The Weekend Navigator Part II Copyright 2008 Coast Guard Auxiliary Association, Inc.

The Weekend Navigator

Part II

Copyright 2008 Coast Guard Auxiliary Association, Inc.

Chapter 14: Tides, Winds, and Currents

Tides, Winds, and Currents

• Tidal heights and currents have a significant effect on navigation

• Tides can affect where you can go and when

• Tides are a coastal phenomenon

• Seiches are found on the Great Lakes

What Causes Tides?

• Influenced by a number of factors– Gravitational pull of the moon– Centrifugal force– Sun

• Earth and moon spin as pair around a common point called a barycenter– Earth orbits sun once per year– Moon orbits earth every 27 1/3 days

What Causes Tides?

• Perigee – moon’s closest approach• Apogee – moon’s farthest remove• Tidal current tables are different every year

– Astronomical pattern for sun and moon repeat every 18.6 years

– Moon’s motion around the earth takes 24 hours and 50 minutes

– Landmasses cause unique local patterns

Tidal Heights versus Tidal Currents

• Tides are only perceptible along shorelines

• Heights of tides are accentuated by near coastal shoaling– Flooding or incoming– Ebbing or outgoing

• Currents are most significant where there is a restriction to flow


• Time of maximum flood current is between time of low water and high water

• Maximum ebb is between high water and the following low

• Greatest ebb and flood velocities are at sites where tides fill and depart from bays and rivers


• You cannot predict tidal currents from tidal heights, requires current tables

• Tidal heights are computed for harbors

• Tidal currents are computed for more constricted areas where current is a factor

Spring and Neap Tides

• Tidal ranges vary by day, month, year

• New moon – moon’s dark side is entirely toward earth (unlighted by sun)– Occurs once very 29 ½ days– Gravitational effects of sun and moon are

additive, resulting in spring tides– High tides are higher and low tides are lower

Spring and Neap Tides

• Full moon – halfway between new moons and moon is fully illuminated

• Halfway between new moons and full moons is the first and third quarters– Sun and moon pull at right (90o) angles to each

other– Their effects are partially cancelled out by one

another– Neap tides or smaller tidal ranges result

Tidal Patterns

• Tides vary in cycle from high to low– Diurnal – daily– Semi-diurnal - twice daily

• Semidiurnal or two tides daily– Two successive highs and lows are not equal in

height– Continents block westward passage of tidal

bulges and cause complex effects

Tidal Patterns

• Diurnal tides common in Gulf of Mexico– Incoming and outgoing tides cancel each other

• Mixed tides common along the Pacific coast and parts of the Caribbean– Daily tides differ significantly

• Diurnal tides common in Gulf of Mexico and coast of Alaska

Tidal Patterns

• Semi-diurnal tides common along Atlantic coast, most of Europe, and Africa

• Semi-diurnal and mixed tides are found along South American coasts

Tidal Ranges and Vertical Datums

• Range of tide – difference between a high tide and following or preceding low– Cape Cod – 1.5 to 3 feet– Boston Harbor – 5.5 to 12 feet

• Important to know the tidal ranges at your location for days and times of interest

Tidal Height and Tidal Current Information

• Primary reference stations – predict tides and tidal currents– Provided by National Ocean Service (NOS)

• Tidal stations are located in harbors or along coast lines


• Tidal current stations are located at points of constricted flow

• Subordinate stations are referenced to nearby reference station by a time difference and a ratio


• Tidal data can be displayed by:– Handheld GPS models– Chartplotters– Navigation software

• WXTide32 (www.wxtide32.com)

• Tideware by Eldridge

• Tide Tool for handheld PDAs


• Printed tide tables

• Simplified current diagrams with set and drift

• Eldridge Tide and Pilot Book– US East Coast with emphasis on Northeast


• Reed’s Nautical Almanac– Covers east and west coasts of North America

and the Caribbean

• NOAA website– (http://co-ops.nos.noaa.gov)


• Tidal current arrows on nautical charts– Indicate direction of flood (incoming) and ebb

(outgoing) currents– Usually a numeric value provides mean of peak

currents in each direction

Adjusting for Tides and Tidal Currents

• Soundings and depth contours are based on vertical datum– Mean lower low water (MLLW) is used as

datum– Minus tide – low tide that registers as a

negative value (actual low tide is predicted to be below the tidal datum)

• Mean low water (MLW) is less conservative than MLLW


• Charted soundings may have been taken years ago

• Bottom conditions change constantly


• Tidal heights-using a subordinate station– Identify station of interest– Calculate time difference– Use factor for determining low and high tide

heights

• Convert times if difference is significant

• Use the Rule of Twelfths method


• Rule of Twelfths – the tide level changes in increments of 1/12 to 3/12s of its total range in each of six (6) equal time periods

• With diurnal tides, time intervals are a little more than two (2) hours each– Minimal in the Gulf of Mexico

• The Rule of Twelfths is an estimate!


• GPS or computer for tide level– Marine units contain tidal data and buoy

locations

• Newer chartplotters contain tide levels

• Local conditions may be different than electronic predictions.

Predicting Tidal Currents

• Tidal currents are driven by tides and have similar cycles

• Incoming currents are called flood

• Outgoing currents are called ebb

• Slack water – interval of zero flow between flood and ebb


• Currents have a profound effect on your course over ground (COG)

• Adjust your heading upcurrent to stay on course

• Currents in narrow channels and rivers tend to flow along the axis of travel– May assist or retard your boat’s progress


• Tide rips – notorious systems of standing waves that look like river rapids at peak ebb or flood

• Current tables express currents in terms of maximum flood (incoming) and maximum ebb (outgoing)


• Subordinate station tables provide time differences for each slack and each maximum, speed ratios, and current direction

• The 50-90 rule can be used and is just as accurate


• Ebb/flood cycle– Slack to maximum flood– Maximum flood to slack– Slack to maximum ebb– Maximum ebb to slack

Using the 50-90 Rule

• Determine the actual duration of each phase from the current tables– Time interval between events is divided into

equal thirds– 50-90 rule is applied to those thirds– Follow the example presented in the textbook

Using a Computer or GPS for Tidal Current Information

• Chartplotters and computer digital charts provide access to tidal current information

• The pop-up box provides tidal current curve and numeric data for:– Slacks– Floods– Ebbs

Ocean Currents

• Prevailing currents in each ocean basin have nothing to do with tides

• In Northern Hemisphere, currents circulate clockwise around each ocean basin

• In Southern Hemisphere they circulate counter-clockwise

Ocean Currents

• Examples include:– Gulf Stream– North Pacific Current

• Pilot charts are an excellent source of information

• MaxSea is a navigation software

Wind, Waves, and Other Unpredictable Effects

• Weather is a major unpredictable factor

• Crosswinds can push a boat off course

• Leeway – the angle between the intended course and actual track of the boat over ground


• TIP – check your wake to see if you are being affected by the wind

• Counter leeway as you would a current by adjusting into the wind


• Yaw – movement of a boat as a result of wave action pushing the bow off course

• Swells are caused by winds in distant weather systems

• Wind waves or “chop” arise locally


• Wavelength – distance between crests– The greater the wavelength, the faster a wave

moves

• Period – measure of how long it takes two successive crests to pass a stationary point


• Wave height is dependent on the amount of wind operating on the water

• Fetch – the distance over which the wind acts on the water– Combination of strong winds and a long fetch

produce higher waves


• Waves are affected by water depth– Breakers result from friction from the sea

bottom causing the wave to “break over”

• Chop tends to have shorter wavelengths– Yawing action is more of a problem in chop

than in swells

Variable Currents and Seiches

• Wind and surface runoff from shore can affect currents

• Seiches – found in fully enclosed bodies of water and is similar to the back-and-forth sloshing of water in a bath tub– Common on the Great Lakes or other enclosed

large lakes

Date post:	18-Dec-2015
Category:	Documents
Upload:	frederick-mills
View:	214 times
Download:	0 times

The Weekend Navigator Part II Copyright 2008 Coast Guard Auxiliary Association, Inc.

Documents