Topics about currents

1. Navigation
NAU 102
Lesson 30

2. Current Sailing
The horizontal movement of the sea
surface.
Current
Expanded Definition
All factors that cause a ship to depart
from its intended course and DR.

3. Current Sailing
Factors
•Ocean Current
•Tidal Current
•Wind-driven Current
•Wind
•Seas

4. Current Sailing
Factors
•Compass Errors
•Steering Errors
•Engine Calibration Errors
•Speed Log Errors
•Fouled Bottom
•Unusual Trim

5. Current Sailing
Current
The combined effect on a vessel of all the
factors.

6. Current Sailing
The art of selecting courses and speeds to
compensate for the effects of current in
order to arrive at your desired
destination.
Current Sailing

7. Current Sailing
•Track (TR) – the intended direction of
travel.
Important Terms
•Set – the direction towards which a
current is flowing.
Speed of Advance (SOA) – the speed
intended to be made good.

8. Current Sailing
•Drift – the speed of a current.
Important Terms
•Speed Made Good (SMG) – speed along the Course Made Good.
•Course Made Good (CMG) – the resultant
direction from a point of departure to a
subsequent position.
estimated position- is the dead-reckoning position corrected for effects of
current, wind, and other factors.

9. Track (TR) is the intended horizontal direction of travel with respect to the
Earth. The terms intended track and trackline are used to indicate the path of
intended travel. See Figure 107a. The track consists of one or a series of
course lines, from the point of departure to the destination, along which one
intends to proceed. A great circle which a vessel intends to follow is called a
great-circle track, though it consists of a series of straight lines
approximating a great circle

10. Track made good (TMG) is the single resultant
direction from the point of departure to point of arrival at
any given time. Course of advance (COA) is the direction
intended to be made good over the ground, and course over
ground (COG) is the direction between a vessel’s last fix
and an EP.

11. Current Triangle
Course & Speed

12. Current Triangle
Course & Speed
If you know any 2 sides, you can solve for
the 3rd side.

13. Current Triangle
Course & Speed
If you know the Course & Speed
And the Set & Drift
You can get the CMG & SMG
Estimated
Position

14. Current Triangle
Course & Speed
If you know the Course & Speed
You can solve for the Set & Drift
And the CMG & SMG

15. Current Triangle
Course & Speed
You can solve for the Course & Speed
And the Set & Drift
If you know the CMG & SMG

16. Current Triangles
Can be solved using:
Charts
Universal Plotting Sheets
Radar Plotting Sheets
Maneuvering Boards

17. Solving for Set & Drift
Problem Type #1 - Determining an
Unknown Current
Given two positions (fixes) and your course
and speed, solve for the current.
At 0919 your position is LAT 37°00.0'N,
LONG 75°30.0'W. You are on course 270°T
at 8.7 knots. At 1000 your position is LAT
36°59.5'N, LONG 75°37.0'W. What was the
current?

18. Solving for Set & Drift
Step 1: Plot the 1st position.
Step 2: DR to the time of the 2nd position.
At 0919 your position is LAT 37°00.0'N, LONG 75°30.0'W. You are on course
270°T at 8.7 knots. At 1000 your position is LAT 36°59.5'N, LONG 75°37.0'W.
What was the current?
0919
C – 270
S – 8.7

19. Solving for Set & Drift
Step 3: Plot the 2nd position.
At 0919 your position is LAT 37°00.0'N, LONG 75°30.0'W. You are on course
270°T at 8.7 knots. At 1000 your position is LAT 36°59.5'N, LONG 75°37.0'W.
What was the current?
0919
C – 270
S – 8.7
1000

20. Solving for Set & Drift
Step 4: Complete the Triangle.
At 0919 your position is LAT 37°00.0'N, LONG 75°30.0'W. You are on course
270°T at 8.7 knots. At 1000 your position is LAT 36°59.5'N, LONG 75°37.0'W.
What was the current?
0919
C – 270
S – 8.7
1000
Set &
Drift

21. Solving for Set & Drift
Direction From DR to Fix = Set
At 0919 your position is LAT 37°00.0'N, LONG 75°30.0'W. You are on course
270°T at 8.7 knots. At 1000 your position is LAT 36°59.5'N, LONG 75°37.0'W.
What was the current?
1000
Set =
137° T

22. Solving for Set & Drift
Distance DR to Fix Converted to Speed = Drift
At 0919 your position is LAT 37°00.0'N, LONG 75°30.0'W. You are on course
270°T at 8.7 knots. At 1000 your position is LAT 36°59.5'N, LONG 75°37.0'W.
What was the current?
1000
Distance = 0.6 nm
0.6 nm
41 min
X 60 min/hr
Drift = = 0.9 kts

23. CMG/SMG
You are underway on course 315°T at 10
knots. The current is 090°T at 2.3 knots.
What is the course & speed being made
good?
Problem Type #2
Given the Course & Speed and a known
current, determine the CMG & SMG
(Estimated Position).

24. CMG/SMG
Step 1: Plot the 1st position.
(If no position given, start in the center of
compass rose or radar plotting sheet.)
Step 2: DR for one hour.
You are underway on course 315°T at 10 knots. The current is 090°T at 2.3
knots. What will be the course & speed being made good?

25. Current
You are underway on course 315°T at 10 knots. The current is 090°T at 2.3
knots. What is the course & speed being made good?

26. CMG/SMG
Step 3: Draw the Set & Drift Vector from the
tip of the Course & Speed Vector
You are underway on course 315°T at 10 knots. The current is 090°T at 2.3
knots. What is the course & speed being made good?

27. CMG/SMG
Step 4: Complete the Triangle
You are underway on course 315°T at 10 knots. The current is 090°T at 2.3
knots. What is the course & speed being made good?

28. CMG/SMG
Step 4: Complete the Triangle
You are underway on course 315°T at 10 knots. The current is 090°T at 2.3
knots. What is the course & speed being made good?
CMG = 329° T
SMG = 8.2 kts

29. Course & Speed
You wish to make good a course of 015°T
and a speed of 8 knots. The current is
330°T at 3 knots. What is the course to
steer and engine speed to turn?
Problem Type #3A
Given a desired CMG & SMG and a
known current, determine the course to
steer & required engine speed.

30. Current
Step 1: Plot the first position.
(If no position is given, use the center of a
compass rose or plotting sheet.)
Step 2: Lay out the desired track (draw the
CMG/SMG vector) for 1 hour.
You wish to make good a course of 015°T and a speed of 8 knots. The current
is 330°T at 3 knots. What is the course to steer and engine speed to turn?

31. Current
You wish to make good a course of 015°T and a speed of 8 knots. The current
is 330°T at 3 knots. What is the course to steer and engine speed to turn?

32. Current
You wish to make good a course of 015°T and a speed of 8 knots. The current
is 330°T at 3 knots. What is the course to steer and engine speed to turn?
Draw the Set & Drift Vector from the 1st
position.

33. Current
You wish to make good a course of 015°T and a speed of 8 knots. The current
is 330°T at 3 knots. What is the course to steer and engine speed to turn?
Complete the Triangle

34. Current
You wish to make good a course of 015°T and a speed of 8 knots. The current
is 330°T at 3 knots. What is the course to steer and engine speed to turn?
Complete the Triangle
Course to
Steer = 034° T
Required Engine
Speed = 6.2 kts

35. Current
You wish to make good a course of 015°T and a speed of 8 knots. The current is
330°T at 3 knots. What is the course to steer and engine speed to turn?
Alternate Procedure – Draw the S&D
Vector Backwards from the CMG Vector

36. Current
You wish to make good a course of 015°T and a speed of 8 knots. The current is
330°T at 3 knots. What is the course to steer and engine speed to turn?
Complete the Triangle
Course to
Steer = 034° T
Required Engine
Speed = 6.2 kts

37. Current
You wish to make good a course of 315°T.
Your engine speed is 8 knots. The current
is setting due north at 3 knots. What is the
course to steer and speed made good along
the trackline?
Problem Type #3B
Given a desired CMG, a known current &
your engine speed, determine the course to
steer & speed made good.

38. Current
You wish to make good a course of 315°T. Your engine speed is 8 knots. The
current is setting due north at 3 knots. What is the course to steer and speed
made good along the trackline?
Draw the CMG/SMG vector an indefinite length.

39. Current
You wish to make good a course of 315°T. Your engine speed is 8 knots. The
current is setting due north at 3 knots. What is the course to steer and speed made
good along the trackline?
Draw the Set & Drift vector from the center.

40. Current
You wish to make good a course of 315°T. Your engine speed is 8 knots. The
current is setting due north at 3 knots. What is the course to steer and speed made
good along the trackline?
Open dividers to engine speed.

41. Current
You wish to make good a course of 315°T. Your engine speed is 8 knots. The
current is setting due north at 3 knots. What is the course to steer and speed made
good along the trackline?
Swing an arc from the S&D vector to the CMG line.

42. Current
You wish to make good a course of 315°T. Your engine speed is 8 knots. The
current is setting due north at 3 knots. What is the course to steer and speed made
good along the trackline?
Complete the triangle.

43. Current
You wish to make good a course of 315°T. Your engine speed is 8 knots. The
current is setting due north at 3 knots. What is the course to steer and speed made
good along the trackline?
Complete the triangle.
Course to Steer
= 300° T
SMG = 9.1 kts

44. Introduction to Navigation
Questions?

45. Example 1: A ship on course 080°, speed 10 knots, is steaming through a current
having an estimated set of 140° and drift of 2 knots.
Required: Estimated track and speed made good.
Solution: See Figure 708a. From A, any convenient point, draw AB, the course
and speed of the ship, in direction 080°, for a distance of 10 miles.
From B draw BC, the set and drift of the current, in direction 140°, for a distance
of 2 miles. The direction and length of AC are the estimated track
and speed made good.
Answers: Estimated track made good 089°,
estimatedspeed made good 11.2 knots.
To find the course to steer at a given speed to make
good a desired course, plot the current vector from the
origin, A, instead of from B. See Figure 708b.

47. Example 2: The captain desires to make good a course of 095° through a current
having a set of 170° and a drift of 2.5 knots, using a speed of 12 knots.
Required: The course to steer and the speed made good.
Solution: See Figure 708b. From A, any convenient
point, draw line AB extending in the direction of the course
to be made good, 095°. From A draw AC, the set and drift of the current.
Using C as a center, swing an arc of radius CD, the speed through the water (12
knots), intersecting line AB at D.
Measure the direction of line CD, 083.5°. This is the course to steer.
Measure the length AD, 12.4 knots. This is the speed made good.
Answers: Course to steer 083.5°, speed made good. Determining an estimated
position. 12.4 knots.

49. Example 3: The captain desires to make good a course of 265° and a speed of
15 knots through a current having a set of 185° and a drift of 3 knots.
Required: The course to steer and the speed to use.
Solution: See Figure 708c. From A, any convenient
point, draw AB in the direction of the course to be made good, 265° and for
length equal to the speed to be made good, 15 knots.
From A draw AC, the set and drift of the current. Draw a straight line from C to
B. The direction of this line, 276°, is the required course to steer; and the
length, 14.8 knots, is the required speed.
Answers: Course to steer 276°, speed to use 14.8 kn.