Chartwork & Compasses

Question 1

What is a dead reckoning (DR) position?

Answer 1

• 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

Question 2

How do you find your estimated position?

Answer 2

• 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

Question 3

What is a running fix?

Answer 3

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.

Question 4

How would you do a course to steer?

Answer 4

• 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

Question 5

What is a VSA and how would you use ones to find your position?

Answer 5

• Vertical Sextant Angle
• The angular separation between three points is measured by using the sextant
• Always 90° - Measured angle

Question 6

What is a HSA and how would you use one to find your position?

Answer 6

• 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

Question 7

What is leeway?

Answer 7

The sideways drift caused by wind on the side of the vessel given in degrees

Question 8

What are the different methods of fixing your position on a chart?

Answer 8

• 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

Question 9

How often should you check your position?

Answer 9

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.

Question 10

What are tidal diamonds?

Answer 10

Symbols on British admiralty charts and others that indicate the direction and speed of tidal streams.

Question 11

What are the compass checks you would carry out prior to departure?

Answer 11

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

Question 12

What are the three types of chart projection?

Answer 12

• Mercator
• Transverse mercator
• Gnomonic

Question 13

What is a mercator projection?

Answer 13

All rumb lines represented by straight lines (a rumb line cuts all the meridians at the same angle)

Question 14

What is a transverse mercator projection?

Answer 14

• Used for large scale charts, harbour plans only
• All parallels of latitude are equally spaced

Question 15

What is a gnomonic projection?

Answer 15

• Used for ocean navigating
• Parallels of latitude are curved
• Great circles are straight lines, rumb lines are curves
• Not to be used for navigation

Question 16

What is variation?

Answer 16

The angular difference between True North and Magnetic North. Variation is named East or West of True North

Question 17

What is deviation?

Answer 17

The angular difference between Magnetic North and Compass North. Deviation is named East of West of Magnetic North

Question 18

When should a magnetic compass be adjusted?

Answer 18

• 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

Question 19

When would you do a compass check?

Answer 19

• Pre-departure
• Once a watch
• After every large alteration of course

Question 20

What are tidal vectors?

Answer 20

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

Question 21

What are spring tides and how are they formed?

Answer 21

• 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

Question 22

What are neap tides and how are they formed?

Answer 22

• 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

Question 23

What is a deviation card?

Answer 23

Tells you the boats deviation error for each heading, unique to each vessel once compass has been swung

Question 24

What is a gyro compass?

Answer 24

• Finds true North
• Unaffected by ferromagnetic metals
• Plot directly onto chart

Question 25

What is an azimuth mirror?

Answer 25

Used in conjunction with a compass to measure bearings of celestial and terrestrial objects. - Point arrow upwards to take bearing of a heavily body - Point arrow downwards to take bearing of terrestrial object

Question 26

What is a compass error check using a transit?

Answer 26

Take a compass bearing of a transit, take the true bearing and then compare them and the difference is the compass error.

Question 27

What is a compass check via amplitude of the sun?

Answer 27

- Find the declination of the sun (nautical almanac) - Note latitude from GPS - Use correct table in Norries, given true bearing of sun - Compare azimuth of sun against gyro or magnetic heading - Sunrise and sunset taken when sun is semi-diameter above horizon

Question 28

What is a compass check via azimuth of a heavenly body?

Answer 28

- Take bearing of the sun and note the time - Find Greenwich hour angle and declination of the sun for that time - Work through chosen longitude and local hour angle - Use sight reduction tables with the argument. LHA, LAT and declination - Exact azimuth of the sun - Compare the azimuth of the sun against your gyro or magnetic compass bearing

Question 29

When should the gyro compass run?

Answer 29

Continuously , if it stops for any reason it should be restarted and regularly checked and only relied upon again once the error is known

Question 30

How do you carry out a chart correction?

Answer 30

- Check which chart in your folio require an update - Check if it is up to date - If the previous correction has not been done, do the old correction first - Implement the correction in magenta ink - Write the correction in bottom left hand corner of the chart - Record the update in the chart update logbook

Question 31

How to make sure your chart is up to date?

Answer 31

- Look up current Cumulative List to check latest edition date against date of chart - Check for corrections that apply to the chart, listed in the Cumulative list - Check Weekly Notice to Mariners published since the Cumulative list for any more recent corrections - Look at bottom left hand corner of the chart to see which corrections have or have not been applied to the chart

Question 32

How do you reduce the errors caused by deviation?

Answer 32

Permanent magnetism - Longitudinal and athwartship magnets are placed inside the compass binnacle Induced magnetism - Soft iron spheres (Kelvins balls) and flinders bar magnets are used. Fit on the side where there is more ship

Question 33

What is Admiralty NP294?

Answer 33

Provides clear guidance on the efficient upkeep and maintenance of official ADMIRALTY charts and publications, whether paper or digital.

Question 34

What is on a deviation card?

Answer 34

- Date and location - Name, CoC and signature of compass adjuster - Location of all correctors - Residuals coefficients - Residual deviation - Heeling magnet - Flinders bar