Tides and Currents for the Cruiser
by Colin Ward
The first twenty years of our sailing experience was largely gained on lakes. We saw the lake levels rise and fall more than 15 feet from the spring rains to the fall drought, but we were still not fully prepared for ocean tides and currents when first started cruising aboard Mandalay.
Here are some practical hints about tides and currents.
Currents are usually caused by downstream flow in rivers or by rising and falling tides. Tidal currents can flow in one direction twice a day and the opposite direction twice a day. Sometimes it is difficult to predict the direction and velocity of current. Conceptually, current flows from the deepest ocean into the shallowest cuts and inlets as the tide rises. In many areas, there are shelves or banks where the water is relatively shallow (Florida Bay or the Bahama Banks would be examples). When the tide is rising, water will flow from the deep ocean onto the shallow banks through whatever land cuts, rivers, or inlets are available. For example, when the tide is rising in the Florida Keys, a current will be flowing from the deep Florida Straits to the south through the cuts between the islands and into shallow Florida Bay to the north. The velocity of the current can be several knots in places where the cut or channel is restricted in width. Tidal current does not reverse direction immediately at high or low tide - in fact in can continue running for a couple of hours beyond.
So what are the effects for the sailor? Well, one effect is that when the current is flowing in the opposite direction from which the wind is blowing, short, steep waves are created which are often uncomfortable for boaters. Perhaps the ultimate example of this situation arises in the Gulf Stream when a north wind opposes a 3 knot northerly flowing current. The wave height will often be two or three times the height just outside the Gulf Stream, plus the wave period is short making the waves very uncomfortable and often dangerous.
If you ever anchor or moor in an area that is near a cut between islands, beware of deep channels. While the depth might be appealing for your deep draft sailboat, that channel was very likely created by the current that rushes through the cut, scouring the bottom of soft sand as it does so. If you can get your anchor to hold in the scoured bottom, you will find your boat dancing all around as the wind pushes it one way and the current drags it another way. A good formula for a restless night! Sometimes two anchors are advised - one for incoming current and one for outgoing. Your two anchor rodes will likely be twisted around each other by morning. During slack periods, the boat will lie to the wind so scout out your anchorage carefully for adequate depths.
Tides and currents can certainly work in your favor. If you run aground on a rising tide, the water will eventually float you free again. Traversing the Atlantic Intracoastal Waterway on a deep draft vessel, it is almost imperative to tackle some stretches at mid tide or better, and hopefully on a rising tide. Unfortunately, the strong tidal currents create many shallow spots in the ICW but with a tidal range of several feet, it is possible to pass those areas unobstructed.
Tides can also help you cross the bar into a river or anchorage. Sometimes, you will look at the depth and turn away from a neat anchorage, but with good timing, you can use a couple of extra feet of water to get in and back out again.
Bucking a tidal current on a slow moving sailboat can be a frustrating experience. A two knot opposing current can reduce your five knot cruising speed by 40%! And if you are trying to beat into a two knot current on your five knot sailboat, you will be lucky to make any forward progress at all. Of course, with careful planning you should be able to travel in the same direction as the current some of the time, which will add those two knots to your speed instead of taking them away.
We recommend a computerized tide program which presents the rising and falling of the tides in graphical form. You can easily see where the tide presently stands relative to its minimum and maximums. You can also calculate manually using the rule of twelfths, but the graphical presentation is ideal.