upon further review

inside game

upon further review…

The smart guys and gals at Tidetech share some of their insight about the AC 17 flip…

The speed of global mainstream and social media being what it is, the world now knows what a 72’ catamaran pitchpoling looks like.

In order to inform those that don’t know how this kind of thing can happen to arguably the best sailors on the planet, Tidetech, technical supplier to the 34th America’s Cup, has analysed the elements that conspired to create conditions capable of landing a cat on its head…

Most sailors know that when current direction is against wave direction the effect is to steepen the wave field and create choppy water. The opposite, when waves and current act in the same direction, results in the waves being ‘smoothed’. This is a feature known as the wave-current interaction.The collision of these two phenomena at a time of maximum ebb current is one likely factor in the capsizing of the Oracle AC72. Here are some facts from the time of the incident:

At 2.50pm on 16 October 2012 the wave buoy Station 46026 – SAN FRANCISCO – 18NM West of San Francisco, recorded mean wind speed of 19.4kt, gusting wind speed of 21.4kt, significant wave height of 5.9 feet, dominant wave period of 13 seconds, average wave period of 6.6 seconds and mean wave direction from the west.

The wind and gust speeds inside the bay itself were higher than this with Predict Wind forecasts showing speeds of around 25kt while the recording station at Crissy Fields on the San Francisco city front showed average wind speeds of around 20kt and gusts up to 28kt. At the same time one of the highest tidal ranges of the year (High Water 1222 PDT 6.6’, LW 1849 PDT -0.9, tidal range of 7.5) resulted in very strong ebb currents heading west out of the Bay. In a nutshell, strong tidal currents were opposed to strong prevailing winds and waves at just the wrong time.

Looking at the animation of the tides through that part of the day shows that only 20 minutes before the capsize, the tidal current in the bear-away zone was only 0.8kts. Between 1430 and 1500hrs the ebb tide increased from 0.8kt to 2kt. This would have rapidly steepened the waves in the critical region where the cat eventually headed downwind. While Oracle may not have had problems on previous circuits, this time around things had changed significantly. Take a look at the tidal video animation.

By 1530 the tide is running at 4.5kt to 5kt just west of the Golden Gate Bridge. This speed of current combined with steep waves no doubt hampered the recovery efforts, which would have become easier around 1930hrs with the onset flood tide.

Any catamaran can pitchpole if buried into a steep wave but it takes something impressive to catch-out a crew of this standard and a boat of this size. Bad luck to Oracle but with no-one injured and time to lick wounds, it’ll hopefully only be a short time before they’re back on the water in their AC72.

Tidetech is offering full access to its OceanView online view to Sailing Anarchy readers for three days. Click here for access. To see tidal currents for 16 October go to the ‘Brown Trousers’ product…