The 7-10-7 Rule: Perfect Spars Without Math

The 7-10-7 Rule — An Old Boatbuilder’s Trick for Perfectly Rounding a Spar

In this episode of Nomad Boatbuilding, I share a simple geometric method that replaces repetitive math when shaping spars by hand.

Some people think wooden boat building requires complicated calculations. That can be true when designing boats — but building them is another story. In practice, geometry is often far more useful than math.

The 7-10-7 rule is a straightforward way to divide the face of a square into proportional sections (7 units, 10 units, 7 units — 24 in total). By removing the corners at those marks, we quickly create an accurate octagon. Repeat the process and we move from square to octagon to 16 sides… and eventually to a smooth, round spar.

In this video I cover:
• How the 7-10-7 rule works
• Why imperial rulers make scaling easy
• Turning square stock into an octagon
• Moving from 8 sides to 16 sides
• Using (and making) a spar gauge
• Applying the method to tapered spars
• Why geometry beats repetitive calculation
• Using the same principle for moldings, banjo necks, plugs, and even walking sticks

This method works for squares, rectangles, trapezoids, and tapered spars. It’s one of those core pieces of traditional boatbuilding knowledge that makes shaping round forms efficient, accurate, and satisfying.

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⏱ Chapters:
00:00 Introduction — What Is the 7-10-7 Rule?
00:43 Geometry vs Math in Boatbuilding
01:01 Turning Square Stock into an Octagon
01:24 How the 7-10-7 Division Works
03:56 Demonstration on a 6x6 Cross Section
06:24 From Octagon to 16 Sides
08:08 Applying the Rule to Rectangles & Trapezoids
11:17 Using the Rule on a Tapered Spar
12:37 Spar Gauges Explained
14:41 Imperial vs Metric Scaling
16:08 Making an Octagonal Plug for a Hollow Mast
17:38 Planing to the Lines by Hand
18:50 A Simple Tape Trick for Divisions
20:19 Beyond Boatbuilding — Banjos & Walking Sticks
21:14 Why This Method Works So Well

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