A boat is said to be turtling or to turn turtle when it is fully inverted. The name stems from the appearance of the upside-down boat, similar to the carapace (top shell) of a sea turtle. The term can be applied to any vessel; turning turtle is less frequent but more dangerous on ships than on smaller boats. It is rarer but more hazardous for multihulls than for monohulls, because multihulls are harder to flip in both directions. Measures can be taken to prevent a capsize (where the boat is knocked over on its beam-ends but not yet inverted) from becoming a turtle (with bottom up).

When a boat is "turned over completely" it has turned turtle. Some sources treat the term "turtle" as synonymous with "capsize" or "keel over". but most others make a distinction. Carrying too much sail or loss of control can lead to broaching—the boat heels too far to one side, or capsizes. While all turtlings involve a capsize, the converse is untrue. Prevention is the first priority.

With the exception of self-righting watercraft, vessels have an angle of vanishing stability (AVS). External forces aside, if they are tilted at an angle less than the AVS, they will pop back upright. If they are tilted at an angle greater than the AVS, they will turn turtle and stay there. In other words, non-self-righting vessels are stable when turtled. Their primary stability and secondary stability are greater than zero, tending to right them, but their tertiary or inverted stability is less than zero, and tends to keep them upside-down.

Capsizing (but not necessarily turtling) is an inherent part of dinghy sailing, and is considered to be "routine". It is not a question of "if" but a question of "when".

For those who prefer to avoid the experience, a keelboat monohull has physics on its side. (See limit of positive stability.) But even yachts can capsize and turtle in extraordinary conditions, so design considerations and suitability for particular tasks, locations, weather, duration and situations are essential queries. "Such events can overcome skill and experience"; boats need to be appropriate for foreseeable conditions. It is a fundamental question of seaworthiness.

Turtling commonly occurs when a boat capsizes and is not righted or attended to in time, allowing it to roll through the approximately 90 degrees of a capsize through to 180 degrees from upright.

Prevention and delay of turtling in dinghies is the highest priority—turtling can be catastrophic in consequences—but it depends on skill, ability and athleticism, which vary greatly. As John Rousmaniere wrote: Testing confirms the Royal Yachting Association's conclusion that "a decisive way to address entrapment is to immediately right the boat by putting weight on the centerboard", daggerboard, (or bilgeboard in a scow). His 2012 report advises that "US Sailing and other organizations should consider establishing this as doctrine." Thus, reliance on boat handling and seamanship may be misplaced.

Sealing masts and attaching flotation are effective preventatives for turtling of dinghies, but not widely utilized. More certain preventatives for this includes various forms of flotation added to the tip of the mast or top of the mainsail. These include floats (e.g., one that looks like a streamlined blimp used on Hobie 16s) or a "sail patch"—a sleeve with built in flotation that fits over the top of the sail, available for example as option on the Wayfarer (dinghy) Mark IV. Another alternative is to seal the mast, thereby increasing its buoyancy.