Nautical Instruments Flashcards
Layout of magnetic liquid compass
1: glass cover
2: fitting for azimuth instrument
3: gasket
4: expansion ring
5: center of rotation (line through taps)
6: float (loading pivot pin with ≈ 100 g)
7: pivot pin (iridium)
8: ring magnet
9: compass card with gradation
10: lubber line
11: jewel/safire
12: bottom cover
13: filler plug
14: bridge for pivot pin
15: lead ring
floating:
- buoyancy
- damping
Earth magnetic field:
Components
Vectors:
T: direction and strength of earth magnetic field
V: vertical component
H: horizontal component
i: inclination
What is the lubber line?
Reference mark on the inside of the compass bowl
Errors that may impair the reading of a magnetic compass?
- Parallax (lubber line close to compass card)
- Collimation (misalignement of compass card and ring magnet)
Requriements for a magnetic liquid compass?
- Stable reading
- Sensitive
How to achieve stable reading of a magnetic liquid compass?
=> compass should not be brought out of balance easily:
1. strong (ring) magnet with poles far apart
2. different rolling periods of compass and ship
3. small friction between pivot pin and jewel
4. compass pivot must be in intersection of cardanic axes
5. spacing between compass card and housing
6. weight on pivot pin ≈ 100g
Types of magnetism on board?
Permanent: high tensile keeps magnetism => doesn’t change with course
Induced: milder steel sensitive to earth magnetic field => changes with heading
Directive force on a magnetic compass
H: horizontal component of earth magnetic field
S: horizontal component of entire disturbing field on board
=> H’ in direction of Nc (compass north)
=> angle H - H’: deviation
H/H’ ≈ 1
high altitudes: H smaller ; S constant => deviation ↑
Compensating a compass
- magnets:
P (fore/aft)
Q (transverse)
r (vertical) - weak iron:
spheres
flinders bar
Reason:
deviation < 5 degree
H’/H ≈ 1
Dutch ships: every 2 years, unless it can be demonstrated in compass book that continuously < 5 deg and ≈ 1
Fluxgate compass: principle
iron bar with 2 coils around it => the earth’s magnetic field induces poles into the bar, which leads to a small current in the coils, which can be read out.
Advantages magnetic compass?
+ independent of electricity
+ no electronics
+ problems easy visible
+ long MTBF
Disadvantages magnetic compass?
- shows Nc => calculation to Nt necessary
- directive forces decreases dramatically due to inclination in higher latitudes
- can not be placed everywhere (steel vessel; engine; …)
Different ways to take a bearing?
- hands
- shadow pin
- notch and wire visor
- PELORUS
- Thomson instrument
Different logs?
- towing log (Cherub log)
- pressure log
- EM log (electromagnetic)
- doppler log
- sailing vessel log
- hand log (Dutchman’s log)
Towing log:
principle and pro/cons
rotation of log rotator is transferred via a line to the clock => distance
+ cheap
+ no electircity
+ easy to repair
- inaccurate at low/high speeds
- only usable in open water
- vulnerable (seaweed, shartks)
- distance measured => speed to be calculated
- distance in water
Pressure log:
principle and pro/cons
measuring pressure differential between dynamic and static pressure in a tube => the pressure differential deflecting a membrane
static tube: ensuring similar pressure both sides of the membrane, if not making way; compensating for draft change
+ very precise
+ can give both speed and distance
- inaccurate at low speed (low pressure differential)
- restless when ship pitching
- vulnerable in shallow waters
- speed in water (not over ground)
- measures only speed ahead (not astern)
Doppler log:
principle and pros/cons
Principle: frequency shift when source and receiver of sound move closer - apart
300 kHz; 60 degree
when transmitting, the receiver is blocked
doppler shift = differential between transmitted and received frequency is measured (almost linear dependency between delta and speed)
Bottom track: upto 200 m depth
Water track: when the dopplerlog loses bottom contact, it will automatically switch to water track, where the water in 10-15m can also reflect the ultrasonic pulses. For this to happen, the receiver is also blocked shortly after receipt of the echo.
+ accurate at low speeds
+ in bottom track: SOG
+ wash does not affect reading
- minimum UKC 1 m
- air under transducer interferes with signal
Echo sounder:
Calculation
s = v * t
D = 1/2 * s
Ultrasound in water
Frequency used: 150-200 kHz
Speed: ~ 1,500 m/s
Depends on:
* Temperature
* Salinity
* (Pressure)
But max deviation: ~ 4%
Going into cold FW: careful!
Echo Sounder:
flow diagram
transmitter = loudspeaker
receiver = microphone
oscillator sends 0 to CPU
Oscillator: Piezo (quarz) or artificial chrystal
Radar:
Azimuthal magnification
Horizontal beam angle (= beamlets = time base) 0.5-2 degrees (depending on quality of scanner)
=> echos will be widened by 1/2 horizontal beam angle on either side
=> dots appear as arcs!
=> total angle magnification is 1/2 beam angle either side = 1 beam angle
=> take bearing through cente of echo
Radar:
Resolution in bearing
Minimum angle between 2 objects in same distance from scanner to give 2 separate echoes
angle between 2 objects <, >, = horizontal beam angle / time base
Radar:
Resolution in range
minimum distance between 2 objects in same bearing, so we can still see 2 separate echoes
Depends on pulse length
=> pulse length = pulse time * 300 m/microsecond
…. e.g.: 30 m (for pulse time = 0.1 microseconds)
=> IF distance < 30 m => return signals of 30 m length overlap
Radar:
Blind Distance
Depends on:
- height of scanner
- vertical beam angle
Radar:
Sea Clutter
Increase the gain at Amplifier depending on time =distance from scanner:
=> Swept Gain
Radar:
Adjusting
- STDBY => ON
- Brilliance
- Gain
- Tune
- FRM / VRM (fixed/variable range marker)
- Sea Clutter (if needed)
- Rain Clutter (if needed)
Radar:
Take bearing
