Planning for Currents

Are you ever curious about what is happening, current wise, at any given time and location on the bay?  There is a great tool on pages 48 through 60 of the Tide & Current Tables.  To use it, you need four pieces of information:

  1. The current velocity in knots of the maximum flood or ebb
  2. The time before or after the maximum flood or ebb
  3. The Factor for Correcting Speeds found in the table on page 48
  4. Speed in Knots taken from the appropriate chart found on pages 49 through 60

Here is an example:

You plan to leave Marina Bay at about 11:00 on Thursday December 18, and sail to Angel Island.  What currents can you expect as you are crossing from the Potrero Reach to Angel Island?

A maximum ebb of 4.3 knots is predicted to occur at 13:07 at the Golden Gate on Thursday December 18, 2014.  Which means your crossing will take place approximately 2 hours before max ebb.

The Factor for Correcting Speeds table on page 48 indicates if the predicted current is between 4.3 and 4.7, you have a Factor of 1.0 to use as a multiplier in the next step.

Turning to the chart titled Two Hours Before Maximum Ebb at Golden Gate, you find a “Speed in Knots” of 1.6  The current you can expect during your crossing is 1.6 x 1.0, or approximately 1.6 knots, moving from right to left as you cross.  Put in other terms, during the 20 plus minutes it will take you to cross, expect to move over a half mile off of your course to the left of your destination.

Here is another thought.  About the time you can expect to arrive in Ayala Cove (One hour before max ebb) expect a 2.5 knot (look it up using the charts) current through Raccoon Straight.  A good piece of that ebb is going to get “caught” on the point of land on the West side of Ayala Cove, and swirl back under the docks from right to left as you approach the slips.  It’s going to be an interesting docking … be prepared for it.

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