Tides and Currents

For South Carolina, Georgia, and Northern Florida – tides on the ICW can range as high as 10 feet (!), and currents are often 2 knots (sometimes 4 knots). This makes for some interesting and complicated navigation, especially when deciding when to travel and how and where to anchor:

THE TIDES --

Low tide – bad time to go through areas with thin (i.e. shallow) water. We have had to avoid passing through some bad areas at low tide. 

Rising tide - If we get stuck hard aground, all we have to do is set an anchor and wait for the rising tide to float us off. [As we did where Catskill Creek joins the Hudson River and dumps its load of silt to form a bar.]

Half-tide and rising – the best of both worlds.  Unfortunately, this happens only twice a day.

High tide – great for passing over the bad (shallow) spots but dangerous: if we get stuck hard aground, we won't be able to get off by ourselves, or maybe even get pulled off by another boat.

Spring tide – when the moon is full or new, the sun and moon are aligned and the tidal range is higher (and lower) than normal.  This can lead to flooding, and if we get stuck we won’t get off until the next Spring tide – two weeks away!

Falling tide – not so good.  If we get stuck, we would have to wait for the tide to drop, then rise back up after low tide until it is higher than when we ran aground. We have to be very careful traveling when the tide is falling.

Half-tide and falling – the worst of both worlds. Thin water, getting thinner, and the problems described above about getting stuck.

Hard aground with a falling tide – if we are stuck on our keel, the boat will tip (heel) over, until the tide reverses.  With a 5-foot draft, this could mean that we will heel enough to take water in over the cockpit combing, possibly flooding and sinking the boat.  The one time we were stuck on the Hudson River, we only dropped a foot or so, with about a 15-degree heel.  Imagine getting stuck at high tide when the water level is going to drop – up to maybe 10 feet.  Ouch!

Hard aground with a rising tide – nice.  Just wait; will float off within a half-hour.

And, to make it as complicated as possible: The tides vary with location – need accurate tide data for the whole route.  This is especially true near inlets to the open ocean.  The further inland, the more delayed the tide will be compared to the ocean.  And then there is the current coming in (or going out) as we pass the mouths of rivers that enter (or leave) the ICW.

Anchoring consideration: We have to put out more rode – sometimes a lot more – to have enough scope for the highest tide level.  This means we need a lot more room to swing (current reversal, remember), and makes crowded or narrow anchorages more of a problem.

MEASURING TIDES -- Our depth gauge gives us the approximate depth under the boat.  The soundings on the charts give us the theoretical mean low water depth. The corrected, local tide tables give us the expected depth above low water. Higher mathematics gives us the actual tide where we are.  But we don't have the time or interest in doing this.  Our keel is a pretty good indicator of the actual depth ...

THE CURRENTS -- These also vary and have to be considered:

Slack current – around the time of, or after, the maximum high or load tide, the current is a minimum; it turns around sometimes pretty quickly; at other times it is a gradual stop-and-start.  

Anchoring consideration:  The boat will probably swing 180 degrees; hopefully the anchor will reset (we bought a Rocna, supposedly great at resetting).

Fair current [tail current] – nice for moving, as we get up to a 2-knot boost. Bad for docking, though, as we have to come in at higher speed to maintain steerage.

Foul current [head current] – slows us down, sometimes a lot. Great for docking at slow speed.

Anchoring considerations - The 180-degree swings.  Stronger pull on the anchor and rode. Might want to med-moor, i.e. set two anchors on the bow, one forward, one back, so we always have one pre-set anchor to hang on, but then we have to unship the dinghy to set the second anchor.

Inlet and Outlets - We can either ride or fight the current when going in or out through a pass to or from the ocean.  This worked perfectly on our outside run from Georgetown SC to Charleston – fair current both ways: Falling tide as we went out in the morning, then rising tide as we came back in, in the late afternoon.  But it could have been the other way around …

MEASURING CURRENT -- Our Raymarine tridata gives us our speed through the water. Our Garmin chartplotter gives us our [GPS] speed over the ground. Subtraction gives us the current.

WIND AND CURRENT --

Coming from opposite directions – bad. Sometimes very bad. Can lead to standing waves (square waves) and extremely rough seas.  NEVER be on the northbound Gulf Stream with a northerly wind!

Coming from the same direction – better.

PUNCHLINE –- A lot to consider when choosing if, when, and where to travel.  And sometimes we just have to go and don't have a choice …