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#1
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Since Booby can't answer the question
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? S.Simon - a Captain who's serious about sailing |
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#2
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Since Booby can't answer the question
The rule of 64ths is used in finance.
"Simple Simon" wrote in message ... 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? S.Simon - a Captain who's serious about sailing |
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#3
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Since Booby can't answer the question
Too easy - but I've wondered if there's a formula for approximated current over time. I
think there's too many variables. 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? S.Simon - a Captain who's serious about sailing |
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#4
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Since Booby can't answer the question
ratio of tidal rise over time.
Rule of Twelfths Shape assumed to be sinusoidal Rise in 1/12ths 1 in first hour 2 in second 3 in third 3 in fourth 2 in fifth 1 in sixth sometimes used as a rule of thumb for tidal current as well On Tue, 2 Sep 2003 11:21:19 -0400, "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? S.Simon - a Captain who's serious about sailing |
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#5
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Since Booby can't answer the question
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. |
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#6
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Since Booby can't answer the question
And, it's only useful in areas with a semi-diurnal tide, right?
"Marc" wrote in message ... ratio of tidal rise over time. Rule of Twelfths Shape assumed to be sinusoidal Rise in 1/12ths 1 in first hour 2 in second 3 in third 3 in fourth 2 in fifth 1 in sixth sometimes used as a rule of thumb for tidal current as well On Tue, 2 Sep 2003 11:21:19 -0400, "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? S.Simon - a Captain who's serious about sailing |
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#7
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Since Booby can't answer the question
Wally, you trying for a world record MEGO factor?
On Tue, 02 Sep 2003 15:43:50 GMT, "Wally" wrote: 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. |
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#8
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Since Booby can't answer the question
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. |
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#9
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Since Booby can't answer the question
Simple Simon wrote:
Excellent Wally! Go to the head of the class. trots to top of class, wearing smug grin You'd think that Bob, with his 8 years experience, would have been able to come up with at least half an answer... -- Wally www.makearatherlonglinkthattakesyounowhere.com Things are always clearer in the cold, post-upload light. |
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#10
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Since Booby can't answer the question
It doesn't work too well for currents. In places like the Cape Cod Canal, Hell Gate, and
Wood's Hole the current ramps up to over half strength in the first hour. Marc wrote: ratio of tidal rise over time. Rule of Twelfths Shape assumed to be sinusoidal Rise in 1/12ths 1 in first hour 2 in second 3 in third 3 in fourth 2 in fifth 1 in sixth sometimes used as a rule of thumb for tidal current as well On Tue, 2 Sep 2003 11:21:19 -0400, "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? S.Simon - a Captain who's serious about sailing |
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