Excellent Wally! Go to the head of the class.


"Wally" wrote in message ...
Simple Simon wrote:
Since Booby is too ignorant to answer the following question,
I'll throw it out to the group as a whole.

What is the rule of 12ths and how would you use it?

In spite of being a sweet, innocent, fresh-faced n00b, I'll have a go at
this...

It's to do with the rate at which the tide goes out or comes in. The key
concept is that the rate is not linear and the purpose of the rule of 12ths
is to allow the sailor to estimate the tidal rate at a given time during the
cycle from HW to LW and back to HW. By knowing the tidal rate at a given
time, and the tidal range for the period concerned, one can estimate the
depth of the water above chart datum.

Lets take an ebb tide as an example. Round these 'ere parts, HW to LW takes
roughly 6 hours and, during springs, the tidal range isn't a kick in the ass
off of 6 metres. We can use the rule of 12ths to estimate the height of the
tide at a given time after HW by referring to the following ready
reckoner...

1 hour after HW - tide has dropped 1/12 of the range.
2 hours after HW - tide has dropped 2/12 of the range.
3 hours after HW - tide has dropped 3/12 of the range.
4 hours after HW - tide has dropped 3/12 of the range.
5 hours after HW - tide has dropped 2/12 of the range.
6 hours after HW - tide has dropped 1/12 of the range.

Add up all of the 12ths, and we get 12/12ths - the full tidal range,
bringing us neatly to LW six hours after high water.

From the above, we can then apply the fractions to the tidal range and
arrive at a fairly good estimate of the height of the tide at a given time.
For my spring tidal range of 6m at, say, 2 hours after HW, I would calculate
the drop from HW - 3/12 - and multiply the tidal range by this fraction...

6 x 3/12 = 1.5

This tells me that the tide will have dropped 1.5m. Subtracting that from
the tidal range of 6m, I find that the height of the tide above chart datum
is 4.5m.


--
Wally
www.makearatherlonglinkthattakesyounowhere.com
Things are always clearer in the cold, post-upload light.