Practical Value of Rangefinders

As optical instruments, rangefinders have finite accuracy and precision. Manufacturers and naval services attempted to identify and quantify sources of error and limitation in an effort to make their instruments more helpful in the fire control process.

This article will focus first on Coincidence Rangefinders, as they are easiest to characterise in optical and mechanical terms.

Practical Use

The first barrier against rangefinders is that they often prove unusable.

At the Battle of the Falkland Islands, "Rangefinders found of little use due to long range, spray from enemy short shots and funnel smoke."

At the Battle of Dogger Bank, "Little use could be made of the rangefinders as few cuts could be obtained, whilst the range was too great for accurate readings to be taken; time and range plotting was impracticable."

At the Battle of Jutland, they seem to have been of little utility to most ships. Their performance is best described by what is not said about them, as the British dreadnought captains often failed to supply Dreyer Table range plots after the battle when directly asked to supply them.

At the Second Battle of Heligoland Bight,

The evidence on the utility of the rangefinders is contradictory. Two ships appear to have got good results, others report few readings were obtained, and one ship reports a total failure.

Ranges were seldom obtained before fire was opened, so that on practically all occasions fire was opened with a guessed range.

Periods of fire were so spasmodic, and visibility so poor, that rangefinder observations generally were of little value for rate keeping. The readings, whenever obtained, provide a check on the gun-range, and were of value.

Cases are reported where one of the rangefinders in a ship got fair results while the other, under identical conditions, failed altogether.

Errors

Sources of Error

The Royal Navy estimated that the human eye can detect misalignment of half images in a coincidence rangefinder when the angle at the eye reaches 12 arc seconds, or 0.0000582 in circular measure. For a rangefinder, this meant that the uncertainty of range estimation was:

error = (0.0000582 * range * range) / (magnification * baselength)

In other words, the error in ranging would vary with the square of the range, and would be reduced linearly by the baselength of the instrument and by its magnifying power.

Errors in Practice

In March 1917, three ships reported their 15-foot rangefinders had given them mean errors of 1,000, 1,450, and 1,500 yards short at ranges between 19,000 and 21,000 yards. Oddly, when this was happening, their 9-foot rangefinders in the top were producing more accurate cuts.

John Brooks cites late-war Grand Fleet reports that

An examination of rangefinder plots obtained at full calibre firings during the third quarter of 1917 shows that small rangefinder spreads were the exception, and that the gun range in many cases differed from the mean of the rangefinders by as much as 1000 yds. In more than one instance, an individual rangefinder altered its divergence from the gun range by 2000 yds. during the firing. Rangefinder rates on the whole were good, but in some cases all rangefinders of a ship indicated a rate of 200 to 300 yds. per minute in error. ...for want of a better explanation, these errors have been attributed to refraction, i.e. the bending of the rays from the target to the rangefinder, due to the opacity of the atmosphere, smoke, or light effects.

Investigation by Barr and Stroud in both cases discounted reports of refraction and placed the blame predominantly on heating of the instruments under direct sunlight.

Bibliography

• Brooks, John (2001). Fire Control for British Dreadnoughts: Choices of Technology and Supply. Unpublished PhD Thesis. London: Department of War Studies. King's College, London.