US Sailing published a report cited on page C2 of the August 9th edition of the San Francisco Chronicle, discussing the loss of the racing vessel Slow Speed Chase in the Farallones earlier this year. The report stated that,
“If the crew of Low Speed Chase had sailed in deeper water, they could have prevented the tragedy”.
The report cited “Failure of seamanship in negotiating shoal waters on a lee shore”, as the main safety issue in which the crew failed to consider the risks associated with waves at that depth of 28 feet. As a result of this failure five people were killed. Yet, what happened to this boat was predictable and foreseeable.
Sailors with many years of experience sailing inside SF Bay often know how to successfully predict winds and currents, and do so quite accurately, including foreseeing counter currents and back eddies. Such experienced racers can predict how the wind will channel around capes and bluffs, and by watching the amount of overcast or clear skies can tell you how windy it will get later that afternoon. Their ability comes from studying the principles of winds and currents and through experience applying these principles to racing on SF Bay.
On the other hand, most of these racing veterans have never taken the time to study and understand the coastal oceanography of swell action in shoal waters on a lee shore. Consequently, when it comes to predicting easily foreseeable rogue wave conditions, they haven’t a clue. They often do not even know that such rogue waves can be so easily predicted and avoided.
To explain this phenomena, which is known to most coastal cruising professionals, I published an article in the August 2004 issue of SEA Magazine, which covers the theory behind all this explaining why 20 foot swells will suddenly appear in shoal water near a lee shore. Understanding the principles behind the phenomena allows easy prediction of rogue waves and the ability to avoid those particular locations where they will occur. That article is entitled BAR CROSSING MADE EASIER: How to Safely Enter and Exit North Pacific Coast Estuaries. (To down load a PDF just click to my WHITE PAPERS page and scroll down to PASSAGE MAKING-CROSSING BARS)
What happened to the Low Speed Chase was identical to normal bar crossings as described in the article. Low Speed Chase was crossing a 28 foot bar in lee shore conditions. Consequently, the rogue waves built up exactly as they do every afternoon in the Potato Patch (Four Fathom Bank) just west of Point Bonita. Four Fathom Bank has only 24 feet of depth. So every afternoon the 6 to 8 foot westerly swell coming in from seaward and deeper water is concentrated by the shallow water into a 12 to 16 foot chop which is closely spaced and which will easily capsize any vessel which takes in on the beam in the Potato Patch.
As the Low Speed Chase rounded close behind Main Top peninsula on Southeast Farallone their course placed them with the westerly swell on their beam in only 28 feet of water. Next, as the water shoaled under their keel, what had previously appeared to be only 6 to 8 foot swells quickly changed into 16 foot rollers on their beam. Suddenly, they were broached to the seas with gigantic rollers on their beam which predictably capsized their vessel.
Several untethered crew members went overboard at that point. The remaining crew, who might still believe that that single large wave had been an isolated fluke, heroically turned around in an attempt to rescue their overboard friends. Yet, in attempting that rescue they inadvertently sailed back onto the shoal. Predictably, the next rogue wave took them up on the rocks.
On the other hand, with a little knowledge all this was foreseeable and predictable, Every skipper who plans to operate under sail or power in coastal waters, even if only for a single race, should make a careful study of what is required to successfully cross bars in all weather conditions. Sailing outside the gate is not at all the same as sailing inside the bay, and a couple hours of studying the oceanography of these wave actions, and learning what quite predictably always happens on any lee shore, will provide the ability to foresee and predict these rogue waves, and do it even more accurately than they predict the afternoon winds, or a counter current along the city front.