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DIVISION SECOND.

DIRECTIONS FOR

"LAYING DOWN" AND "TAKING OFF" SHIPS

ON

THE MOULD-LOFT FLOOR.

INTRODUCTORY OBSERVATIONS.

1. Laying down consists in drawing the lines of the ship on the mould-loft floor, to the full size. The body-plan forms the most prominent part, and the sections which are shown in it are moulding-lines or outside the timber, and are measured with a scale from the drawing or model, and with these measures the lines are laid down. When a model is used in laying down, a series of vertical lines representing generally every fourth frame, or at least a sufficient number for ascertaining correctly the figure of the vessel, are to be drawn upon the vertical side of it. `The measurements for constructing the sheer-plan can now be obtained by taking the height of the several sheer and water-lines from the base-line or lower edge of the model, and the distances forward or aft of some given vertical line on each water and sheer-line, of the fore-edge of rabbet of stem, and after-edge of rabbet of stern-post and centre-counter timbers. The next operation is to take the model apart, and draw upon each lift, ordinates at the stations of the frames already marked. These ordinates, being measured by the proper scale, give the half-breadths at the points where the several frames intersect the several water-lines and sheer-lines; and all the dimensions as taken off are to be written in a Table prepared for that purpose and intended to be used in transferring the lines to the full size on the mould-loft floor. These Tables are known as "Tables of Ordinates." In many cases, the desired form of the vessel is first drawn on paper, the lines proved and all the necessary calculations made before making a model, the necessary dimensions are then taken off and written in the Tables as before described. The reader is referred to the "Tables of Ordinates" for laying down a steam screwcorvette of the Antietam class, which were taken from a drawing and show exactly what data is required before commencing the work in the mould-loft. A great deal of trouble may be saved in the fairing-in of the lines, by reading with the scale on the drawing as accurately as possible, so that when these measures

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are laid down on the floor, and a batten passed through, the curve drawn along its edge will form a fair curve.

Unless due attention be paid to laying-down, considerable injury may be done to a good design, by deviating from the drawings. Accuracy in making the moulds is also necessary, in order to insure the economical appropriation of timber; and it also facilitates the execution of the workmanship.

In proceeding with the explanations required for laying down the body of a ship, it is taken for granted that the student is conversant with the names of the various timbers of the frame, and the manner in which they are combined; if not, it is necessary to study these points first, which may be done by referring to the Third Division of this work on Shipbuilding.

2. The laying-down upon the mould-loft floor should be proceeded with in such a manner as to be least subject to error, and, as far as practicable, that the conversion of the timbers may go on at the same time, to prevent any delay in proceeding with the structure. The moulds, therefore, should be made for the different timbers as they are laid down, beginning with the stem, stern-post, forward and after deadwoods and fore-foot; which, with a plan of the shift of deadwood, should be sent at once into the shipyard for providing the pieces. In order to economize time, the keel-scarphs are laid down and a mould made for them; the after-piece of the scarph always overlaps the foremost piece; the scarphs are what are termed "plain horizontal scarphs" and should be long enough to admit of the fastening of four frames through each, the nibs of the scarphs should come under a futtock or filling timber. Two long battens, having the distance between the joints of the square frames painted upon them, called "room and space battens," are accurately marked; one is sent to the foreman of shipwrights to be used in laying off the stations of the frames on the keel, the duplicate is retained and used in the mould-loft in laying down the vessel.

In laying down a vessel there are three principal plans made use of which are distinguished by the names of sheer, body and half-breadth plans. These combined constitute what is termed the draught of a ship. We will describe each separately.

SHEER PLAN.

3. The sheer-plan (Plates I. and II., Fig. 1) is the representation of an imaginary longitudinal section, dividing the ship into two

equal parts, by a vertical plane passing through the middle of the keel, stem, and stern-posts. This section is bounded by the forepart of the head, under-side of the keel, aft side of the rudder, rake of the stern, and sheer of the upper part of the top-side.

Besides this plan being a section of the ship amidships, showing the sheer of the decks, rails, port-sill, cutting-down or throating-line, bearding-line, &c., on it are also projected, in lines perpendicular to the aforesaid longitudinal section, the stations of the frames, the ports, cathead, head-rails, side countertimber, &c.

From this we see that upon this plan all lines are projected as to height and length; it gives the vertical longitudinal form, and from it lines are transferred to the half-breadth as to length, and to the body as to height.

The lines of the sheer-plan are transferred to the body-plan by means of their heights taken at various parts on the sheer-plan, and those same heights measured off as heights on the body-plan from the base-line. All horizontal lines on the sheer-plan will be horizontal on the body-plan, as the water-lines when parallel to the base-line.

BODY-PLAN.

4. The body-plan (Plates I. and II., Fig. 2) is simply a representation of the form of the ship by transverse sections perpendicular to the keel, before, at, and abaft the greatest transverse section, which is termed "dead-flat," and usually denoted by the symbol . These transverse sections are transferable to the halfbreadth plan as to breadth, and to the sheer-plan as to height.

HALF-BREADTH PLAN.

5. The half-breadth plan (Plate VII., Figs. 1 and 2) is that on which is shown the form of the body by horizontal and diagonal longitudinal sections, from which are obtained other sections in the body-plan,-such as the intermediate transverse sections, and the oblique sections as the cant timbers. Besides the above the form of the decks, port-sill and rail-lines, may be also delineated on the half-breadth plan.*

*

*The lines of the half-breadth plan are transferred to the body-plan by means of their distances taken at various parts on the half-breadth plan, and those same distances measured off from the centre line of the body-plan, either in a horizontal or diagonal direction. All lines parallel with the centre

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