Compass Flashcards
Properties of a Free Gyroscope
Gyroscopic Inertia
Precession
What does this image represent?
Gyrocompass damped in tilt.
Easterly Drift (D) is countered by Gravity-controlled Precession (Pg).
Upward Tilt (T) is countered by Damping Precession (Pd).
The settling point is to the East and upward of the meridian and horizon respectively as the forces must be in equilibrium.
What does this image represent?
Gyrocompass damped in tilt.
Easterly Drift (D) is countered by Gravity-controlled Precession (Pg).
Upward Tilt (T) is countered by Damping Precession (Pd).
The settling point is to the East and upward of the meridian and horizon respectively as the forces must be in equilibrium.
What does this image represent?
Gyro damped in Azimuth
Upward Tilt (T) is zero as there is nothing to oppose it at the meridian so the settling point is above the horizontal.
If the Gravity-controlled Precession (Pg) outweighs the natural Drift (D), then a Azimuth Damped Precession (Pd) is “added” to the drift to bring the net value to zero.
Thusly, an Azimuth-damped Gyro will always settle exactly on the meridian.
Where will a Gyro damped in Tilt settle in the southern hemisphere?
West of meridian
Below horizontal
Main advantage of a Gyrocompass damped in Azimuth?
NO DAMPING ERROR
Damped in Tilt will settle east (Northern hemisphere) or West (Southern Hemisphere) of the meridian. This leads to an error.
How to calculate Damping Error?
1.5 x tan(Latitude)
Magnetic compass card not rotating - possible causes?
Card has fallen off bevel
No fluid in bowl
Whilst covering a watch for the Third Mate, you decide to take a compass error.
The magnetic heading reading is way off.
Actions?
Call an additional officer to bridge to maintain situational awareness, conduct proper handover.
Consult reverse of deviation card and note magnet positions.
Proceed to check magnets in binnacle against record.
Check fluid in bowl.
If all okay, begin to question why this wasn’t noticed sooner.
Notify Master to arrange for compass swing.
If the compass card has fallen off the Bevel, how can you repair it?
You can’t/shouldn’t (?)
Instead, swap the compass bowl with spare carried as per SOLAS V.
What is variation?
The angular difference between Magnetic and True North
Converting a Free Gyroscope to a North-pointing Gyrocompass
Due to the earth rotation the spin axis of a free gyroscope will trace out a circular path as it remains pointed in a fixed direction in space.
If a free gyro was oriented North, it would immediately begin tracking East and upward to start the circular path.
Step One: GRAVITY CONTROL (liquid ballistic)
Two pots containing heavy fluid, joined by an air duct, will counter the vertical component. As the free gyro tracks East and up, the fluid will flow from the high-side to the low-side pot, inducing a downward torque and a horizontal precession.
This makes the path of a gravity-controlled gyroscope follow an elliptical pattern indefinitely, with a reduced vertical component and large horizontal component.
Generally, the period (time taken to complete an elliptical cycle) of a gravity-controlled gyro is 1-2 hours.
Step 2 DAMPING
Damping in tilt is the most common method of stamping applied to commercial compasses and targets the vertical component. Only one component needs targeting as the other will follow in reduction naturally.
