Secondary port time differences.

[laika]

Last time this was asked on Scuttlebutt (googling turned up an old thread), lots of people explained how to do secondary port calculations and no-one answered the question the OP had actually asked. Let's be clear: I know how to do secondary port calculations and do them every time I go sailing. That's not the question. What is the question *is* is the one I was asked the other day:

What causes the difference in the difference of high or low water time at the secondary port according to what time of day high/low water occurred at the standard port.

If I was the sort of person who doesn't like to admit when they don't know something I would have done some hand waving, mumbled something about the gravitation effect of the sun and which way the earth was pointing at high/low water, and distracted everyone with an amusing youtube video of a kitten. Actually that roughly *was* my response but with an additional acknowledgement that it wasn't a real answer and I'd research a better explanation. Remarkably, of all the resources I've found about secondary port calculations, all explain how to use tabulated data without explaining how the content of the tables is calculated.

Anyone want to have a go at explaining?

Again: THIS IS NOT A QUESTION ABOUT HOW TO DO SECONDARY PORT CALCULATIONS!

 

[Deleted member 36384]

The shape of the land, seabed and relative position to the direction of flow from the standard port. With the same geographical area the tide doesn't just go up and down the water also flows in and out. The ratio is not linear because drag/friction is involved which conforms to a power law, so time differences vary as the rate of water flow varies through the monthly tide cycle. Or something like that.

 

[nigelmercier]

What causes the difference in the difference of high or low water time at the secondary port according to what time of day high/low water occurred at the standard port.

Just to be clear, you mean why the times for 0000 and 1200, or 0600 and 1800 below make a difference.

Last time I had to do this was when I took my YM exam, and I can't remember if I knew then!
Are the times always 0000 and 1200 etc?

.

 

[laika]

The shape of the land, seabed and relative position to the direction of flow from the standard port. With the same geographical area the tide doesn't just go up and down the water also flows in and out. The ratio is not linear because drag/friction is involved which conforms to a power law, so time differences vary as the rate of water flow varies through the monthly tide cycle. Or something like that.

Fair call and certainly mostly agree, but the time differences are based on time of high/low water in the day rather than tidal range on the day as you'd expect if it was a lunar tidal cycle thing.

Just to be clear, you mean why the times for 0000 and 1200, or 0600 and 1800 below make a difference.

Yes

Are the times always 0000 and 1200 etc?

Not always but often. In any case, the times when the differences seem to be always 12 hours apart: If the maximum time difference occurs when high tide is at 2am, it will also occur when high tide is at 2pm

 

[Buck Turgidson]

The mass of water has to flow in and out. If your secondary port is "upstream" the flow arrives their earlier. If it's "downstream" it arrives later. Up and downstream are in relation to the mean flow of water which will depend on the geography of the area. The water doesn't rise and fall on the spot. It flows in and out or along.

 

[nigelmercier]

... the times when the differences seem to be always 12 hours apart: If the maximum time difference occurs when high tide is at 2am, it will also occur when high tide is at 2pm

Must be sun related then, but don't understand why not 0000 and 1200 all the time.

Note to the other posters in this thread: we know why the secondary port has different times, but not why they are different at different times of day.

 

[Buck Turgidson]

Must be sun related then, but don't understand why not 0000 and 1200 all the time.

It's a function of the sun and moon. Their forces are not the same all the time as the geometry is changing on daily, monthly and yearly cycles. Hence the time at which the total force is highest is constantly changing as is the force itself.

 

[Bilgediver]

Must be sun related then, but don't understand why not 0000 and 1200 all the time.

Note to the other posters in this thread: we know why the secondary port has different times, but not why they are different at different times of day.

Cos the moon has a mind of its own and is the primary influence?

 

[nigelmercier]

It's a function of the sun and moon. Their forces are not the same all the time as the geometry is changing on daily, monthly and yearly cycles. Hence the time at which the total force is highest is constantly changing as is the force itself.

Please re-read the original question.

 

[laika]

Must be sun related then, but don't understand why not 0000 and 1200 all the time.

I think we can do some acceptable handwaving there and say that because of topological effects (as BlowingOldBoots references) what matters is sometimes not so much where the sun was at high tide at the standard port but where it was at high tide somewhere else. But that still doesn't make me feel much wiser about the actual difference cause.

To clarify again (I think Buck Turgidson slightly misinterpreted what I was trying to ask) I'm asking for a clear* explanation of why the difference in tide times varies according to time of day high/low tide occurs at the secondary port (as illustrated in nigel's post), not why there's a difference (which hopefully should be clear to most of us).

*Edit: "clear" here being something more enlightening than my own fluffy handwaving "It's to do with the gravitational effect of the sun..."

 

[Buck Turgidson]

Please re-read the original question.

I understand the question.
It's simply a matter of geometry. Just think of where the focus of the combined forces is during daytime/night time and with the cycles of the moon. Is it more aligned before or after the Primary port and therfore is it closer or further from the secondary at the time of high tide at the prime.

 

[webcraft]

.
HW Springs and HW Neaps occur in most places at roughly the same time every cycle.

For example, in Oban on the 16th of this month HW Springs is at 07.37

The next HW Springs at Oban on the 30th April is only one minute different at 07.36

I'm staying out of this now - I got worse abuse than in your average ColRegs thread last time.

- W

 

[Buck Turgidson]

It takes nineteen years for the geometry to be the same. So roughly the same isn't exactly the same. The harmonics of all the different cycles will only be the same every nineteen years. The harmonics are different at the two (if you have two) High waters each day and hence the offsets are different too.

 

[laika]

.
HW Springs and HW Neaps occur in most places at roughly the same time every cycle.

Consistency certainly: HW springs portsmouth is roughly a bit after midnight *or* a bit after 5am this year looking at my almanac, but those are nearly at opposite ends of the difference differences (if you see what I mean) for solent secondaries using portsmouth as standard port (relatively earliest hw occuring when hw portsmouth at 00:00/12:0000, latest when hw portsmouth 06:00/18:00)

 

[nigelmercier]

I understand the question...

I don't think you do. It is quite difficult to explain the question, why is why I asked for clarification.

Possibly easier with the example: why does HW at 1200 not have the same time difference between the ports as when HW is at 1800?

In other words: how does the tide know what time it is! The answer, as I suggested above, must surely be due to the sun.

 

[Buck Turgidson]

I don't think you do. It is quite difficult to explain the question, why is why I asked for clarification.

Possibly easier with the example: why does HW at 1200 give a different time difference between the ports from when HW is at 1800?

Because the various forces which cause the HW at 1200 are not the same as those which make the HW at 1800 and therefore the rate and volume of flow is different and thus the time it arrives at a fixed distance from the standard port is different.

 

[nigelmercier]

Because the various forces which cause the HW at 1200 are not the same as those which make the HW at 1800 and therefore the rate and volume of flow is different and thus the time it arrives at a fixed distance from the standard port is different.

Now that makes sense, but see my post that I was editing while you quoted it.

 

[Buck Turgidson]

Now that makes sense, but see my post that I was editing while you quoted it.

But it isn't just the sun. The little graphic I posted shows the various forces which each have an effect at a slightly different frequency and magnitude. It is the combined effects of all of these. Note they are not synchronised hence the height and rate of flow is constantly changing and the time offsets are just an average value which has been fine tuned by local measurement.

 

[LadyInBed]

Buck, everything you say is true, but as Nigel says, it is only the Sun's harmonic that is a constant to the 0000 0600 1200 1800

 

[laika]

But it isn't just the sun. The little graphic I posted shows the various forces which each have an effect at a slightly different frequency and magnitude. It is the combined effects of all of these.

My understanding of the graphic is that it's about the combination of forces affecting the tide which is as we would expected: I presume "M" is "Moon" and has the biggest effect*. Yet the difference in secondary port time *difference* (in the almanac) is corrected for only by time of day. If the difference in difference were a function of *tidal range* (the influences of which I believe is what your graphic might be showing) I wouldn't be confused.

*Although I've presumed "M" is "moon" and "S" is "sun", if the letter "M" can sponsor an episode of sesame street it might have some influence all its own

EDIT: (which is what LadyInBed has just posted in far fewer words)