The lines drawing: what it is and how to read it

Years ago, the first step to design a boat was to carve a small-scale hull model in wood. This model was then sawn into vertical and horizontal sections that were ultimately used to generate the drawings that allowed manufacturers to build the full-size hull. This process was later inverted, and designers started to draw the lines of the hull first. A small-scale model of the hull was still built, based on the drawings, to check the design’s suitability. Today, computer-aided design software is used to develop the three-dimensional shape of the hull.

The lines drawing, also referred to as the lines plan, is the conventional graphical representation of the hull. Besides the hull, it sometimes also includes the keel and the rudder/s.

In the lines drawing, the shape of the hull is represented by three different views, which can be considered the projection of the hull onto three orthogonal planes:

Profile plan: is the view of the hull from the side and is usually placed at the top left of the lines drawing. This view is very important to appreciate the appearance of the yacht since it shows the shapes of the bow, the stern, and the sheer line;
Body plan: is the view of the hull from the front, and it is usually placed at the top right of the drawing;
Plan: is the view of the hull from the top. As the hull is typically symmetric about its centerline, only half of the hull (half-breadth plan) is usually shown. This view is typically located under the profile plan, at the bottom of the drawing.

Lines drawing in white and black of a sailing yacht

To better appreciate in a two-dimensional drawing the three-dimensional shape of the hull, a whole set of different lines are generally also drawn:

Waterlines are horizontal cuts through the hull parallel to the waterplane. They are the lines representing the intersection between the hull and horizontal planes parallel to the still water surface. The shapes of the different waterlines curves are seen in the plan (or half-breadth plan) view;
Sections are transverse vertical cuts through the hull perpendicular to the centerline. These lines are the intersection between the hull and vertical planes perpendicular to the hull’s symmetry plane. The shape of the sections lines can be seen in the body plan. As the hull is usually symmetrical port and starboard, the hull’s forebody is commonly represented in the body plan to the right, and the afterbody to the left;
Buttocks are vertical, longitudinal cuts through the hull parallel to the centerline. They represent the intersection between the hull and vertical planes parallel to the symmetry plane. The shape of the buttocks is fully appreciated in the profile plan, and their position is indicated at the plan or half-breadth plan view;
Diagonals are shown in the plan or half-breadth plan. They are obtained by cutting the hull longitudinally in different inclined planes, which are, as much as possible, at right angles to the surface of the hull. Diagonals are the paths the water theoretically follows when the yacht is moving through the water surface and they are, therefore, an indication of the hull’s longitudinal smoothness. Uneven diagonals curves will increase the resistance and thus reduce the speed of the yacht.

Sailing yacht hull - 3D model — 3D model of a hull

Sailing yacht hull - 3D model with sections — Sections

Sailing yacht hull - 3D model with buttocks — Buttocks

Sailing yacht hull - 3D model with waterlines — Waterlines

Sailing yacht hull - 3D model with sections, buttocks, and waterlines — Sections, buttocks, and waterlines

Model with sections, buttocks, and waterlines

The three-dimensional hull can be fully defined just with waterlines and sections. However, buttocks and diagonals are also employed to help in the visualization of its shape.

The traditional rule for positioning the sections is to divide the designed waterline, D_WL, into ten equal parts. The sections are then numbered from the forward perpendicular, FP, backward, from section 0 to section 10. However, additional and/or intermediate stations can also be drawn. Sections located before section 0 are usually named with negative numbers (e.g., section -1), while sections located aft of section 10 are designated with the following available numbers (e.g., section 11, section 12, etc.).

The reason why the waterline is divided traditionally into ten evenly spaced stations is that it makes the calculation of the hull’s hydrostatic characteristics such as the prismatic coefficient (C_P), the longitudinal position of the center of buoyancy (LCB), the longitudinal position of the center of flotation (LCF), or the range of static stability easier (e.g., by manually integrating the curves with the Simpson’s rule). Today, these characteristics are calculated by computers, and the number of sections and their locations depend more on the hull’s manufacturing process.

Whatever the case, as a general rule, the total number of waterlines, sections, buttocks, and diagonals to be drawn will depend on how rapidly the hull shape changes. The more abruptly the curvature of the hull changes, the more lines will be necessary to define it.

Lines drawing in color of a sailing yacht

Another curve is sometimes also included in the plan or the half-breadth plan view. It is the curve of sectional areas. The value of this curve at each station represents the area of that section under the designed waterline (D_WL), i.e., under the water. The total area under the curve represents the volume displacement (∇) of the yacht.

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