Avoiding Capsizing And Swamping

A study of five years of capsizes and swampings reveals how they happen and what you can do to avoid them.

Capsized vesselPhoto: United States Coast Guard

It was July 4, 2012, when a 34-foot cabin cruiser headed out into Long Island Sound for the fireworks show. The boat, called Kandi Won, was packed with 27 people — eight of them high on the flybridge — and when the fireworks ended, the boat headed home along with many others, all jockeying for position and all creating wakes. Shortly after, the boat, probably rocked by several large wakes, rolled over, spilling passengers into the water. Tragically, three children were trapped in the cabin and died. While capsizes are far more common on smaller boats, it's a sobering reminder that even large boats can capsize under the right conditions.

What causes boats to capsize? In a word, instability. Boats are inherently stable until something causes them to become unstable (see Stability sidebar). And that something is weight — where it is and how much it is determines when a boat will tip over far enough to capsize or fill with water. BoatUS Marine Insurance analyzed five years of capsizes and swampings, to find out what you can do to prevent your own boat from capsizing.

A capsize is defined as a boat rolling over onto its side or completely over; swamping typically means that a boat fills with water (often from capsizing) but remains floating. So to simplify, we'll use the term capsize from here on. As mentioned, the study showed that boats capsize because they become unstable, but there are three main reasons for that instability: too much or unbalanced crew or equipment weight; leaking water, which also creates too much weight; and bad weather, which causes instability as a boat is rocked and filled with water. First, we'll look at which boats have the highest capsize risk, then we'll look into what exactly makes them unstable enough to capsize.

Hope It Floats

There is always a very real possibility of injury when passengers unintentionally go in the water with nothing to hold onto. The U.S. Coast Guard (USCG) has addressed this by requiring monohull powerboats built after 1972 under 20 feet in length to float when filled with water. This is a good thing, because without it, most of the small boats in the study would have sunk out from under the crew, leaving nothing to hang onto while waiting for rescue. The bad news is that boats larger than 20 feet that don't have built-in flotation will eventually sink if capsized, and even smaller boats with flotation can still sink if grossly overloaded. (Note: Boats up to 26 feet built to the American Boat & Yacht Council (ABYC) standards adopted by the National Marine Manufacturers Association (NMMA) also have flotation). Inboard and sterndrive boats have less rigorous basic flotation requirements than outboard-powered boats. If your boat was built before 1972, it wasn't required to — and probably won't — have flotation at all. (See Sidebar, Does it Float?).

Which Boats Capsize

One thing that stood out is that the majority of incidents — not surprisingly — were small boats (see chart below). Almost 10 percent were 8-footers, mostly dinghies, and capsizes here often didn't cause much damage. But the biggest group, as the chart shows, comprise the 15-19 footers, representing 41 percent of all capsizes. These boats were typically fishing boats, often with large, hard-to-drain cockpits, sometimes out in poor weather, and were sometimes overloaded.

The next most common group are boats in the 20-24 foot range, representing a quarter of the total; half of those were outboard-powered 22-footers. Larger boats tend to be more stable and rarely capsize, though there were several boats over 38 feet that capsized.

Capsizes By Boat Length

Capsizes by Boat Length pie chartSource: BoatUS Marine Insurance Claim Files

Why They Capsize

Nearly all capsizes can be assigned one of three causes. The most common is too much or poorly distributed weight. Small boats are much more susceptible to an extra person or two or a couple of heavy coolers aboard than larger boats. Older boats especially may have gained weight over the years as more gear is stored aboard. On boats with cockpit drains, an extra beefy friend or a second cooler might be all it takes to make the water come back in through the drains, filling the boat. While most of these under-20 foot boats are required to have flotation, they also must have a capacity plate that states how much weight and how many people can safely be aboard. Pay attention to this number, and keep in mind that the number of seats in a boat is not always an indication of the number of people it can carry safely. Exceeding the capacity limits, even in calm water, is asking for trouble; and in many states, operators can be ticketed for it. All it takes is a stiff wind, a large wake, or an unbalanced load to flip over. Safe passenger loading is also controlled by the number of "underway seating positions." For example, two slim adults taking four or five small children out for a spin on a boat designed to seat four would be taking a big safety risk, even though the total passenger weight might be well under the boat's stated capacity. Where the weight is distributed is almost as important as the amount. Too many people on one side of the boat (Hey, look at that whale!) forces the gunwale too far down, potentially allowing water to pour in. Boat manufacturers often provide a diagram for each boat showing where passengers can safely sit so as to not upset the boat's balance.

Swamped powerboatMost power boats built before 1972 have no flotation and will sink out from under the crew if swamped.

Some boat manufacturers, such as Sea Ray, label upper decks on their larger boats to indicate how many passengers can be on upper decks. Weight that is substantially above the waterline raises the boat's center of gravity and makes the boat less stable and more likely to capsize (The ABYC is working on recommending upper deck capacity labels for larger boats.) Another thing to keep in mind is that people have gotten larger over the years. The Centers for Disease Control and Prevention (CDC) says that the average weight of an individual in the United States has increased to 185 pounds, up significantly since commercial passenger-carrying regulations went into effect in the 1960s, when the average American weighed 160 pounds. Those extra pounds can quickly add up, causing your boat to be unbalanced. While you may not want to carry a scale with you, it's good practice to eyeball your guests and try to get a ballpark weight before you take out a larger group.

Large wave capsizes boatLarge waves can overwhelm a boat and cause it to capsize. Keeping the bow into the waves can prevent the boat from rolling.

Installing a four-stroke engine on an older boat can add 10-15 percent more weight that the designers, who built the boat for lightweight two-strokes, didn't anticipate. Water can backflow into the cockpit and cause the boat to take on water.

Fishermen and hunters in small boats can make the boat unstable simply by standing up because their center of gravity rises. Some of the most severe injuries from capsizing are from this group of boaters during the winter; immersion in cold water seriously shortens your chance of survival, especially if you're alone.

Too much weight topsidePontoon boats are generally more stable than monohulls, but even they can be capsized by an unbalanced load. (Photo: John Silver)

The bottom line is that loading too much cargo or too many passengers in one part of the boat can affect its stability, even if the total load is within the boat's maximum capacity. Weight needs to be evenly distributed, especially in smaller boats. One other thing worth mentioning is that capsizes can also be caused by modifications that affect the stability of the boat. Even a small tuna tower can severely change the center of gravity, especially on a smaller boat. Contact the manufacturer or a naval architect if you're unsure how a modification will affect stability.

Leaky Vessels

The second major cause of capsizing is leaks. Sometimes it's as simple as forgetting to put the drain plug in; other times it's leaking fittings. Water sloshing around in the bottom of the boat affects stability and waves or a wake can cause it to flip. Tying the drain plug to your boat key is a simple way to remember the plug. On the other hand, leaking fittings that can fill the boat with water are usually out of sight, often in livewells and bait boxes. Several claims were reported when an owner installed a livewell fitting using cheap PVC pipes and valves, and at least one livewell had no shut-off valve at all with no way to stop the ingress of water once it began leaking. Any fitting that penetrates the hull needs to be closeable and should be made from stainless steel, bronze, or Marelon. One more thing the claims revealed: Some livewells are plumbed in such a way that they'll flood the boat if the valve is left open while underway.

Many older outboard-powered boats have low transom cutouts that can cause the boat to flood simply by slowing down too quickly, especially with excess weight in the stern. Newer outboard boats have a well that reduces the risk.

Some boats have cockpits that drain into the bilge (generally considered a poor design), requiring the use of a bilge pump to even stay afloat. Bilge pumps are designed to remove nuisance water only, not to keep a boat from sinking. If your boat's cockpit drains into the bilge, be aware that if the bilge pump fails, your boat can fill with water and capsize or sink.

Bad Weather

Weather is another major cause of capsizes, sometimes in concert with overloading. Small boats are easily overwhelmed by modest waves or even wake, especially if they've got a full load and sit low in the water. A sudden squall can flip even a larger boat. Check the weather forecast before you go out, and keep a weather eye on the sky. In most areas, NOAA broadcasts continuous weather via VHF radio. If you're within range, smartphone apps can show you detailed weather maps, including radar, which can indicate approaching storms. Weather changes quickly on the water, so at the first sign of bad weather, head back to the dock. If you're caught out in a squall, have your passengers stay low near the center of the boat to maintain stability.

Capsized boat in a storm Storms can overwhelm small boats. Always check the weather before you leave the dock.

Life Jackets

Life jackets can buy extra time until you're rescued, but they have to be worn to work. BoatUS Foundation tests showed that even modest waves can make it very difficult to don a life jacket when you're in the water, a job made harder still if you have to search for one after capsizing. 

— Published: August 2016


Seaworthy, the damage-avoidance newsletter, is brought to you by the BoatUS Marine Insurance Program. For an insurance quote, please call 1-800-283-2883 or apply online at BoatUS.com.


Dave Gerr, nautical architect and author of (among other books) The Elements of Boat Strength for Builders, Designers and Owners, believes that all powerboats should carry capacity plates, and he has recommended that the ABYC adopt the requirement as a standard. Gerr, with a group of other naval architects, investigated the sinking of Kandi Won.

"It was immediately apparent to all of us that this was crazy," he says. "You don't put 27 people on this boat." So a bunch of us said, "OK, that's our intuition, it seems like we all agree. Let's take a look at the stability numbers, and see if the numbers prove that conclusively."

The numbers, it turned out, were the easy part. The hard part, Gerr says, was reconstructing the boat. Silverton was in bankruptcy, and plans for the older boat were difficult to come by. He had to start from scratch, developing a mathematical model as well as a computer drafting simulation to analyze the boat. Once he had those models, he was able to determine the stability of the vessel when it was loaded with people.

"My original results were surprising," Gerr says, "so I had a couple of people look at it. Basically, it was quite a nice boat. Quite safe and stable. But with 27 people onboard, including eight on the flybridge, the stability was very questionable. It had almost no reserve stability at all."

His conclusion: "A boat that size was probably safe with 15 or 16 people onboard. Maybe even a few more."

A real-world test of his finding was fairly simple. Marine consultant Eric Sorenson piled weight on the rail of a similar Silverton 34 and measured the angle of heel. In a detailed article in Soundings, Sorenson was more conservative in his preliminary findings. "Some feel that 15 [passengers] would be permissible in calm waters in daylight if half of them are kids," he writes, "but I think this is excessive because of the complications so many people create in the event of an emergency."


Does It Float?

Inside an old World War II-era aluminum building in Solomons Island, Maryland, that was once used to train troops, visitors are surprised to find a 25-by-10-foot pool of deep blue water. Even more surprising perhaps, is finding a 19-foot boat floating in the pool while a group of engineers and technicians gather round.

This odd building, and the people in it, are making boating safer by making sure that the USCG-mandated flotation regulations do what they are designed to do — keep boats afloat while swamped — and making sure that when they're swamped, they don't roll over easily.

Most powerboats under 20 feet in length (multihulls are an exception) are required by the USCG to have a plate affixed to them that states the maximum number of passengers they can carry, along with the maximum horsepower rating of the boat's engine. These are usually based on manufacturer calculations and not necessarily real-world usage; unfortunately some boats don't meet the regulations, despite the plate. With thousands of registered boat manufacturers, and hundreds more new ones every year (and hundreds more that go out of business every year) the USCG wants to make sure that builders aren't just slapping a number on a plate and selling the boats to an unsuspecting public. This is where the old aluminum building and the workers come in to play. For years, the USCG has contracted with a company called PPG to test a sampling of boats each year, to verify the capacity plate is accurate.


Every year, PPG visits boat manufacturers, dealers, and boat shows, looking at boats, especially new models. With a practiced eye, employees spot boats that may not pass the tests, and then anonymously buy them — about 35 boats every year. The engines are removed (just in case), the boats are put in the pool, and weights are added to represent the missing engines. More weights are added to simulate the maximum capacity shown on the plate, and then a large pump begins to fill the boat with water until it's full to overflowing. If the boat doesn't sink and makes it this far (65 percent of them don't), it's left for an 18-hour "soak test" to simulate a capsized boat waiting for rescue. If all is still well the next day, weights are then placed near the sides of the boat to represent some of the passengers collecting on one side of a swamped boat. Boats with the correct amount of flotation in the right places won't capsize and will still float relatively level. If they don't, they fail the test.

Flotation testing

It's a fair bet that manufacturers don't want the USCG knocking on their door, telling them that their boat failed flotation tests. That's because it almost always results in a recall, forcing the manufacturer to fix the problem on every boat of the type that failed, usually by adding more or better-placed flotation. As the USCG finds more boats that fail, word gets out, and builders take more care to insure their boats won't be a part of an expensive recall campaign.