1

Current Lesson 11:

Learning Objectives:

– Comprehend the causes and general types of currents.

– Apply correct procedures in the use of the Tidal Current Tables to construct a complete current table for a locality of interest and to find the expected current for a particular time or period of interest.

– Know the information on currents contained on pilot charts and in current diagrams and charts.

2

Current Current - The horizontal movement of water. It is of particular

interest to the navigator because it continually affects ship’s movement. Causes of current include:– The primary cause of currents is the rise and fall of the

ocean caused by the gravitational effects of the sun and the moon.

– Heating and cooling caused by the earth's atmosphere– Winds blowing across the surface of the ocean.– Evaporation and rain dilute and salt the ocean which has an

effect on current.– The Coriolis force which is associated with the earth's

rotation. This force causes currents to form giant patterns of rotation called “gyres” in each of the major ocean basins

These gyres move in a clockwise direction in the northern hemisphere, and in a counterclockwise direction in the southern hemisphere.

– First type of currentFirst type of current = majormajor ocean currentsocean currents which are like rivers in the oceans, but they far surpass any continental river in size and strength. Ex. Gulf Stream - at its strongest point is 40 miles wide and 2,000 feet deep, and its carries 100 billion tons of water at a velocity sometimes approaching five knots.

– There are also deep ocean currentsdeep ocean currents which are caused primarily by water density differences. The deep current

3

Current system is as extensive as the surface current system and is

believed to have been instrumental in shaping the ocean floor in many areas of the world.

Predicting Ocean Currents - Although Planning Guides and Sailing Directions contain some information on normal locations and strengths of currents, the pilot chart is the most valuable reference for use in predicting the direction and velocity of ocean currents.

– The direction in which the current flows is called setset and its velocity is referred to as driftdrift.

– Predicted set and drift can be determined by observing the green color-coded arrows on the chart. The direction of the arrows indicate the set while the figures printed nearby indicate the drift.

Second type of current - The major ocean currents are of concern to the navigator while out at sea; however, when coastal waters are entered, tidal currentstidal currents. This current caused by primarily by the rise and fall of the tide; coastal waters affected by tidal currents are often referred to as tidewater areas.

Flood current - tidal current that flows toward shore. Ebb current - tidal current that flows away from shore. Slack water - period when there is no horizontal movement

of the water. This period corresponds to the period when there is no vertical movement of the water which is known as the stand of the tide.

4

Current The change in tidal current always lags behind the turning of

the tide by an interval of varying length, depending upon geographic location.

Reversing current Flood current and ebb current are exactly opposite.

Rotary current Current flows continuously with no clearly defined ebb or flood (its set moves completely around the compass during each tide cycle).

Prediction of tidal current - It is important to be able to predict currents when operating in piloting waters because currents significantly effect a ship’s movement. Approaches into some harbors are impossible for a deep-draft vessel to negotiate when a current of any appreciable velocity is flowing. Tools available to the navigator when predicting the effects of current:

– Tidal Current Tables are used extensively for current predictions in the coastal waters of the United States.

– A series of tidal current diagrams and charts are also available for U.S. waters.

– British Admiralty-produced coastal approach charts often contain a graphic known as a current diamond which can be used in conjunction with the applicable volume of the Tidal Current Tables.

– The Tidal Current TablesTidal Current Tables consist of two volumes, one titled Atlantic Coast of North America and the other the Pacific Coast of North America and Asia.

5

Current

6

Current

7

Current The appearance of the Tidal Current Tables are similar to

that of the Tide Tables.

– The Tidal Current Tables contain daily predictions of the times of slack water and maximum flood and ebb current velocities for each of several reference stations, and they list time and velocity difference ratios for each of several hundred subordinate locations.

Each Tidal Current Table is divided into several numbered tables; the West Coast edition has four tables, while the East Coast edition has five, one extra for rotary current predictions.

– Table 1 lists, in chronological order, the times and maximum velocity of each flood and ebb current and the time of each slack water for all reference stations used in that volume. Because tidal currents are caused mainly by tides, there arethere are usually 4 maximum usually 4 maximum currents during the tidal daycurrents during the tidal day with a slack water with a slack water between each currentbetween each current. As in the Tide Tables, times given are based on the reference station’s standard time zone

– Table 2 consists of:

– time differences for flood currents, and for the minimum currents preceding them (usually but not always slack waters).

8

Current– speed ratios, average speed and true direction of

flood and ebb currents, and other useful information for each subordinate location.

– As in the Tide Tables, the subordinate locations data is arranged in geographic sequence, with an alphabetic index to Table 2 located in the back of the volume.

– Table 3 is used to find the velocity of current for a given time at a reference station or subordinate location. Note that table 3 actually consists of two tables. Table 3A is used for most computations while Table 3B is used for locations listed in Table 2 that use Cape Cod Canal, Hell Gate, or the Chesapeake and Delaware Canal.

– Table 4 is used to find the time frame around a slack water or minimum current within which the current velocity will be below a desired maximum. It is made up of two parts, A and B, which are divided in the same manner as Table 3 .

– Table 5 appears only in the Atlantic Coast volume. It is for use in predicting the set and drift of an offshore rotary current at 46 locations off the Atlantic seaboard.

Solving the tidal current prediction problem: standard current table form is utilized with three specific sections:

9

Current The first section is for constructing a complete current

table for a given location of interest in conjunction with Tables 1 and 2.

The second section is for computing the velocity of current for a given time utilizing Table 3.

The third section is for use with Table 4 in finding the earliest and latest times on each side of a slack water within which the velocity of current will be below a specified limit.

– Example Problem : Compute the set and drift of the current at Brooklyn Bridge, NY for 08 OCT utilizing Tables in Hobbs..

Solve for velocity of current at 0900 Solve for duration of slack with desired maximum

velocity of 0.5 kts

10

Current Current diagrams and charts: In addition to tabulated data,

there are several types of graphic aids available to help predict the set and drift of tidal current. Two of the more widely used of these are a set of current diagrams included in the Atlantic Coast Tidal Current Tables and Tidal Current Charts issued by the NOS.– Atlantic Coast Tidal Current Tables - table is entered by

aligning a parallel ruler along the appropriate ship’s speed line located at the monogram to the right. The other edge of the parallel rule is moved until it intersects the vertical line that corresponds to the time difference between the desired time of transit and the time of ebb or flood current adjacent to the area of interest.

– Tidal Current Charts are sets of twelve small-scale charts of a particular bay, harbor, or sound, which depict the normal set and drift of the tidal current for each hour after occurrence of a particular current event at a designated reference station. The charts show the predicted current at various locations for each hour after a tidal event; the arrows represent the set while the adjacent numbers represent drift.

Boston Harbor, Long Island Sound, Block Island Sound, Charleston Harbor, etc….

Tidal current diamonds: For predictions of the set and drift in foreign coastal waters, the applicable Enroute volume of the Sailing Directions, which contains tidal current diagrams and

11

Current

charts for selected ports, foreign-produced equivalents of Tidal Current Tables and Charts, or charted current information, can be used.

– On charts produced by the British Admiralty, lettered symbols called Tidal Current Diamonds are printed at various locations along major channels of interest. The letters within the diamonds refer to small tables printed on the chart, which give hourly predictions of set and drift based on the occurrence of a certain tide event at a designated reference station, usually high water.

Wind driven currents: When using current predictions from any source, the navigator must always bear in mind that the predictions are subject to error during unusual wind or current conditions. The navigator may wish to combine estimated wind-driven current vectors with predicted ocean or tidal current vectors in order to obtain a more accurate prediction of the actual current.

– Table 2 of Current Tables

12

Current

Wind: 15 knots

Current: 1 knot

Ship’s speed: 0 ----- Ship will move in which direction??

13

Current

Homework

– Read Chapter 13 - Current Sailing

– Workbook problems Chapter 12, Section 2, 1A