part 7. tide and tidal current problems
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The Cutterman's Guide to Navigation ProblemsTRANSCRIPT
The Cutterman’s Guide to Navigation Problems
Part Seven: Tide and Tidal Current Problems Calculating tides and currents manually involves determining the height of tide or state of current at a reference station, determining offsets to your desired location, and then applying offsets to the reference station to find your exact values. Problems are based on the 1983 tide tables and 1983 tidal current tables. Descriptions of these tables are located in the preface. Frequently utilized tables from the Tide Tables and Tidal Current Tables are reprinted at the end of this Part.
Tide Problems Problem 7-‐1 (CG-‐703). The following question is taken directly from the USCG test bank and illustrates how to solve height of tide problems. On 5 March, 1983, at 0630 EST (ZD +5), what will be the predicted height of tide at Ocracoke, Ocracoke Inlet, NC. Answer: 0.1 foot.
Step 1: Locate the desired port in the Tide Tables and note the reference station and any offsets required.
For Ocracoke, Ocracoke Inlet, NC (station 2459), the reference station is Hampton Roads, Virginia. The offsets are: Time High Water Low Water Height High Water Low Water -‐1:23 -‐1:00 *0.40 *0.40
Step 2: Find the tidal information for the reference station for the desired
date.
For 5 March, 1983, the tidal information is:
0139: 2.5 feet 0800: 0.2 feet 1358: 2.0 feet 2003: 0.2 feet
Step 3: Determine if daylight savings time is in
effect.
The problem states that times are to be determined in EST, which means daylight savings time is not in effect, and no correction to the tidal times is required.
Step 4: Create a table to compute tides at the desired location near the
desired time.
Time High Water Low Water Height High Water Low Water Reference 01:39 08:00 2.0 feet 0.2 foot Offsets -‐1:23 -‐1:00 *0.40 *0.40 Ocracoke 00:16 07:00 0.8 feet 0.1 foot
Step 5: Answer the required questions regarding the state of tide.
The question asks the state of tide at 0630, which is 30 minutes before the time of low water. Refer to the “Height of Tide at any Time” table in the Tidal Tables (reprinted at the end of this Part). The duration of rise or fall is 07:00 – 00:16 or 5 hours, 44 minutes. The time from nearest high water is 30 minutes.
Entering the table with this data yields a height correction of 0.0 foot. Therefore there is no offset to the calculated tide, and the correct answer is 0.1 foot.
Problem 7-‐2 (CG-‐695). The following question is taken directly from the USCG test bank and illustrates how to solve height of tide problems. On 10 August 1983 you will dock near Days Point, Weehawken, on the Hudson River at 1800 DST (ZD +4). The charted depth alongside the pier is 24 feet (7.3 meters). What will be the depth of water when you dock? Answer: 23.9 feet or 7.2 meters.
Step 1: Locate the desired port in the Tide Tables and note the reference station and any offsets required.
For Weehawken, Days Point, NJ (station 1521), the reference station is New York. The offsets are:
Step 2: Find the tidal information for the reference station for the desired date. For 10 August, 1983, the tidal information is: 0322 -‐0.9 foot 0931 5.3 feet 1536 -‐0.5 foot 2150 5.9 feet
Step 3: Determine if daylight savings time is in effect.
Time High Water
Low Water
Height High Water
Low Water
+0:24 +0:23 -‐0.3 0.0
The problem states that times are to be determined in DST, which means daylight savings time is in effect, and one hour must be added to determine the correct times. The new tidal information is:
0422 -‐0.9 foot 1031 5.3 feet 1636 -‐0.5 foot 2250 5.9 feet
Step 4: Create a table to compute tides at the desired location near the
desired time.
Time Low Water High Water Height Low Water High Water Reference 16:36 22:50 -‐0.5 foot 5.9 feet Offsets +00:23 +00:24 0.0 -‐0.3 Weehaken 16:59 23:14 -‐0.5 foot 5.6 feet
Step 5: Answer the required questions regarding the state of tide.
The question asks for the height of water at the time of mooring (1800 DST). The height of water is MLLW + the height of tide. Using the Height of Tide at any Time table: The duration of rise or fall is 14:59 – 21:14 or 6 hours, 13 minutes. The time from nearest high or low water is 1 hour, 1 minute. The range of tide is 6.1 feet. Entering the table with this data yields a height correction of 0.4 feet. Since the nearest tide is low, the correction is applied from low water. 1659 to 1800: -‐0.5 foot + 0.4 feet = -‐0.1 feet. Since the height of tide at 1800 is -‐0.1 foot and the charted depth alongside is 24 feet, the correct depth of water at the pier at the time of mooring is 23.9 feet.
Tidal Current Problems Problem 7-‐3 (CG-‐1470). The following question is taken directly from the USCG test bank and illustrates how to solve tidal current speed problems. What is the predicted velocity of the tidal current 2 miles west of Southwest Ledge for 2330 DST (ZD +4) on 7 September 1983? Answer: 1.1 knots.
Step 1: Locate the desired location in the Tidal Current Tables and note the reference station and any offsets. The reference station for Southwest Ledge (station 2211) is The Race. The offsets and tabular data are:
Minimum before Flood
Flood Minimum before Ebb
Ebb Speed ratio (Flood)
Speed ratio (Ebb)
Minimum before flood
Max Flood Minimum before Ebb
Max Ebb
-‐0:33m -‐0:33m -‐0:10m -‐0:08m 0.5 0.5 0/-‐ 1.5kts/321°T 0/-‐ 2.1 kts/ 141° T
Step 2: Determine the tidal current
information for the reference station. The reference station data are:
The Race Maximum Velocity Slack Water 0212 4.8 Ebb 0525 0814 4.2 Flood 1119 1439 4.7 Ebb 1748 2037 4.3 Flood 2342 The Race
Step 3: Determine if daylight savings time is in effect and adjust times as required. The problem states that DST is in effect. To obtain DST times, one hour must be added. The corrected data are:
The Race Maximum Velocity Slack Water 0312 4.8 Ebb 0625 0914 4.2 Flood 1219 1539 4.7 Ebb 1848 2137 4.3 Flood 0042 The Race
Step 4: Create a table to calculate the required information at the desired
location. The problem asks for the velocity of the tidal current, so only relevant data are included in the table.
Max Flood
Before Desired Time
Velocity Desired Time
Velocity Slack After Desired Time
Velocity
Reference Station 21:37 4.3 kts Flood
00:42 0 kts
Offsets -‐0:33m *0.5 -‐0:10 0 kts Southwest Ledge 21:04 2.2 kts 2230 TBD 00:32 0 kts
Step 5: Answer the required questions.
The question asks for the velocity at a specific time (2330 DST). Table 3 in the Tidal Current Tables gives the velocity of the current at any time (Table 3 is reprinted at the end of this Part). The interval between slack and desired time (2330) is: 2330 to 0032 = 1 hr, 02 m. The interval between slack and maximum current is: 2104 to 0032 = 3 hr, 28 m. Entering Table 3 yields a correction factor (f) of 0.5. Per the instructions in Table 3, the factor (f) is multiplied by the maximum tidal current velocity to yield the tidal current at the desired time: 0.5 x 2.2 kts = 1.1 kts at 2230 DST
Problem 7-‐4 (CG-‐2066). The following question is taken directly from the USCG test bank and illustrates how to solve tidal current problems for specific time windows. You want to transit Hell Gate on 23 July 1983. What is the period of time around the AM (ZD +4) slack before ebb when the current will be less than 0.5 knot? Answer: 0939 to 0957.
Step 1: Determine the tidal current information for the desired location. Since Hell Gate is it’s own reference station no offsets are required. The data are:
Hell Gate Maximum Velocity Slack Water 0252 0552 3.3 Flood 0848 1146 4.4 Ebb 1503 1809 3.5 Flood 2107 Hell Gate
Step 2: Determine if daylight savings time applies and adjust times as required. The problem states that DST is in effect. To obtain DST times, one hour must be added. The corrected data are:
Hell Gate Maximum Velocity Slack Water 0352 0652 3.3 Flood 0948 1246 4.4 Ebb 1603 1909 3.5 Flood 2207 Hell Gate
Step 3: Answer the required question.
a. The question asks for the period of time around the AM slack with
current less than 0.5 knots. The AM slack is at 0948 (technically it could also be at 0352 but in this case the question seeks the 0948 slack).
b. To find the duration of slack water (the period around slack with a given current speed), utilize Table 4 in the Tidal Current Tables (Table 4 is reprinted at the end of this Part).
The maximum current is 4.4 knots for the nearest flood, and 3.3 knots for the nearest ebb. The period sought is for a window of current less than 0.5 knots. Utilize table B, because the question deals with Hell Gate. The duration of slack (less than 0.5 knots) based on the nearest flood is 15 minutes. The duration of slack (less than 0.5 knots) based on the nearest ebb is 20 minutes. The average duration of slack based on both the nearest flood and nearest ebb is: (15 min + 20 min) ÷ 2 = 18 minutes. Therefore, if slack is at 0948, the window of time with current less than 0.5 knots is 0948 +/-‐ 9 minutes, or 0939 to 0957.
Additional Problems and Answers All of the following questions were taken directly from the 2013 USCG test bank and illustrate the concepts in this Part. Note – not all problems have been worked and are subject to occasional errors in the database. For more problems and answers, see the USCG database of questions (database information located in the preface). Tide and tidal current problems are located on the navigation general test. Problem CG-‐107. Determine the height of the tide at 1430 EST (ZD +5) at New Bedford, MA, on 10 April 1983.
a) 1.1 feet b) 1.2 feet c) 1.4 feet-‐ correct d) 1.7 feet
Problem CG-‐408-‐ Determine the height of the tide at 2045 EST (ZD +5) at Augusta, ME, on 8 March 1983.
a) 1.4 feet (0.5 meter) b) 1.9 feet (0.6 meter)-‐ correct c) 2.3 feet (0.7 meter) d) 2.6 feet (0.8 meter)
Problem CG-‐439. Find the height of the tide at Port Wentworth, GA, on 5 October 1983, at 1840 DST (ZD +4).
a) 3.0 feet b) 3.5 feet c) 4.0 feet d) 4.4 feet-‐ correct
Problem CG-‐447. For 3 November 1983, at 0830 EST (ZD +5) at Catskill, NY, what is the predicted height of tide?
a) +0.1 foot b) -‐0.6 foot-‐ correct c) +0.9 foot d) -‐1.3 feet
Problem CG-‐697. On 2 November 1983 at 1630 EST (ZD +5), what will be the predicted height of tide at Fulton, FL?
a) 2.8 feet-‐ correct b) 3.4 feet
c) 4.2 feet d) 5.6 feet
Problem CG-‐1517. What will be the height of tide at Three Mile Harbor Entrance, Gardiner’s Bay, NY, at 0700 (ZD +5) on 14 November 1983?
a) 1.1 feet b) 1.7 feet-‐ correct c) 1.9 feet d) 2.2 feet
Problem CG-‐336. At what time after 1400 EST (ZD +5), on 4 January 1983, will the height of tide at Port Wentworth, GA be 3.0 feet?
a) 1612 b) 1630 c) 1653-‐ correct d) 1718
Problem CG-‐695. On 10 August 1983 you will dock near Days Point, Weehawken, on the Hudson River at 1800 DST (ZD +4). The charted depth alongside the pier is 24 feet (7.3 meters). What will be the depth of water when you dock?
a) 23.5 feet (7.1 m) b) 23.9 feet (7.2m)-‐ correct c) 24.9 feet (7.5m) d) 26.3 feet (8.0m)
Problem CG-‐2001. You are to sail from Elizabethport, NJ on 22 May 1983 with a maximum draft of 28 feet. You will pass over an obstruction with a charted depth of 27 feet. The steaming time from Elizabethport to the obstruction is 1h 40m. What is the earliest time (ZD +4) you can sail on the afternoon of 22 May and pass over the obstruction with 3 feet of clearance?
a) 1407-‐ correct b) 1331 c) 1303 d) 1242
Problem CG-‐1470. What is the predicted velocity of the tidal current 2 miles west of Southwest Ledge for 2330 DST (ZD +4) on 7 September 1983?
a) 1.3 knots-‐ correct b) 1.6 knots c) 1.9 knots d) 2.2 knots
Problem CG-‐1515. What will be the direction and velocity of the tidal current at Provincetown Harbor, MA at 1045 DST (ZD +4) on 5 May 1983?
a) 0.0 knot at 135° T b) 0.2 knot at 135° T c) 0.4 knot at 315° T-‐ correct d) 0.6 knot at 315° T
Problem CG-‐1524. What will be the time of maximum flood current at the Sagamore Bridge on the Cape Cod Canal during the morning of 6 December 1983 (ZD +5)?
a) 0708-‐ correct b) 0712 c) 0716 d) 1020
Problem CG-‐1526. What will be the velocity and direction of the tidal current at Old Ferry Point, NY, at 1340 EST (ZD +5) on 5 February 1983?
a) 0.8 knot at 060° T b) 0.8 knot at 240° T c) 1.0 knot at 076° T d) 1.4 knot at 076° T-‐ correct
Problem CG-‐1536. What will be the velocity of the tidal current at Port Royal, VA at 1505 DST (ZD +4) on 4 June 1983?
a) 0.0 knot b) 0.1 knot c) 0.4 knot-‐ correct d) 0.7 knot
Problem CG-‐1537. What will be the velocity of the tidal current in Bolivar Roads, Texas, at a point 0.5 miles north of Ft. Point, on 23 November 1983 at 0330 CST (ZD +6)?
a) Slack water-‐ correct b) 0.8 kt c) 1.2 kts d) 3.4 kts
Problem CG-‐2069. You will be entering the Mystic River in Connecticut. What is the current at the Highway Bridge at 1900 EST (ZD +5) on 24 January 1983?
a) 2.2 knots flooding
b) Slack water c) Slight ebb-‐ correct d) 2.5 knots ebbing
Problem CG-‐2066. You want to transit Hell Gate on 23 July 1983. What is the period of time around the AM (ZD +4) slack before ebb when the current will be less than 0.5 knot?
a) 0939 to 0957-‐ correct b) 0943 to 0953 c) 0844 to 0852 d) 0348 to 0356
Problem CG-‐2068. You want to transit Pollack Rip Channel, MA on 6 April 1983. What is the period of time around the 0955 (ZD +5) slack water in which the current does not exceed 0.3 knot?
a) 0911 to 0955 b) 0940 to 1010 c) 0955 to 1044 d) 0935 to 1017-‐ correct
Problem CG-‐409. Determine the time after 0300 CST (ZD +6) when the velocity of the tidal current will be 0.5 knot on 16 April 1983 at Port Arthur Canal Entrance, TX.
a) 0436 b) 0507-‐ correct c) 0538 d) 0554
Problem CG-‐405. Determine the duration of the first PM slack water on 3 March, 1983, east of the Statue of Liberty, when the current is less than 0.1 knot?
a) 10 minutes b) 13 minutes-‐ correct c) 16 minutes d) 19 minutes
Problem CG-‐1469. What is the period of time from around 1008 FST (ZD +4) at Canapipsit Channel, MA on 7 August 1983, in which the current does not exceed 0.4 knots?
a) 0945 to 1031 b) 0950 to 1026 c) 0955 to 1021-‐ correct d) 1000 to 1024
Problem CG-‐1148. The predicted time that the flood begins at the entrance to Delaware Bay is 1526. You are anchored off Chestnut Street in Philadelphia. If you get underway bound for sea at 1600 and turn for 8 knots, at what point will you lose the ebb current?
a) Billingsport b) Marcus Hook c) Mile 63-‐ correct d) Mile 52
