A question about calculating tides

[mcchaddy]

Hi All,

I am heading to inskip for a week this weekend and have a question about tides in the area.

I bought a Great Sandy Strait(South) boating chart today and it displays all depth soundings at Lowest astronomical tide(LAT). If a particular area shows a depth of 6mtrs and the high tide for that day will be 2.11mtrs. Does that mean the actual depth in this spot will be 8.11mtrs at high tide.

The reason I ask is that I want to get into a particular little creek/cutter to drop some pots. The entrance depth is only about .5mtr at LAT, does this it will be 2.61mtrs at high tide

Or have I got this completely wrong

Cheers

Michael

[aussiefool]

Try this web site,
http://tide-times.com.au/state_QLD.html

may help
Aussiefool
ANdrew

[tinman42]

Tide heights can be estimated using the rule of twelfths. Tides change every 6 hours. Assuming for the purpose of this exercise the high tide is 1.8mtrs and low tide is .6mtrs the tidal movement for this tide is 1.2mtrs.

In the first hour 1/12th of the total tidal movement will occur (.1mtrs), in the second hour after the change 2/12th of the total tide movement will occur (.2mtrs), in the third hour 3/12th will occur, in the 4th hour 3/12th will occur, in the 5th hour 2/12th and in the 6th hour 1/12th of the tidal movement will occur.

Look at the charts of the area, observe the LAT (lowest astronomical tide) measurement, add the height of the high tide and apply the rule of 12ths, and you will have a fairly accurate water depth indicator for an area at a given time.

Pretty complicated but I hope it helps.

cheers

Bob

[dinky]

very handy info tinman42.cheers Dean

[mcchaddy]

Hi Tinman,

I will try your method this weekend. Will let you know how I go.

Cheers

Michael.

[madman1]

Shouldn't the variance between high and low be divided by 6 as there is generally 6 hours between high and low?

[tinman42]

Shouldn't the variance between high and low be divided by 6 as there is generally 6 hours between high and low?

You would think so, however the amount of water moves by 1/12th of the total water to be moved (difference between high and low water) in the first hour, then 2/12ths in the second hour, then 3/12ths, then 3/12ths again, then 2/12ths, and finally in the 6th hour the last 1/12th.
It always thought the tide or movement of water was equally divided over the 6 hours or so between tide changes. However as you can see the tidal flow starts slowly, building up till midway in the tide, then begins top slow before stopping (at the turn of the tide) and the process starts again.