The waters of San Francisco Bay are constantly in motion. As the tides rise and fall in the Bay, enormous amounts of water move in and out through the Golden Gate and reach as far inland as Stockton and San Jose.
By CaptaIn Ray
Published: July, 2013
The waters of San Francisco Bay are constantly in motion. As the tides rise and fall in the Bay, enormous amounts of water move in and out through the Golden Gate and reach as far inland as Stockton and San Jose. The Bay’s tidal range, from the lowest of the low tides to the highest of high tides, is about eight and a half feet. While that is the extreme, even a quite moderate three-foot tidal range means that more than three million cubic yards of water pass through the Gate in approximately six hours. With a six-foot tidal change (not unusual), that figure doubles.
In addition, the entire western slope of the Sierra drains into the Bay through the Delta. The fresh river water, being lighter, wants to float on top of the salt water. But three or four million cubic yards of denser, heavier salt water is a force to be reckoned with. Sometimes these tidal currents just push the fresh water out of its way, sometimes they flow under the fresh water, sometimes they mix and sometimes they swirl around each other in a powerful dance of giant eddies. The velocity of the Bay’s tidal currents varies from almost nothing to a little more than six knots (about seven miles per hour).
Sailing on this ever-changing and very complex series of watery interactions can be quite challenging. The current carries everything floating in the water with it (including the boat onboard which you may be reading this). The result of this is that a boat is not always traveling in the desired direction or speed. Depending on the current’s orientation to the boat’s course, it could speed it up, slow it down or move it to either side of its intended course. The trick for the navigator is to have the course "made good" (where the boat actually goes) match the "intended track" (where you wanted the boat to go). There are several ways to do this.
Some people just let the current have its way and deal with it at the end of their passage. This plan, arguably not the best, calls for you to do the hardest part (head straight into the current) at the end of your trip. There are also ways, both mathematical and graphical, to pre-calculate how much offset will be necessary to compensate for the current.
These methods are commonly used on longer voyages where you are not altering course frequently. Inside the Bay, courses are much shorter and are changed quite often. All that pre-computing can be quite a hassle. Therefore, a simpler, quicker method of staying on course is necessary.
"Ranging" is the name of the technique we employ. All we want to do is move the boat in a straight line in the direction it is pointing. As we learned in high school geometry, any two points define a line. So, look out in front of the boat to the land off in the distance and select two in-line objects, one behind the other. You don’t have to identify them and they don’t have to be on the chart; you just have to see them aligned in front of the boat. As long as they remain in alignment, the boat is moving down that straight line. If the rear object appears to be moving to the right in relation to the front object, the boat is moving to the right of the line. Now we just adjust our course until they realign and remain realigned and the effects of current are accounted for.
Ray Wichmann, is a US SAILING-certified Ocean Passagemaking Instructor, a US SAILING Master Instructor Trainer, and a member of US SAILING’s National Faculty. He holds a 100-Ton Master’s License, was a charter skipper in Hawai’i for 15 years, and has sailed on both coasts of the United States, in Mexico, the Caribbean, and Greece. He is presently employed as the Master Instructor at OCSC Sailing in the Berkeley Marina.
