Secondary Ports - differences data?

webcraft said:
.
Here's an annotated illustration of the way secondary port data is presented (HW only):

Secondary-Port-layout-expained.jpg



NOW LET'S LOOK AT THE EXAMPLE GIVEN IN THE OP
I'll add the missing info to help:

STANDARD PORT
.........................High Water
.........................0000 0600
.........................1200 1800
SECONDARY PORT DIFFERENCE
........................+0015 -0010

These are purely time differences - nothing to do with springs and neaps as people were suggesting

SO WHAT DO THEY MEAN?

If HW at the standard port is 00.00 or 12.00 then the time difference at the secondary port is +15 minutes.

If HW at the standard port is 06.00 or 18.00 then the time difference at the secondary port is -10 minutes.

WHAT ABOUT INTERMEDIATE TIMES?

All intermediate values can be worked out by interpolation, by creating a table or by drawing a graph, whichever works best for you.

e.g. HW at sec. port is 03.00
Total difference 00.00 - 06.00 = -25min
half of this is -12.5 min

Therefore if HW at the standard port is 03.00 HW at the secondary port is two and a half minutes before that. The figures for low water are presented and calculated in a similar manner.

CALCULATING TIDAL HEIGHTS AT SECONDARY PORTS

Thre is no allowance made in the time differences for springs and neaps, but there is (obviously) in the depths given. Depth differences for the secondary port are given at HWS, HWN, LWN and LWS and again if it is between springs and neaps you perform an interpolation.

The best explanation of this I have found, and the one I use when teaching this myself, can be found on the COCKPIT CARDS website. This is slightly simplified as it only gives the HW data, but the low water data and calculations are exactly the same.

- W
Click to expand...

Nobody said the times in the almanac correspond exactly to Springs & Neaps. For a start it would be weird for them to always occur on the hour.

However if you look at a few examples you'll see there's an approximate correlation to the typical times of Springs & Neaps at the primary port, which is no coincidence



Oh, and for the differences in depths it is wise to extrapolate for big springs and small neaps, not just interpolate between the means.
 
dt4134 said:
Nobody said the times in the almanac correspond exactly to Springs & Neaps. For a start it would be weird for them to always occur on the hour.

However if you look at a few examples you'll see there's an approximate correlation to the typical times of Springs & Neaps at the primary port, which is no coincidence



Oh, and for the differences in depths it is wise to extrapolate for big springs and small neaps, not just interpolate between the means.
Click to expand...

Ah, you caught me in the middle of editing. I had made the mistake of thinking that people were saying the columns corresponded specifically to Springs and Neaps.

Good point about extrapolation - might have been handy for all the ships that went aground at the recent big spring.

- W
 
webcraft said:
STANDARD PORT
.........................High Water
.........................0000 0600
.........................1200 1800
SECONDARY PORT DIFFERENCE
........................+0015 -0010

These are purely time differences - nothing to do with springs and neaps as people were suggesting

Thre is no allowance made in the time differences for springs and neaps, but there is (obviously) in the depths given.
Click to expand...

We'll have to agree to disagree then because I think it's self evident that those values are approximations of springs and neaps and I can't think of any other reason they'd be used.
 
Last edited:
Mark-1 said:
I'll take the bait.

So I'm not sure I can count five people who are materially wrong. Put me out of my misery.
Click to expand...

It was a misunderstanding on my part of what people were meaning by what they were saying. I now suspect that it was pretty clear in most peoples' minds.

Right, that's about as much apologising as you are likely to get out of me in a week :eek:

- W
 
Mark-1 said:
We'll have to agree to disagree then because I think it's self evident that those values are approximations of springs and neaps and I can't think of any other reason they be used.
Click to expand...

I certainly wouldn't confuse people new to the subject by bringing this into it, but I suspect you are absolutely correct. You are of course referring to a part of my post that was quoted while I was deleting it.

I've already said that I misunderstood some of the responses Mark. I haven't got an almanac here to see how closely the values given for various standard ports correspond to actual times of springs and neaps, but I suspect you are right. Of course springs and neaps are not at the same time every month, the variation being over an hour from month to month in some locations.


- W
 
Last edited:
webcraft said:
I certainly wouldn't confuse people new to the subject by bringing this into it, but I suspect you are absolutely correct. You are of course referring to a part of my post that was quoted while I was deleting it.

I've already said that I misunderstood some of the responses Mark. I haven't got an almanac here to see how closely the values given for various standard ports correspond to actual times of springs and neaps, but I suspect you are right. Of course springs and neaps are not at the same time every month, the variation being over an hour from month to month in some locations.


- W
Click to expand...

Yes, all understood.
 
Despite having been taught how to do tides from droggy's tide tables back in 64, these days I take the easy route using Tides Planner on my Iphone or Bellfield on my pc. Life is too short to get bogged down in tides.

Anyway the real tide is rarely an accurate reprentation of the tidetable because of weather impacts on the tide so in reality precise prediction is impossible.
 
alahol2 said:
Is there such a thing as a set of differences between STANDARD ports. That way I would only ever need to buy one set of tide tables and could keep my almanac till it falls apart?
Click to expand...

I tend to rely on the time differences on Dover shown for each location in any copy of 'Reeds, in conjunction with the range data.
An old "Reeve-Fowkes" also helps to eke out the data from my annual purchase of the Dover tables.

I am ashamed to say my secondary port calculations take the form of shabby guesswork.
 
Last edited:
doug748 said:
I tend to rely on the time differences on Dover shown for each location in any copy of 'Reeds, in conjunction with the range data.
Click to expand...

Be vary careful with that. The time differences from Dover can vary. I once found there was an extra 60 minutes difference between the time of HW based on Dover with an offset & that based directly on the local primary port. I presume the other boats in the race didn't notice that as they got their tidal strategy completely wrong. :)
 
dt4134 said:
Be vary careful with that. The time differences from Dover can vary. I once found there was an extra 60 minutes difference between the time of HW based on Dover with an offset & that based directly on the local primary port. I presume the other boats in the race didn't notice that as they got their tidal strategy completely wrong. :)
Click to expand...

I also have an entertaining half hour every year trying to remember if: +5h 35 means add 5.35 to Dover, or subtract it.
 
You must log in or register to reply here.

Share this page

Top