Chartwork & Compasses Flashcards
What is a dead reckoning (DR) position?
- Derived from the course steered by the vessel and her speed through the water
- Used as rough estimation
- Shown on chart as cross and the time
How do you find your estimated position?
- Most accurate position fix utilising all known factors such as leeway, wind current and tidal streams.
- Leeway applied first
- Tidal vector applied at the end of the course line
- Shown on the chart as a triangle
What is a running fix?
Method of fixing your position if only one or spaced apart reference points are available. Use any start point on the first bearing line and use your heading to do an EP as usual then transfer the first position line across to the second bearing through the EP and where it crosses is your fix.
How would you do a course to steer?
- In the future, usually given a start position and waypoint
- Tidal Vector applied first
- Leeway applied at the end but isn’t drawn on the chart
- Lastly convert the course to steer to compass
What is a VSA and how would you use ones to find your position?
- Vertical Sextant Angle
- The angular separation between three points is measured by using the sextant
- Always 90° - Measured angle
What is a HSA and how would you use one to find your position?
- Horizontal Sextant Angle
- Take a bearing of the object and measure the angle between the top of the object and the waterline
- Use Norie’s to calculate the range from the table
What is leeway?
The sideways drift caused by wind on the side of the vessel given in degrees
What are the different methods of fixing your position on a chart?
- GPS
- 3 radar ranges
- 3 point fix
- Radar range and bearing
- Position line from soundings
- Dipping distances
- Horizontal sextant angles
- Vertical sextant angles
- Running fix
How often should you check your position?
Frequency of fixing should depend on the distance from navigational hazards and the time the ship would take to run into danger before the next fix. This depends mainly on the ships speed.
What are tidal diamonds?
Symbols on British admiralty charts and others that indicate the direction and speed of tidal streams.
What are the compass checks you would carry out prior to departure?
The performance of all magnetic compasses, including spares should be checked as follows:
- Freedom of movement of the gimbal
- Card floating freely, level and rotating without any friction
- Liquid free of bubbles and clear
- Compass card clear and sharp (able to read) with no distortion or discolouration
- Optical system (if any) correctly adjusted and clean
- No liquid leaks around seals or filler plugs
What are the three types of chart projection?
- Mercator
- Transverse mercator
- Gnomonic
What is a mercator projection?
All rumb lines represented by straight lines (a rumb line cuts all the meridians at the same angle)
What is a transverse mercator projection?
- Used for large scale charts, harbour plans only
- All parallels of latitude are equally spaced
What is a gnomonic projection?
- Used for ocean navigating
- Parallels of latitude are curved
- Great circles are straight lines, rumb lines are curves
- Not to be used for navigation
What is variation?
The angular difference between True North and Magnetic North. Variation is named East or West of True North
What is deviation?
The angular difference between Magnetic North and Compass North. Deviation is named East of West of Magnetic North
When should a magnetic compass be adjusted?
- When they are first installed
- They become unreliable
- The ship undergoes structural repairs or alterations
- electrical or magnetic equipment close to the compass is added, removed or altered
- A period of two years has elapsed since the last adjustment
- When the compass shoes physical defects
- Collision or major change of latitude
When would you do a compass check?
- Pre-departure
- Once a watch
- After every large alteration of course
What are tidal vectors?
Made up of two parts, set and drift.
Set: Direction of a given current or tidal stream is moving given in degrees
Rate: Velocity of the current or tidal stream and is expressed in knots
What are spring tides and how are they formed?
- Higher highs and lower lows (greater range)
- Twice every lunar month when the Moon and the Sun are inline with each other and the Earth
- When the Moon is passing between the sun and the Earth the Moon and Sun are said to be acting in CONJUNCTION
What are neap tides and how are they formed?
- Lower highs and higher lows (less range)
- Twin every lunar month when the Moon and Sun are at right angles to each other
- The moon and the sun are said to be in QUADRATURE
What is a deviation card?
Tells you the boats deviation error for each heading, unique to each vessel once compass has been swung
What is a gyro compass?
- Finds true North
- Unaffected by ferromagnetic metals
- Plot directly onto chart
