Titanic’s Masthead Light

Lights on the horizon can be very deceiving at times. Is it a rising star, or is it the mast light of a steamer coming your way? Certainly if you wait long enough the light will be seen to move relative to the background stars, the telltale of an approaching vessel. But how far off can this vessel be for its mast light to be seen?

At the American Inquiry Titanic’s surviving Fourth Officer, Joseph Boxhall, had this to say:

The requirements concerning navigation lights in effect at the time specified:[1]

Art. 2. A steam vessel when under way shall carry –

(a.) On or in front of the foremast…a bright white light, so constructed as to show an unbroken light over an arc of the horizon of 20 points of the compass, so fixed as to throw the light 10 points on each side of the vessel…from right ahead to 2 points abaft the beam on either side, and of such a character as to be visible at a distance of at least 5 miles.

(b.) On the starboard side a green light so constructed as to show an unbroken light over an arc of the horizon of 10 points of the compass, so fixed as to throw the light from right ahead to 2 points abaft the beam on the starboard side, and of such a character as to be visible at a distance of at least 2 miles.

(c.) On the port side a red light so constructed as to show an unbroken light over an arc of the horizon of 10 points of the compass, so fixed as to throw the light from right ahead to 2 points abaft the beam on the port side, and of such a character as to be visible at a distance of at least 2 miles.

(d.) The said green and red side lights shall be fitted with inboard screens projecting at least 3 feet forward from the light, so as to prevent these lights from being seen across the bow.

(e.) A steam vessel when under way may carry an additional white light similar in construction to the light mentioned in subdivision (a). These two lights shall be so placed in line with the keel that one shall be at least 15 feet higher than the other, and in such a position with reference to each other that the lower light shall be forward of the upper one. The vertical distance between these two lights shall be less than the horizontal distance.

Art. 10. A vessel which is being overtaken by another shall show from her stern to such last-mentioned vessel a white light or a flare-up light. The white light required to be shown by this Article may be fixed and carried in a lantern, but in such case the lantern shall be so constructed, fitted, and screened that it shall throw an unbroken light over an arc of the horizon of 12 points of the compass, viz., for 6 points from right aft on each side of the vessel, so as to be visible at a distance of at least 1 mile. Such light shall be carried as nearly as practicable on the same level as the side lights.

The 5 mile visibility requirement for the mast light was a minimum visibility distance. It was written to allow for oil lamps to be used. As Boxhall admitted, he believed he could see mast lights more than the regulation distance. But was he correct in believing that mast lights could not be seen at 14 miles?

The Titanic carried a single masthead light on her foremast about 145 feet above the water. To be seen at a distance the light had to be above the horizon of the observer. It also had to be bright enough to be seen and recognized for what it is. For example, consider a person standing on the deck of a typical tramp steamer with a height of eye above the water of 45 feet. If the Titanic’s masthead light were bright enough, that observer should notice the light coming over the horizon when it is 22 miles away, the maximum geographic range between light and observer. But would Titanic’s masthead light be noticed at that distance? Was it bright enough?

According to author and TRMA trustee Bruce Beveridge, Titanic’s electric masthead light developed a total of 32 candlepower (CP) from 2 filaments of 16 CP each. One candlepower is equivalent to 12.57 lumens. Therefore, the filaments produced a total of 402 lumens of light intensity, about as much as a typical 40W incandescent bulb. Now it happens that a single candle seen at a distance of 1098 meters, or 3602 feet, will appear as bright as a 1^st magnitude star.[2] Since brightness falls off as the square of the distance, something that is 32 times as bright as a single candle should appear just as bright when seen at a distance of 5.66 times further away. Therefore, without any lens amplification, a 32 CP light would appear as a 1^st magnitude star at a distance of about 20,400 feet on a clear dark night, a distance of 3.4 nautical miles.

But now we have to take into account the amplification of the masthead light lens; i.e., its light gathering power. The recovered masthead light from the Titanic is seen at right in a photograph from Denise Hunyadi taken in 2002 at the Great Lakes Science Center in Cleveland, OH. From what was described by Denise, Titanic’s masthead light lens is about 8 inches in height.

Using the principal of reciprocity, the gain of a lens in transmitting light is the same as the gain of a lens in receiving light. Normally the increase in light gathering power of one lens relative to another is the ratio of their areas. However, a masthead light lens only had amplification in the vertical plane which concentrates the light into a beam along an arc in the horizontal plane extending 112.5° to both sides of dead ahead. Since the amplification of the lens was only in the vertical plane, the amplification factor we have to use is the ratio of the height of the lens to the diameter of the lens opening of the human eye, which is about 0.32 inches on a very dark night. Therefore, the ratio we are interested in is 8/0.32 = 25. This concentration of light in the optics of Titanic’s masthead light lens is shown in the diagram below.