Answer Construction Question 10

Question 10: With the aid of sketches, Explain various lines plan.

Answer: Lines plan
The hull form of the ship is delineated on a scale drawing known as a Lines Plan. A set of Lines consists of three views as follows:

Profile or Sheer - side elevation: Sheer Plan which is usually placed at the top left hand side of the lines plan drawing represent the shape of the ship hull looking from the side of ship at several buttock lines. Buttock line is the vertical plane that cuts the ship along its length, creating the buttock line curves as indicated in Figure. The middle buttock line is the plane that cuts the ship along its centre line which creates the profile curve of the  ship. Other buttock lines are drawn outward (offsets) of ship’s centre line and normally at equally spaced distance. The stations and waterlines grids are shown in this sheer plan drawing.

Half Breadth Plan - plan view : The same hull form if it is viewed from top will produce the plan view of the ship. However since the hull shape is complex and unique, the plan view must be made at several waterline planes. Thus Half Breadth Plan is a lines drawing that represents the shape of the ship hull looking from top view at every waterlines of the ship. Waterline is the horizontal plane that cut the ship along its vertical axis, thus creating the waterlines curves as shown in figure. Waterline is normally equally spaced, although half waterline may also be used at the lower region of the ship. Since the hull is symmetry about its centre line, only half of the hull is shown in this plan as shown in Figure. Apart from waterline curves, the deck line curve needs to be drawn on this plan. If the ship has bulwark, chines or/and knuckles lines, these curves have also to be shown in the drawing. In this plan, the grid lines shown are the stations and buttock lines of the ship.

Body Plan - cross-sectional view: It represents the shape of the ship hull when viewing from the front or rear of the ship at every ship stations as shown in  Figure. Station is a transverse cross-section along the ship length which normally equally spaced. A ship is normally divided into 11 or 21 stations from after perpendicular, AP (Sometimes noted as station 0) until forward perpendicular, FP (or noted as station 10 or 20). Half or even quarter station may also be used especially at the region with high curvature. Body plan is normally placed at the top right hand side of the drawing although it can also be placed at the middle or on top of the sheer plan drawing depending on the size and type of ship. Since most ships have symmetrical shape for both port (left  side  looking  from  rear) and starboard (right) sides, only one side can also be in  the  drawing. Therefore, it is almost a standard practice to show the stations of the rear region of the ship at the left side of body plan while the right hand side of the body plan represents the stations at the forward region of the ship. The curve on the body plan is also call station curve. The centre line of the body plan represents the centre line of the ship. Apart from showing the station curves, the body plan also shows the waterlines and the buttock lines grid. These grid lines are essential not only for reference lines but also used for transferring and checking data from one plan to another.

A preliminary version of the Lines Plan will be prepared at the time of the conceptual design to give the required capacity, displacement etc. and is subsequently refined during the preliminary design stage to obtain the desired propulsive and seakeeping characteristics. The finished Lines Plan must be fair i.e. all the curved lines must run evenly and smoothly and there must be exact agreement between corresponding dimensions of the same point in the different views. When the small scale Lines Plan was manually drawn and faired the draughtsman would compile a 'table of offsets', i.e. a list of the half breadths at given heights above a base line to define each of the drawn cross-sections. These 'offsets' and the Lines Plan were then passed to loftsmen for full size or 10 to 1 scale fairing, or to a computer centre for full scale fairing. The loftsmen or computer centre would prepare a full set of faired offsets for each frame of the ship which would be utilized in its construction. With the use of integrated design systems on the shipyards computers, the conceptual creation of the hull form and its subsequent fairing for production purposes is accomplished without committing the plan to paper. The hull form is generally held in the computer system as a 3-dimensional 'wire model' which typically defines the moulded lines of all structural items so that any structural section of the ship can be generated automatically from the 'wire model'.