OUPV / Six Pack Captains License Course

NAVIGATIONAL GENERAL - Tides & Currents

Navigation General

Tides & Currents

Tide is the vertical rise and fall of water caused by the gravitational “pull” of the moon and sun on different parts of the earth as the earth rotates.  A Tidal Current is the horizontal movement of water caused by the tides as they rise and fall.  The Tidal Range is the vertical distance water moves from low water to high water, or the difference between the high and low water.  This varies depending on geographic location.  If an area has a Diurnal Tide it has one high tide and one low tide each day.  The North shore of the Gulf of Mexico has diurnal tides.  If an area has a Semi-Diurnal Tide, then it has two high tides and two low tides each day.  The U.S. East Coast tides are semi-diurnal.   As a tide turns, the water moves slowly at first, then speeds up until it reaches its maximum speed halfway between a high and low (or low and high) tide.

– Stand is the period at high or low water when there is no noticeable rise or fall of the tide. 

– Ebb Tide is the tide moving away from shore, water level falling. 

– Flood Tide is when the tide is moving in towards the shore, water level rising.

– Spring Tide
 occurs when the moon and sun are in line with each other and exert a gravitational pull together on the earth’s surface.  These occur twice each lunar month during new moons and full moons.  High and low tides are higher and lower than normal during a spring tide.

– Neap Tide
 occurs when the sun and moon are 90 degrees to each other so that their tidal affects oppose each other.  This occurs twice each lunar month.  High tides are lower than normal and low tides are higher than normal during a Neap Tide.

– Slack water
 occurs when there is no horizontal movement in a tidal current. 
Offshore currents like the Gulf Stream move consistently in the same direction due to the Coriolis Effect, while near shore currents can change due to tidal flow, extreme weather conditions, drought and flooding.

Tidal Datum
 on a chart gives the level from which all heights and depths are measured.  Most US Charts utilize depths at Mean Lower Low Water, which is calculated from observations taken over a period of 19 years.  This is not the lowest water can be due to other factors.  Mariners should take into account that strong offshore winds lower the level of water shown in tide tables and strong onshore winds raise the level of water shown in the tide tables.  Heights on a chart are usually given at Mean High Water to permit safe passage under bridges and cables.

Tide Tables
 contain the prediction times and heights of the high and low waters for each day of the year.  A plus (+) sign indicates that you add the values and a minus (-) sign indicates that you will be subtracting the value.

Practical Exercise: According to the Tide Table for Key West for July 2012, answer the following questions:
  1. What time was morning low tide on July 3rd, 2012 and what was the height of the tide?
    A. 1:57 am, +.3   
    B.  9:51 am, +2.2   
    C.  2:52 am, +.26   
    D. 4:39 pm, -.5

  2. On July 1st, 2012 you are anchored in Key West Harbor at 2:58 pm and weather conditions are normal.  You plan to dock your boat at Ocean Key House.  The charted depth at the entrance is 6 feet.  Your sailboat is fully loaded and draws 5.7 feet.  What is the depth at 2:58 pm and would it be safe to enter? 
    A. 6.5 feet, yes   
    B. 5.57 feet, no   
    C.  6 feet, yes   
    D.  1 foot, no

  3. On July 7th, 2012, you are anchored in Key West Harbor at 1:23 am.  Your geographic position of your chart indicates the depth is 11 feet.  Under normal conditions, what is the actual depth for your position?
    A. 12.8 feet   
    B.  12.34 feet   
    C.  11.2 feet   
    D. 10.9 feet

Answer: 1. C; 2. B; 3. B