Position Fixes

Question 2

What is Dead Reckoning position?

Answer 2

Calculated from course steered & speed through water (log/engine revolutions). Marked as a cross with time.

Question 3

What is an Estimated Position?

Answer 3

Position corrected for all known factors: leeway, tide, current. Marked as a triangle with time.

Question 4

What is leeway?

Answer 4

Sideways drift from wind on ship’s side, applied to course steered.

Question 5

Define Set and Rate.

Answer 5

Set = direction current/tide is moving.
Rate = speed in knots (distance drift per hour).

Question 6

How to plot an EP?

Answer 6

Lay off water track (incl. leeway).
Mark DR.
Apply tidal vector.
Result = EP (triangle).

Question 7

What is ground track?

Answer 7

Line from start to end of tidal vector → actual movement over ground.

Question 8

How to calculate a Course to Steer (CTS)?

Answer 8

Draw intended ground track to destination.
Apply tidal vector.
Measure distance/time to run.
Draw water track back to ground track.
Apply leeway last.
Convert to compass course.

Question 9

What are the main methods of fixing a position?

Answer 9

GPS.
3 radar ranges.
3-point visual bearings.
Radar range + bearing.
Soundings + bearing.
Sextant angles (HSA/VSA).
Running fix.

Question 10

How are soundings used in position fixing?

Answer 10

Compare echo sounder reading with chart depth → gives position line.

Question 11

What is horizon range?

Answer 11

Distance off light when it appears/disappears on horizon, based on observer’s eye height + light’s height.

Question 12

How to fix with HSA?

Answer 12

Measure angle between 3 charted objects with sextant horizontally → construct position circles → fix at intersection.

Question 13

What is a station pointer?

Answer 13

Metal plotting tool for fixing position by 3-point sextant angles.

Question 14

How to fix with VSA?

Answer 14

Measure angle between top & base of an object → use Norie’s to calculate range → combine with bearing for fix.

Question 15

What is a running fix?

Answer 15

Fix from two bearings of the same object taken at intervals. First bearing transferred using course, speed, tide, leeway → intersect with second bearing.

Question 16

What causes tides?

Answer 16

Gravitational pull of Moon & Sun.

Question 17

Difference between tides & tidal streams?

Answer 17

Tides = vertical rise/fall of water.
Tidal streams = horizontal flow caused by tide.

Question 18

What is tidal height vs range?

Answer 18

Height = water level above chart datum.
Range = difference between high and low water.

Question 19

What are springs and neaps?

Answer 19

Springs = higher highs & lower lows, greater range.
Neaps = lower highs & higher lows, smaller range.

Question 20

What is a Dead Reckoning (DR) position?

Answer 20

Position calculated by course steered & speed through water, ignoring wind/tide.

Question 21

How is DR shown on a chart?

Answer 21

Cross with time (×).

Question 22

What is an Estimated Position (EP)?

Answer 22

Position corrected for all known factors (tide, current, leeway).

Question 23

How is EP shown on a chart?

Answer 23

Triangle with time (△).

Question 24

What is leeway?

Answer 24

Sideways drift due to wind pressure on vessel.

Question 25

Define Set and Drift.

Answer 25

Set = direction of tidal stream/current; Drift (Rate) = speed of current in knots.

Question 26

Describe how to plot an EP with tide.

Answer 26

Plot water track → DR → apply tidal vector → mark EP.

Question 27

What is ground track?

Answer 27

Actual track over the seabed (course made good).

Question 28

Explain how to calculate a Course to Steer (CTS).

Answer 28

Draw intended track → apply tidal vector → back-plot water track → apply leeway → convert to compass course.

Question 29

List main methods of position fixing.

Answer 29

GPS, 3 visual bearings, 3 radar ranges, radar range & bearing, soundings, sextant angles, running fix.

Question 30

How would you fix with soundings?

Answer 30

Compare echo sounder depth with charted depth line.

Question 31

What is a running fix?

Answer 31

Fix from two bearings of same object at intervals, transferring first bearing.

Question 32

What is a Horizontal Sextant Angle (HSA)?

Answer 32

Sextant angle between two charted objects, used for fix with 3 objects.

Question 33

What is a station pointer?

Answer 33

Plotting tool used for 3-point sextant fixes.

Question 34

What is a Vertical Sextant Angle (VSA)?

Answer 34

Sextant angle between top and base of object, gives distance off via tables.

Question 35

How do you obtain a fix with a VSA?

Answer 35

Use sextant angle + object height from chart → distance off → combine with bearing.

Question 36

What is Dipping Distance?

Answer 36

Range at which a light appears/disappears over horizon due to Earth’s curvature.

Question 37

What factors affect horizon range?

Answer 37

Observer’s eye height and light’s elevation.

Question 38

What’s the difference between Tides and Tidal Streams?

Answer 38

Tides = vertical rise/fall of sea level; Tidal streams = horizontal movement of water.

Question 39

Explain springs and neaps.

Answer 39

Springs = Sun & Moon aligned → higher highs/lower lows, greater range. ## Footnote Neaps = Sun & Moon at 90° → smaller range, lower highs/higher lows.