And Then There Were Tides

As we left the southernmost lock on the Mobile River, we entered tidal waters whose levels and currents are affected by gravitational forces from the sun and moon. For us Great Lakes boaters, ocean tides are like adding a fourth dimension to reality. There is a new vocabulary and new factors to consider in daily boating.

There are high tides, low tides, rising tides, and falling tides. There are neap tides (with a relatively low difference between high and low tide, that happen when the sun and moon form a right angle), spring tides (with a relatively large difference between high and low tide, that happen when the sun and moon are lined up), and slack tides (relatively still water at low tides.) And there are diurnal (daily) tide cycles (one high and one low per day) and semi-diurnal (semi-daily) tide cycles (two highs and two lows per day). A semidiurnal cycle is normal for most of the earth, but the Florida Gulf Coast – with very few other areas on earth – has diurnal tides.

Here is a sample two-day tides chart for Mobile Bay, showing about a half-foot range from high to low tides:

Skinny water. Our boat needs about four feet of water. There are shallow areas that are less than four feet deep at low tide, so we will need to pass through those areas on rising or high tides to avoid running aground.

Fender Placement. Before we arrive at a freshwater marina we usually check if the docks are fixed or floating, so that we know where to hang rubber fenders to protect the boat. In tidal areas we have seen many fixed docks attached to pilings. In those situations we need to consider the tidal rise and fall of the boat during the day and overnight. Cylinder-shaped fenders hung horizontally seem to work best.

Tidal Currents. Between low tide and high tide, changing water levels can cause strong tidal currents, making docking, undocking, and just plain driving more challenging. We will try to arrive at, and leave, these areas at high or low tide, to minimize currents.

We are excited to learn these new nature and boating lessons!