Build a small boat’s mast from the straightest softwood pole you can find, 8-10 cm across at the base and tapering toward the top, joining two lengths with a long scarf if no single pole is long enough, and check it against the wind load by bend-testing an offcut of the same wood. In the worked example a mast 9 cm across at the partner sees about 7.5 MPa of bending stress at the top of force 5, against about 30.6 MPa in a bend test, a margin of roughly 4 to 1 that clean wood can lose to knots, slope of grain and moisture.
Part of Small Sailboat Construction
A wooden mast is a tapered pole, solid or scarfed from two lengths, that must resist bending from the sail while staying light enough to raise by hand.
A Mast Fails Where The Wood Is Weakest, Not Where The Load Is Highest
The carrier asks for a pole that is strong, straight and light. In practice the three requirements fight each other, and the failure is decided by defects. A knot or a slope of grain in the highest-stress zone halves a pole’s strength, and a spiral grain twists and splits under load. Pick the pole by looking down its length from the end, then rolling it: it should be straight, with grain that follows the axis, and free of large knots, above all in the middle of its length where bending is greatest.
Seasoned wood is lighter and stiffer than green wood (see Wood Seasoning), but a pole dried too fast splits along its length, so it should dry slowly, protected from sun and wind, and with the cut ends sealed with pitch or wax.
What You Need
- A straight softwood pole (spruce, pine or fir), at least the boat length, with a base of 8-10 cm
- A drawknife and spokeshave to round and taper the section (see Hand Tool Making)
- Lashing cord for scarf joints (see Lashing Techniques)
- An offcut of the same wood, about 5 cm across and 1.2 m long, for a bend test
- Weights and a measuring tape
Method: Shape, Join And Test
Step 1: Round and taper. Shave the base to a clean 8-10 cm circle, then taper toward the top (the 4.5 m mast in Mast & Rigging goes from 9 cm to 6 cm), keeping the taper smooth so there is no step where stress concentrates.
Step 2: Scarf if needed. Cut each end to a long, shallow wedge so the joint is far longer than the pole is thick. A ratio of about 8 to 1 or flatter is the figure usually quoted in boatbuilding (recalled from practice, not from a source in this corpus), so test your own. For a 9 cm pole that means a joint at least 72 cm long (8 × 9 = 72). Glue it and then lash it every 10 cm or so; hide glue (see Hide Glue) softens in water, so the lashing, not the glue, is what must keep the joint closed on a wet mast.
Step 3: Bend-test the offcut. Support it on two points 1.0 m apart, hang increasing weight from the middle until it breaks, and record the load P. The bending stress at failure is 8 × P × L ÷ (π × d³), with L the span and d the diameter.
Step 4: Compare with the working stress. Compute the mast’s stress as shown below and keep the ratio well above the safety factor you adopt (see Safety Factors).
Worked Example: Stress At The Partner
The sail example in Spritsail Rig is 5.6 m². At 38 km/h the force is 68.2 N/m² × 5.6 m² = 382 N. Take the sail’s centre 1.4 m above the partner, an assumed layout. Bending moment at the partner is 382 × 1.4 = 535 N·m. The section modulus of a 9 cm circle is π × 0.09³ ÷ 32 = 7.16 × 10⁻⁵ m³, so the stress is 535 ÷ 7.16 × 10⁻⁵ = 7.5 MPa.
For the test, suppose a 5 cm offcut on a 1.0 m span breaks at P = 1,500 N. That is an illustrative reading, not a property of any species. The stress is 8 × 1,500 × 1.0 ÷ (π × 0.05³) = 30.6 MPa, so the margin over 7.5 MPa is 30.6 ÷ 7.5 = 4.1. Knots, a sloping grain, and wet wood all take from that; if your own test breaks at half that load, the margin falls to 2.0 and the mast needs a shroud (see Standing Rigging) or a smaller sail.
Common Mistakes
- Trusting a stiff pole — wood that does not flex can still be weak at a hidden defect.
- Cutting the taper as a notch — a sharp change of diameter makes a stress raiser that fails first.
- Skipping the lashing on a glued scarf — see the glue note above.
- Testing one offcut and averaging — use the weakest of several tests, since the mast breaks at its weakest point.
Safety
A broken mast falls toward the crew
A mast that snaps at the partner drops the sail and the broken pole across the boat. Test before you step the mast, keep a spare line to lash a splint to a cracked section, and never sail with a crack that runs along the grain.
What’s Next
- Learn next: Standing Rigging — shrouds and stays that relieve the mast of bending
- Combine with: Mast & Rigging — stepping and partner details