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A '''Dumaresq''' (sometimes the initial D is capitalised, though the practice subsided) is an mechanical analog [[Fire Control|fire control]] instrument helpful in related the relative motion of one's [[Own Ship|own ship]] and a target ship to the corresponding [[Range Rate|range rate]] and [[Speed-Across|speed-across]].  It was designed and patented by Lieutenant [[John Saumarez Dumaresq]] between 1902 and 1904.<ref>Brooks, ''Dreadnought Gunnery and the Battle of Jutland: The Question of Fire Control'', p. 42</ref>

There were a considerable number of dumaresq Marks, but all answered to the same underlying geometric principles.

==Geometric Principle==

It is often easiest to apprehend mechanical devices through visual means, such as [http://vimeo.com/986479 this online video].

A dumaresq functions by mechanically implementing a flexible means of working the geometry of two independently moving ships with a relative bearing from one of the other and being able to relate this to motion along and across the line of bearing (range rate and speed-across, respectively).  Specifically, it permitted you to subtract the motion vector of your own ship from that of a target ship to yield the target's relative motion vector which could then be projected onto a Cartesian coordinate graph oriented along the line of bearing, permitting the range rate and speed-across to be read off as a coordinate pair.

Users commonly set up a dumaresq in this manner:

# set their own ship's speed and heading
# set the target ship's speed and heading (by estimation)
# rotate a pair of coordinate axes along the line of bearing to the target
# read the range rate off one axis of the graph and the speed across off the other

However, the dumaresq was not a modern computer which used a given set of inputs to a given set of outputs.  It was wholly indifferent to which of the variables it represented in its workings were inputs and which were outputs.  Altering one variable by rotating a knob or by positioning a slider caused other variables to change to continually satisfy the relationship between two ships moving around and the range rate and speed-across their relative motion would imply.  This meant that the sequence of use described above which derives range rate and speed-across from the speed and heading of both vessels and the target bearing could be worked flipped on its head to find the enemy speed and heading from a given range rate and speed-across.  This was called a "cross-cut".

==Cross-Cut==

When making a '''cross-cut''', a dumaresq operator would manually move the indicator stem expressing the difference of own ship's and target ship's motion and permitting the enemy heading ring and speed slider to re-orient and slip as needed to follow your demands.  Upon placing the indicator at the range rate and speed-across you thought proper, the dumaresq would then indicate the enemy speed and heading that would be implied by your hypothesis of range rate and speed across (own ship's speed and heading and the target bearing being held unchanged in this case).

Steps to make a cross-cut on a dumaresq:

# set their own ship's speed and heading
# rotate a pair of coordinate axes along the line of bearing to the target
# drag the relative velocity indicator stem to the estimated range rate and speed-across
# observe the enemy speed and heading which are then indicated

==Dumaresq Marks==

Dumaresqs existed in a variety of Marks, differing in their features and in some odd particulars, such as which parts rotated and which were fixed in position.  They were manufactured by Elliot Brothers.

===Mark I===

This model was marked for range rates according to the original Royal Navy practice, not in yards per minute but in time interval required to change the range by 50 yards.  The design was later adapted by adding a rifle-like peep sight to the dial plate so it could be rotated to match the line of bearing by finding the target in the sight from an aloft position.  Presumably in such a mode of use, the dumaresq would have its "own course" arm left aligned with own ship's keep, making all headings and bearings on the instrument relative ones.  Some found their way onto the new [[H.M.S. Dreadnought (1906)|H.M.S. Dreadnought]].<ref>Brooks, John.  ''Dreadnought Gunnery and the Battle of Jutland'', pp.42-43.</ref>

===Mark II===

An enlarged model which also saw service in Dreadnought<ref>Brooks, John.  ''Dreadnought Gunnery and the Battle of Jutland'', pp.42-43.</ref>.    It was considered obsolete by 1920.<ref>Schleihauf, Part I. p. 8.</ref>

===Mark II*===

Presumably, this is also larger than the Mark I, but it is not clear how it differs from the Mark II.  It may possibly be identical to the Mark III*.  It was considered obsolete by 1920.<ref>Schleihauf, Part I. p. 8.</ref>.  

===Mark III===

This dumaresq had a fixed fore-and-aft bar for own heading, though an inner ring marked with compass headings would be rotated.  Common practice kept it locked in place so its markings would become relative bearings.  Likewise, the enemy heading ring had a compass wing that could be rotated, but was usually locked down to match the own heading bar.  The result of this was that the enemy heading would be expressed as a relative heading.

The dial plate retained the rifle peep sights.  This model was still in service in 1920, but noted as "practically obsolete."<ref>Schleihauf, Bill. ''The Dumaresq and the Dreyer, Part I.'' pp. 7-8.</ref>

===Mark III*===

The particulars of this model yet await my discovery.

===Mark IV===

This model was used in turrets, and therefore it was essential to permit its fore-and-aft bar to swivel.  In practice, the fore-and-aft bar was swiveled back to the keel, and the fixed angle was the line of bearing on the dial plate which faced the turret face.  The markings for aligning the fore-and-aft bar were arranged such that the operator indicated the turret's relative training angle, not own ship's heading.  The enemy heading ring was geared to rotate as the bearing was set with the consequence that the operator spun the enemy heading bar to an indicated [[Inclination|inclination]] (rather than heading) &mdash; convenient!<ref>Schleihauf, Bill. ''The Dumaresq and the Dreyer, Part I.'' pp. 8, 11.</ref>. 

===Mark V===

This model was used in spotting and control towers that did not revolve<ref>Schleihauf, Bill. ''The Dumaresq and the Dreyer, Part I.'' pp. 8.</ref>.  From this, we may infer that the own heading arm was fixed or generally kept locked in line with own keel and that bearings were relative.  It was smaller than the Mark III, presumably to avoid cluttering the busy platform on which it was sited.  It retained the rifle sight characteristic of dumaresqs intended for standalone use in positions that can directly observe the enemy.<ref>Schleihauf, Bill. ''The Dumaresq and the Dreyer, Part I.'' pp. 11.</ref>.

===Mark VI===

"The chief difference between the Mark III and the Mark VI is that the setting of the enemy's inclination is kept on automatically during a turn of own ship.  In this instrument there is a revolving ring between the dial plate and the fixed ring, and this revolving ring is geared to the holder carrying the enemy bar.  It is also geared to the dial plate.  Any movement of the revolving ring is directly transmitted to the enemy bar, which will move by the same angular amount in the same direction as the revolving ring.  The dial plate can be moved independently of the revolving ring by means of two small pinions. On the dial plate of the Mark VI are screwed 4 thin bras strips.  Two of these strips are graduated in degrees of change of bearing at 10,000 yards, the other two are plain.  Two celluloid strips are supplied with the instrument, and these strips can be placed so as to slide between the brass strips.  If one of these celluloid strips is placed at the degree of rate of change of bearing at the range of the target and the other is placed at the rate in use, the point of intersection of the black lines on the celluloid strips will be the actual point over which the pointer-of-the-enemy bar should be, and thus the course and speed of the enemy can be checked.

To set the instrument: set the speed of own ship and enemy's speed and inclination in the usual manner. Align sights of the dial plate on the enemy by means of the small pinions, the worm shaft being connected up. The rate, deflection and bearing can then be read off.  If own ship alters course, the bearing must be kept on by turning the small hand-wheel on the vertical shaft, or by unclutching the worm-shaft and turning the intermediate ring.  The latter method is quicker than using the hand-wheel.  During the alteration, the revolving ring being geared to the enemy bar will revolve the ring and preserve its inclination to the line of bearing."<ref>Schleihauf, Bill. ''The Dumaresq and the Dreyer, Part I.'' pp. 12.</ref>

===Mark VI*===

This is the variant which was used by the Mark III Dreyer Table.

===Mark VII===

"Similar to the Mark VI, with the addition of an extra ring, graduated from 0 to 180 on either side fo the fore and aft line mounted outside the original ring on the enemy bar carrier as fitted in the Mark VI.  This ring facilitated the setting of the angle between keels, but enemy inclination is estimated rerelative to the line of bearing, or line of fire, in modern practice, and all later dumaresqs are graduated accordingly,  In other respects the operation of the instrument is similar to the Mark VI, except that for change of bearing dial is revolved by two small knobs, no pinions being fitted for this purpose."<ref>Schleihauf, Bill. ''The Dumaresq and the Dreyer, Part I.'' pp. 13.</ref>

===Mark VII*===

"# Three rings are fitted on the enemybar carrier: one graduated for inclination to line of fire; one for inclination to own fore and aft line; and one as a compass ring.
# A double enemy bar is fitted, the upper portion to be used for setting speed and inclination and the lower portion carrying the pointer for reading off rate and deflection.
# The dial plate is fitted with gearing driven by a milled head under the fore and aft bar."<ref>Schleihauf, Bill. ''The Dumaresq and the Dreyer, Part I.'' pp. 14.</ref>

===Mark VIII===

"No outer compass ring is fitted on this instrument, and only one ring, graduated for inclination to line of bearing, is fitted on the enemy bar carrier.  This ring is geared to the dial plate so that it is always kept with its zero radius parallel to the line of bearing.  The enemy bar is normally free of this inclination ring and does not revolve with it, so that for ordinary alteration of  bearing the "angle between keels" is preserved; a pedal clutch permits clutching the enemy bar to the inclination ring when own ship alters course, thus preserving enemy inclination to line of bearing.  The clutch consists of two serrated rings, one on the underside of the inclination ring and the other in the upeper side of the enemy bar.  Depressing the pedal raises the enemy bar until the two rings mesh, when enemy bar will be locked to, and will revolve with, the inclination ring."
<ref>Schleihauf, Bill. ''The Dumaresq and the Dreyer, Part I.'' pp. 15.</ref>

The Mark VIII continued to be used in aloft sitings, as indicated by its appearance in a post-World War II handbook<ref>B.R. 1534 (1), ''Handbook on Minor Fire Control Instruments''</ref>.

===Mark IX===
===Mark X===
===Mark XI===

The extremely Spartan Mark XI was probably the simplest dumaresq ever built.  It was suitable to armed merchant ships and likely  saw service throughout World War II<ref>B.R. 1534 (2), ''Handbook on Minor Fire Control Instruments''</ref>.  It was so reductionist in its design that the speed of own ship was fixed at 12 knots.  Moreover, the dial plate did not even have markings for taking range rate &mdash; it indicated a speed-across only.  The speed-across obtained was intended to be used directly as the [[Gun Deflection|gun deflection]], as the vessels mounting such a dumaresq were expected to fight at such short ranges that times-of-flight were roughly proportional to range and so speed-across was a fair estimate of deflection.

===Mark XII===

The Mark XII was very similar to the Mark XI in design and intended deployment, and only a little fancier; its dial plate ''did'' indicate a range rate<ref>B.R. 1534 (3), ''Handbook on Minor Fire Control Instruments''</ref>.  This implies that it was probably used in concert with a [[Vickers Range Clock]].

==Obsolesence==

The dumaresq was first eclipsed from service when its integrated application within the [[Dreyer Fire Control Table]] was not succeeded by an appearance with its heirs and cousins such as the Royal Navy's [[Admiralty Fire Control Table]] or the American Navy's [[Ford Rangekeeper]]. Both these evolutionary paths of development features "rate solvers" similar in mechanical design to the one found in the [[Argo Clock]]s.  The rate solvers in the Argo clocks were much more compact and intrinsically supported the Argo's inventive visual display which clearly represented the relationship of own ship, target ship, and the relative bearing.  The functionality was largely the same between these two designs, but the dumaresq looked and felt clumsy relative to the alternative.

The dumaresq enjoyed a longer reign in aloft positions for moderately equipped ships which paired simple dumaresqs with a standalone range clock.  The Mark VIII, XI and XII were prominent examples, finding representation in documents created after World War II<ref>B.R. 1534 (1-3), ''Handbook on Minor Fire Control Instruments''</ref>.

==See Also==
*[[EU/SV Anzeiger]]
*[[Wind Dumaresq]]

==Footnotes==
<small>
<references/>
</small>

==Bibliography==
<small>
*{{BibBrooksDreadnoughtGunnery}}
*{{BibHandbookMinorFCInstruments1946}}
</small>


[[Category:Fire Control]]
[[Category:Shipboard Equipment]]