Formula & assumptions

Wind pressure = 0.00256 × wind speed² (psf). Side force = pressure × sail area × side-force coefficient. Heeling moment = side force × (CE height + CLR depth).

Righting arm ≈ GM × sin(heel). Righting moment = W/B total weight × righting arm. HCG is the W/B weight-weighted station from the bow and is shown for load audit; it is longitudinal and does not create transverse righting. VCG/KG is the W/B weight-weighted height above the hull bottom (lowest keel) and is incomplete until every weight has a height. KB uses that same hull-bottom plane, not Adory y = 0. GM is calculated per W/B case as KB + BM − KG at the floating freshwater waterline (and trim) from the drawn hull, and is labeled calculated. BM uses I_T = (2/3)∫ z³ dx over the waterplane, with z equal to each section's half-breadth at the trimmed waterline cut (not the gunwale). If any height is missing, or the boat does not float within the hull, Sail falls back to the typed GM and flags that source. Check “Override with typed GM” to force the typed value even when a calculated GM is available.

Steady heel in the design wind is where heeling moment equals righting moment W × GM × sin(θ), using the calculated GM and that small-angle arm. Gunwale immersion (deck-edge / downflooding angle) is the smallest station heel at which the leeward sheer point reaches the trimmed floating waterline; extra sinkage is not modeled. Margin is immersion minus steady heel. A warning is shown when that margin is under 10°, or when steady heel exceeds the editable 15–20° target line (drawn on the tipping display with the immersion angle).

Limits: steady wind, centered loads, and the small-angle GM approximation. The typed GM is a fallback or manual override, not a substitute for measured stability data. This planning check does not model gusts, crew movement, flooding, or a large-angle GZ curve. It is guidance in the spirit of ISO 12217-2 / ABYC good practice, not a certification check.