O.k., here's a question I've been wondering about, related to tides and currents: Take here in the SF Bay. The tide chart shows that the high and low tides at the golden Gate are as follows: Low at 0145, high at 0724, low at 1311 and then a high at 2122. Slacks are as follows: 0400, 0921, 1633 and 2249. So you can see, the slack following the first low is over two full hours later. Same for the rest of the slacks. So, several questions: Am I right that the slack is so much later than the high or low because while this is the high at the Gate, it takes two full hours for that high (or low) to reach all the way back to the end of the Bay? Therefore, the water keeps running through the Gate, even though it doesn't get any higher at the Gate. Is this the explanation? Also, there's another question: Our boat is docked at a marina where the highs and lows are about a half hour later than at the Gate. But how about the slacks - are they also a half hour later than the slack at the gate (which is about two hours after the high or low there)? What I'm driving at is this: If I want to time my departure by the tides, do I go by the time of the high at the gate and leave a half hour after that high, or do I go by the time of the slack at the Gate and then leave a half hour after the slack that follows the high at the Gate? Is that just as clear as mud?
Tidal changes & slacks
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Where are you going?
Tide is the up and down height of the water which you use when navigating shoal waters, bridges, and anchoring. Current is the horizontal motion of the water; Ebb, Slack, and Flood, which you use to make the best time, avoid tidal boars, and turbulent eddies. You keep mixing the two. As for your first question the answer is yes. The Oceans tidal rise builds up at the gate creating a high and that height begins to diminish as the water backed up at the gate continues into the bay. It happens in reverse when the water in the bay reaches its equilibrium and the ocean tide drops below the level in the bay. If you are worried about clearing sand bars use the tide tables. If you are trying to make time or play eddies, use the current tables and ignore the tide tables. Joe S
Tide is the up and down height of the water which you use when navigating shoal waters, bridges, and anchoring. Current is the horizontal motion of the water; Ebb, Slack, and Flood, which you use to make the best time, avoid tidal boars, and turbulent eddies. You keep mixing the two. As for your first question the answer is yes. The Oceans tidal rise builds up at the gate creating a high and that height begins to diminish as the water backed up at the gate continues into the bay. It happens in reverse when the water in the bay reaches its equilibrium and the ocean tide drops below the level in the bay. If you are worried about clearing sand bars use the tide tables. If you are trying to make time or play eddies, use the current tables and ignore the tide tables. Joe S
John, read Kimball Livingston's book
"Sailing the Bay," it'll explain all of that, and he takes a whole chapter to do it, so I won't copy it here. Also explains the tide table as well as south bay and north bay affects on the cycles you ask about. Short answer, leave at high water from Fortman which is shown on the tidal differences and current differences in the front of the tide book. Park Street Bridge and various substations in Oakland Inner Harbor are also shown.
"Sailing the Bay," it'll explain all of that, and he takes a whole chapter to do it, so I won't copy it here. Also explains the tide table as well as south bay and north bay affects on the cycles you ask about. Short answer, leave at high water from Fortman which is shown on the tidal differences and current differences in the front of the tide book. Park Street Bridge and various substations in Oakland Inner Harbor are also shown.
high water vs. slack
Thanks Stu. What I am gathering from what you write is that in deciding when to leave, I go by the tide vs. the current. If, for instance, high water is at 1134 and slack (when the current stops going in) is at 1313, and as you say where I am the high water is a half hour later, then ideally I would leave at 1204 (half hour after high water at the Gate), not at or after 1343 (half hour after slack at the gate).
Thanks Stu. What I am gathering from what you write is that in deciding when to leave, I go by the tide vs. the current. If, for instance, high water is at 1134 and slack (when the current stops going in) is at 1313, and as you say where I am the high water is a half hour later, then ideally I would leave at 1204 (half hour after high water at the Gate), not at or after 1343 (half hour after slack at the gate).
Tides and Currents
John, it appears that you are confusing the two. If I'm reading your first post correctly, you mention tides and then slacks. Slacks are current related only. What you need to do, in addition to reading the suggested book by Livingston (he calls the Tide & Current booklet "The Most Misunderstood Book in Town") and learn how to read the tides & currents book, is to look at pages 6 to 8 for tidal corrections and pages 9 to 11 for current differences from the Golden Gate in your Tide & Current Tables little book. Tides: page 6 shows the tidal difference at Park St. Bridge as +0.38 for high water and +0.44 for low water with respective height differences. That means that the times of high and low water are later (the + sign) than at the Gate, and differences in elevations are as stated. Currents: page 9 shows the High St. Bridge current differences as -1.36 flood and -1.42 ebb, plus the low and high slacks with different speed ratios for the floods and ebbs. So the currents start EARLIER than at the Gate. What this means is that the times of high and low water (tide heights) are LATER than the times shown in the book for daily occurrences for those listed at the Gate, AND that the currents START earlier. Part of this apparent anomaly is because Alameda is an island and the water pushing up from the south bay on an ebb is going around the island. You really need to learn and study the tables and check some observations. For instance, I regularly go through the four bridges, what I call "going out the back way" and have learned to read the book and see what is happening. You don't have to go through the bridges, either, just motor down to Park Street one day, and make some observations compared to the book. Do it once in the morning and once 6 hours later and you'll see. There is also the Bay Model in Sausalito, worth the trip to learn how it all works around here.
John, it appears that you are confusing the two. If I'm reading your first post correctly, you mention tides and then slacks. Slacks are current related only. What you need to do, in addition to reading the suggested book by Livingston (he calls the Tide & Current booklet "The Most Misunderstood Book in Town") and learn how to read the tides & currents book, is to look at pages 6 to 8 for tidal corrections and pages 9 to 11 for current differences from the Golden Gate in your Tide & Current Tables little book. Tides: page 6 shows the tidal difference at Park St. Bridge as +0.38 for high water and +0.44 for low water with respective height differences. That means that the times of high and low water are later (the + sign) than at the Gate, and differences in elevations are as stated. Currents: page 9 shows the High St. Bridge current differences as -1.36 flood and -1.42 ebb, plus the low and high slacks with different speed ratios for the floods and ebbs. So the currents start EARLIER than at the Gate. What this means is that the times of high and low water (tide heights) are LATER than the times shown in the book for daily occurrences for those listed at the Gate, AND that the currents START earlier. Part of this apparent anomaly is because Alameda is an island and the water pushing up from the south bay on an ebb is going around the island. You really need to learn and study the tables and check some observations. For instance, I regularly go through the four bridges, what I call "going out the back way" and have learned to read the book and see what is happening. You don't have to go through the bridges, either, just motor down to Park Street one day, and make some observations compared to the book. Do it once in the morning and once 6 hours later and you'll see. There is also the Bay Model in Sausalito, worth the trip to learn how it all works around here.
I am sure the current charts exist for SF bay just as they do
for Chesapeake Bay. Mine for the Chesapeake contain a page for each hour of the tide cycle with the title on each page reading slack High water at Baltimore harbor. the next page with be marked one hour after ebb starts at Baltimore Harbor, 2 Hours, 3 hours etc until you get to 1 hour before high tide at Baltimore Harbor. If you can find such a chart for SF Bay it will enhance your education.
for Chesapeake Bay. Mine for the Chesapeake contain a page for each hour of the tide cycle with the title on each page reading slack High water at Baltimore harbor. the next page with be marked one hour after ebb starts at Baltimore Harbor, 2 Hours, 3 hours etc until you get to 1 hour before high tide at Baltimore Harbor. If you can find such a chart for SF Bay it will enhance your education.
yes...
yes, I understand what you are saying, Stu, about the difference between the tide (vertical motion of the water) and the current (horizontal motion). What I'm asking is this: Assume the book shows that my departure point shows high and low tides to be a half hour later than it is at the gate. Does this mean that the current change (including the slack current) is a half hour later than the current change at the gate also? Clearly, when the tide is high at the Gate, the current is continuing to run through the gate on a flood (i.e., entering into the Bay) for about two more hours. This is why the slack current is about two hours after the high (or low) tide. So, does the current continue to run all the way down the bay for two more hours after the high at the Gate also? If so, then if the high at my marina is at 0754, for instance, then the flood current would continue running until about 0954, after which time it would start to reverse. In between the flood and the ebb current would be the slack, and I should time my departure for this slack or later. If the slack current at my marina comes immediately after the high tide at the marina, then it would be about two hours earlier (O954).
yes, I understand what you are saying, Stu, about the difference between the tide (vertical motion of the water) and the current (horizontal motion). What I'm asking is this: Assume the book shows that my departure point shows high and low tides to be a half hour later than it is at the gate. Does this mean that the current change (including the slack current) is a half hour later than the current change at the gate also? Clearly, when the tide is high at the Gate, the current is continuing to run through the gate on a flood (i.e., entering into the Bay) for about two more hours. This is why the slack current is about two hours after the high (or low) tide. So, does the current continue to run all the way down the bay for two more hours after the high at the Gate also? If so, then if the high at my marina is at 0754, for instance, then the flood current would continue running until about 0954, after which time it would start to reverse. In between the flood and the ebb current would be the slack, and I should time my departure for this slack or later. If the slack current at my marina comes immediately after the high tide at the marina, then it would be about two hours earlier (O954).
NO
U wrote: "Does this mean that the current change (including the slack current) is a half hour later than the current change at the gate also?" No, no, no. While you're right about the differences between tides and currents, John, you seem to be missing the whole point. Each of the odd and even pages in the book are one for tide and one for current. Read page 2 and then the pages 6 to 11 I mentioned in my last post. Each (the tide and the current) has a different correction time and a different point in time at the Gate from which to apply the corrections. At your marina (Park St and High St bridges) the tides have a half hour time difference, the currents have a one and a half hour difference, but each is from the time of the tide and current from the Gate on the odd and even pages of the book for the daily occurrences. The time of highest tide and max currents at the Gate are different, too, that's why there are two different pages for each day: tides on the left page, currents on the right page, for each month.
U wrote: "Does this mean that the current change (including the slack current) is a half hour later than the current change at the gate also?" No, no, no. While you're right about the differences between tides and currents, John, you seem to be missing the whole point. Each of the odd and even pages in the book are one for tide and one for current. Read page 2 and then the pages 6 to 11 I mentioned in my last post. Each (the tide and the current) has a different correction time and a different point in time at the Gate from which to apply the corrections. At your marina (Park St and High St bridges) the tides have a half hour time difference, the currents have a one and a half hour difference, but each is from the time of the tide and current from the Gate on the odd and even pages of the book for the daily occurrences. The time of highest tide and max currents at the Gate are different, too, that's why there are two different pages for each day: tides on the left page, currents on the right page, for each month.
John, Stop talking about tides
You keep trying to relate times of current to times of tides. Tides and currents are only incidentally related as the rise in tide causes the push that causes the currents. You should be concerned with tides only if you want to know you anchored in enough water when the tide goes out, or if you want to go over a bar that might be too low at some tide. To make transits you should be concerned only with currents. Use the current tables and forget the tide tables. Just completely forget about the time of the tides at the gate. Please! Joe S
You keep trying to relate times of current to times of tides. Tides and currents are only incidentally related as the rise in tide causes the push that causes the currents. You should be concerned with tides only if you want to know you anchored in enough water when the tide goes out, or if you want to go over a bar that might be too low at some tide. To make transits you should be concerned only with currents. Use the current tables and forget the tide tables. Just completely forget about the time of the tides at the gate. Please! Joe S
John, maybe an easier way for you to come to
an understanding of this is to separate the tides & currents in your mind, and do the corrections of tide height first, then current differences separately. Once you do a few of those exercises, then you can begin to see a pattern between them, but don't start out doing that. The key, it seems, from your posts and Joseph's replies, are that you are trying to analyze the two at the same time. Also, it is very important to know that the two opposing pages of tide and current on the monthly pages start with different times, for example, for high tide and max currents. Stop at West Marine and buy Kimball's book. It's all in there in great detail. Heck, you could sit in West Marine in the Alameda store and read that chapter...
an understanding of this is to separate the tides & currents in your mind, and do the corrections of tide height first, then current differences separately. Once you do a few of those exercises, then you can begin to see a pattern between them, but don't start out doing that. The key, it seems, from your posts and Joseph's replies, are that you are trying to analyze the two at the same time. Also, it is very important to know that the two opposing pages of tide and current on the monthly pages start with different times, for example, for high tide and max currents. Stop at West Marine and buy Kimball's book. It's all in there in great detail. Heck, you could sit in West Marine in the Alameda store and read that chapter...
tide booklet
Just get a free tide booklet at WM. The last few pages show currents one and two hours before and/or after the tide for the entire bay through the Carquinez Straight. Great information if you want to get washed up the river vs. motoring in place. No rule of thumb as each geographic location will have it's own particulars.
Just get a free tide booklet at WM. The last few pages show currents one and two hours before and/or after the tide for the entire bay through the Carquinez Straight. Great information if you want to get washed up the river vs. motoring in place. No rule of thumb as each geographic location will have it's own particulars.
I used this site when in San Francisco..... then I just went sailing
Raccoon Straight I never could predict. http://www.deltanine.com/SSS%20Races.html paulj
Raccoon Straight I never could predict. http://www.deltanine.com/SSS%20Races.html paulj
Stu, you are right...
I was thinking of tide and current as being related in the sense that they go together. In a sense, I guess you could say that the tide causes the current, in that higher water at the Gate causes a current to flow through the gate into the area where the water is lower, and the reverse for lower waters (low tide). But they really are two separate things, as you point out. Also, I never really looked at the tide booklet closely enough. (It's the American in me - we never read the instructions; we just get going.) So, thanks for suggesting I read the first few pages of the tide book more closely. However, as they say, no helpful deed ever goes unpunished, so here goes: A closer reading of the booklet reveals that "high slack" (booklet's term) is 1 hour and a quarter earlier at the Webster St. tunnel than it is at the Gate. (I assume this means the slack that follows a flood current.) This is counter intuitive. One would think that the reversal of the currents at this location would come AFTER the current reverses at the Gate.
I was thinking of tide and current as being related in the sense that they go together. In a sense, I guess you could say that the tide causes the current, in that higher water at the Gate causes a current to flow through the gate into the area where the water is lower, and the reverse for lower waters (low tide). But they really are two separate things, as you point out. Also, I never really looked at the tide booklet closely enough. (It's the American in me - we never read the instructions; we just get going.) So, thanks for suggesting I read the first few pages of the tide book more closely. However, as they say, no helpful deed ever goes unpunished, so here goes: A closer reading of the booklet reveals that "high slack" (booklet's term) is 1 hour and a quarter earlier at the Webster St. tunnel than it is at the Gate. (I assume this means the slack that follows a flood current.) This is counter intuitive. One would think that the reversal of the currents at this location would come AFTER the current reverses at the Gate.
counter intuitive?
At the risk of repeating myself, John, read Kimball's book. Basically, the south bay ebbs and floods before the north bay, and the water going around Alameda (island) does some very strange things. If you look at all of the rest of the current information for Webster Street, you'll note that they are ALL earlier, around an hour and a half. Mick, I disagree, there's a reason for everything, and in this case there are good, documented reasons, and are explained fully in the reference book I've been suggesting this morning.
At the risk of repeating myself, John, read Kimball's book. Basically, the south bay ebbs and floods before the north bay, and the water going around Alameda (island) does some very strange things. If you look at all of the rest of the current information for Webster Street, you'll note that they are ALL earlier, around an hour and a half. Mick, I disagree, there's a reason for everything, and in this case there are good, documented reasons, and are explained fully in the reference book I've been suggesting this morning.
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