Sources of info

Open and print Tides Practice Guide

Use SF Tides for your calculations. This is a traditional set of tables, much like the ones you'll see on the test. The link to Tide Tables above is a bit more modern but leaves out some data on current that you need to know.

We're going to keep this pretty straight forward and simple enough for us English majors. For anyone who wants the details - or wants to design their own world replete with tides - I commend you to Bowditch Chapter 8

For us English majors it boils down to Up/Down; Back and Forth. And in this part of the world, it doesn't do any of that very spectacularly.

But you don't have to cruise very far to find tides that will leave you on the bottom or tidal currents that flow faster than our boats usually sail.

How do you know what you're in for tide-wise?

You gotta ask and/or do some research: On line tide tables are the easiest, but beware that many are unofficial and have lengthy disclaimers in their headers. There are also "official" sites on line, but they are set up only for major ports and - oddly, also have disclaimers that they're not fully augmented, vetted etc. Finally there are tide tables, official and un, that provide a compact way of transporting the data. They take some practice, which you will get since that is what the ASA test tests.

Frequency is semi-diurnal (two highs, two lows each day) in most parts of the world. One exception close to home is Seattle. It enjoys a "mixed tide." Bowditch explains on p. 146.

The greatest tidal range is during so-called Spring Tides, which have nothing to do with the season, but refers to the linear alignment of sun and moon. That occurs twice a month: new moon and full moon, when tidal range (difference between high and low) can be 20% above the mean.

Tidal current is the horizontal movement of water caused by gravitational interaction between the sun, moon, and earth. Tidal currents are a part of the vertical rise and fall of the sea which we refer to as tide. But it is not entirely correct to refer to ebb tide or flood tide. Without being too much of a dilettante, it would be more accurate to say Flood Current and Ebb Current.

We may also come across non-tidal current, which include river currents, ocean currents and wind-driven currents.

Set: the direction toward which a tidal current flows.

When we talk about a south set, we're talking about water moving south. This is opposite of how we talk about a south wind, which comes out of the south and carries us north.

Drift: the speed of a tidal current, which is normally expressed in knots and measured to the nearest 10th of a knot in most tide tables. Probably for the same reason that harbor speed limits are in mph, so, too, are River currents measured.

Ebb: tidal current moving away from land or down a tidal stream.

Flood: tidal current moving toward land or up a tidal stream.

Slack, or Slack Water: the state of a tidal current when its speed is near zero, prior to reversing direction. The term is also applied to the entire period of low speed prior to and after the turning of the current when it is too weak to be of any practical importance in navigation. Not to be confused with stand of the tide.

In ports that experience significant current, slack water is when we try to tie up or get underway. I have a personally embarrassing story concerning Raccoon Straits in SF Bay, slack tide, and mooring.

Stand of the tide: the point when vertical movement stops at both high and low tide. It is not slack water. In fact, the current is usually pretty sporty at the stand of the tide.

Tidal current can be 4 kts or more (as fast as we are in our Hunters most days)

Tidal current is strongest in constricted channels (Golden Gate, Morro Bay Marina)

Where currents collide, expect turbulence (potato patch): races and rips Ð swirling choppy water.

Every "Potato Patch" I know such as the one between Santa Cruz Island and Santa Rosa and the one just outside the Golden Gate are just such areas. The shallow water over SF's potato patch shoal just exacerbates the problem.

You can see less intense versions of conflicting currents off Anacapa's Arch Rock.

When waves oppose tide, expect bigger waves- a current of 1kt can double size of oncoming waves. Can also set up standing waves. I have another story about this.

Lunar day is 24.8 hours, tides every 6 hrs and 12 minutes, give or take.

Wind and barometric pressure have some effect on tidal movement. In fact, wind can change the whole picture.

Before we head into this morass, I'll confess I mostly use unofficial sites on the web for my tidal information. For instance, when I head to Santa Cruz I check. Prisoners Harbor

That said, click on link to right. SF Tide

Select this year's San Francisco Bay tides

The chart shows tides for the Golden Gate.


Q: What time is the second low tide on 23 Feb?

Q: How low is it?

Q: What time is the second low tide at the Oakland pier? Instructions follow.


On the left side of the page, select Tidal Differences for the Golden Gate.

Look down the chart to San Francisco Bay, south

See Oakland pier? The time difference for low tides is +048. So add 48 minutes to the time you got at the Golden Gate.

In that same chart, find the tide height difference....it's +.2 ft. For high tides, 0 for low. So the height of the low tide is the same here as at the Golden Gate.

In ascertaining water depth we had two charts - a reference point chart, which told us what the time and depth of water were at high and low tide AND a differential chart which allowed us to compute the values at various other points in the Bay.

Same deal with Currents.

Click back to the Main Tide Page


Select this years Golden Gate Current table

Note the headers: Slack Max Curr Slack Max Curr Slack Max Curr Slack Max Curr Slack


Q: On 23 Feb when is slack water nearest noon?

Q: What is the time and speed of the friskiest Flood Tide?

Now select Current Differences for the Golden Gate

Find TC 309 Oakland Harbor, High St Bridge

Calculate the time of maximum flood at its velocity.

Note the time calculation is the same as the tide - just add a + or subtract a minus.


The current is presented as a ratio. If the ratio is 0.4 and the current is 5 knots at the Gate, then the current at the selected spot is 0.4X5=2.0

You might have noticed that the time zone is at the top of the page. Pretty convenient. But the test uses a table so old that you have to calculate that for yourself. You will screw up all of the tide problems if you are not aware whether you are in daylight savings or standard time.

OK, I'll give you an even better hint - they'll ask for a calculation in the summer. You're on PDT, but the chart isn't. To convert from PST to PDT add 1 hour.

If you forget to check the tides on your way out and you have a modern GPS/chartplotter aboard, you'll probably be able to find the info you need right there. Slew your cursor to a major port near your destination. Zoom in. If you find a "T" put the cursor on it and click. That's a tide station and all will be revealed. One caveat: make sure the system has the time and date set accurately.