kudu
One last example – taken from an old textbook. The quotes are the book notes and the attached image is the figure referenced in the notes. The other referenced articles (art.) are not included.
Quote:
(7) LINES DRAWING
The sheer and deck plans are always drawn with the bow to the right. The origin of this convention is probably a desire to show the starboard side in preference to the port in the outside profile. Before the adoption of rudders, ships were steered by a large oar or "steer board" and the side on which it was secured acquired a certain precedence that it still retains Where there is a choice, the starboard alleyways and accommodation, etc., are allocated to the yacht owner and his guests or to the officers of a warship or liner.
The various features of a lines drawing are shown in fig.1. The terms used have been defined in art. (6). It will be noted that there are two sets of intersecting planes of major importance: (1) the water planes and (2) the transverse sections. There are also two sets of planes of lesser importance: (3) the bowline and buttock planes and (4) the diagonal planes. Except for the diagonals, these planes show as straight equidistant lines in two of the three views comprising a lines plan and as curves in the remaining view. The horizontal water planes show as curves known as water lines in the half-breadth plan and as straight lines in the sheer and body plan.
The transverse sections show as curves in the body plan and as straight lines (the ordinate stations) in the other views. They are parallel to a vertical datum plane at the midship section. The water planes are all parallel to a horizontal datum plane or base parallel to the L.W.L. and usually passing through the top of the keel amidships. The third datum plane is the longitudinal vertical plane on the centre line of the ship from which the half breadths are taken.
The moulded surface is completely defined by the intersections of planes (1) and (2) All offsets appearing in the displacement table are the half-breadth measurements and these can be taken from the body plan alone after this has been properly faired. See art (56) for a description of the displacement table.
(8) FAIRING THE LINES
In practice it would be impossible to fair the lines from the intersections of the water planes and transverse sections alone. The bow and buttock lines and the diagonals have to be added for this purpose.
The process of fairing the lines consists in removing any local humps or hollows in the curves and making sure that the various intersections agree in position on all the three views. This is always a case of trial and error. Not only have all the curves to be sweet and fair with the volume of displacement as required but the centre of buoyancy must be in the designed position. In addition the curve of areas must be fair and of a suitable type. This should be re-checked after the lines have been faired. It is sometimes assumed that if the different form coefficients are all as designed and the form is fair the curve of areas must be satisfactory This is a fallacy. Quite often a final check will reveal an objectionable shoulder, especially at the commencement of the run, This should, of course, be faired out,
(9) SHEER LINE
The character of this line may make or mar the whole appearance of a vessel, so that it is a matter of considerable importance for yachts and liner. The sheer on ordinary commercial vessels is usually that given in the free-board tables (arts, 72~3), as otherwise penalties are incurred, The sheer on warships is normally extremely flat to facilitate the fitting of gun mountings, torpedo racers, etc, and to save weight. The centre line of a destroyer's upper deck is often parallel to the LWL so that the sheer at the stern is only the amount of the camber amidships. This very flat sheer may look alright on paper where it is shown as a projection and foreshortened from the real line, On the actual ship the increased length and the apparent disappearance of the camber at the stern will make the deck line appear to droop at the stern and give an ugly hogged appearance,
The point requires great attention on yachts, especially on those with wide flaring bows. The sheer should first of all look right as a projection on a small profile and then additional amounts should be added at bow and stern. This compensating sheer will be noted in fig. 1. The reason for specifying a small profile is that the ordinary lines plan is so long that the eye is unable to focus the whole length at once and has to move to take in both bow and stern. A small profile should be drawn so that the eye can judge and compare both ends at the same time.
The conventional laying-off of lines and the template preparation of plates are likely to be superseded by optical and electronic methods, at all events in the larger yards. The simplest method is to project an ordinary small-scale drawing direct on to the whitewashed surface of a plate that is placed in a darkened booth. The camera must be high enough above the plate to ensure a full size enlargement without appreciable distortion. The correctness of this enlargement is checked from reference dimensions and spots on drawing and plate that must exactly correspond, The projected lines, etc., are then scribed on in the usual way.
More complicated methods enable plates to be flame-cut to size automatically and without scribing and hand guidance. In the Schichau-Monopol machine, the control of the burning head is obtained from an “electronic eye” that scans a glass negative, prepared from a one-tenth scale drawing. In the British Oxygen-Ferranti process, the burning head receives instructions from a magnetic tape that has been produced by a computer. All drawing dimensions must be given as positions relative to the axes about which the machine works.
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These notes were written from a warship point-of-view and the section numbering from forward to aft is standard except that the overwhelming practice on commercial ships is the opposite. It really does not matter except that care should be taken to ensure which system is adopted on any drawings etc that you view. Much more important is that all the views are the
same scale and both the profile and the waterlines run in the
same direction.
The last two paragraphs of the notes have been retained mainly out of interest because they describe the changes that were underway in the late 50’s and early 60’s. The change from manual lofting to 1/10th scale lofting and the early computer controlled burning. At first it was simply sets of offsets and manual definition of points to be followed by the burning head. It took another 10 years for easier coding and 10 years afterwards for 3D modelling to become the norm in the major yards.
The notes do not fully describe how to begin. Obviously the grids were prepared but the line
endings are often critical depending on the type of vessel. In the attached image, all the waterlines, except the bulwark in the fore-body, simply continue in almost straight lines and stop at the centreline. This is
not normal and can cause later problems on many ship types. If there is a stem bar of some type then the moulded line must stop on the bar not the centreline. The same thing will occur in the sectional views if there is a bar keel. This is not apparent on small scale drawings and does not become clear until large scale details are made.
Hope this has helped
Michael
01-26-2003, 10:27 PM
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