In spite of a documented history of poor stability, the SS Eastland capsized dockside in Chicago in 1915, killing more than 800 passengers and crew.
Graphics by Patrick Bray
Editor’s Note: Stability considerations are at the core of designing and building buoyant seagoing vessels. Yet with centuries of naval architecture and boatbuilding experience behind us, stability failures continue to plague our industry. It’s understandable, looking back at the infamous 1628 capsize and sinking of the newly launched Swedish warship Vasa, that the state of the boat design at the time was not adequately advanced enough to predict such a failure. The same can’t be said of the 2014 capsize and sinking of the Northern Marine-built 90′ (27.4m) expedition yacht Baaden at her launching in Washington; the tragic sinking of the 184′ (56m) sloop Bayesian at anchor during a squall in August 2024; and the September 2025 capsize and sinking of the 24m (79′) Turkish-built yacht Dolce Vento immediately after her launch. Together, these contemporary failures point to disturbing deficits in understanding of, or attention to, vessel stability. They prompted us to revisit the relevant core design skills.
For naval architects, stability is the potential of a vessel to return to a previous situation or orientation—usually a balanced upright position. Positive stability is the ability to return to upright, while negative stability results in the vessel overturning. Calculated stability in its most basic form quantifies the relationship between the center of all floatation in the hull—known as the center of buoyancy (CB)—and the center of all weight, commonly referred to as the vertical center of gravity (VCG).
In short, it’s an accounting of the downward pull of gravity and the uplifting force of buoyancy working against each other to achieve an equilibrium. Beyond these primary factors, all other forces play minor roles in determining vessel stability.
Static Stability
This is stability at rest with no influence from active external forces. The VCG is a relatively fixed point in space at any given loading condition. It reflects the location of major weights within the boat, including the cargo as well as the fixed superstructure and multiple decks.
Patrick Bray The shape of the hull determines the CB, which is not fixed. Its location changes with the motion of the vessel to balance the loads and keep the forces in equilibrium. The shape of the hull has a major effect on the path that the CB will take as the vessel heels or trims. At rest, the CB and the VCG align vertically. That doesn’t mean a boat will rest even on her waterline, but rather she will list until CB and VCG come into vertical alignment and the vessel achieves equilibrium.
Form Stability
As forces such as propulsion, wind, or waves are applied to the vessel, its attitude changes in relation to the water’s surface. As the boat heels, sides of the hull previously above water are immersed and bottom sections previously underwater are now exposed. This causes the CB to move, resulting in righting arm forces (GZ) to bring the vessel into equilibrium (see Figure 1). If the CB can’t create adequate counterforce to right the vessel, it will capsize.
It’s the sort of thing best not left to chance—or to discover at launching. Fortunately, the required counterforce at various angles of heel can be determined mathematically from the designer’s drawings and then verified on the finished boat with an inclining experiment. (For more on inclining experiments, see Butch Dalrymple-Smith’s “Best of Inclinations” in Professional BoatBuilder No.162, page 24). The first step is the naval architect completing a weight calculation to establish the VCG. This extensive analysis accounts for every item on the boat, down to the weight of the paint and screws, along with the vertical and fore-and-aft position of each item in the vessel. (Transverse measurements are considered less critical, as most boats are largely symmetrical from side to side.)
With a fixed VCG calculated, the next step is to look at where the CB moves for each angle of heel and loading condition. This is a far more complex calculation best done by an experienced naval architect (and their computer). Hullform and ballast, if any, play major roles. Both will affect overall stability.
Patrick Bray Dynamic Stability
Once a vessel is under way, the effects of wave train, bottom pressure, and change of trim can add or detract from the amount of stability. Many planing hulls designed to exceed displacement speed gain stability on plane, due to the change in pressure distribution across the hull bottom (see Figure 2). Because of the V-bottom and the spray rails outboard, unequal pressure builds up on the lower side in a turn, which helps restore the vessel to upright when the heeling forces of the turn abate. At higher speeds, these pressures can become unequal or unstable, potentially leading to porpoising or chine walking.
Stability Requirements
International regulatory organizations set minimum standards of stability for various types of commercial and pleasure craft, depending on their use and area of operation. However, these are minimums, and any additional margin will provide a higher factor of safety. Also, these standards do not address comfort, which is of course subject to personal interpretation.
A vessel designed for offshore passages will often feel tender initially and exhibit a long, easy rolling motion—one that’s unlikely to unbalance crew or dislodge gear. For recreational boaters, the period of this roll can seem disconcertingly long and the angle extreme. However, this is frequently a feature of the design, not a flaw, for seagoing vessels.
If a boat is very stable at the dock—great for liveaboards in flat water—it will often have a quick, jerky motion at sea. Consider a 110′ (33.5m) powerboat that runs harbor charters and seats 95 passengers for dinner cruises. When taken to sea, it has such a quick motion in a seaway that the crew go around on their hands and knees to avoid being knocked off their feet.
Compare that to a 60′ (18.3m) model with a low GM—the distance between the metacenter, where a vertical line through the heeled CB crosses the vessel’s centerline, and the vessel’s VCG (see Figure 3). The boat may noticeably shift under your weight as you step aboard, which doesn’t instill faith initially. Having it heel under the press of a strong breeze may further erode confidence; yet a vessel with low GM can have an excellent sea motion, long range of stability, and be extremely comfortable offshore once you get your sea legs.
Patrick Bray A naval architect must strike a compromise between stability for dockside comfort and a suitable motion for safe operation in an average seaway. The specifics of that compromise depend on many details, including the vessel’s intended use and the prospective owner’s level of experience.
Design Considerations
Let’s look at the effects various elements of design have on the stability characteristics of a vessel.
Ballast
When we look to increase stability, we often think first of adding ballast. This will help, but not in all cases. If ballast reduces the freeboard to the degree that the deck edge will enter the water at a much lower angle of heel, overall stability is drastically reduced. Again, it’s the relationship between the hullform and the overall center of weight that tells the story. In this situation, removing top weight from as high up as possible will have a greater effect than adding ballast low down.
Ballast does little to increase initial stability (stability at very low angles of heel). Its useful potential is to improve seakeeping ability at heel angles exceeding 45º.
Beam
Wide beam yields high initial stability but low ultimate stability at high angles of heel. It’s great for liveaboard space and comfort at the dock, but when taken to extremes, it will produce a boat with quick, uncomfortable motion in a seaway as it works to keep itself level with the water surface. The combination of very wide beam and low freeboard in a monohull can be dangerous as the high initial stability peaks at a low angle of heel, and once overturned the vessel becomes very stable in that inverted position.
A catamaran cruiser is a classic example of wide beam and no ballast (see Figure 4). The interior volume and low heel angles make these vessels very comfortable to live aboard but with low ultimate stability. To compensate for this, specific ocean-going hullforms have been developed that allow high immersion of one hull before the other hull leaves the water.
Patrick Bray Inertia
One way to improve the motion of an existing vessel is to alter the inertia. If you have a boat with a quick motion and want to slow it down, try moving major weights horizontally outboard from the centerline. This will increase inertia and dampen out the motion. By moving the weight horizontally rather than vertically, you do not affect overall stability. If your vessel has a slow roll you aim to quicken, move heavy weights inboard or down to reduce the inertia and/or increase the stability.
Superstructure
Once a deck edge is immersed, the shape and size of the superstructure become important factors in determining the CB. For this reason, trunk cabins and deck houses must be well built, with minimal openings that could allow water to enter. If a vessel is fitted with wide side decks, the house will enter the water later, reducing its effect on the stability curve. On a flush-deck vessel, all stability must come from the hullform itself.
Another potential stability influence of a large superstructure is windage, allowing a strong breeze to heel the boat. If a hull’s form stability is adequate, this shouldn’t be a problem. Indeed, in some seagoing boats, the press of wind on the superstructure can act to dampen rolling motions. Unfortunately, it is not a sail area that can be reduced as the wind pipes up.
Waves
All vessels will suffer some loss of stability on a wave face. If the waves are short and steep, the situation becomes worse. Running with the waves, a vessel spanning between two crests will be supported by the buoyant ends where there is little waterline width (see Figure 5). As the vessel heels, the buoyant midsection can fall into the trough. This can induce deep rolling motions and potential capsize.
Even a beam sea can create serious problems if the wave train coincides with the vessel’s natural rolling motion. While a minor change in direction or speed can avert a progressively worsening stability situation, that depends on operator ability and experience—factors a designer can’t count on.
Patrick Bray Active Stabilizers
These are usually automated hydraulically activated fins installed amidships. Their dynamic forces return the vessel upright when a heeling force is detected. They can dampen rolling motions by more than 80% under way, but don’t increase the vessel’s actual stability. While fin stabilizers are most effective under way, they can provide limited dampening at anchor when a zero-speed system is installed. Alternatively, gyro stabilizers have no appendages and offer better stabilization at anchor.
Passive Stabilizers
These passive systems vary from paravanes to fixed bilge keels to anti-rolling ballast tanks. Their primary feature is that they have few to no moving parts and are mechanically simple and, therefore, relatively reliable. While they do nothing to increase actual stability, they can dampen the effects of external forces acting on your vessel, but not as effectively as an active system.
Downflooding Considerations
All the stability in the world will be of no use if there are low openings in your hull or superstructure that will let water flood your boat. Loading doors and engine room intakes located low in the hull sides and leading directly below decks are classic design vulnerabilities. Even if a boat has a good range of stability, it won’t do much good if water is pouring in through a large opening. Penetrations for interior ventilation need to be placed carefully, and non-watertight hatches to hull compartments should be checked during design to ensure water won’t enter at low angles of heel.
The effects of uncontained water inside a boat are dramatic. Water always runs to the low side, making matters worse. As a vessel rolls, the water shifts from side to side, amplifying motion. Even uncontained water on deck has the effect of raising the VCG and reducing stability. Large freeing ports in the bulwarks are necessary to get the water off the boat quickly before the next wave adds to the burden on deck.
Once a hull has been damaged and water is pouring in, there is little that can be done if there are no watertight subdivisions in your boat. Commercial vessels are required to have watertight bulkheads and must be designed to remain afloat and stable with one and sometimes even two compartments completely flooded. It’s a good standard to meet when designing offshore cruising pleasure craft as well.
Stability Isn’t Accidental
Good stability can and should be achieved during the design phase of building or refitting a new boat. Careful placement of heavy items down low, and knowledgeable design of hullform, superstructure, and hull penetrations that takes stability characteristics into account will produce a sound vessel with a comfortable motion. Attention to the numbers that determine the vessel’s final characteristics will assure a safe boat long before it is launched. Stability is not something that just looks good on the sales sheet; it will prove itself when needed the most in trying conditions on the water.
For the Number Crunchers
A monohull motor yacht intended for extended coastal cruising, open-ocean passages, or any vessel expected to be caught out in really bad weather, must meet the international standards requiring a minimum of the following:
- Maximum righting arm (GZ) must be over 25° and preferably over 30°
- Downflooding angle must be in excess of 40°
- Initial GM must be over 6″ (152mm)
In my opinion, these characteristics are inadequate for offshore safety. A Seaworthy motor vessel should hope to achieve:
- Maximum righting moment (GZ) that occurs after 45° and preferably after 60°
- Minimum range of stability to 60° of heel and over 80° preferred
- GM of at least 1.5′ (457mm) and preferably around 3.5′ (1,067mm) for comfort
For sailboats expected to cruise open waters, the international standards require a minimum of the following:
- A positive range of stability in excess of 90° (In vessels over 45m/147.6′), a lesser range may be considered.)
- Downflooding angle must be in excess of 40°
- An “angle of steady heel” calculation must exceed 15°
About the Author: Patrick Bray is a naval architect with more than 50 years of experience in design and new construction, including three years as on-site naval architect for Crescent Yachts (Surrey, British Columbia) doing shop drawings and ensuring compliance with the classification societies. He filled a similar role at Trites Marine Services, another B.C.-based aluminum commercial vessel builder. Doing business as Bray Yacht Design and Research Ltd., he has worked directly with many other yards as designer and project manager.
