Currents are caused by a number of factors including gravitational (flowing from high point to a low point and effects of gravity on tides), winds, and physical properties of different water bodies like temperature and salt content. Some of these currents are permanent and some are intermittent; winds across large water surfaces may change surface currents with wind direction, some currents are seasonal, some currents change direction with tidal flow, etc.
Terms Associated With Currents
The SET is the direction a current is flowing toward (this is opposite of how we describe winds. Winds are described for where they are blowing from. Current sets are where they flowing to.) For example, a current with set of 90 degrees is described as an easterly current; flowing towards the east.
The DRIFT is the speed of a current. Drift is described in knots, with the exception of river currents which are described in mph in the U.S. Velocity and drift are not interchangeable. Velocity is a term used to describe strength and direction and is not typically used to describe currents. Drift and set are more appropriate.
A tidal current is described as FLOOD when it flows in from the sea resulting in higher tidal stages. When the tidal current flow is seaward, it EBBs as the tidal stage falls.
SLACK WATER, or SLACK is when there is no discernible flow prior to the reverse of a tidal current.
Low and high water STAND is the time when the water has reached it’s highest or lowest point in the tide cycle, just prior to reversing. There may be no visible flow at the point of a tide datum, but a careful consideration must be made to understand slack water time and low or high water stand time is not always the same. High tide at the mouth of an inlet occurs prior to high water at points up any tributary or river where the water continues to flood and will begin to ebb at a later time than at the mouth. When navigating through any narrows you need to clearly understand your timing and the corresponding time intervals between low or high water stand and slack water, especially when the interval is hours instead of minutes. The current you travel through will be significantly different than planned if your timing is off.
Most boaters who tend to travel in near-coastal areas are affected the greatest by tidal currents (you may also hear the term “tidal stream” which at times is used interchangeably even though the meaning is different).
In most bays and rivers you get a reversing current, one which flows alternately in one direction and then changes to the other direction as the tide rises and falls in the area. The strength and duration of the current depends on factors such as the change in water height, geography of water flow (narrow vs wide inlets, etc.,) and timing of high and low water. You may also encounter a hydraulic tidal current resulting when two large bodies of water are both connected by a narrower waterway and both bodies are affected by tidal changes. The connecting waterway current is described as the hydraulic tidal current. This current will change direction and speed as each body of water goes through rising and falling tide levels.
In offshore areas you may also encounter a rotary current where land does not influence tidal flow and the current rotates through all compass points each tidal cycle.
Currents and Navigation
Any current along our intended course will affect our route in some manner; speed, heading choice, departure and arrival timing, etc. Knowing the set and drift can make certain we use current to our advantage or minimize the impact of the current on our intended course and travel time. If we are in a boat that only travels at 8-10 knots, even a small opposing current will significantly impact our travel time and fuel needs to reach our destination. For some ocean inlets, currents are dangerous and impassible at certain times; so timing is critical to navigating safe passage.
Precise current prediction for some coastal tidal currents is difficult, but learning to read current tables and understanding tidal actions will assist in safe navigation planning.
For a very unique view of ocean currents, including the Gulf Stream and a number of large rotary currents, check out this site: https://www.nasa.gov/topics/earth/features/perpetual-ocean.html
Tidal Current Tables
For the U.S. Puerto Rico, and the windward islands, tide and current information can be found on the NOAA site; https://tidesandcurrents.noaa.gov/
You can purchase a hard copy book of tide current tables (an NOAA publication) at this link: Tidal Current Tables 2019 (this is an affiliate link)
Let’s discuss how to read and use these tables when navigation planning. (Coming Soon!)