Interpreting the Lines Plan

What is lines plan? What can you figure out from the image given below?

Lines Plan


At first sight, it may seem to you like modern art, but before you conclude this let’s try and break up the drawing in smaller parts.

Profile or Sheer plan.

Consider a loaf of bread and now look at it sideways. Now imagine and cut a plane through the center along its length. The section along this plane gives the profile of the loaf/ship.

Now let’s imagine a few more equally spaced planes parallel to center plane on both the sides. The outlines that you will get from these planes are called as buttock lines. These lines run along the length of the ship.

Profile/Sheer Plan


Half-Breadth or Waterlines Plan

Now let’s look at the loaf of bread from the bottom and cut a very thin slice along a horizontal plane parallel to its base. Once again if we cut a set of equally spaced horizontal planes we will get an outline of each plane and what we get after projecting them on a single plane is called as the half-breadth plan or waterlines plan. These profiles are known as waterlines and the planes are known as water planes.


Half-Breadth Plan


Since the ship is symmetric about the centerline we only draw one-half of the outlines in the half-breadth plan.

Body Plan

Now if very thin slices of the loaf are cut at regular intervals as shown below. Now take the outlines of all the slices and project them on a plane. This kind of a representation is termed as body plan. Planes at which the slices are cut are referred as body plan stations. The body plan stations are generally evenly spaced, but towards the end, a few extra stations are drawn as the shape of the ship changes rapidly.


Body Plan


  • The horizontal lines (blue lines) in the body plan represent the height/waterplane/the z coordinate.
  • There are a few vertical lines (red lines), the one in the center is the center line from which all horizontal (y-coordinates) are measured. Out from the center line, we see evenly spaced vertical lines These are the buttock lines we saw as curved lines in the sheer profile and straight lines in the half breadth.
  • The sections to the right of the center line represent the sections from forward to the midship, while sections to the left represent sections from midship to the aft.

These lines are used to represent the shape of ship’s hull. And since the shape of the hull has a direct effect on the ship’s resistance we try to make the hull as smooth as possible. To make the hull smooth we generally avoid sharp corners or a sudden change in curvature. These sharp corners or sudden change of curvatures can be spotted if one closely observes the lines plan. Once these problematic areas are identified the designer redraws the curve to eliminate it, this process is known as fairing of the lines plan.

P.S. when a curve is being faired for example in body plan its effect will also be observed in half breadth and sheer plan. Thanks to modern technology if you use softwares like Maxsurf for fairing you can instantly see the effect on the other two plans. The designer should also note that while fairing other parameters like displacement and coefficients related to hull geometry are also affected and should keep in mind that these parameters are also adhered to.

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2 thoughts on “Interpreting the Lines Plan

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  1. Nice to see the effort put behind making the concepts easy to understand. Please continue keeping up the good work.

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