Dumaresqs

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Dumaresq Mark VI

This is the dumaresq which was used on the Dreyer Fire Control Table Mark III.  Prior to this model, most dumaresqs were pretty darned simple.

Notice the considerable gearing used to permit a flexible shaft which comes in from the top to apply gyrocompass influence to maintain the heading of our own ship.  But it gets nastier.  Because the Mark VI has a range clock beneath it, and this could not be made to revolve with the dial plate, the plate was fixed and this required a pretty clumsy ballet of opposing motions (I think... it hurts to think about it).

On the lower right is the handle to set the range clock, whose indicating pipper on the slotted dial plate is bluish in this rendering. To its left is a manual knob for adjusting the bearing (it has a spring-loaded clutch to disengage the bearing clock).  Not shown below the bearing clock speed gauge at the bottom is the handle which sets its speed.

Wind Dumaresq, ca. 1918

This was a special type developed after Jutland and retrofitted onto the Dreyer Tables.  It calculated the amount of lateral deflection the wind's influence would exert on the shell as it flew downrange, allowing a correction to be applied.  It used enemy bearings in relative form (unlike the Mark VI), but lacked a bearing clock, which required the operator to update this setting sporadically.  Overall, it was a high maintenance busybox, as he was also required to enter the present gun range, own speed, and the "wind you feel".

Wind Dumaresq

As on the Mark VI, you'll note that the Wind Dumaresq's dial plate (the cartesian graph) is fixed and the outer brass ring revolves about it.  Unlike the Mark VI, however the scale for own speed on this device extends toward "Forward", and not "Aft".  This is because, unlike a regular Dumaresq, where we want to subtract our own ship's motion from that of the enemy ship, here we want to subtract the wind induced by our own ships motion from the "wind you feel", and the first quantity is the OPPOSITE of our ship's motion vector.

Wind Dumaresq, front view (which places operator behind the Dreyer Table)

This view shows where the operator would input the present gun range for the weapon in use.  As the knob spun, a cylinder under the dial plate (extending laterally from red 90 to green 90 in this render) would spin, with numbers written on it to indicate the number of knots of deflection that would be needed as a correction.  The number to use would be that in line with the pipper.  It would naturally be the placement of the numbers on the drum under the dial plate that recorded the subtle relationship of how far the shell would be displaced as it travelled downrange.  When you consider this, you'll grasp that the amount of this displacement will be a function of the time of flight (which is what the gun range dial is truly expressing, as time of flight is a function of gun range) and the component of the "wind you feel" which crosses the line of sight.