Knockdowns and Capsizes

What Causes a Knockdown

A knockdown occurs when the boat is heeled to 90 degrees or beyond — mast horizontal or in the water. There are two primary causes: extreme wind gusts and breaking waves. Of the two, breaking waves are far more dangerous.

Wind knockdowns happen when a sudden, extreme gust heels the boat past its ability to resist. A boat sailing with too much sail in gusty conditions can be knocked flat by a gust that exceeds the sail plan's design range. These knockdowns are usually recoverable — the wind passes, the boat rights itself, and the damage is limited to pride and perhaps some broken crockery below.

Wave knockdowns are far more serious. A breaking wave hitting the boat beam-on delivers its energy as a rolling force. The wave doesn't need to be enormous — a breaking crest height that exceeds the beam of the boat contains enough energy to roll it past 90 degrees. In the 1979 Fastnet Race, boats were knocked down and rolled by breaking waves of 5–7 meters — waves well within the normal range of a North Atlantic storm.

The physics: A breaking wave applies force high on the hull (at waterline or above). This force creates a rolling moment around the keel. If the rolling moment exceeds the righting moment at that angle of heel, the boat continues to roll. The critical factor is not wave height but breaking wave steepness — a steep, plunging breaker delivers far more energy than a smooth, rolling swell of the same height.

What Happens During a Knockdown

A knockdown is one of the most violent events a person can experience on a boat. Understanding what happens — physically, structurally, and psychologically — prepares the crew to respond rather than freeze.

At 90 degrees: The mast is horizontal. The keel is horizontal. Everything not secured below goes to the leeward side — which is now the 'floor.' Water enters through any opening — the companionway, cockpit drains (which are now above the waterline on one side), dorade vents, open ports. The crew on deck are hanging from whatever they're clipped to. The crew below are thrown against the leeward side of the cabin.

The rig: The mast is in the water or near it. The sails are in the water. The rigging is under extreme, asymmetric load — the windward shrouds are taking the full weight of the mast in a direction they weren't designed for. If the rigging holds, the mast may survive. If a shroud, chainplate, or mast fitting fails, the mast breaks.

The cockpit: If the companionway is open, water floods below. If the cockpit drains are submerged (they are, at 90 degrees of heel), the cockpit fills rather than drains. A cockpit full of water at 90 degrees dramatically shifts the centre of gravity and can prevent the boat from righting.

Crew on deck: Anyone not clipped on goes overboard. Anyone clipped on may be dragged through the water if the tether is too long. Short tethers (under 2 meters) are critical — a 3-meter tether allows a person to be submerged under the hull.

Recovery: If the boat's AVS exceeds 90 degrees (which it should for any offshore-capable boat), the righting moment at 90 degrees is positive — the boat wants to come back upright. As the wave passes, the boat rolls back. The recovery may be sudden and violent — crew should brace for the return as well as the initial knockdown.

180-Degree Inversion and Self-Righting

A 180-degree inversion means the boat is upside down — keel pointing skyward, mast pointing at the seabed. This is the ultimate capsize and occurs when a breaking wave has enough energy to roll the boat past 90 degrees, past the AVS, and fully inverted.

Self-righting: A boat with an AVS above 120° will self-right from most inversions — the keel weight and hull buoyancy create a righting moment that eventually rolls the boat back upright. The time inverted may be seconds to minutes. During this time, the crew is upside down, water is entering through every opening, and loose items are projectiles.

Boats that may not self-right: Boats with low AVS values (under 110°) may remain inverted for extended periods. Multihulls (catamarans, trimarans) have very low AVS values — an inverted catamaran is stable upside down and will not self-right. This is the fundamental safety trade-off of multihull design: they are initially more stable but once inverted, they stay inverted.

After an inversion: Check crew for injuries. Check the hull for structural damage (cracks, separated joints, broken fittings). Check the rig — a dismasting during inversion is common. Check below for flooding — every hull opening has been submerged. Start the bilge pumps immediately. Assess whether the boat is still seaworthy or whether the situation requires abandoning to a life raft.

Damage Assessment After a Knockdown

After a knockdown, the immediate priority is crew safety — injuries, MOB check, and ensuring everyone is conscious and accounted for. The second priority is assessing the boat's condition.

Hull integrity: Look for cracks in the hull laminate, especially around high-stress areas: chainplates, keel stub, rudder bearings, and hull-deck joint. Any water ingress that wasn't present before the knockdown indicates structural damage. Run the bilge pumps and monitor the rate of water ingress.

Rig: Check standing rigging for broken strands, bent turnbuckles, cracked swages, and damaged chainplates. Check the mast step and partners. A mast that appears intact but has shifted at the step may be compromised. If the mast is broken (dismasting), the immediate job is securing the broken rig to prevent it from holing the hull.

Rudder and steering: Check that the rudder is intact and the steering responds. Rudder stocks can bend or break in a knockdown. If steering is lost, assess emergency steering options.

Below decks: Check batteries (shifted?), engine (oil level — if oil is on the ceiling, it came out of the sump), through-hulls (any leaking?), and structural bulkheads (delamination at hull joints).

The decision: After assessment, the crew must decide: is the boat seaworthy enough to continue, to divert to the nearest safe port, or to abandon ship? This decision must be made calmly and with accurate information — which is why the structured damage assessment comes before the decision.