The Drift of Wreckage

In July 2007, in my on-line article “Collision Point,” I showed how the average drift and set of the local current in the area where Titanic sank could be derived based on the now known location of the wreck site, a reported noontime fix for the SS Californian, and the dead reckoning (DR) position of the wreckage that was seen when Californian departed the area at 11:20am, Californian time. The derived current had a drift 1.09 knots and a set of 196.7° True.[1] The derivation was a refinement of the method used by the accident investigators of the Marine Accident Investigation Branch (MAIB) of the British Department of Transport as part of their 1992 reappraisal of evidence relating to the Californian affair. However, there are some individuals who assert that the total drift of wreckage seen was due more to the action of the wind that came up in the morning, and not by the local ocean current in the area. In fact, there is one who made the bold claim that “the experience of Californian suggests there may not having been a current that night or one so slight as to be almost indiscernible.”[2] It will now be shown why these claims cannot be supported.

In maritime drift there are two factors that come into play. One is the total drift of the water current consisting of two components, ocean current and wind current. The other factor is the leeway drift caused by the action of wind on a floating object. The total drift is the vector sum of all these components.

Ocean current is the large scale flow of the ocean in a given region. Wind current is a component caused by wind action on the sea surface over a period of time. It is taken to act at an angle of 30° to the right of the downwind direction for latitudes north of 10°N, 30° to the left of the downwind direction for latitudes south of 10°S, and in the downwind direction for latitudes between 10°N and 10°S. The magnitude of the wind current is equal to 1/28 (0.036) the magnitude of the wind speed that is blowing over the area.[3]

Leeway drift is generally in the downwind direction, but as both the shape and the exposed portion of the object are factors which contribute to the direction and rate of drift, leeway due to wind action on the object will not always follow the downwind direction. The object will tend to drift somewhat to the left or right of downwind creating an angle of divergence to either side of the downwind direction.

All of this goes into consideration when determining search areas in air/sea rescue operations.[4] In the diagram below we see the three components of drift that affect the total drift of an object in the northern hemisphere (>10°N latitude) from its last known position. The ocean current vector in this generic diagram is somewhat arbitrary, pointing SSW. The wind current component vector is shown pointing 30° to the right of the downwind direction which is taken as southward, and the two leeway drift components are each separated by a divergence angle from the downwind direction. The magnitude of the leeway component vectors are dependent on the type floating debris, as is the divergence angle from the downwind direction. The radius of the two dotted circles at the end of the leeway vectors are equal to a probable error of position, and the dotted rectangular area represents the calculated search area.

In the case of Titanic’s wreckage, we already know where the wreckage was last seen, and we also know the last known position of Titanic. We also know that the wind first came up as dawn was first breaking,[5] and that it came more or less out of the north.[6] Before that, there was a dead calm. It is clear from looking at photographs of Titanic’s lifeboats as they approached Carpathia during the early morning hours that the wind conditions during most of the rescue operation ranged between “light” to “gentle” breeze conditions on the Beaufort scale, averaging perhaps 6 to 7 knots with 1 foot waves. It was only after 8:30am, the time Californian arrived alongside Carpathia, that the wind had reached “moderate breeze” conditions according Carpathia’s Captain Arthur Henry Rostron.[7] (A moderate breeze is defined as a Force 4 on the Beaufort scale; 11-16 knots with 3 foot average wave heights.[8])

Is it at all possible that the drift of wreckage from Titanic could have been caused mostly by the action of wind as some have suggested? To explore this possibility let us look at some of the established facts.

Titanic sank about 2:20am at 41° 43.5’N, 49° 56.8’W. The estimated position of her wreckage 9 hours 15 minutes later, when Californian departed the area, is at 41° 33.9’N, 50° 0.6’W,[9] a distance of 10.01 nautical miles from the wreck site. Seen amongst the wreckage was overturned Collapsible boat B, some deck chairs, a few lifebelts, and other small debris. From leeway studies that led to models used by search and rescue (SAR) teams,[10] we find that for boating debris such as deck chairs and lifebelts and other small stuff, a leeway drift multiplier equal to 0.02 and a divergence angle of 10° are used.[11] (A capsized shallow-ballast marine life raft has a multiplier of 0.017 and a divergence angle of 8° which are close to the values for typical debris that can be expected from a boat that is sinking and/or breaking up, and probably close to what we would expect for an overturned collapsible lifeboat.) Using this information, and the 0.036 multiplier for the wind current component, we can develop a simple equation to give us the required sustained wind speed needed to cause wreckage to drift 10 miles from the time the wind came up in the morning. The equation is:

W = D / [ T(0.036 + m) ]

where W is the wind speed, D is the drift distance, T is the time that the wind is acting on the object, and m is the leeway drift multiplier for the type of wreckage seen (in this case m = 0.02).

We also know that there was a flat calm in the area until dawn began to break. Assuming the wind first came up 2 ½ hours after Titanic went down, about 4:50am when the sky was brightening in the east, then we have 6 hours and 45 minutes for the wind to act on the floating wreckage, or T = 6.75 hours, to cause it to drift D = 10.0 nautical miles. Plugging these parameters into the above equation, what we find is that W = 26.5 knots of sustained wind needed to cause the wreckage to drift those 10 miles in 6 hours and 45 minutes to the time Californian departed the area of wreckage. This wind speed is at the high end of a Force 6 wind on the Beaufort scale which is listed as a “strong breeze” with average wave heights of 9 feet (max 12 feet), and described as large waves that begin to form with white foam crests that are “more extensive everywhere (probably some spray).” These conditions never took place.

Looking at photographs of Titanic’s lifeboats as they approached Carpathia that Monday morning, it is quite obvious that the prevailing wind conditions alone were incapable of causing all that wreckage to drift as far south as it did. The major contributor to the southerly drift had to be the cold Labrador current that dominated the region of the disaster, the same current that set all that pack ice and icebergs as far south as 41° 16’N latitude as reported from Carpathia later that day. But can we estimate how much drift may have been caused by the Labrador current versus how much drift may have been caused by wind current and leeway?

For this we can only form rough estimates from the conditions seen in the photographs of lifeboats as they approached Carpathia. As we noted before, for most of the pickup time, it appears that the wind ranged between light to gentle breeze conditions. Therefore, let us take 7 knots over a period of 6 hours 45 minutes for the average wind conditions that affected the drift of wreckage. Under this assumption, the drift of wreckage works out to 2.63 nautical miles due to wind current and leeway. This leaves 7.37 miles of drift due to ocean current acting over a period of 9 hours and 15 minutes, or an average ocean current drift of 0.8 knots.

It is also interesting to consider the drift of a person in the water wearing a personal flotation device such as a life belt. According to SAR models, a person will assume the classic fetal position with legs drawn up and arms huddled across the flotation device, a position a conscious or unconscious person takes on, especially in cold water, when wearing offshore lifejackets, horse-collar lifejackets, or inflatable vests. The leeway drift multiplier for this condition is equal to 0.012 with a divergence angle of 18°.[12] If we apply these parameters, what we find is that persons in the water, conscious or unconscious, would drift an average of 2.25 nautical miles in 6 hours 45 minutes due to an average wind of 7 knots over that period of time. Again, this is the drift due to wind only. Compared to the boat debris model, the drift of an unconscious body in the water will be somewhat less than the drift of floating wreckage, but with a wider divergence angle. This is shown to scale in the figure below.

It is interesting to note that during the American inquiry, first class passenger Arthur G. Peuchen, a major in the Canadian militia and an experienced yachtsman, was questioned by Senator Fletcher about why there were no bodies seen amongst the wreckage:

When Californian arrived on the scene, Captain Stanley Lord was asked to continue to search for survivors. Carpathia’s Captain Arthur H. Rostron:

According to Captain Lord:

Californian searched to leeward of the floating wreckage and saw nothing but the abandoned lifeboats that Carpathia left behind and the floating wreckage from Titanic.[13] According to Captain Rostron, he received a wireless message from Captain Lord the next day saying: “Have searched position carefully up to noon and found nothing and seen no bodies.” We know Californian departed the area of wreckage at 11:20am, Californian time, and got a noontime sight of the sun to establish their noontime position 51 minutes later. According to James Bisset, Carpathia’s second officer at the time:[14]

We know from Captain Rostron that Carpathia came near the wreckage as he was picking up the last lifeboat. However, it appears that Carpathia never went to windward of the wreckage that Rostron identified. In fact, it appears that he purposely avoided the area northward of the observed wreckage. It is quite obvious that there were hundreds of bodies floating around as James Bisset said, after all the Mackay-Bennett found them amongst some of the wreckage five days later, including overturned Collapsible lifeboat B.[15] Despite Rostron's claim that he saw but one floating body, it seems he decided that it was not a good idea to take any bodies on board as he was on a rescue mission, not a recovery mission.[16]

In conclusion, we have shown that most of the wreckage drift had to be due to the cold Labrador current in the vicinity of the wreck on the morning of 15 April 1912. This drift was enough to account for about 75% of the total drift that was seen 10 nautical miles S by W ½ W True of the wreck site location 9 hours 15 minutes after Titanic sank. The drift due to wind would only have accounted for about 25% of the total drift. The wind only started up at daybreak, about 2 ½ hours after Titanic disappeared beneath the surface, and only reached moderate breeze conditions shortly before Carpathia left the scene. The downwind drift of bodies was not as much as the downwind drift of the floating debris that was observed from Carpathia and Californian. Neither of these vessels appears to have bothered to search to windward of the debris that was seen, and only one body was reported seen close up from Carpathia that morning.