Lunitidal Intervals

[nickbam]

I've just bought this fancy new Casio watch which does everything except steer the boat and make the tea, but which requires me to enter the 'lunitidal interval' for the port I want to set as my home port.
I sail out of the Exe and the nearest town the handbook offers is London which gives entirely the wrong tidal readings....
Apparently the lunitidal interval is the time between the moon crossing ther meridian and High Tide, so I tried calculating the halfway point betwen a few moonrises and sets in the almanac and comparing that to the high tide times for the same dates - that didn't work either - the figure is not consistent...
Any suggestions as to how I find, or work out the lunitidal intervals for the Exe - and indeed any other ports aound the channel I might sail in and out of....?
All suggestions gratefully received....
Nick Bamford

 

[Guest]

Are you referring to High Tide Greenwich or Exeter?

This may help

"lunitidal interval
The interval between the Moon's transit (upper or lower) over the local or Greenwich meridian and the following high or low water. The average of all high water intervals for all phases of the Moon is known as mean high water lunitidal interval and is abbreviated to high water interval (HWI). Similarly, mean low water lunitidal interval is abbreviated to low water interval (LWI). The interval is described as local or Greenwich according to whether the reference is to the transit over the local or Greenwich meridian. When not otherwise specified, the reference is assumed to be local. When there is considerable diurnal inequality in the tide, separate intervals may be obtained for the higher high waters, lower high waters, higher low waters, and lower low waters. These are designated respectively as higher high water interval (HHWI), lower high water interval (LHWI), higher low water interval (HLWI), and lower low water interval (LLWI). In such cases, and also when the tide is diurnal, it is necessary to distinguish between the upper and lower transit of the Moon with reference to its declination. Intervals referred to the Moon's upper transit at the time of its north declination or the lower transit at the time of south declination are marked a. Intervals referred to the Moon's lower transit at the time of its north declination or to the upper transit at the time of south declination are marked "

 

[DanTribe]

A "Looney Toonal" watch? This sounds like a wind up to me.
Dan

 

[halcyon]

Could be, but I have a tide clock in the study, it shows whether the tide is rising or falling, and the state.
It's realy a Lunar clock i.e. 13 months to a year, as the tide follows the moon, and the clock follows the moon, etc.

To set it you wait till the max spring tide local, at the time stated in local tide table for high tide you set the clock to read 12 o'clock, from the on it approximatly follows the tides.


Brian

 

[HaraldS]

You can get the lunitidal interval if you have the harmonics for the port you are interested in. Using just this, will always give you an approximation for the time of high tide or low tide, as it leaves out the effect of the sun. It works as good or as bad as a tidal clock.

The harmonic analysis describes the water height f at the time t as follows:

f(t) = Z0 + M2 * cos (28.9841°/h * t - m2) + S2 * cos (30°/h * t - s2) + R

The second part of the equation is the effect of the sun, the part that relates to the moon is: M2 * cos (28.9841°/h * t - m2). The BIG M2 describes the amplitude effect of the moon, the small m2 the phase (lag) relative to the moon. It is the thing you need to find for your port, then you can use the formula:

I = m2/28.9841°/h - Z + L/15°/h to get the lunitidal interval.

Z is the time zone offset, for you it would be 0. L is the longtude east being positive and west being negative.

Example Plymoth Devonport: m2 = 154, lon = 4.18W


I = 154°/28.9841°/h - 4.18°/15°/h = 8.458 h

Don't know what port you want, I might be able to find closer harmonics.

But as I said, it's quite some simplification, the error will be smallest when sun and moon are in line, but again only approximately.

See if this gives you a reasonable result.

 

[Mirelle]

High water full and change.

Known in former years as the establishment of the port.

You will find the information in any good pilot book, or at least the older editions of pilot books.

There is no "magic" to this. Many old longcase clocks dating from the eighteenth century show the state of the tide at the port which they were made in.

 

[Joe_Cole]

I've got the tidal constants which you need if you want to follow up on HaraldS's note. I have them for Exmouth Dock, Exmouth Approaches, Starcross and Topsham.

However, they are all on my boat and I could bring them back next week if you want.

Incidently, I am moored up at Starcross. Where is your boat?

Joe Cole