Thank you, David! Now I would know.One of the classic mistakes in researching Titanic is to take automobile experience to sea. A ship is as different from a car as a rhino is from a clam. But, we have all grown up with cars for transportation while few have had serious "bridge time" at sea.
This "car to sea" mistake is not confined to amateurs. When I presented the Grounding White Paper to the Society of Naval Architects, one of the august members described how by turning to the left Titanic had pulled its bow clear and the stern naturally followed. While I'm sure the man knew Froude's Law and how to calculate G-Z, he hadn't a clue about how his creations acted in their natural element.
Navigation lights have the primary purpose of preventing collisions at sea. They are not designed to illuminate anything. The lamps in them and the lenses are all designed to shine a narrow beam with little or no "spill" light, not provide general illumination.
Ever since Ralph Nader's book, "Unsafe At Any Speed," cars have been equipped with small lights that can be seen from the side. These illuminate nothing, but do provide information about the size and direction of the car to another driver approaching an intersection. On a less formal level they do the same job as navigation lights.
On land, a car is confined to a relatively narrow roadway. It is necessary for the driver to have as much information about the twists, turns, and pot holes of that road as possible. In rural areas drivers need to know when a stray cow is lounging on the pavement. Headlights are the answer. The driver's night vision is not as important as illuminated a narrow stretch of highway for a few hundred feet down the road.
Night vision is the name of the game when operating a vessel at night on the open sea. There is no confined highway, only trackless ocean. Dangers can approach from well outside the field of view provided by headlights. The only way to be certain of seeing such dangers is the properly dark-adapted eyeball of a trained observer (i.e. a "lookout") And, in this case training is critical. Not many people have honed the ability of seeing at night. After all, there is little need in our everyday, car-infested life style.
If the looks had constantly been switching from an illuminated patch of ocean to the darkness of the surrounding swells, they would have constantly compromised their night vision. Worse, even the best nautical "headlight" (a carbon arc spotlight used on America's western rivers) has only limited range. Lookouts in Titanic's crow's nest would have been effectively blind to objects dead ahead, but beyond the illuminated area. Robbed of their night vision, they would have been less able to spot a danger on the horizon, so the officers in charge would have had less time to choose the proper course of action to avoid an extremis situation. Bottom line, a ship with headlights "on" would be in more danger, not less, than Titanic with its well dark-adapted lookouts in the crow's nest.
Those carbon arc spots ... They are used for three primary purposes. One is to illuminate lock entrances when approaching with a stick of barges in tow.
The second is to illuminate the river bank to one side of the ship. By keeping the circle on where water meets land, the towboat skipper can stay fixed distance off the mud even though the river twists and turns.
And, the third use is to mortify small pleasure boats who tend not to realize the danger of getting in front of a few million pounds of coal or grain.
I've never seen a carbon arc spot used to illuminate the river like headlights on a car.
-- David G. Brown