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Positive Stability: A Weighty Concern

M- Metacentric Center, G- Center of Gravity, B- Center of Bouyancy, K- Keel, WL- Waterline

M- Metacentric Center, G- Center of Gravity, B- Center of Bouyancy, K- Keel, WL- Waterline

Positive stability is a vessel’s tendency to resist capsizing. Negative stability? Not so much.

Anyone who has ever made a good-faith effort to understand stability has also been baffled by it. The dynamics of stability are invisible, yet the consequences are real. You can’t walk over to the center of gravity, give it a rap and then watch it interact with the center of buoyancy to form a righting moment. You can’t just place a measuring tape on your GM (the distance from the center of gravity to the metacenter) to see how it’s holding up. Instead, you have to imagine these things. Terms like metacenter, GM and righting moment don’t help.

“Make sure that your vessel is not sitting deeper in the water than designed, and by no means change the waterline to accommodate a new reality.”

But it may not be necessary to understand those things to make informed choices for your boat. If you have ever been in a skiff, then you already know that weight up high is bad and weight down low is better. A skiff also shows how the center of gravity follows a weight that is moved, as when someone leans over the side, causing a list. You know that the center of gravity moves toward a weight added and away from a weight removed. These same forces are at work in bigger boats, but are harder to spot.

Stiff and tender are relative terms that describe a vessel’s natural roll period. A stiff vessel has a quick, snappy roll. It appears to resist heeling forces. It zips from side to side, sometimes causing people to vomit. A tender vessel has a long, slow roll period; it may seem to hang before returning to the upright position. The wait may cause people to feel nervous, and then vomit.

At small angles of heel — normal everyday conditions — a stiff vessel gives the impression that it is stable, whereas a tender vessel seems less so. The resistance to heeling forces at small angles is referred to as initial stability. But initial stability does not tell the whole story about how a vessel will hold up to more extreme angles such as those brought on by a powerful squall, heavy weather or an exceptionally large wave. These are conditions that test a vessel’s ultimate stability, the angle at which heeling forces overcome resistance, and a vessel capsizes. A given vessel may reach ultimate stability at 40, 50, 60, 90 degrees or more. The tipping point depends on the design. It also depends on how the original vessel may have changed over time.

One notion of seaworthiness is that a vessel should be designed and built for its intended purpose, with a margin of safety for the unexpected. That margin of safety can accommodate the limits of wisdom and skill up to a point. Boats designed for protected waters are not intended to meet the test of open water, though we may get away with such cruises from time to time. Boats designed to carry six people may sometimes carry seven, or even eight, if conditions are right. A fair-weather craft may handle the occasional squall. When a person is wide-eyed to the fact that he is pushing the bounds of intended purpose, he is more likely to manage those risks with care. When pushing the boundary is routine, risk becomes invisible, and people become careless.

One way a boat’s original stability can be unwittingly undermined is through the gradual accumulation of weight on board. A yacht is designed with a waterline that allows adequate reserve buoyancy for expected conditions. Reserve buoyancy is the watertight volume of a vessel above the waterline. Essentially it is the freeboard, plus the volume of any cabin or superstructure designed to keep out water. Each time a vessel heels, that reserve buoyancy acts as a righting force to bring the boat up again. And again. And again.

Whether for utility, convenience or fun, there is a tendency to put more things aboard a boat than there is to remove them: tools, spare parts, hardware, another anchor, sails, extra line, oil, hydraulic fluid, glues, goops and paints, a blender, a kayak, scuba gear and a compressor. In addition to holding useful spare parts, boats are often laden with worn-out or broken parts squirreled away for some imagined heroic moment — or worse, components for things that haven’t been aboard in years. This accumulation usually becomes evident only when you decide you need a bigger boat. Meanwhile, your vessel is sitting a little deeper in the water with a little less reserve buoyancy for that time when ultimate stability is tested.

Bigger changes may come in the form of adding larger holding tanks, a watermaker, an inverter or greater battery capacity, or replacing a small kerosene stove with a more substantial propane one, with the propane tank situated on deck, thus raising the center of gravity. On a longer cruise, we might lash jugs of fuel and water on deck to extend the range. This too adds weight and raises the center of gravity. Individually, these changes are beneficial, but collectively, they can undercut original seaworthiness. All things being equal, a bigger boat is less affected by the addition of a weight than a smaller one.

Captain Daniel S Parrott photo

Capt. Daniel S. Parrott

Make sure that your vessel is not sitting deeper in the water than designed, and by no means change the waterline to accommodate a new reality. Periodically take everything off the boat and make a disciplined evaluation of what should be aboard and what should not. Look for trade-offs. Research major modifications that add or shift weight, with an eye to how they might affect stability. Changing the rig or increasing sail area can also have consequences for stability.

Pose this question to yourself: Am I trying to make this boat into something it was never intended to be? If so, then maybe you need a bigger boat.

This article originally appeared in the July 2018 issue.



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