Wind & Current
Homework
Q&A
Junior Navigation
Chapter 12
1
Objectives:
■ Distinguish between leeway and current.
■ Know the meaning of the terms: DR track,
intended track, speed of advance, course and
speed over ground, course and speed made
good, drift angle.
■ Determine course to steer, allowing for
current/leeway.
■ Determine set and drift of current by
observation.
■ Determine leeway by observation.
■ Understand elements of estimated reckoning.
2
1. The actual path of a boat over the
bottom is:
a. course made good.
b. course over ground.
c. DR track corrected for drift angle.
d. course over ground corrected for
drift angle.
Ref.: Table 12 - 1
3
2. Current is:
a. the horizontal motion of water.
b. the horizontal motion of a vessel
through the water.
c. the net difference between COG and
CMG.
d. the net difference between COG and
DR track.
Ref.: ¶ 18
4
3. Given the same wind, leeway will be
greatest for:
a. a deep-keeled boat sailing
downwind.
b. a deep-keeled boat sailing with the
wind abeam.
c. a shallow-draft boat sailing
downwind.
d. a shallow-draft boat sailing with the
wind abeam.
Ref.: ¶ 19
5
4. Speed over ground is best measured
by:
a. knot meter
b. GPS
c. measuring the distance run between
fixes an hour
apart.
d. measuring the distance run between
fixes several
hours apart.
Ref.: ¶ 25
6
5. The navigator of a boat crossing the Gulf
Stream plots a DR position and a fix at
1130. The distance measured between the
two positions represents:
a. the drift angle.
b. the drift of the Gulf Stream current over the
last hour.
c. the average drift of the Gulf Stream current
since the last fix.
d. the net distance that the boat has been set
off the DR track by current, wind, and/or
other factors.
Ref.: ¶ 44
7
6. You wish to maintain a rhumb line course of
010°T, and anticipate a current setting 040° at
1.5 kn. Your optimum cruising speed is 9 kn.
a. What is the true course to steer?
Drawing a current diagram and measuring
with a protractor shows a drift angle of 5° and
CMG of 010°T.
To maintain a course of 010°T, steer a course
of 010° - 5° = 005°T.
b. What would be the course to steer allowing for
4° of leeway in a strong northwest wind?
Wind will set the vessel to the east of the
desired course. To offset leeway, the steering
course is 005° - 4° = 001°T.
Ref.: ¶ 46, 47
8
7. Consult the pilot chart from your student
material kit. Departing from the entrance to
the Straits of Gibraltar (L 35° 56'N,
Lo 6° 15'W) enroute to Anegada Pass.
a. What is the initial true course?
Measuring a rhumb line on the pilot chart from
Gibraltar on the route to Anegada Pass, the
initial steering course is 265°T.
9
Problem 7. Continued from previous slide
b. Allowing for current (set 180°, drift 0.5 knots),
what would be the true course to steer at a
boat speed of 5.5 knots?
Drawing a current diagram indicates a drift
angle of 5° to the south.
To offset the current, the course to steer is
265° + 5° = 270°T.
10
Problem 7. Continued from previous slide
c. Allowing for 3° of leeway, what would be the
magnetic course to steer assuming no
deviation?
The pilot chart suggests the leeway would be
southerly.
Adding leeway, the magnetic course to steer
would be 270° + 3° + 6° = 279° M
(Variation is 6°W, from the pilot chart).
Ref.: ¶ 34 – 39 & the pilot chart
11
8. The course steered from 0800 to 1200 has been
095°M; the course made good from an 0800 fix
to a 1200 fix has been 103°M.
If you want to maintain a rhumb-line course of
090°T, what magnetic course do you steer
now? Variation is 5°W.
_____________
087° M
Solution:
Steering Course
T
090°
V
5°W
M
095°
Course Made Good
T
098°
V
5°W
M
103°
Working with true courses, drift angle is 098° – 090° = 8° to the
south. To offset this drift angle, steering course should be
090° – 8° = 082°T.
Convert this to magnetic course.
T
082°
V
5°W
M
087°
12
Ref.: ¶ 27 – 29 & Advanced Piloting
9. Your steering course is 075°. From the pilot chart
you see that the average current in this area is
setting to the northeast at 0.3 knot. The wind is
steady from the southwest at Force 4. Your speed
by knot meter has been 6.0 knots but regular plots
of the DR track and fixes indicate SMG is 5.3 to 5.5
knots.
What is the most probable explanation?
a. The current is stronger than expected.
b. The wind is setting up a surface current that offsets
the
normal average current.
c. The knot meter is inaccurate.
d. There is an unanticipated current setting to the
southeast.
Ref.: ¶ 34, 35
13
10. Make a similar analysis for the same passage
as Example 3 in reverse, from Cristobal to
Puerto Rico. Assume you have a motor
vessel with optimum cruising speed in calm
water of 9 knots, making negligible leeway
at this speed.
a. What current would you probably encounter
first upon departure from Cristobal?
The first major current you would encounter
after departure from Cristobal would be setting
to the east at about 0.8 knot.
14
Problem 10. Continued from previous slide
b. From a point on the course at Lo 75°W, what compass
course would you have to steer to stay on the course?
For this part of the problem, use the average current
determined in Example 3, and consider only this current
and average boat speed.
At about 75°W, you would begin to encounter
the strong west-going current flowing between
the Caribbean islands and the north coast of
South America. The average current would have
the same set and drift as determined in
Example 3: Set 270°, Drift 0.9 knot
(See Figure 12-5 with Example 3, paragraph 32).
Graphically, this results in a current offset of 3°
and a course to steer of
054° + 3° = 057°T
15
10. Continued from previous slide
c. Given the resulting SOA, how long will it take to cover
the remaining distance from this point to Puerto Rico?
Answer: 62.65 hr or 2 days, 15 hr
Solution:
Click to view Fig. 12 – 8
Draw a current vector diagram for this set and drift, with
boat speed of 9 knots.
(See Figure 12–8)
Speed of Advance (SOA) is 8.3 knots.
From the point at which the course crosses the 75ºW
meridian, the distance to Puerto Rico is approximately
520 nautical miles by measurement on the pilot chart.
Therefore, the time required to reach Puerto Rico from
this point is:
T=D÷S
T = 520 nm ÷ 8.3 kn
T = 62.65 hr or 2 days 15 hr
16
Problem 10. Continued from previous slide
d. Discuss the probable effects of wind on the boat.
How will leeway and waves affect the course and
speed you could make over the ground?
The wind roses at L 12º30´N, Lo 72º30´W, and
L 17º30´N, Lo 67º30´W on the pilot chart show that
over 90% of the time winds can be expected to be
from the east or northeast at Force 4 to 5 (11 to 21
knots). You could expect to be heading into
moderately high waves much of the time. Indeed
wave heights ranging up to 12 feet can be expected
occasionally in part of this area. These waves may
require a reduction of speed or a course change to
achieve an easier ride. Expect to use more fuel than
usual and have an uncomfortable passage. When the
wind is from the east, the boat would probably make
some leeway, particularly at slower speeds.
Ref.: ¶ 35 - 39
18
Wind & Current
End Of
Homework
Q&A
Junior Navigation
Chapter 12
19