown on the same plan as in Fig. 9. Other waterlines are struck at convenient intervals above and below the L.W.L., as shown by the dotted lines on Fig. 7. These are also shown on the " waterline plan," but to avoid confusion they have not been inserted on Fig. 9.

Again considering our original deckline in plan, if a line is drawn parallel to the centreline, as in Fig. 10 the result is a plane shaped as in Fig. 11. This line is known as a "buttock line." In order to save space, this is shown on the sheer plan, as in Fig 12 Other buttocks are taken out, being struck as shown by the dotted lines in Fig. 10. These also appear on the sheer plan, but for the sake of clarity, only one buttock has been shown in the diagram.

Once more, starting from our original deckline in plan, a line is struck at right angles to the centreline, as shown in Fig. 13. The boat is again cut through, and we get a shape as in Fig. 14.

As the boat is being shown as a half-model, only one half of this " section " will appear on the plans. Other sections are taken out as shown by the dotted lines on Fig. 13. These sections are plotted on the " body plan " (or " section plan "), as shown in Fig. 15. For the sake of clearness, only about half the full number of sections have been shown in the diagram. It will be noticed that on the waterline and sheer plans the sections appear as straight lines, and as curves on the section plan. Similarly, the buttocks, though not shown on Fig. 15, appear in the section plan as straight lines. The waterlines also appear as straight lines on the section plan, but only the L.W.L. is shown in Fig. 15

As will appear in the chapters on designing, the diagonals are the most important lines in the boat. The diagonals can be shown in three ways. The first is in plan in their own plane as shown in Fig. 16.

This plan is usually shown on the opposite side of the centreline to the waterline plan. It is plotted by measuring on the section plan up the diagonal to the centreline on each section. The diagonals can also be shown in elevation on the sheer plan. In this case the vertical distances above and below L.W.L. are measured on the section plan and plotted on the sheer plan. The third way is to show the diagonals in plan. To do this, the distance the diagonal is horizontally away from the centreline on each section is measured on the section plan and plotted on the waterline plan. Although not shown in the figures illustrating this chapter, the deck camber can be shown on the sheer plan by means of a line representing the centreline of the deck. This line can be seen on the plans of racing class models included in this book and is just above the sheer line. This line runs into the sheerline at the stemhead, and in a canoe sterned boat also at the sternpost head. This completes the catalogue of what are known as the "lines" of a yacht and as such represent her actual shape.


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