I wrote a blog the other day about the findings of a report into the IMOCA 60 BT’s near sinking (here) and the conclusion that a rogue wave, or ‘hydrodynamic event of significance’ was at least partially to blame.

Inevitably when such conclusions are reached, some readers cynically think: well, the designers would say that wouldn’t they? That may or may not be so but it doesn’t change the fact that big waves, almost incalculably powerful waves, have indeed existed.

So how can you allow for the dynamic loads placed on a boat hit by a liquid equivalent of a falling wall of concrete? The answer, apart from allowing a reasonable margin, is that you can’t. And of course the other big variable – the speed and loading of the yacht at the time – is something of a moving goalpast.

If you have a lightly bult, less durable boat an experienced crew may back off early or shape a different course. Whereas if you have a strongly built boat proven in heavy weather they may press on. So no matter how strongly you build a boat, a racing crew will always be inclined to push it to what they believe it can withstand.

There are lots of examples of this.

But first, the question of rogue waves. Are they a commonplace reality or more of a convenient myth?

There are many anecdotal examples of such waves, which seem to be characterised not only by  overall height but also by their particularly steep faces and/or breaking tops – in other words, the force they are carrying to sweep something in their path aside or asunder.

When Miles and Beryl Smeeton were pitchpoled in the Southern Ocean in their boat Hang Tzu in 1957, Beryl vividly described it as ‘a completely vertical face, down which ran white ripples of water, like a waterfall’.

A similarly steep wave, probably compressed and shortened by its meeting with the continental shelf of South America, pitchpoled Canadian skipper Derek Hatfield’s 40ft Spirit of Canada in the Around Alone race in 2003. Like Hang Tzu, the boat was dismasted when she rolled stern over bow.

Proving huge waves exist

The power of these waves has been well documented in the shipping world, too. In the last 20 years, around 200 supertankers and container ships over 200m have been lost to severe weather conditions, and rogue waves are thought to account for many of these losses.

We know that huge waves exist because, very occasionally, they have been measured. On New Year’s Day 1995, a monster of 26m (measured by laser) hit a North Sea oil rig when the surrounding significant wave heights were at 12m.

A better picture emerges all the time from data gathered by synethic aperture radar installed on weather satellites. They have a sufficiently fine resolution to identify some very large waves that break the pattern of surrounding wave heights.

But it may not be the overall height that makes a wave dangerous. There is a lot of evidence to show that it’s the shape and the steepness that is so destructive. And this can be contributed as much by a deep trough ahead of a wave as by its height above the sea surface.

These heaped, crested or breaking waves, as most offshore sailors know, tend to build up where there is wind over a strong ocean current – such as the fearsome Agulhas Current off the coast of South Africa – or where the passage of a front and a veering or backing wind has heaped wavetrains on top of one another.

Lost overboard

A rogue wave produced by wind against current struck Rod Brigg’s 40ft cruising yacht in the Agulhas Current in 1990, making what he described as ‘a huge impact on the starboard side of the vessel’ and rolling her.

Two crew were violently thrown overboard in the capsize and were never recovered.

The phenomenon of new wavetrains running through a previous group at different wave periods can create these differently wave shapes with much more destructive power. French sailor Alain Gautier memorably described one he encountered in the Bay of Biscay shaped “like a pyramid”.

It stormed out of a 10m swell, rose beneath his 60ft trimaran Foncia in 2002 and snapped one of the main beams as if the boat were a toy. Gautier had to be rescued. He commented later that he had been sailing “delicately” because of the cross sea. Although he hadn’t spotted this monster in the dark, “I had seen several waves of that type during the day.”

The same heaping of waves and foreshortening of the wave period was one of the principal reasons why the 1979 Fastnet Race proved so fatal so suddenly.

The IMOCA 60 BT had gone through a storm and was in the path of another, so it is possible a rogue wave was an accumulation of two wave trains, one set propagating from the oncoming storm hundreds of miles away.

Blindsided by a wall of water

Going on first-hand accounts, the biggest danger from large, steep waves is of being smacked beam on, or of the wave and its breaking crest toppling onto or over the deck of a boat. Yacht stability experiments have proven that even quite small breaking waves have sufficient dynamic power to roll a boat so what must a huge breaking wave be capable of?

When it comes to the sheer might of these devastating waves you have to have sympathy for designers and engineers. There are, undoubtedly, conditions in the oceans of the world that no yacht could withstand.

One of the most sobering examples of their crushing power was damage caused to the 72ft steel Global Challenge yacht Veritas in 2001. The boat was blindsided by a huge wave in the Bass Strait during the dark hours of the night and tonnes of water crashed over the topsides and into the cockpit.

The wave swept one crewmember into the wheel of the boat, legs first, causing such severe fractures that he has, to this day, never fully recovered. The steel boat had been designed for upwind Southern Ocean conditions, yet was later found to be dented underneath and along the topsides between the ring frames to a depth of 8mm all the way from the mast to the sail room forward.