Analyze key boat stability metrics like GM, Roll Period, and Comfort Ratio.
A well-balanced coastal cruiser.
A stiff, performance-oriented boat.
A heavy, comfortable bluewater boat.
"My Boat Plans" is a collection of over 500 boat plans, from simple dinghies to larger cruisers. It includes step-by-step guides, video tutorials, and more.
Download NowInitial stability describes how firmly a boat resists the first few degrees of heel, and it is governed by the metacentric height, or GM. A boat with healthy GM feels planted and returns to upright quickly; one with too little GM feels tender and slow to right, which is dangerous under sail or in a seaway.
GM is not something you can read straight off a drawing. It comes from the relationship between the centre of gravity and the centre of buoyancy, and builders estimate it from beam, displacement and ballast. This tool turns those inputs into the roll period and comfort ratio that sailors actually use to compare designs.
Enter your beam, displacement, ballast weight and an estimated GM, and the calculator returns the ballast ratio, the natural roll period, and a Brewer-style comfort ratio so you can judge whether the hull will be stiff, cranky or seakindly.
Ballast Ratio = Ballast ÷ Displacement × 100; Roll Period (s) = 0.44 × B ÷ √GM| GM (ft) | Roll Period (s) | Typical Feel |
|---|---|---|
| 0.5 | 6.2 | Very stiff, slow, seakindly |
| 1.0 | 4.4 | Cruising comfort |
| 2.0 | 3.1 | Brisk, responsive |
| 4.0 | 2.2 | Cranky, quick snaps upright |
| 6.0 | 1.8 | Racing-stiff, uncomfortable offshore |
Roll period for a 10 ft beam at a range of GM values.
Formula verified against standard naval-architecture references · Updated October 2026
GM, or metacentric height, is the distance between a boat's centre of gravity and its metacentre, and it measures how strongly the hull wants to stand up when it heels. A larger GM gives a quicker, stiffer motion; a smaller GM gives a slower, tenderer one. For a DIY builder the important point is that GM is driven by beam, low ballast and keeping heavy gear as low as possible.
Most cruising sailboats sit between 35 and 45 percent, trading a little performance for volume and comfort. Offshore and bluewater designs often go higher, sometimes toward 50 percent, for extra righting power. Daysailers and light-displacement boats can be lower. Match the ratio to where you sail, not to a magazine number, because a heavy boat also needs a heavier structure to carry it.
Beam has an outsized effect on initial stability because a wider hull moves the centre of buoyancy further out as it heels, creating a bigger righting lever. That is why a beamy catamaran-like scow feels stiff at small angles. However, a beam that is too wide for the displacement can make the boat snap upright and slam, so designers balance beam against displacement and keel depth.
The comfort ratio is a figure popularised by designer Ted Brewer that estimates how a boat moves at sea from its displacement and waterline beam. A higher number indicates a slower, more comfortable roll suited to heavy offshore cruising, while a low number flags a tender or quick-motioned boat. It is a comparative index, so use it to line your design up against boats whose behaviour you already know and trust.
Not necessarily. Very high initial stiffness feels reassuring but can produce a jerky, unpleasant motion and, more importantly, a boat that reaches its angle of vanishing stability sooner. A moderate, balanced GM with good form stability and a deep keel usually gives both a comfortable motion and strong resistance to being knocked down, which is what offshore sailors actually want.
Yes, mostly by managing weight. Adding lead low in the keel raises GM directly, while removing heavy gear from the mast, rigging and cabin top does the same without adding displacement. Just adding ballast high, or filling the bilge with loose gear, can make things worse, so focus on the vertical position of mass as much as the amount.