Before Casting Off - Route Planning -. Homework Q & A. Junior Navigation Chapter 10. Objectives: ■ Interpret information on pilot charts and use them for ocean route planning. ■ Understand the advantages of the Mercator and Great Circle projections.
Q & A
■ Interpret information on pilot charts and use them for ocean route planning.
■ Understand the advantages of the Mercator and Great Circle projections.
■ Identify charts and publications needed for offshore navigation
■ Lay out the route for an offshore voyage.
Follow the Student Manual for guidance
a. is counterclockwise in the ocean basins of the northern hemisphere.
b. is a factor driving the prevailing ocean currents.
c. is consistently from west to east.
d. results in a band of strong westerlies circling the globe north of 60°N.
Ref.: ¶ 17
a. Force 3
b. Force 4
c. Force 5
d. Force 6
Ref.: Table 10-1
4. On a pilot chart, lines of equal magnetic variation, called isogonic lines, are shown as:
a. heavy, solid or dashed lines with numbers.
b. solid green arrows, some with numbers.
c. solid gray lines with numbers.
d. heavy black lines.
Ref.: ¶ 30 - 36
5. For the following questions, use the pilot chart provided with your student materials.
a. Note the mean maximum iceberg zone extending south from the Labrador Sea. Icebergs are masses of ice that break off from pack ice to the north. Why do you think icebergs are found so far to the south?
Answer: Icebergs are carried south by the southerly current in the western part of the Labrador Sea (the Labrador Current). This current extends south of Newfoundland.
Ref.: ¶ Pilot chart
b. Note the large values for magnetic variation in the Labrador Sea. What do you think causes this pattern?
Answer: Proximity to the north magnetic pole.
Ref.: ¶ Pilot chart
6. In 1982, Steven Callahan set out from the Canary Islands. His boat sank in a severe storm near L 23.5°N, Lo 31°W, southwest of the Canary Islands. Callahan took to his life raft and survived until he was within sight of land, when he was rescued by fishermen.
a. Given the prevailing wind and current as shown on the pilot chart, estimate where Callahan would have ended up and how long it would have taken him to get there. Assume the raft could make 0.5 knot through the water in Force 4 winds, in addition to current drift.
Answer: Callahan ended up near the island of Guadaloupe in the Lesser Antilles, approximately L 16°N, Lo 61.5°W, after 76 days adrift in the raft.
b. Callahan had a pilot chart on his raft and improvised a crude sextant with which he occasionally made observations of Polaris. With no sight reduction tables or calculator on the raft, what position information could he obtain from these observations?
Answer: Observations of Polaris provided latitude (review Chapter 1).
c. He was concerned lest he drift north of 18ºN. Why?
Answer: He wanted to stay south of 18°N because at that latitude the Caribbean island chain turns to the west. He would have drifted longer before reaching land, or worse, he might have ended up being carried northwest and then north by both current and wind, and never reached land. It is worth noting that during his drift Callahan sighted nine merchant vessels and signaled with flares, but to no effect. This story is told in Adrift, by Steven Callahan, (Houghton Mifflin, 1986).
Ref.: ¶ 29 - 43
7. A boat bound from Bermuda to Norfolk, Virginia, is crossing the Gulf Stream and has found the net set and drift are consistent with pilot-chart values. At L 35ºN, Lo 70ºW, she runs into strong northeast winds. After a day of this, the navigator checks the course made good and finds that despite being in the Gulf Stream the boat is being set to the southwest. What is the probable minimum wind speed required to offset the Gulf Stream drift? What would you expect the sea conditions to be like in these circumstances?
Answer: Wind speed is probably a minimum of 30 knots. Over a 24-hour period a wind-driven current of 2% of 30 knots, or 0.6 knots, could become established, counteracting the Gulf Stream at this position. Depending on the nature of the vessel, leeway might result in her being set to the southwest at an even lower wind speed. A wind of upwards of 30 knots would be Force 6 or 7 on the Beaufort scale, causing high waves. The fact that the wind opposes the Gulf Stream current would lead to the formation of very steep breaking waves—a bad situation.
Ref.: ¶ 29 - 43
a. using the latitude scale at the middle latitude of the chart.
b. using the latitude scale at the same latitude as the line being measured.
c. using the longitude scale, since it is constant at any latitude.
d. using the longitude scale at the same longitude as the line being measured.
Ref.: ¶ 65
9. Islands of identical shape and size at 20N and 50N would appear on a Mercator chart as:
a. identical in shape and size.
b. the same size but different shapes.
c. the same shape but different sizes.
d. nearly identical, but the island at 20N will appear slightly larger.
Ref.: ¶ 64
Ref.: ¶ 91 - 92
Before Casting Off- Route Planning -
End Of Homework Q & A