I have a few questions about tides & currents, especially why there is such a difference in time between high or low tide and slack current. For instance: The tide book shows that tomorrow, Sat., 3/22, at the Golden Gate Entrance to the Bay there will be a low tide at 0638, and a slack at 0848 - more than two full hours after the high. What I can't figure out is why there is this difference. It would seem to me that the point at which the tide gets as high as it's going to get would also be when the flood tide starts to stop; if it continued, then why wouldn't the water get higher? In other words, why is the slack not around the same time as the high? Another question: My boat is docked at a marina where the tide changes about 25 minutes after it does at the Golden Gate. Does this mean that the slack (as well as the max current) is also 25 minutes later? My reason for asking, obviously, is I'm trying to figure out when it's best for me to leave my marina in relation to the tide and the current (aside from simple curiosity).
Tides & currents
- Thread starter John
- Start date
- Status
Inlets make a huge difference .....
Inlets and narrow 'entrances' to bays, etc. delay the tides and tidal flow by creating a 'restriction' for the ocean driven tide to pass through. The tide has to rise well above the normal inlet 'mean' to give flow to the water into and behind the inlet ... hence the delay in tide 'behind' the inlet. Ditto the reverse, when the tide is on its way out through the inlet, It will back-up and delay due to the 'restriction' of the inlet.
Inlets and narrow 'entrances' to bays, etc. delay the tides and tidal flow by creating a 'restriction' for the ocean driven tide to pass through. The tide has to rise well above the normal inlet 'mean' to give flow to the water into and behind the inlet ... hence the delay in tide 'behind' the inlet. Ditto the reverse, when the tide is on its way out through the inlet, It will back-up and delay due to the 'restriction' of the inlet.
I'll give it a try. Slack and high are
probably the same at the end of a body of water, but not the same at any point along the way. On a flood tide, the current keeps going past any given point until the area that the water flows into, gets to a height where the energy that pushed it from the sun and the moons gravity is dissipated (like a wave sloshing on a beach). At that point it starts to go the other way. The point under the Golden Gate sees the water continue to pass but doesn't get any higher because the volume of water passing that point isn't staying there. It's the reverse on the Ebb. Hope that helps and doesn't confuse.
probably the same at the end of a body of water, but not the same at any point along the way. On a flood tide, the current keeps going past any given point until the area that the water flows into, gets to a height where the energy that pushed it from the sun and the moons gravity is dissipated (like a wave sloshing on a beach). At that point it starts to go the other way. The point under the Golden Gate sees the water continue to pass but doesn't get any higher because the volume of water passing that point isn't staying there. It's the reverse on the Ebb. Hope that helps and doesn't confuse.
John , I wrote a long explanation that Father Phil's
server lost in cyber space. So you will have to make do the the other answers. Suffice to say you can't fill San Francisco Bay as quickly as you can fill the golden gate. That is the reason you get high tide before slack water at the Gate.
server lost in cyber space. So you will have to make do the the other answers. Suffice to say you can't fill San Francisco Bay as quickly as you can fill the golden gate. That is the reason you get high tide before slack water at the Gate.
and inside the Bay?
So is there the same time difference between high (or low) and slack inside the Bay - say where high and low are about a half hour later than at the Gate?
So is there the same time difference between high (or low) and slack inside the Bay - say where high and low are about a half hour later than at the Gate?
Not necessarily! There will be some point in the bay where high water and slack
happen at the same time. On this end of the country in my cruising waters maximum ebb occurs 10 miles down river when high tide happens at the tide line at Port Deposit. As a result I try to sail about an hour before high tide at Perryville in order to catch the ebb at Spesutia Island. That is about six miles. There is not much current at Perryville but at Specutia Island the ebb runs close to 2 knots.
happen at the same time. On this end of the country in my cruising waters maximum ebb occurs 10 miles down river when high tide happens at the tide line at Port Deposit. As a result I try to sail about an hour before high tide at Perryville in order to catch the ebb at Spesutia Island. That is about six miles. There is not much current at Perryville but at Specutia Island the ebb runs close to 2 knots.
SF Bay Graphical Current and Tide Website
John: I don't know enough of the science to answer your question, but if you haven't found this site (or another that is similar) take a look: http://www.sailwx.info/tides/tidemap.phtml?location=5654 On the web page, scroll down below the map of SF Bay and you will see a list of locations for both tide and currents within SF Bay. Click on one you want and up pops a graph of time vs current or time vs tide. My location is Sausalito. Somedays, I will print out the current graphs for (say) GG Bridge, Racoon Straight, and Alcatraz. The info helps me plan the best time to go out and on which route I'm likely to have neutral or following currents for most of the trip if possible. I find the graphical presentation much easier to look at than figuring out from the tide/current book.
John: I don't know enough of the science to answer your question, but if you haven't found this site (or another that is similar) take a look: http://www.sailwx.info/tides/tidemap.phtml?location=5654 On the web page, scroll down below the map of SF Bay and you will see a list of locations for both tide and currents within SF Bay. Click on one you want and up pops a graph of time vs current or time vs tide. My location is Sausalito. Somedays, I will print out the current graphs for (say) GG Bridge, Racoon Straight, and Alcatraz. The info helps me plan the best time to go out and on which route I'm likely to have neutral or following currents for most of the trip if possible. I find the graphical presentation much easier to look at than figuring out from the tide/current book.
Buy Kimball Livingston's book
"Sailing The Bay" - he explains it all. If you sail SF Bay and don't have this book, you should get it imemdiately. It's also a great, fun read.
"Sailing The Bay" - he explains it all. If you sail SF Bay and don't have this book, you should get it imemdiately. It's also a great, fun read.
- Status