Tidal diamonds

[john_morris_uk]

In reply to the original question - the correct answer "by the book" is interpolate.

A couple of comments about theory and accuracy. The theory course requires accuracy because you need to be able to do it accurately before you start the guesstimating games that most of us use most of the time in real life.

There are lots of jobs where even professionals don't use all the theory all the time - but they know how to do it when necessary. A good navigator knows when as accurate as possible is needed and when its not and knows the difference - and still knows all the other factors that help decide what is going on.

I think Searush says it quite well:
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Accurate enough, with minimum effort means you get a quick and simple result that can be easily verified by observations.


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It doesn't mean that you shouldn't know how to do it as accurately as possible when necessary.

 

[Sailfree]

You missed my point.

I appreciate its all approximate but the tidal curves for most ports had a straight line with only small curves for 1st and last 1/2hr.

A straight line is a linear amount of tide not variable as the rule of 1/12s gives with only the 3rd and 4th hour being near linear.

I gave up trying to find a tidal curve that was near the rule of 1/12s.

Are there Ports that obey the rule of 1/12s albeit approx or is this rule so wildly inaccurate that it is only good for suggesting the reason for slack water.

 

[john_morris_uk]

Try doing some tidal height calculations using the rule of 12ths and then using the tidal curve for any port that doesn't have a double high etc. You might be suprised at how close the answers are.

You say at the beginning of your question 'I know its only an approximation', but then go on to find holes in the fact that it is an approximation. I don't understand your reasoning.

The rule of 12ths gives you a quick rough and ready way to estimate the height of tide and it works very well in 99% of places.

What point are you trying to make about linearity of the tidal curve and the 12th's rule? The 12ths rule is linear in the middle two hours because the tidal curve approximates to a straight line for this very period.

 

[DavenHelen]

Have you tried plotting the rule of twelfths on graph paper? Looks good to me.

 

[Sailfree]

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You say at the beginning of your question 'I know its only an approximation', but then go on to find holes in the fact that it is an approximation. I don't understand your reasoning.



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The reasoning is that if the slope is a staight line apart from the 1st & last 1/2 hr it is not an approximation it simply has no relevance as a straight slope is a constant amount of water for almost 5hrs - not a varying amount as indicated by the rule of 1/12s.

Maybe I was unlucky in trying to illustrate the rule of 1/12s that I could only find steep straight tidal curves. Will look at Reeds again more carefully when I am on the boat. I was tending to use CI & French Ports as I was also explaining the tidal coefficient method to calculate currents.

 

[john_morris_uk]

You've still lost me. Tidal curves are approximately a sine wave. The rule of 12ths approximates a sine wave as well (sort of). The rate of rise and fall (or the rate of the current) is at a maximum for the middle two hours of the 6 hour tidal cycle, as indicated on the graph or in the rule of 12ths.

Can you explain how you understand the rule of 12ths is used - perhaps with a rough working?

 

[Sailfree]

If you look at St PP , CI its only a curve for 1/2hr then last hour and its linear for 4.5hrs - so no rule of 1/12s there!

I used the high tide areas as I wanted to go on to explain cill heights with a drastic change so calcs are important but perhaps other areas are more like a true sine wave than a steep straight line for almost 5hrs.

 

[john_morris_uk]

St PP is an interesting case. Firstly, I wonder why you would consider using the rule of 12ths for something that needs to be as accurate as calculating the depth over a cill. I agree that because of its situation in the Little Russell Channel, the tide is not as sinusoidal as other ports. A hydrogropher tried to explain it to me in detail, but I lost the will to live after the first ten minutes!

Secondly I have delayed replying as I wanted to do some comparison figures to see how far out the 12th rule would be in St PP and I haven't had access to an almanac for a few days.

I will try and find one in the next day or two and I will do some calcs and post the results.

 

[Sailfree]

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St PP is an interesting case. Firstly, I wonder why you would consider using the rule of 12ths for something that needs to be as accurate as calculating the depth over a cill.

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I wasn't. I was explaining tides and currents to someone that was interested in studying for his yachtmaster and just found that each curve I picked (and yes they were extreme ones) they did not correspond very much to the rule of 1/12s. Made me wonder if it was such a generalisation as to be almost worthless.