Friday, January 22, 2016

When is high tide?

Nerd alert—The word of the day is, TIDE!  Say it with me, T-IIIII-DDD-EEEEE!

It all started with a simple question--when is high tide in Georgetown, Bahamas?  On our journey south, Mother Nature gently taught us land-locked Missourians about tides.  In Annapolis, MD the mean range was 0.97 ft with fixed docks at most of the marinas.  By the time we reached Brunswick, GA the mean range was 6.6 ft with most marinas having floating docks.  In Florida, the mean range goes back down to 2.5 ft.  In the Bahamas, it is 3 ft (mostly fixed docks).  (The largest tides in the world are 38 ft in Nova Scotia!)

According to Chapman’s the definition of tide is “…the rise and fall, the vertical movement, of bodies of water as a result of the gravitational pulls of the moon and sun…Tides originate in the open oceans and seas, but are only noticeable and significant close to shore...Curiously, the effect of tides may be more noticeable a hundred miles up a river than it is at the river's mouth because water piles up higher in the river's narrower stretches.”  (versus current which is the horizontal flow of water.)  

The effect of the moon:  When the moon is full, it is called “Spring tides” and have a greater range.  When the moon is quarter, it is called “Neap tides” and have less range.  As we learned in Special Report: What the heck is a perigean spring tide?, "When the moon is closest to the earth, it is called a lunar perigee.  The gravitational pull of the moon is the strongest.  Three to four times a year, the lunar perigee and full moon coincide.  This is the perigean spring tide (or unofficially, the supermoon or "king tide" in New Zealand) and can affect tides up to 20 percent (usually just a few inches).  Apparently, we experienced this once in Annapolis--when it's perigean spring tide, wind blows onshore from the bay and it storms (barometric pressure drop), they get flooding downtown." (See weather below)

The effect of weather:  According to NOAA, “we cannot predict the effect that wind, rain, freshwater runoff, and other short-term meteorological events will have on the tides.”
In an example, Explorer Chart's stated, "...the tides were running at least ½-meter higher than predicted schedules—that would be 1.5 meters above chart datum—Mean Low Water Springs. This was due to the Derecho that occurred in the Exumas on January 6, 2016."  In other words, the high winds had driven more water into the Bahama Banks than usual.

Explorer Chartbook further explains tides by stating, “[it is] part science, part art…Tides are seldom exactly like the predictions.”  and further explaining that tide is affected by local features with differences at cuts.  “Generally, the farther from the ocean, the later the tide.”

The tidal flow in our part of the world is generally a 6-hour cycle between high tide and low tide and changes daily.  There are generally, two high tides and two low tides a day.

I learned the term “rage tide” from the National Park Service kayaking website, while anchored at Cape Lookout Bight just outside Beaufort, NC. 
“Most paddlers will want to avoid the rage tide, the time when the current flows the hardest, as ½ of the total water volume passes through inlets during the third and fourth hours of tide flow. During this time, flatwater will turn to whitecap waves and intermediate terrain can become expert.”

This is further explained by the Rule of Twelfths:  Per Nigel Calder's Cruising Handbook, you can use the Rule of Twelfths to approximate the state of tide.  "This says that in the first hour after high or low tide, the water falls or rises by one twelfth of the total tide; in the second and fifth hours, it is two twelfths; and in the third and fourth hours, it is three twelfths."  This also means that the hour before and hour after, you are still relatively close to high/low tide.

What are tide tables:
According to Chapman’s, The National Ocean Service (NOAA) is responsible for surveying and publishing the computer-generated Tide Tables which are “…PREDICTED times and heights of high and low waters for each day of the year at a number of important points known as ‘reference stations.’”.  This information is available for free at

More examples of tide information:
Navionics e-charts:  “Our Tide & Current data are predictions based on the government Tide & Current Stations (including many different sources, which are proprietary information)…Please note that the Tide & Current information do not take Daylight Saving Time (DST) into account, so during this time period the predictions may be off by one hour if the device settings have not been adjusted for DST.”
Garmin BlueCharts: source unknown
Explorer Charts:  pg 72 Bahamas Exumas.  Nassau tide tables, “NOTE:  this data may differ slightly from the Bahamas Dept of Meteorology tables due to different source data.”  They also give a correction range for the Bahamas of Nassau -0:13 to 0:30 (source:  Bahamas Department of Meteorology)
A Cruising Guide to:  The Southern Bahamas by Stephen Pavlidis
A Yachtsman's Guide to the Bahamas
Bahamas Department of Meteorology ( daily weather forecast
The local net (usually references local knowledge)
OR:  the book Passages South, The Thornless Path to Windward by Bruce Van Sant, "Near open ocean you can assume high tide at 8 o'clock local time on the day of a full moon.  Add 52 minutes a day thereafter and do without tide tables forever!"

So when is high tide in Georgetown, Bahamas?  
It depends on who you ask.  For example, on January 16, 2016 (departing Red Shanks anchorage):
11:15 am Navionics chart plotter gave high tide at “Exuma Harbour” 
12:18 pm Pavlidis Appendix E, Nassau 12:38 - 0:20
12:37 pm Dept of Meteorology (“Exumas”)  = Nassau -0:01 
12:38 pm Yachtsman’s Guide to the Bahamas = Nassau
12:45 pm Garmin at “Steventon” (“14 miles from chart center”—North, Nassau + 7)
12:53 pm  (Nassau 12:38 + 15)
12:58 pm local & charter captain (Nassau + 20)
1:21 pm The Net says Nassau 12:38 + 45 
(Thornless:  The next full moon is Jan 24, so high tide should be 8:00 if "near open ocean". NOAA has high tide at Nassau as 7:51)
That’s a 2-hour difference!  A local told me that high tide can be different at either end of the harbor. If you use the Thornless explanation of "near open ocean" this makes sense. The Northern entrance is the Exuma Sound (very close to the ocean) and the southern cut is very shallow water ("the banks", 11 ft) 

Why do I care?
Anchoring:  While anchoring, we need to know where we are in the cycle. Do we need to factor an additional 3 ft into our depth while calculating our 7:1 scope?  Or will it be 3 ft less under the keels than what our depth finder is reading? If there is a Supermoon or derecho (1 meter = 3 ft x 7 = 24 ft) it could be the difference between dragging anchor or not dragging (ask the catamaran next to us in Lake Worth!)

Skinny water:  Many cuts can be shallow, requiring a high tide to safely navigate through. (Others can have a significant current, requiring slack water to be navigable--another topic, but you definitely do not want to transit during the rage). The ICW also has areas that silt in, requiring high tide to avoid going aground (until it can be dredged again). Some excellent anchorages have a shallow entrance, requiring high tide to enter, but deep water once inside (RedShanks).

At a dock:  If you are at a fixed dock, and tie up at low tide, lines have to be secured with the idea that the boat will rise 3 ft at some point (usually in the middle of the night) possibly straining lines, stanchions or fenders.

Clearing bridges:  Our catamaran (47 ft mast) was in winter storage at a primarily-powerboat marina, with a bridge height just before the entrance of 53 ft. Our visit to the Albemarle Sound had us scooting under the Plymouth, NC 50 ft bridge to ride out the remnants of a Tropical Storm at the sheltered and free town dock.

The moral of the story:
Question your chartplotter.
Question the tide tables.
Confirm the location of the tide info given.
Confirm the source of the info.
Take into consideration the moon phase and weather (especially barometric & wind)
Take into consideration the range of tidal swing (2 ft or 8 ft?)
Compare the depth finder to your charts
Compare the depth finder to tide table
Keep a log
Look around--not just while anchoring but frequently throughout the day to get a feel for the area.
Ask a local fisherman or charter captain:  I've tried this and I get a lot of shoulder shrugs. "It is, when it is." Here's how they do it: visually.  Look at landmarks, pilings, limestone (water marks), sand bars/reef, slime on the side of buildings (visible?  low tide.  Not visible?  high tide)

We've also been known to stand on our stern step and attempt to "stick the bottom" with the boat hook.  We've also snorkelled down to take a look--in very shallow water, we can approximate how much water is under our keels or even stand up!

Don't get me started on low tide vs slack water.

No comments:

Post a Comment