A lines plan is the drawing that defines the shape of a ship’s hull. It looks intimidating the first time you see one: three views covered in crossing curves and straight lines. Once you understand that every one of those lines is just a slice through the hull, reading a lines plan becomes fairly logical. This guide explains each view, how they relate, and how to use the plan to make bulkheads or frames.
What a lines plan is
Imagine slicing a hull in three directions:
- across it, like slicing a loaf of bread, to give stations (sections)
- horizontally, parallel to the water, to give waterlines
- vertically along its length, parallel to the centreline, to give buttock lines
Each set of slices shows up as curves in one view and as straight lines in the other two. A lines plan uses three views to show the hull from the side, from above and from the end, so that every slice appears as a curve in one of them.
The three sets of reference lines
| Line | What it is | Appears as a curve in | Appears as straight lines in |
|---|---|---|---|
| Stations (sections) | Vertical slices across the hull | Body plan | Sheer plan and half-breadth plan |
| Waterlines | Horizontal slices | Half-breadth plan | Sheer plan and body plan |
| Buttock lines | Vertical slices along the length | Sheer plan | Half-breadth plan and body plan |
Stations are usually numbered or lettered along the length of the hull. Waterlines are usually labelled by height above the baseline, and buttocks by distance from the centreline.
The sheer plan
The sheer plan (or profile) is the side view. It shows:
- the outline of the hull from the side, including the stem, keel and stern
- the sheer line, the curve of the deck edge from bow to stern
- buttock lines as curves, showing the hull shape at set distances from the centreline
- stations as vertical straight lines and waterlines as horizontal straight lines
The sheer plan is where you take the keel, stem and sternpost shapes, and the height of each station at the deck edge.
The half-breadth plan
The half-breadth plan is the view from above or below. Because a hull is symmetrical, only one half is drawn, from the centreline outwards. It shows:
- waterlines as curves, each showing the hull’s width at a particular height
- the deck edge (sheer) in plan
- stations as straight lines across the hull and buttocks as straight lines parallel to the centreline
If you build a hull bread-and-butter style, from stacked layers, the waterline curves on the half-breadth plan give you the shape of each layer.
The body plan
The body plan is the end view, and it is the view modellers use most for bulkheads and frames. It shows each station as a curve, the cross-section of the hull at that point.
Because a hull is symmetrical, the body plan normally shows only half of each section. By long-standing convention, the sections for the forward half of the hull are drawn on one side of the centreline and those for the after half on the other. On most plans the forward sections are on the right and the after sections on the left, but always check the station numbers rather than assuming.
The largest section, usually near the middle of the hull, is the midship section. Sections get smaller towards both ends.
How the three views cross-check each other
The views must agree with each other, and checking that they do is a good way to spot errors in a plan or in your own copy:
- The width of a section at any waterline in the body plan should match the width of that waterline at the same station in the half-breadth plan.
- The height of a section at any buttock in the body plan should match the height of that buttock line at the same station in the sheer plan.
- The deck edge height at each station in the body plan should match the sheer line in the sheer plan.
Use dividers or a strip of paper to transfer measurements between views. If a measurement does not match, one of the drawings is wrong, or the copy has been distorted.
Other marks you will see
- Baseline: the horizontal reference line from which heights are measured, often at or near the bottom of the keel.
- LWL (load waterline or designed waterline): the waterline at which the ship is designed to float.
- Perpendiculars: vertical reference lines at the bow and stern (forward and after perpendiculars), used to locate stations along the length.
- Midship mark: the midship station is often marked with a special symbol.
- Rabbet line: on wooden ships, the line along the keel, stem and sternpost where the planking meets them.
- Diagonals: lines drawn at an angle in the body plan and developed as curves in a separate view. They help fair the hull at the turn of the bilge, where waterlines and buttocks cross the surface at shallow angles.
Moulded lines and planking thickness
Lines plans are drawn to a particular surface of the hull, and it matters which one. For wooden ships, the lines are commonly drawn to the outside of the frames, which is the inside of the planking. These are the moulded lines. Some plans, especially those drawn for modellers, are drawn to the outside of the planking.
Check the notes on the plan. If the lines are drawn to the outside of the planking and you plan to plank your model, your bulkheads need to be smaller than the drawn sections by the thickness of your planking. Getting this wrong produces a hull that is slightly too large or too small all over.
The table of offsets
Some plans include a table of offsets: a grid of numbers giving the half-breadth of each waterline and the height of each buttock at every station. Offsets let you redraw the lines at any scale with precision. Historically, offsets were often written in feet, inches and eighths, so read the notes to see the convention.
To use offsets:
- Draw the station, waterline and buttock grid at your chosen scale.
- Mark each offset on the corresponding view.
- Join the points with a fair curve, using ship’s curves or a flexible spline.
- Cross-check the views as described above.
If you are redrawing at a new size, our guide on scaling model boat plans up or down covers the calculations.
Using the lines plan to make bulkheads
To turn the body plan into bulkhead patterns:
- Copy the body plan at your building scale.
- Trace each section onto paper, then flip the tracing along the centreline to make a full, symmetrical outline.
- Mark the deck line using the sheer plan heights, and the waterline as a reference.
- Reduce the outline by the planking thickness if the lines are drawn to the outside of the planking.
- Add the keel slot to match the real thickness of your keel material.
- Add any deck or building-board allowances your construction method needs.
Our guide to transferring model boat plans to wood picks up from here, covering how to get these patterns onto plywood and cut them accurately.
For an overview of where lines plans and other ship plans come from, see our main guide to model ship plans.