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

Tidal Heights versus Tidal Currents • 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

Tidal Heights versus Tidal Currents • 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 (90 o ) 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 Height and Tidal Current Information • 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 Height and Tidal Current Information • Tidal data can be displayed by: – Handheld GPS models – Chartplotters – Navigation software • WXTide32 (

www.wxtide32.com

) • Tideware by Eldridge • Tide Tool for handheld PDAs

Tidal Height and Tidal Current Information • Printed tide tables • Simplified current diagrams with set and drift • Eldridge Tide and Pilot Book – US East Coast with emphasis on Northeast

Tidal Height and Tidal Current Information • Reed’s Nautical Almanac – Covers east and west coasts of North America and the Caribbean • NOAA website – (

http://co-ops.nos.noaa.gov

)

Tidal Height and Tidal Current Information • 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

Adjusting for Tides and Tidal Currents • Charted soundings may have been taken years ago • Bottom conditions change constantly

Adjusting for Tides and Tidal Currents • 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

Adjusting for Tides and Tidal Currents • 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!

Adjusting for Tides and Tidal Currents • 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

Predicting Tidal Currents • 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

Predicting Tidal Currents • 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)

Predicting Tidal Currents • 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

Predicting Tidal Currents • 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

Wind, Waves, and Other Unpredictable Effects • 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

Wind, Waves, and Other Unpredictable Effects • 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

Wind, Waves, and Other Unpredictable Effects • 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

Wind, Waves, and Other Unpredictable Effects • 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

Wind, Waves, and Other Unpredictable Effects • 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