Nautical Instruments and Their Principles of Operation
This domain tests the physical principle behind each bridge instrument: what quantity it senses, what its display is referenced to, and which errors follow from that principle. The examiner rarely wants a description of a box; the question is usually what the instrument actually measures, and why the reading can be wrong.
The measurement chain
Every instrument is a sensor, a signal processor and a display. Error can enter at any stage: an echo sounder transducer offset, a log fouling factor, a GNSS datum setting. First identify the sensed quantity, then the reference point of the readout.
Principles to recognise
- Magnetic compass: a magnet in the earth's field feels a torque proportional to the horizontal component H and indicates magnetic north.
- Gyrocompass: a fast-spinning rotor, free to precess, settles its spin axis in the meridian through gyroscopic inertia and gravity.
- Echo sounder: a transducer transmits a pulse and times the echo; depth = c x t / 2, with c about 1500 m/s in seawater.
- Electromagnetic log: seawater moving through a magnetic field induces an EMF proportional to speed through water.
- Doppler log: frequency shift of a beam backscattered by particles, giving water track or bottom track.
- GNSS: ranges from signal travel time, position by multilateration, output SOG and COG over the ground.
Worked example
A sounder shows a round-trip time of 0.133 s. With c = 1500 m/s the one-way distance is 1500 x 0.133 / 2, about 100 m below the transducer. The transducer is 1.0 m below the waterline and the draft is 8.0 m, so depth below keel = 100 + 1.0 - 8.0 = 93.0 m. If the charted depth is 90.0 m, under-keel clearance is 3.0 m. The sounding is referenced to the transducer face, not to the keel or waterline.
Traps
- Treating the sounder reading as depth below keel when no offset has been set.
- Using 1500 m/s as a universal constant; sound speed varies with temperature, salinity and pressure, roughly 1450-1550 m/s.
- Believing a log gives ground speed; EM and pitometer logs give speed through water.
- Confusing heading, where the bow points, with COG, where the ship moves over the ground.