In order to avoid capsizing, you need to understand what causes vessels to capsize.
What causes boats to capsize?
In a word, instability. Boats are inherently stable until something causes them to become unstable. 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.
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, 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.
We 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.
Which Boats Are More Likely to Capsize?
Small boats are most likely to capsize. Almost 10 percent were 8-footers, mostly dinghies, and capsizes here often didn't cause much damage. But the biggest group, according to a BoatUS study were 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.
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.
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.
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.
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.
Based on an article in BoatUS