How to Scale Model Boat Plans Up or Down Without Losing Accuracy

Plans are rarely at exactly the size you want. A magazine plan may be drawn for a 60 cm model when you want a metre-long hull, or a museum plan may be at 1:48 when you want to build at 1:96. Scaling plans is straightforward in principle, but there are a few traps that can spoil accuracy. This guide covers the arithmetic, the main methods, how to check your results, and the parts of a model that should not simply be scaled with the drawing.

The basic formula

To find the enlargement or reduction percentage:

Percentage = (old scale number ÷ new scale number) × 100

Here “scale number” means the second figure in the ratio: the 48 in 1:48.

Worked examples:

• 1:96 to 1:48: 96 ÷ 48 = 2, so print at 200%.
• 1:48 to 1:64: 48 ÷ 64 = 0.75, so print at 75%.
• 1:72 to 1:35: 72 ÷ 35 ≈ 2.057, so print at about 205.7%.

If you know the length you want rather than a scale, use lengths instead:

Percentage = (desired model length ÷ length on the plan) × 100

For example, a hull drawn 600 mm long that you want to build at 900 mm needs printing at 150%.

Methods for resizing plans

Photocopier or print shop

Most copiers enlarge and reduce by a set percentage, which makes them the quickest option for small plans. For large sheets, a print or copy shop with a large-format printer can scan and print the whole plan in one piece.

Points to watch:

• Copiers can introduce slight distortion, particularly along the length of the sheet. Check every copy (see below).
• Repeated enlargements of enlargements compound errors. Go from the original to the final size in one step where possible.
• Make all copies on the same machine at the same settings so parts match.

Scanning and scaling digitally

Scanning a plan lets you resize it precisely in image-editing or drawing software and print it in tiles on a home printer.

• Scan at a high enough resolution that fine lines stay sharp after enlargement.
• Check the scan is square and not skewed before resizing.
• When printing, set the printer to actual size or 100%, not “fit to page”, or the printer will rescale it for you.
• If you redraw the plan in vector or CAD software, you can scale it with no loss of line quality at all.

The grid method

The grid method needs only a pencil and ruler. It is slower but useful for simple drawings or when no copier is available.

1. Draw a grid of equal squares over a copy of the plan, for example 1 cm squares.
2. Draw a second grid on your new sheet with squares scaled by your ratio. For 200%, that would be 2 cm squares.
3. Copy the drawing square by square, marking where each line crosses the grid lines and joining the points.

For curves like hull sections, use a flexible curve or ship’s curves to join the points smoothly.

Checking accuracy after scaling

Before cutting any wood, check every scaled sheet:

• Measure the scale bar on the copy. If the plan has one, it should measure exactly what it says at the new scale.
• Measure in both directions. Check a length dimension and a width or height dimension. If one is right and the other is not, the copy is distorted.
• Overlay copies. Lay matching parts from different sheets over each other on a light box or window to confirm they agree.
• Cross-check the views. The hull length should be the same on the profile and half-breadth plan, and section widths should match the half-breadth plan. Our guide on how to read a model ship lines plan explains how the views relate.

What does not scale with the drawing

This is where most scaling problems come from. Some dimensions on a plan represent the materials you build with rather than the ship itself, and these must be redrawn rather than enlarged:

• Material thickness. If the original plan used 3 mm plywood bulkheads and you scale it to 200%, the plan will now show 6 mm. Decide what material you will actually use and redraw accordingly.
• Keel and bulkhead slots. Slots must match the real thickness of the material that fits into them, not a scaled version.
• Planking allowance. Bulkheads are usually drawn inside the hull line by the planking thickness. If your planking thickness does not change in proportion, adjust the bulkhead outlines. Our guide to transferring model boat plans to wood covers this.
• Commercial fittings. Propellers, motors, shafts and fittings are bought in standard sizes. Check what is available at the new size.
• Rigging line sizes. Rope diameters on a scaled rigging plan need to be matched to real thread sizes.
• Notes and text. Labels may become tiny or huge. It does not affect accuracy, but it can make sheets hard to read.

Scaling a working model: weight, stability and speed

If the model will float or sail, scaling changes more than its size:

• Weight and displacement scale with the cube of the size change. Doubling the length of a model multiplies its displacement by roughly eight. Halving it reduces displacement to about one-eighth.
• Area scales with the square. Sail area, deck area and the cross-section of the hull increase by four times when the length doubles.
• Scale speed for a realistic-looking model follows the square root of the scale. A 1:16 model looks right at about one-quarter of the real vessel’s speed, since √16 = 4.

In practice, a larger model can carry more ballast and gear and usually behaves more like the real thing, while a smaller model has less room for equipment and less margin on weight. If you are scaling down a working design, check that the motor, batteries and receiver will still fit and that the hull can carry their weight. Our guide to radio control setup for model boats covers what that gear involves.

A simple workflow

1. Decide the final scale or length.
2. Calculate the percentage.
3. Resize from the original in one step.
4. Check every sheet for size and distortion.
5. Redraw material thicknesses, slots and planking allowances.
6. Check fittings and gear for the new size.
7. Only then start making templates.

For more on finding and judging plans in the first place, see our main guide to model ship plans.