The Binnacle.
The
binnacle is designed to correct for horizontal and vertical induced fields
affecting the mean directive force at the compass. (a,b,d,e & k rods).
The
quadrantal correctors comprises two pairs of soft iron scissor arrays arms,
each pair mounted radially outwards from a binnacle
and spaced 180º apart on a ring that can be rotated around the binnacle.
Each
scissor array reduces the vertically induced field at the compass. The amount
of reduction can be varied by increasing the span of the scissor array.
Each pair
of scissor arrays increases the horizontal magnetic field strength along the
direction of their axis and each pair alone creates quadrantal deviation. If
the axes of the paired arrays are perpendicular, the horizontal field strength
is increased uniformly on all headings. By adjusting the angle of the paired
array’s axes the increase in horizontal field can be biased along any axis to
correct for quadrantal deviation.
Using the
modern post panamax vessel’s observations, the
wheelhouse compass site was measured to have vertically induced fields causing
a multiplier of the vertical field (µ) of 120% and horizontally induced fields
causing a multiplier of the horizontal field (λ) of 77%. This is an increase of 20%
in the vertical field and a decrease of 23% in the horizontal field.
To have
both the vertical and horizontal fields with the same percentage of the earth’s
field, the vertical field can be reduced until it is 77% of the earth’s field
((77-120)/120 = -35.8% decrease), the horizontal field can be increased until
it is 120% of the earth’s field ( (120-77)/77 = 55.8% increase ) or somewhere in-between.
On a graph,
the required increase in the horizontal field alone can be put on the X axis
and the required decrease in the vertical field alone is put on the Y axis. A
line can be drawn from the percentage of only changing the horizontal force on
the X axis to only changing the vertical force on the Y axis. This line
represents the corresponding values of increasing the horizontal force to
decreasing the vertical force in order to deliver the same multiplier of the
vertical and horizontal force to the ambient magnetic field.
The
intersection of this line and the performance curve of the Quadrantal Corrector
indicates the required setting of the span of the
array to deliver equal multipliers to the vertical and horizontal fields.
From figure
1, it can be seen that the span of the array should be set to decrease the
vertical induced field by 17½ % which will provide a corresponding increase in
the horizontally induced field of 28½%. The effect of these settings is to
reduce the vertical field to 99.3% of the earths field ((1-0.175) x 120 =
99.3%) and increase the horizontal field to 99.3% of the earth’s field ((1+
0.285) x 77 = 99.3%). As the vertically
induced force from the correctors is directly proportional to the span of the
array opening, the setting of the opening can be taken directly off the graph’s
second Y axis. In this example, the span of the arrays should be set to 64 mm.
(If the
shorter arrays were used it can be seen that the span of the arrays would be
set at 113mm delivering a mean directive force of 83½ % of the earth’s field.)
Figure 1
Continuing
with the example from the modern post panamax, the
observed quadrantal error was a minus co-efficient D
of 1⅓º. By
going to the Quadrantal Correction graph (figure 2) the required bias of the
array axes can be derived. In this case, the array axes each need to be brought
towards fore & aft line by 3½º to bring them 7º from perpendicularity to
increase the fore & aft directional force.
(If the
shorter arrays were being used, they would each be set at 7º towards the fore
& aft line to bring them 14º from perpendicularity.)
Figure 2
Soft Iron
Arrays set for correct compensation on the post panamax
bulkcarrier.
Long Arrays delivering a multiplier of 99.3% Short Arrays
delivering a multiplier of 83%
