Master Mariner Remigiusz Dzikowski
Dean of Non-stationary Studies
Department of Marine Navigation
Room.236
Lecture Topics:
Magnetic compasses requirements according to SOLAS V / 12 (b),
12 (c):
1. Vessels of 150 gross tonnage and upwards shall be equipped
with:
a. the main magnetic compass, except in the case specified in
3.2.4;
b. a magnetic steering compass, if the course information from the
main magnetic compass required by 3.2.1.1 is not available and
clearly displayed for the helmsman at the main steering position;
2. adequate means of communication between the main compass
station and the position of navigation and manoeuvring;
4. measures to ensure visibility when taking bearing, as far as
practically possible, around the entire horizon in the 360 ° range.
Adjustment of Compasses
Each magnetic compass required to be carried by the Regulations shall be
properly adjusted and its table or curve of residual deviations available at all
times. Magnetic compasses should be adjusted when:
a.) they are first installed;
b.) they become unreliable;
c.) the ship undergoes structural repairs or alterations that could affect its
permanent and induced magnetism;
d.) electrical or magnetic equipment close to the compass is added,
removed or altered; or,
e.) a period of two years has elapsed since the last adjustment and a
record of compass deviations has not been maintained, or the recorded
deviations are excessive or when the compass shows physical defects.
Adjustment of Compasses
Masters are advised that it is essential to check the performance of
magnetic compasses particularly after:
a.) carrying cargoes which have magnetic properties;
b.) using electromagnetic lifting appliances to load or discharge;
c.) a casualty in which the ship has been subject to severe contact or
electrical charges; or,
d.) the ship has been laid up or has been lying idle - even a short
period of idleness can lead to serious deviations, especially for small
vessels.
Compass performance should be monitored by frequently recording
deviations in the compass deviation book. Compass errors should be
determined after every large alteration of course, and at least once
every watch when there have been no major course alterations.
Checking the compass deviation regularly may show the need for
repair, testing or adjustment. In addition, compasses should be
inspected occasionally by a competent officer or compass adjuster.
Magnetic Compass construction
Compass direction- Deviation and its changes Deviation δ positive
The magnetic compass of the ship does not only affect the magnetic field of the Earth, but also the magnetic field of the
ship, making the needle deviate from the magnetic N-S line and aligning along the Compass N-S line . This direction on
the plane of the observer's horizon is called the N-S compass line,
NM
NC
A vector representing the ship's magnetic field (red) can be decomposed into a
horizontal component acting in the plane of the observer's horizon and the vertical
component. The horizontal component of this vector composed with the horizontal
component of the Earth's magnetic field (green vector) result resultant force vector
along a compass line. That determines the direction of the N-S compass line. The angle
between N Magnetic and Compass is called deviation δ. When the Compass line
Assuming they are constructed well, compasses on ships fail to point to true
(geographic) north due to two factors:
1. Magnetic variation(or magnetic declination), the angle between magnetic
north and true north due to the local direction of the Earth’s magnetic field,
and
2. Magnetic deviation (δ), the angle between the compass needle and
magnetic north due to the presence of iron within the ship itself.
Two kinds of ships iron:
1. Soft iron that has a low carbon content and is easily magnetized and
demagnetized with a small hysteresis loss(2,5% aluminium)
2. Hard Iron or steel which is not readily magnetized by induction, but which
retains a high percentage of the magnetism acquired.(5% wolfram)
Hard Iron Magnetic Forces
Q
M
R F
Hard Iron Magnetic Forces semicircural deviation
B and C components
Soft iron instantly becomes magnetized on being
placed in a magnetic field, and just as rapidly loses those
properties on being removed from it.
Note that when magnetized by induction, iron receives:
Blue (S) polarity where the lines of force enter and
Red (N) polarity where they leave.
The mathematical description of compass
deviation – Poisson Equations
𝑋 ′ = 𝑋 + 𝑎𝑋 + 𝑏𝑌 + 𝑐𝑍 + 𝑃
𝑌 ′ = 𝑌 + 𝑑𝑋 + 𝑒𝑌 + 𝑓𝑍 + 𝑄
𝑍 ′ = 𝑍 + 𝑔𝑋 + ℎ𝑌 + 𝑘𝑍 + 𝑅
This total magnetic force is made up of:
1. The magnetic force due to the Earth X, Y, Z magnetic force components -
positive to the bow, starboard side and keel respectively,
1. The magnetic force due to the permanent magnetism of the ship P, Q, R
positive to the bow, starboard side and keel respectively,
1. The magnetic force due to the induction in the soft iron roads comprising the
fore and aft component (aX+bY+cZ) positive if directed to the bow, the
athwarthship component (dX+eY+fZ), positive if directed towards the
starboard side and the vertical component (gX+hY+kZ), positive if directed to
the keel.
The mathematical description of compass
deviation – Poisson Equations
The mathematical description of compass deviation:
δ = 𝐴 + 𝐵𝑠𝑖𝑛𝐶𝐻𝑑𝑔 + 𝐶𝑐𝑜𝑠𝐶𝐻𝑑𝑔 + 𝐷𝑠𝑖𝑛2𝐶𝐻𝑑𝑔 + 𝐸𝑐𝑜𝑠2𝐶𝐻𝑑𝑔
Where:
A – permanent coefficient due to misalignment compass north to the ships
keel line or asymmetrical arrangements of the soft iron horizontally in the ship
B – semi-circular forces Fore and aft components of the permanent magnetic
field due to hard iron and the induced magnetism in asymmetrical vertical iron
forward or aft of the compass(P force and c – iron)
C – semi-circular forces Athwartship component of the permanent magnetic
field due to hard iron and the induced magnetism in asymmetrical vertical iron
port or starboard of the compass(Q force and f iron)
D – Quadrantal forces Induced magnetism in symmetrical arrangements of
horizontal soft iron(e and a iron)
E – Quadrantal forces Induced magnetism in asymmetrical arrangements of
horizontal soft iron(b and d iron)
Magnetic Compass deviation factors
N + E + S + W
A=
4
E − W
B=
2
N −S
C=
2
NE − SW SE + NW
D= −
4 4
N + S E + W
E= −
4 4
Magnetic Compass heeling deviation (pitching dev.)
Generally there exist three principal causes of heeling error,
all
giving deviation of a semicircular type. They are:
(i) The vertical component of the ship's permanent magnetism
(force R).
(ii) Induced magnetism in vertical structure below or above
compass level.
(iii) Induction by the earth's vertical force Z m transverse
soft iron, i.e. the iron which is the main cause of coefficient
D when upright.
Magnetic Compass heeling deviation (pitching dev.)
R + (e − k ) Z
= cos KK
H
Compensation of the Magnetic Compass deviation
THE mechanical correction of that part of the deviations caused
by the ship's permanent magnetic field is effected by means of
permanent magnets, and is in practice straightforward, but in the
case of the soft-iron correctors, complications arise due to the fact
that the induced magnetism in the correctors may not be wholly
due to the earth's field. It may in part be due to the permanent
correcting magnets or due to the compass needle system itself.
According SOLAS: Each magnetic compass referred to
in subparagraph (i) shall be properly adjusted and its
table or curve of residual deviations shall be available at
all times.
Compensation of the Magnetic Compass deviation
The principle underlying successful compass adjustment
can be stated very briefly as Like cures Like, by which is
meant:
1. Deviation due to hard iron must be compensated by
permanent magnets.
2. Deviation due to V.S.I(vertical soft iron), must be
compensated by V.S.I. (Flinder's Bar).
3. Deviation due to H.S.I(Horizontal soft iron), must be
compensated by H.S.I, (soft iron spheres).
Compensation of the Magnetic Compass deviation
Four Steps to compensate semicircural magnetic deviation
(B,D components):
1. Keep E Heading. Check value of deviation. Compensate to
0° with fore and aft situated magnet.
2. Alter the heading to the W. Check value of deviation.
Compensate to the half of the value with fore and aft
situated magnet.
3. Keep N Heading. Check value of deviation. Compensate to
0° with Athwartship situated magnet.
4. Alter the heading to the S. Check value of deviation.
Compensate to the half of the value with Athwarthship
situated magnet.
Compensation of the Magnetic Compass deviation
The END
