The Bounty, Sandy and Storm Avoidance Techniques. The NTSB Report

Bounty (Wikipedia photo)

The National Transportation Safety Board has released it's report on the sinking of the Bounty. (here pdf).  There were lots of questionable practices with regard to maintenance, training and procedures  however with regards to the actual navigation in relationship  to the storm  there has been questions about the decision to pass west of the storm. There is a good graphic of the track of both the ship and the storm here.

Possibly the captain was using traditional low-tech  hurricane avoidance techniques that were not applicable to the situation.

From the  NTSB report:

 It is possible that the captain may have focused too narrowly on the position of the storm’s eye instead of on Sandy’s total expanse (winds associated with the storm spanned more than 1,000 miles in diameter, and the area into which the Bounty was heading was already under tropical storm warnings, with conditions  forecasted to worsen).Still, the captain seemed to believe that he could outrace the storm.

A traditional method of hurricane avoidance  is to stay a safe  distance from the eye on a track that passes the hurricane using  the so-called  navigable  semi-circle.

 On the navigable side,(the west side in the case of Sandy) as the storm approaches,  the winds and seas will be astern (important for a square rig sailing vessel) while also  pushing the vessel out of the path of the storm. Also the wind speeds are less because the winds are the cyclonic so the total wind speed is the  winds minus the storm direction.   On the dangerous semi-circle side (east side) the winds and sea tend to push the vessel into the path of the storm and also the cyclonic winds are added to the storm direction.

It seems plausible that Capt. Walbridge would have had high confidence in traditional low-tech avoidance methods.

The problem is that in the case of Sandy at the time of the encounter  the concepts of navigable/dangerous semi-circle were invalid.

This is because  when the Bounty encountered  Sandy it was transiting from a tropical cyclone(warm core) to an  extratropical cyclone (cold core). The meteorology is complex but the take-away for the mariner is that the wind field will expand and the field will no longer have the same characteristics.

Here is  tropical cyclone Katrina:

Katina (From NASA)

The dark colors represent the area of highest winds.  The strongest winds are NE of the center, the so-called dangerous semi-circle.

From NASA: 

Katrina was a textbook tropical cyclone, with a compact, symmetrical wind field that whipped around a circular low-pressure center.

Sandy, at about the time of the encounter on the other hand looks quite different:

Sandy (from NASA)

  In Sandy  the strongest winds are in south and west of the center and further from the center. Tragically this is where the Bounty was when it encountered Sandy.

 So when tropical cyclones become extratropical, their wind and cloud fields expand dramatically. Their strongest winds generally weaken during this process, but occasionally a transitioning storm retains hurricane force winds, as was the case with Sandy.

 From the NHC on the 28th.

THE MOST RECENT AIR FORCE HURRICANE HUNTER AIRCRAFT MISSION DID NOT
FIND WINDS OF HURRICANE FORCE NEAR THE CENTER OF SANDY.
HOWEVER...EARLIER DROPSONDE DATA INDICATED THAT WINDS TO HURRICANE
STRENGTH WERE OCCURRING WELL TO THE SOUTH AND SOUTHWEST OF THE
CENTER.

What  the Bounty encountered may have  the highest wind speeds in the system. The Bounty  was about 100 miles from the center in what would be considered (in a hurricane) the navigable  semi-circle. What was encountered instead was  not only the highest wind speeds but also the roughest seas, made short and steep  by the contrary (north bound) Gulf Stream current.

In spite of the complex situation the models used by meteorologist  for the Sandy forecast were  accurate.  There is a good discussion of the forecast here.   Had the captain used the NHC (National Hurricane Center) forecasts he may have been able to avoid the worse of the storm. Of course with few hurricane  encounters under his belt the inexperienced  Walbridge likely had more confidence in his own skills then the NHC forecasts.  Walbridge's confidence was misplaced.

A better approach is to set wind speed  and/or sea height  limits before the voyage and to use forecasts to ensure those limits are not exceeded. 

Even better then dodging bad weather  of course to find a good hurricane hole, put out extra lines and stay put till the storm passes.

K.C.


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The NWS warns that the 1-2-3 Rule  "does not account for the typical expansion of the wind field as a system transitions from hurricane to extratropical gale/storm."  Nonetheless the rule holds up quite well.

Sandy and the 1-2-3 Rule from David Burch Navigation

Source for the info on tropical system and extratopical is from: Comparing the Winds of Sandy and Katria