The Great Lakes produce a distinctive wave pattern known to sailors as the three sisters. It consists of a set of three large waves arriving in rapid succession, each one larger than the last. The pattern is especially hazardous because the waves hit ships faster than the vessels can recover, which can cause catastrophic structural failure.

The physics behind this hazard involve wave period, the time between successive waves. In the open ocean, storm waves typically arrive 10 to 15 seconds apart, giving a ship time to recover and shed water. On the Great Lakes, the wave period during a storm is often only four to six seconds. This compressed interval is the result of fetch, the distance wind travels across open water. The lakes are large enough to generate powerful waves but not large enough for the waves to spread out into longer, more regular intervals.

A ship experiencing these conditions is subjected to repeated mechanical stress. The hull flexes in two directions: hogging, where the middle of the ship is lifted, and sagging, where the bow and stern are supported while the middle drops. Under normal conditions, a ship has time to transition between these states. When waves arrive within seconds of each other, the metal does not have time to recover, and the hull is worked back and forth in rapid succession.

The danger is compounded by resonance. Every structure has a natural frequency at which it vibrates. The natural frequency of a steel freighter of the type that dominated Great Lakes shipping in the mid-20th century closely matches the wave frequency during a storm. When the two align, each wave amplifies the structural stress left by the previous one. A series of moderate waves can generate more cumulative stress than a single much larger wave.

Wave shoaling also plays a role. When waves encounter shallow water, the bottom of the wave drags on the lake bed, causing the wave to slow and compress vertically. The swell becomes a steeper, taller wall of water. The Great Lakes contain numerous underwater ridges and reefs that create these conditions. The combination of compressed wave periods, shoaling effects, and resonance is what makes the three sisters uniquely dangerous.

Two of the most famous ships lost under these circumstances are the Edmund Fitzgerald and the Carl D. Bradley.

The Edmund Fitzgerald sank on November 10, 1975, in Lake Superior. The ship was 729 feet long, carried 26,000 tons of taconite pellets, and had a crew of 29. At 7:00 p.m., the captain radioed a nearby ship and stated, “We are holding our own.” Ten minutes later, the Fitzgerald disappeared from radar. No distress call was received. The ship was found later in two pieces on the lake floor, consistent with sudden structural failure rather than gradual flooding.

The Carl D. Bradley sank on November 18, 1958, in Lake Michigan. At 639 feet, it was the longest ship on the Great Lakes at the time. It had been in service for 31 years and had a crew of 35. The ship was traveling about 30 miles southwest of Gull Island when it encountered a storm. The crew felt a violent shudder, and the ship began taking on water rapidly. The captain sent a distress call, and the crew attempted to abandon ship. The vessel broke in half. Two crew members survived after spending approximately 15 hours in the water before being rescued.

Other ships have been lost under similar circumstances. The Daniel J. Morrell broke in half on Lake Huron in 1966, killing 28 of 29 crew members. The Henry B. Smith disappeared on Lake Superior in 1913 and was found a century later in two pieces. The Kamloops sank near Isle Royale in 1927 with no survivors.

The timing of these disasters is a central factor. When a ship breaks apart due to these forces, the sinking can occur in less than 60 seconds. There is no time to launch lifeboats, don survival suits, or send distress calls. The water temperature in November is around 38 degrees Fahrenheit, which makes prolonged survival unlikely even for those who reach the surface.

Modern safety measures have reduced, but not eliminated, the danger. After the Bradley sank, regulations required more frequent structural inspections and ultrasonic testing of older vessels. After the Fitzgerald sank, hatch covers were redesigned, and weather routing became mandatory. Newer ships are built with stronger steel alloys and more flexible hulls.

Despite these improvements, the underlying hazard cannot be engineered away. The lakes are a fixed geographic feature. The fetch, the water depth, and the storm patterns remain constant. In November 1995, the crew of the James R. Barker, a modern 1,000-foot freighter, reported taking three large waves over the deck in succession on Lake Superior. The ship survived, but the encounter demonstrates that the phenomenon remains active.

As a result, shipping companies now generally avoid November voyages when storms are forecast. The economic cost of tying up a ship is lower than the risk of losing the vessel and its crew. When conditions are favorable for the three sisters to form, the ships remain in port until the weather passes.