A dugout’s stability depends on its hull shape and on how high the crew’s weight sits: for a hull whose cross-section is a circular arc the metacentre lies at the centre of that circle, so the initial stability is GM = R − KG, the arc radius minus the height of the centre of gravity. For a half-round hull of radius 0.30 m and a 75 kg paddler this gives +0.018 m kneeling, which a 5 cm lean of the body overturns, -0.327 m standing, which capsizes at once, and +0.073 m with 40 kg of ballast stones on the floor; a flatter bottom, a thicker wall that lowers the freeboard, or an outrigger float changes the picture.
Part of Coracle & Small Watercraft
Shaping for stability is the choice of hull shape, wall thickness and ballast that decides whether a dugout canoe stays upright under a moving crew.
Initial Stability And Reserve Are Different Things
The carrier says a round-bottomed dugout is fast but tippy, and for utility advises leaving the bottom slightly flat, tapering both ends upward so the bow and stern ride above the water, placing the widest point slightly behind centre, and, if it is too narrow and tippy, attaching outrigger floats (logs or sealed containers) on one or both sides, joined by poles lashed across the gunwales. Each piece of advice has a number behind it.
Distinguish the two ideas. Initial stability is the hull’s resistance to small tilts, measured by GM. Reserve or range is how far it can tilt before it loses its righting effect, which for an open hull is set by the freeboard: the angle at which water reaches the gunwale. A wide, flat boat has high initial stiffness and a range limited by shallow freeboard; a round boat has low initial stiffness and a long, shallow righting curve. The dangerous case is the one that feels fine until it does not: a loaded boat with shallow freeboard in a chop.
What You Need
- The hull’s dimensions: outer radius, wall thickness, length
- The weights and heights of crew and gear, measured above the floor
- Ballast: stones in bags, lashed in, or sealed containers of water
- For outriggers: sealed floats, poles and lashing (see Lashing Techniques)
- A calm pool for a real inclining test (see Loading for Stability)
Method: Compute The Metacentre
Step 1: Find the centre of the circle. For a half-round hull with a flat opening at the diameter, the centre is at gunwale level, R = 0.30 m above the lowest point.
Step 2: Find the combined KG: the mass-weighted height of hull, crew and ballast above the bottom.
Step 3: GM = R − KG. Positive means stable at small angles, negative means it capsizes.
Step 4: Check the range: the gunwale reaches the water at sin φ = freeboard ÷ R.
Step 5: Test it afloat with a known weight moved across the hull, as in the inclining test.
Worked Example: A Half-Round Dugout
Hull: radius 0.30 m, wall 4 cm, length 3.5 m, shell mass 55 kg (from Dugout Canoe). The shell’s centre of gravity lies 2R_m ÷ π below the circle’s centre, where R_m = 0.28 m is the mid-wall radius: 2 × 0.28 ÷ π = 0.178 m, so the shell’s KG is 0.30 − 0.178 = 0.122 m. A 75 kg paddler’s centre of gravity above the floor is assumed: 0.30 m sitting on the floor, 0.40 m kneeling, 1.0 m standing (measure your own).
| Case | KG (m) | GM = 0.30 − KG (m) |
|---|---|---|
| Sitting on the floor | (55 × 0.122 + 75 × 0.30) ÷ 130 = 0.224 | +0.076 |
| Kneeling | (55 × 0.122 + 75 × 0.40) ÷ 130 = 0.282 | +0.018 |
| Standing | (55 × 0.122 + 75 × 1.0) ÷ 130 = 0.627 | −0.327 |
| Kneeling, 40 kg stones at 0.05 m | (6.7 + 30 + 2) ÷ 170 = 0.228 | +0.073 |
Standing is a capsize: a negative GM means the hull rolls over until something else supports it. Kneeling is positive but thin. If the paddler leans 5 cm sideways, the whole centre of gravity shifts 75 ÷ 130 × 0.05 = 0.029 m. The righting arm at the gunwale angle is GM × sin φ. Spreading the 130 kg over a 3.5 m prismatic hull needs a submerged area of 130 ÷ (1,000 × 3.5) = 0.0371 m², which is a draft of 0.113 m and a freeboard f = 0.187 m, so sin φ = 0.187 ÷ 0.30 = 0.622, φ = 38.5°, and the kneeling righting arm is 0.018 × 0.622 = 0.011 m. The shift 0.029 m exceeds the 0.011 m available, so the kneeling paddler is overturned by a 5 cm lean. With the stones (170 kg) the draft is 0.137 m, f = 0.163 m, sin φ = 0.544 and the arm is 0.0725 × 0.544 = 0.039 m, which allows a lean of 0.039 ÷ (75 ÷ 170) = 0.089 m, about 9 cm. (A prismatic hull overstates the freeboard because real ends taper, so treat these as upper bounds.) Ballast adds weight and reduces freeboard, but is a cheap way to gain GM; tie it down.
Flat bottom. For a flat-bottomed section of waterline width b, BM = b² ÷ (12 d), with d = 130 ÷ (1,000 × 3.5 × b) the draft, KB = d ÷ 2 and the kneeling KG of 0.282 m. At b = 0.4 m GM is −0.09 m, at 0.5 m +0.036 m, at 0.6 m +0.23 m: GM goes with roughly the square of the waterline width. These are rectangle-limit values, which a real rounded dugout reaches only if the log is wider than 0.6 m.
Outrigger. A sealed 30 L float of 1.5 kg (assumed) has 30 − 1.5 = 28.5 kg of buoyancy, 280 N when fully submerged; at 1.0 m from the centreline it gives a righting moment of up to 280 N·m. The 5 cm lean of the paddler is a heeling moment of 75 × 9.81 × 0.05 = 37 N·m, so the float has 280 ÷ 37 = 7.6 times the margin, at the cost of drag and a structure that must stay lashed.
Common Mistakes
- Standing in a round canoe — GM goes negative.
- Loose ballast — stones that slide are worse than none; tie them in bags.
- Thick walls — they cut the freeboard and the range.
- Floats lashed loosely — an outrigger that moves cannot right the hull.
Safety
Feels stable until it is not
A round dugout can feel quiet at rest and still overturn with a small shift of weight. Kneel low, keep the weight central, do not stand, and wear flotation. If it overturns, stay with the hull; see Safety & Capsizing Recovery.
What’s Next
- Learn next: Canoe Paddling — staying upright while moving
- Combine with: Loading for Stability — the same formulae for a wide hull