The writers have long been of the opinion that a new work on the subject of "Laying Off," adapted for the use of students, had become necessary. It came to their knowledge that both were engaged in preparing such a text-book, and this led to a collaboration, the present work being the result.

One difficulty in the preparation of such a book is that practice varies considerably in different building yards and in different shipbuilding centres. The principle adopted has been to describe processes and methods which are known to the authors and which are found to be satisfactory in practice. If other methods come under the reader's notice they may be compared with those indicated herein.

It has been thought desirable to provide an introductory chapter on the principles of "descriptive geometry" in order to lay a satisfactory foundation of knowledge for the subsequent work.

Although the work of "laying off" is mainly done by a special class of skilled workmen, it is considered very desirable that ship draughtsmen and students of Naval Architecture should have an acquaintance with the subject. In many cases it is not found possible for such students actually to do work themselves on the mould loft floor. It is hoped that the present work will be found useful to this large class.

It is strongly recommended that students, in addition to reading the text, should draw out for themselves the various examples that are scattered throughout the book.

The drawings illustrating the text are very numerous, and have been very carefully drawn, and care has been taken in the lettering, etc., in order to make the illustrations self-explanatory as far as possible. Students requiring a knowledge of the special and difficult laying-off problems connected with wood shipbuilding are referred to Dr. Thearle's valuable work on "Naval Architecture."

Mould Loft Floor - All shipbuilding yards have a special building called the "mould loft," on the floor of which vessels are laid off. This floor is a large clear space, well lighted, and the lines of the ship are drawn in upon it in chalk and faired. Where a floor is specially constructed for the purpose it is usually built up of planks about 6 in. wide and 3 in. thick, each plank being tongued and grooved at the edges. The butts of the planks are fitted as shown in Fig. 22; and all nails or screws should be sunk below the surface, the hole being filled in with a wood dowel similar to that in a wood deck. The floor should be laid on a substantial level foundation, and the surface well smoothed and covered with a flat black paint. In one yard the planks of the floor are laid at an angle, and this is stated to be a very convenient arrangement, as the ordinates are then quite clear of the plank edges and no confusion in this respect can arise.

Along each of the sides of the floor a fixed straight batten is secured from which measurements may be taken by simply forcing the end of the scale or batten against it. Considerable labour may be saved in this way as compared with having to set the end of the scale or batten to the base line on each occasion. The base line would be drawn at some even distance away from the fixed edge-batten. In one yard a sloping blackened board is fixed behind the edge batten on which numbers of stations can be chalked for convenience in placing the end of the scale or batten when lifting off distances from the edge, to ensure that the scale or batten lies along the ordinate.


WORK ON THE MOULD LOFT FLOOR

Sheer Drawing - To delineate the form of a ship three planes of reference are employed. These are:

(a) The, Sheer, which gives the general outline of the profile of the ship, the position and sheer of decks, also the position of the L.W.L. Sometimes the main portions of the structure, as the transverse bulkheads, frames, vertical keel, decks, etc., are shown.

(6) The Half-breadth, which gives the shape of the decks and the shape of lines given by the intersection with the ship's surface of horizontal planes.

(c) The Body Plan, which gives the shape of lines given by the intersection with the ship's surface of transverse vertical planes. The forward sections constitute the fore body on the right and the after sections the after body on the left.

These three plans together form the sheer drawing, a specimen one for a small tug being given in Plate I.

The lines A, B, and C in the half-breadth and body are called low lines forward and buttock lines aft, and the shape of the intersection of vertical planes through these lines with the surface of the ship appear as dotted in the sheer. (Although these lines are used in fairing the sheer drawing, they are not usually inked in in the finished plan.) The three plans are mutually dependent; thus at No. 4 station the breadth at number 3 level line is given by oa in the half -breadth and also by oa' in the body plan, and these must be equal.

The bow and buttock lines must cross the level lines in the sheer at points which are squared up from the corresponding intersections in the half-breadth. They must also cross the section s at points which are squared across from the corresponding intersections in the body plan.

The work of designing the shape of a ship is a matter of "ship design." It will be sufficient here to say that displacement, fineness of form, shape of midship section, position of the centre of buoyancy, both fore and aft and vertically, and position of the transverse metacentre, all have to be considered. The process is often a matter of trial and error before a form having the desired qualities is obtained, and when obtained the form must be "fair," i.e. all the curved lines must run evenly and smoothly and the various portions of the sheer drawing must correspond as above described. The work of making a good sheer drawing can only be accomplished after much practice.

The sheer drawing issued from the drawing office has now to be enlarged to full size on the floor. This drawing for a large ship will usually be drawn to the scale of 1/4 in. to the foot. Measurements are sometimes sent out on a skeleton displacement sheet which define the shape of each displacement section and which will result in a form giving the desired qualities. It must be mentioned that the usual practice is to draw the sheer drawing to represent the "moulded" surface, i.e. to the frame lines, but for Admiralty work the drawing is to an imaginary line representing a mean thickness of plating added to the frame line, and this thickness must be allowed for when drawing in the frame lines on the floor. In the case of a sheer drawing for a "sheathed" ship the thickness of the planking as well as the plating must be allowed for. Methods of making these allowances will be discussed later.

In some yards the preliminary work of fairing is done with the displacement sections, and it is not until the body is approved as regards displacement, position of centre of buoyancy and metacentre, that the plating (or plank) is "taken off" and the moulded surface faired.

It is now a very usual practice in merchant shipyards to fair the ship on drawing paper stretched on a boards the drawing being made on a large scale (say f in. to the foot for a large ship), and by skilful draughtsmanship exceedingly good results are obtained, the measurements from this drawing when completed being taken on diagonals direct to the scrive board. As a check on the fairness, it is desirable to transfer measurements on level lines and diagonals to the mould loft floor and have lines run through the spots thus obtained. The body plans which show each frame on this large scale are found of great assistance in the subsequent drawing office work, as exact drawings can be made, giving the shape of the ship at any desired position. For Admiralty work it is usual to do the whole of the fairing on the mould loft floor.

Length of Ship - The length of the ship is usually defined as the "length between perpendiculars." The fore perpendicular (F.P.) is a line square to the designed load water-plane at its intersection with the fore edge of the stem (see Fig. 30). The after perpendicular (A.P.) is a line square to the designed water-plane, usually through the centre line of the rudder head in war vessels. In merchant ships the A.P. is usually the after edge of the stern post, as in Plate I., and if the stern post has a rake, the point where the after edge produced meets the upper deck beam at centre, is taken as the A.P. For the F.P. if the stem has a rake the run of the fore edge of stem is continued up from below the water-line until it meets the upper deck beam at centre.

It is usual in merchant-ship practice to number displacement sections and frame stations from aft, but in Admiralty practice this is done from forward, except in some special cases. The number of displacement sections on the sheer drawing will vary with the size of ship. It is usual in Admiralty practice to use twenty-one ordinates, dividing the length between perpendiculars into twenty equal parts. The conditions will vary for different types of ships; thus the length for displacement in a single-screw merchant ship would be from the fore side of the body post to the after side of stem, and this length would be divided up as convenient for obtaining the displacement (see Plate I.).

Laying off on the Floor - When transferring the measurements given on the skeleton displacement sheet or taken from the sheer drawing on to the floor full size, it is evident that inaccuracies which are not apparent on the small scale of the sheer drawing will be revealed, because of the great enlargement. The process by which a fair surface is obtained is called fairing the body.

Besides the sections, water-lines, and bow and buttock lines above mentioned, diagonals are freely employed for fairing the body. A diagonal is the shape of the intersection with the surface, of a plane which meets the middle line plane in a line parallel to the load water plane, and which is inclined both to the sheer and the half-breadth. A diagonal will appear as a straight line in the body plan. Diagonals are of great use in fairing because we obtain with them square definite intersections with the sections. For instance, it will be noted in Plate I. that the intersections of the lower water-planes with the sections in the body plan are very acute and not easy to locate with perfect accuracy.


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