Estimate initial stability from a weight-shift test. Move a known weight across the boat, measure the heel, and enter the numbers; the page returns GM and the small-angle righting moment per degree.
Use this when you want a number for initial stability after a refit, a ballast change, or added weight aloft — or simply to understand how stiff your boat is at small angles.
Do the test in calm water with mooring lines slack, tanks either full or empty (half-full tanks slosh), and crew in fixed positions. Move a known weight a measured distance across the boat and read the steady heel, either with a digital inclinometer or a pendulum hung in the companionway.
Displacement should be the boat’s weight at the time of the test, including the test weight.
Moving a weight w a distance d across the boat creates a heeling moment w × d. The boat heels until the righting moment balances it. At small angles the righting arm is GM × sin(heel), so displacement × GM × tan(heel) ≈ w × d, and GM = w × d ÷ (displacement × tan(heel)). This is the same relationship naval architects use in a formal inclining experiment.
A pendulum gives tan(heel) directly: the sideways deflection at the bob divided by the pendulum length. Long pendulums give bigger, easier-to-read deflections. The righting moment per degree is displacement × GM × sin(1°), useful for comparing before and after a change.
tan(heel) = measured, or pendulum deflection / pendulum length
GM = (weight × distance) / (displacement × tan(heel))
righting moment per degree ≈ displacement × GM × sin 1°
Each row below starts from the default inputs and changes only the columns shown. Recompute any row by hand with the formula above.
| Disp lb | Weight | Shift ft | Method | Heel | GM ft | RM per ° |
|---|---|---|---|---|---|---|
| 10000 | 200 | 8 | Angle | 1.50° | 6.11 | 1,066 |
| 10000 | 200 | 8 | Angle | 1.00° | 9.17 | 1,600 |
| 10000 | 200 | 8 | Angle | 2.50° | 3.66 | 640 |
| 10000 | 200 | 8 | Pendulum | 1.51° | 6.06 | 1,058 |
| 6000 | 150 | 6 | Angle | 2.00° | 4.30 | 450 |
| 18000 | 300 | 10 | Pendulum | 1.49° | 6.40 | 2,011 |
With the calculator defaults (Displacement incl. test weight (lb) = 10000; Weight moved (lb) = 200; Distance moved athwartships (ft) = 8; Heel measured by = Angle; Heel angle (°) = 1.5), the arithmetic runs like this:
tan(heel) = tan 1.5° = 0.02619
heeling moment = weight × distance = 200 × 8 = 1,600 ft·lb
GM = moment / (displacement × tan heel) = 1,600 / (10,000 × 0.02619) = 6.11 ft
righting moment per degree ≈ Δ × GM × sin 1° = 10,000 × 6.110 × 0.017452 = 1,066 ft·lb
Result: 6.11 ft GM. Heel 1.50° from 200 lb moved 8 ft. Small-angle righting moment about 1,066 ft·lb per degree.
Now change Heel angle (°) to 2.5 and leave everything else alone:
tan(heel) = tan 2.5° = 0.04366
heeling moment = weight × distance = 200 × 8 = 1,600 ft·lb
GM = moment / (displacement × tan heel) = 1,600 / (10,000 × 0.04366) = 3.66 ft
righting moment per degree ≈ Δ × GM × sin 1° = 10,000 × 3.665 × 0.017452 = 640 ft·lb
Result: 3.66 ft GM. Heel 2.50° from 200 lb moved 8 ft. Small-angle righting moment about 640 ft·lb per degree.
The same weight shift producing 2.5° of heel instead of 1.5° means GM is about 40% lower — a tender boat. Comparing a before-and-after test is often more useful than the absolute number.
Take readings with the weight on one side, then the other, and back to center. Averaging port and starboard readings cancels a small list and gives a cleaner result. If the readings drift, wind or a mooring line is interfering.
Free surface in tanks reduces effective GM. Test with tanks pressed full or empty, and note it, so a later test can be compared fairly.
GM describes stability only at small angles. Two boats with the same GM can behave very differently at 30° of heel, where hull shape, freeboard, and ballast position matter far more.
Units: Pounds for weights, feet for distances and GM, degrees for heel, inches for pendulum deflection.
Where the model stops: Small heel angles, calm water, and a correctly known displacement. Free surface, wind, mooring-line tension, and large-angle stability are not modeled.
Not a substitute: This is not a stability certificate. Use a naval architect or surveyor for stability assessments, especially after structural or ballast changes.
| Measured heel | GM (ft) |
|---|---|
| 0.75° | 12.22 |
| 1° | 9.17 |
| 1.5° | 6.11 |
| 2° | 4.58 |
| 2.5° | 3.66 |
| 3° | 3.05 |
| 4° | 2.29 |
Small heels mean a stiff boat (large GM). Doubling the heel halves the GM.
GM from this test describes initial stability only, at small angles. It says nothing about the range of positive stability or how the boat behaves in a knockdown. Before major changes such as a heavier rig, a flybridge, or ballast removal, consult a naval architect.
Enough to produce 1–4° of heel. Too little and the reading is lost in noise; too much and the small-angle assumption weakens.
Yes — crew moving across the deck is a common test weight. Weigh them and keep everyone else still.
The boat’s total weight during the test is what the righting moment acts on.
No. A large GM means a stiff boat with a quick, sometimes uncomfortable roll; a small GM means a tender, slow roll. It is one number among many.