30 July 2004
Getting the Drift in Current Vocabulary
by Capt. Alan Hugenot
It is surprising how many people do not know the difference between the stages of the tide (as in high tide or low tide) and the tidal currents (as in flood and ebb). It is a commonly held belief that word ebb is synonymous with low tide and flood is synonymous with high tide. I often hear the phrase ?The tide was at max ebb?, being used to describe the time of low tide. When they should have simply said, ?It was low tide? or ?the tide was all the way out?. Saying that the tide was at its ?max ebb?, actually means that the tide moving out at its fastest current, and this maximum current occurs when the tide is only about half way out.
Learning the best time to take a boat through the Golden Gate, is impossible without first learning the basic vocabulary. Avoiding the worst conditions requires timing your crossing to suit the tides and currents, and that means knowing the difference between a tide and a tidal current.
EBB AND FLOOD DO NOT REFER TO LOW AND HIGH: They are instead directional adjectives, which indicate the direction of the tidal movement. ?The tide was at ebb?, means that the current was going out. ?it was during the ebb?, means that it happened while the tide was going out. Flood, on the other hand means that the current was going in, and maximum flood is the time when the incoming current reaches its maximum velocity. Maximum flood occurs about half way between the time of low tide and the time of the next high tide. Maximum flood is not, as many people assume, a condition that occurs when all the tide is in. That condition of the tide being all the way in is known as high tide. At the time of high tide, or shortly thereafter the current becomes slack, which means it is neither ebbing nor flooding but just standing still. High slack usually occurs 30 minutes to an hour after high tide. The tide then turns and begins to ebb. It continues to ebb until low tide, which occurs about six and a half hours after the time of high tide. Once the basic vocabulary is understood boaters can begin to figure out when these various stages of the tides will occur, and with that information they can plan their crossings for the best conditions.
Unfortunately, most of the forces exerted on the tides are caused by the moon, which complicates the time calculation because the moon takes 24 hours and 56 minutes to make a complete transit. Normally this means that tomorrow the tides will be about an hour later than they were today, and about an hour later than that the following day. Added to that gain of 56 minutes a day, the sun also exerts an influence on the tides, but the sun makes a transit in 24 hours. The lesser pull of the sun retards or advances the times of the tides slightly. The combination of these forces gives us the varying heights of the tides. Once, we understand all the terminology it should be simple enough to look up the time of high or low tide in a tide table, and the times of maximum flood and maximum ebb in a tidal current table.
But, if you have ever been to the Bay Model and watched that demonstration of the tidal currents flooding into the bay and ebbing out again, then you probably know that the highs and lows arrive at different places in the bay at different times. At some locations the highs and lows can be as much as two hours later than they are at the Golden Gate. Added to that is the fact that the maximum depth of the tide could be quite a bit higher or lower that it was at the Golden Gate. To calculate these times, heights and current velocities for the various locations around the bay requires using the tidal offset tables. These were developed by years of observation at each tidal reference point listed in the chart. The Coast and Geodetic Survey, a government agency that preceded NOAA, collected and compiled all this data and created the tidal offset tables.
However, after we understand this calculus we don?t need to actually figure it out any more. Instead we can go on the Internet and find it already calculated for us, at the two sites listed below. Many of the savvy racing skippers on the bay use this info which shows the currents in every area of the bay, to plan their strategy before each race. By looking at the predicted current flows and knowing the bays current patterns from experience they can determine which side of the race course will be favored with an assist from the tidal current if the predicted wind direction occurs during the race.
