Tide and Current Question

[Cmdr Pete]

I wonder if it's possible to predict what the current will do in a body of water based on the time of current change and the times of high/low water.

Suppose there was a Bay like the one below.

You'll notice the current is slack/beginning to flood at the Inlet at 11:00, two hours after low water at the Inlet.

Could you expect the same to happen at various points inside the Bay two hours after low water there?

I realize that there are many geographic and other variables that will affect the current. I'm just trying to grossly oversimplify things to start with.

Assume the time of max ebb and flood is 3 hours after they start at the Inlet, if it matters.

What do you think the current would be doing at 2:00 at Near Bay, Mid Bay and Far Bay?

3:00?

[Delezynski]

Pete,

Take a look at: http://en.wikipedia.org/wiki/Rule_of_twelfths for the rule of 12th. In regard to tide. Then you would need to know a lot about the size of the body of water, that will give you the amount and sped it must move in and out. Way more math than I want to do. MOST chart plotters give you some tide / current data for selected places.

You can look at: http://www.mobilegeographics.com:81/locations/5545.html
for a chart that shows the relationship for the San Francisco Bay area. When we were sailing there we learned FAST that just because it was high tide it was NOT slack current. Or the same for low tide. They follow each other and it's much easier to get a set of tables, or a program for your netbook. Like the free OpenCPN that has the current built in.

Hope this helps! or did I miss the question?

Greg

[Tim]

Pete, I have been hesitant to say anything as the basis is entirely intuitive with no scientific or mathematical backing.

I even went through all my current atlases to find a similar example but nothing of course is that simple.

My first suggestion is that it would mostly depend on geographic features, so in your drawing currents would increase nearer the mouth at any point during tidal change. Once it hit midpoint currents would ease as water would move to the center as well as the mouth.

So the simple off the top of the head guess is that near bay current would be twice as strong as mid or far. The degree of current would depend on size of bay and tidal differential.

For a more complex explanation I suggest you pose it to the Ariel forum and let you know who have a crack at it 

 

[Cmdr Pete]

Thanks guys

I guess I'm asking because I often trailer my Melonseed to new and unfamiliar Bays.  I have nothing to go on to predict the current but a chart and tide tables.

If I time the currents right, I can ride a fair current out and back again. Otherwise, a pleasant sail can turn into a long and difficult slog back to the launch ramp.

They have tidal current diagrams for a few areas, like New York Harbor, where they show direction and speed of the current for each hour after high/low tide. I'm trying to see if I can make an educated guess about other areas where there are no current diagrams.

Then I can factor in some other variables like where the Bay is narrow, where it's deep, where there might be eddys etc.

Does the OpenCPN show current direction and speed, with arrows or something?

Thanks again

 

[newt]

Pete, there are calculations for all sorts of water. I use tables in the San Juans that were figured out along time ago, and just has a little update each year to use the charts for the current year. When were sailing I pull it out often, it is the most important tool I use next to the paper chart the the current forcast.

[Delezynski]

Pete,

Yes, OpenCPN shows the current (in certain areas) with arrows. The size of the arrow denotes the speed of the current.

Another thing to take into consideration is ?where? the bay is!

When we departed the SF Bay, we made sure to stage at the opening so we could catch the outgoing tide. Then, months later we were about 5 miles out of San Diego bay (almost as big as SF, and I went into panic mode as I had not looked up the currents for the entrance. Once I did, I found out that there was almost NONE!

I'd like to say I planned it that way  BUT....

Greg