Navigation Flashcards
What are the different methods for obtaining a single position line?
- Compass bearing/radar bearing
- Transit of 2 fixed objects
- Horizontal angles between two charted objects
- Radar range
- Range by vertical or horizontal sextant angle
- Astronomical observation
What is a spheroid?
A sphere flattened at the poles
What is a small circle?
The intersection of a spherical surface and a plane which does not pass through the centre of the sphere
What is a great circle?
The intersection of a spherical surface and a plane which does pass through the centre of the sphere
Define Latitude?
A place on earths surface which is the angle that the perpendicular at that position meets with the plane of the equator
What are the 4 terrestrial sailings?
- Parallel sailing
- Plane sailing (Used up to 600Nm)
- Mercator sailing
- Great Circle sailing
How do you measure your position from a vertical sextant angle?
Choose an object on the shoreline with a charted height and correct for Height of tide. Measure the vertical sextant angle. Distance = cotan x height
How do you measure a position line using horizontal sextant angles?
- Measure the horizontal angles between 3 charted objects. A,B,C.
- Join the charted objects together, measuring the distance (d) between each.
- The centre of the position circle from each object is found by the formula
X = 0.5 (d) / tan Horizontal sextant angle
The centre of the circle (O) is found by marking a line of length (x) drawn from a position halfway along and perpendicular to the length d
- Draw each position circle with radius (the distance between the charted object and O) and where they intersect is the position.
Angle of cut between two widest objects should be as close to 90 degrees as possible
What should be consulted for the appraisal part of the passage plan?
- Rhumb line or great circle route?
- Are there TSS or shipping lanes to consider?
- ETD/ETA and required speeds
- Weather forecasts for the route / Mariners handbook
- Heights of tide/UKC and Draught limitations / Load line areas / squat
- Ship Reporting areas
- Navigational warnings / T & P Notices / Notices to mariners
- Any time zone changes?
- Ocean passages for the world
- Relevant pilot books (admiralty sailing directions)
- Tidal streams and currents on route
- Ports/ areas of refuge/anchoring areas
- Pilotage requirements/pilot boarding areas/instructions
- No Go/Danger areas
- ALRS - Radio channels/pilot instructions/port operations
What are the four stages of passage planning?
Appraisal
Planning
Execution
Monitoring
What are the 6 scales of navigational chart?
- Berthing (Largest scale)
- Harbour
- Approach
- Coastal
- General
- Overview (Smallest scale
Where are the pressure zones around a ship which cause interaction?
- Pressure zone around the bow
- Suction zone amidships
- Pressure zone around the stern
How can a ship’s interaction be limited?
Reducing speed. The pressure zones vary with the square of the water velocity. Halving the speed reduces the pressure by 75%
What are the 4 types of interaction?
- Shallow water effect
- Squat
- Canal Effect
- Ship proximity
What actions should be taken prior to entering restricted visibility?
- Note bearings, ranges and courses of ships in sight. Plot on radar.
- Position fix
- Reduce to a safe speed
- Helmsman on standby
- Extra lookout on the bridge
- Call the Captain
- Monitor the echo sounder
- Sound fog signals
- Engines ready for immediate manoeuvre and inform engine room
- Close watertight doors
