Laying-off, or Laying-down
The act of delineating the various parts of the ship to its true size upon the mould-loft floor, from the draft given for the purpose of making the moulds, is performed as follows:
In order to do this, the vessel is represented as being cut in sections, usually three ways: First, crossways, Fig. A; second, on level or parallel lines with the water line, Fig. B, and third by vertical section or parallel lines with that through the keel, stem or stern post, Fig. C. These sections are, first, the body plan, which forms the most prominent part and represents the vessel as being cut crossways of the keel. Second, the breadth plan, which represents the vessel as being cut by parallel lines with the water line. Third, the sheer plan or a longitudinal view, supposing a vessel to be divided or cut right through center, and looking at one of the halves where divided. These three plans combined are what is generally known as the draught of a ship. Without due attention and accuracy in laying down, it is almost impossible to obtain a good form of vessel and insure fairness; nor could the architect without them give any statement of the probabilities of the behavior or speed, or estimate accurately the weight of the structure, carrying, capacity or cost.
To commence to lay a vessel down, the first thing necessary is a level (or nearly so) floor. It need not be so long that the whole length of the vessel may be represented at full length, but must be wide enough to take the full width and depth of the vessel.
Some of the Government navy yards have lofts of sufficient size for the full lengths of the largest vessels, but in general use this is not the case.
Next, a measuring batten or pole is to be made, and this pole is to be the standard for all measurements both in this work and the building yard. It is very important to establish this at the very outset. Now make a long batten or pole about 2 1/2 inches by 3/4 inch thick, that will extend the entire length of the floor, to be used for sheer line, or those not requiring much curve, and known as the sheer batten. Also one about 1 1/4 inches by 3/4 inch thick, suitable for being used on greater curves and known as the water line batten. Also one about 3/4 inch by 1/2 inch thick and known as the body plan batten.
It will be found convenient to have at hand several smaller battens, with the sizes reduced and the ends tapered for use on special curves. A number of large awls should now be made for holding the battens in place; some loft men, however, prefer nails sharpened to a point for this use; also have chalk of two or more colors, and wide pencils.
Having decided on a satisfactory plan or model, to transfer the same to the floor, you will begin, first taking the measurements from the plan or model by a scale in the same manner as they are to be placed on the floor. In doing so it is customary to make a "table of ordinates." Much labor will be saved in the loft room by using great care and accuracy in reading the scale and compiling the table, as well as in the setting off in the loft. The table is usually made to read feet, inches and eighths of an inch. If measurement is a trifle more than 1/8 the sign + is added and if less the sign--is added; thus 12, 3, 7 + would be 12 feet 3 inches and 7/8, full.
Now strike or mark on the floor a straight line DD, as shown on body plan. This is called the base line. Next erect a line at right angles to base FF on body plan. This line will be the center line of vessel. Make parallel lines with base line DD 2, 4 and 6 feet apart. These are termed water lines. Now set off parallel lines on each side of center line. They are usually dotted on the body of plan. Also put a line each side of center for breadth of ship and then set off FG, which represents rise of floor.
Next we strike a level line EE, which will represent center of ship, and run parallel lines at 2 feet, 4 feet and 6 feet apart. Of course we assume in this case only 3 sections, lines 2 feet apart, but number and spacing may be lessened or increased to suit size of vessel. At base line on plan B set off frame sections 0, 1, 2, 4, 5, 6, 7, 8, at right angles to base line. They generally represent every sixth frame, but can be varied according to shape of ship. The 0 and 8 in this case are called the after and forward perpendiculars. In plan C, sheer plan, we mark off DD, and draw lines parallel to base 2, 4 and 6 feet asunder; they are termed water lines same as on plan A, body plan. Next set off frame sections on vertical lines, 0, 1, 2, 3, 4, 5, 6, 7, 8, at right angles to base; space and number similar to those on half breadth plan. When this is finished, all the fixed lines are established on floor.
It is customary in laying off the sheer and half breadth plans, to make base line DD serve as the base line EE for the half breadth plan (or in other words, having sheer plan C and hair breadth plan D made as one plan). By this means half the floor space only is required.
Where mould loft is rather short to admit of vessel being laid down full length, the vessel may be contracted--that is, frame sections are placed half the proper distance asunder. In vessels with very fine lines it is advisable to contract the length, especially at the ends, as it causes the lines to show more of a fullness or curve, and consequently the battens will give a quicker curve, and detect errors in setting off any uneven places in the lines.
From "table of ordinates" mark sheer spots at all the section lines on sheer plan C, and with long batten draw the sheer line fore and aft, using a piece of chalk. From "table of ordinates" mark points for stem at each water line, and measuring forward from frame section 7, place a batten on the points obtained and draw in curve for shape of stem; also from table mark off stern points at certain level lines parallel with water line and draw in curve by the aid of a batten. From the table, set off on body plan A half breadth at 2 feet water line, also half breadth at 4 feet and 6 feet water line, and half breadth and height of rail for midships frame section 4. Nail or pin a batten down to the points already obtained and see that the curve is fair, and then draw it with a chalk or pencil. Next take frame section 5, mark half breadths at the various water lines and rail, and draw through points with batten as before.
Proceed and line in all the other frame sections on body plan in similar way as 4. If batten does not pass exactly through spots do not force it back to spots, but see that the curve is fair; height of foot of 7 section line to be taken from 7 section line on sheer plan. Again take the half breadth of 6 feet water line at the various frame sections on body plan A, and transfer them on to half breadth plan in this method; lay batten on frame section 4 and mark off water line points, then transfer batten onto 5 frame section line and mark off water line points, and so on until you have points on each frame section transferred on half breadth plan. Pin batten down to pass through points and mark in water lines. Transfer all other water line points from body to half breadth plan in like manner, and run in lines; also the width of deck lines to be treated similar to water lines. The ending of water lines on half breadth plan are transferred from plan where each water line intersects stem and stern lines.
Now fair and prove the water lines by the vertical longitudinal sections, already marked by dotted lines on body and half breadth plans. Transfer heights from the base line of body plan A, where the transverse sections intersect line on the same frame station. Points may also be transferred from half breadth plan to sheer plan for the development of the vertical longitudinal section. When water lines on half breadth plan intersect dotted vertical longitudinal section lines, these points would be squared up to the sheer plan, and where they intersect the corresponding water lines a batten should be fastened down and a line run in. Any change of the lines in one plan makes a corresponding change in the two other plans.
It is customary to test correctness of form by running diagonal lines as shown on body plan and marked XX. The process of fairing the ship by diagonal lines instead of water lines is much the same. Take offsets from diagonal in the same way as with water lines on the body plan and transfer them onto the half breadth plan. The difference in running these lines from water lines is principally in establishing the point from which measurements are to be taken.
Where the batten does not pass through points marked, the new distance should be transferred back to the plan from which it was taken, and a new line drawn so that all the points when transferred from one plan to another will agree. After having proved that all water lines, frame section lines and diagonals meet their corresponding marks in the several places, the vessel has been faired.
Up to this point we have only obtained the form of the vessel and shape of certain frames. The intermediate frame lines can now run in on the half breadth and sheer plans, stern cants, longitudinals, keelsons, rib bands and floors. Deck lines and edges of outside plating require to be marked off on the various plans and faired, and the method to be pursued is similar to that already described.
Some draughtsmen use a shortened or contracted method for fairing. Thus, a few frames evenly spaced are put in the body plan on the floor from model or table at once. They then take a board about 8 inches wide by 3 feet long, and square marks across the same about 1/2 inch apart, each mark representing a frame section. On this set-off from the edge of board, the edge being used as the first water line (not as a center line), the first line run will represent the second water line, or, in other words, the difference in the increase in width of water line. This is continued up to the widest water line.
Beveling of frames is a term often alluded to, which means the amount of angle which must be put on the outside of frames, in order that the same, if extended, would meet the adjacent frame. These angles are taken from the floor, and placed on small boards, known as "bevel boards."
There are many other lines employed in the mould loft work not referred to herein, such as buttock and bow lines, deadwood lines, bearding or stepping lines, rabbet lines and rib band lines, but they are used for special purposes, and have reference to some local portion of the structure of the vessel, such as the shaping of the knightheads, counter frames, hawse pieces, cant frames, etc.--C. H. Simonds, Naval Architect, by permission.