Tides, Currents, Shoaling and Skinny Water

Tides - Currents - Shoals

The area  from St Augustine to northern Georgia has a history of tidal swings of up to 8.5 ft.  Tides produce associated currents, and shoaling.  To add further concern this area, also has skinny (shallow) waters.  Many vessels run aground  in this area.  There is a saying 'It is not a question of if you run aground, but, when'  HHmmmm.  There are many contributing factors to someone running aground.  Obviously, depth is the major factor.  But timing the tides, captain inattention and following the traditional magenta line instead of the markers (which themselves are repositioned as needed, and are not on the charts) are factors too.

While is St Augustine there was a full moon, and a 7.2 ft tidal swing.  Look at the images above, this is the SAME boat, on the same lift.
On the left the waters are touching the cross beam, on the right the low tide is very evident.  Impressive.

Here is another set of images taken at Hilton Head Island.  Compare the posts at the end of the dock with the height of the canvas.  The tidal swing here has been just under 12 ft recently (Apr 2015,  typical tide is  8-9 ft). The current has been just under 5 Knts.

While not as dramatic, the pictures above are pictures taken at Isle of Hope Marina (near Savannah) The pics demonstrate the tides (about 7.5 ft) in a different manner. Observe the pole, angle of the walkway, and the shore.

 Here is what grows on any sub water surface....  These molusks are very hard, and difficult to remove.  Evidence of their starting to attach (to anything , including a dingy) can occur in as little as 12-14 days.

Shoaling is a concern too, as the tides do their thing they also create currents and shoaling.  The image above is a pic from Hells Gate near Isle of Hope.  The orange colored squares are warnings that a shoal is present.  This is an image that represents about a mile maximum from top to bottom, and there are five shoaling hazards!

Tidal currents are a factor while cruising too.  If you have a current of say 2.5 knots it either pushes you that amount, of fights you that amount.   We typically cruise at 7.5 knts.  so a 2.5 knt current will put us at 5 or 10 knots per hr.

Here is a video of a three Knt tidal current:  Click
The video above was taken at the stern of Blue Willow, AND the current is flowing into the stern!
We worry about Daisy falling into the waters when these currents are present.

Another concern, is when two sounds are connected inland, and the tidal current 'meets' each other from opposing directions.  In these areas, the concern is shoaling with silt, as this is when the flow stops in each direction, and the silt drops.

If one runs aground, there are services to come pull you off.  We use BoatUS, another service is SeaTow.  Insurance is inexpensive, while the bill is very high.

During this trip I have been in total awe of this facet of nature.  The volume of water that moves in and out each day (sometimes twice a day) is totally beyond belief for a boy who has only known waters around Michigan.

FYI:
Here is what grows on any sub water surface....  These molusks are very hard, and difficult to remove.  Evidence of their starting to attach (to anything , including a dingy) can occur in as little as 12-14 days.

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