Secondary Port outside range

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Galwad

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A question on 2ndry ports.
Based on pbo almanac has 2 columns for HW..0300 and 1500
0900 and 2100.

are these 2 columns for neaps ans springs? If so should you interpolate between the two? Eg hen between heaps and springs?
Also, if Hw time is outside these ranges, should you extrapolate (hw time of 2300).

Not been able to find an answer on this so any advice appreciated.
 
The neaps/springs issue is irrelevant when it comes to those time columns. Yes you do need to interpolate between, say, 2100 and 0300 (using your 2300 example for HW) to find the difference in time for the secondary port. You also interpolate for the height difference, you don't extrapolate. you can do it graphically or by inspection.

There are several online guides which may be useful. Here is one example chosen for no particular reason. Secondary Ports – DS & YM - Penzance Sailing Club
 
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Thanks for your reply.
Unfortunately the links don't solve the question which is not about hie to calculate secondary ports.

1. The almanac has two HW columns.
A. With high tided at 0300 and 1500.
B with high tides at 0900 and 2100.

This leads to some questions.
1. Why 2 columns.
Eg if you have HT at 1200 you could use either column , but they give different answers.
I could only assume one is heaps, the other springs.

2. If the above is resolved, what do you do with actual HT is out side the time of the columns. Eg a time of 2300? Do you have to extrapolate, which does not seem a correct mathematical approach.

As mentioned this is not about basic secondary ports, but about the PBO almanac, which most explanations do not cover (ie 2 tidal columns)
S04 said:
The neaps/springs issue is irrelevant when it comes to those time columns. Yes you do need to interpolate between, say, 2100 and 0300 (using your 2300 example for HW) to find the difference in time for the secondary port. You also interpolate for the height difference, you don't extrapolate. you can do it graphically or by inspection.

There are several online guides which may be useful. Here is one example chosen for no particular reason. Secondary Ports – DS & YM - Penzance Sailing Club
Click to expand...
 
Can you post a page from the almanac? The first row not column covers the period 0300 to 0900, the second row covers the period 1500 to 2100. So if your HW is at 1200 you interpolate between the difference at 0900 and the difference at 1500. There is no confusion.

Here's a trivial example for Braye using St Helier as the standard

St Helier Time differences

HW

0300 0900
1500 2100

Braye

+0050 +0040

Time difference at Braye for HW St Helier at 0900 is +0040

Time difference at Braye for HW St Helier at 1500 is +0050

I want the time difference for HW at 1200 St Helier I interpolate between the two. The difference is increasing between 0900 and 1500 so for 1200 it must be between those two values but later than +0040. Interpolating by inspection you can see that the difference is +0045.
 
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Galwad said:
1. The almanac has two HW columns.
A. With high tided at 0300 and 1500.
B with high tides at 0900 and 2100.

This leads to some questions.
1. Why 2 columns.
Eg if you have HT at 1200 you could use either column , but they give different answers.
I could only assume one is heaps, the other springs.
Click to expand...
If HW is at 1200, which is half way between 0900 and 1500, the time difference will be half way between the value in column B and the value in column A.

if for example the difference in column A is +0005 and the difference in column B is +0015 the figure you need to apply for1200 is +0010

Forget about springs and neaps .

.
 
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Galwad said:
A question on 2ndry ports.
Based on pbo almanac has 2 columns for HW..0300 and 1500
0900 and 2100.

are these 2 columns for neaps ans springs? If so should you interpolate between the two? Eg hen between heaps and springs?
Also, if Hw time is outside these ranges, should you extrapolate (hw time of 2300).

Not been able to find an answer on this so any advice appreciated.
Click to expand...

1. Not Np or Sp, just time at the port for HW
2. You interpolate between time range in each columns

03:00__________09:00
and____________and
15:00__________21:00

You would usually interpolate to establish the correction for 23:00 between 15:00 and 09:00

Is this a real example, or is the 23:00 just selected to make this point? An 18 hour period is large.

Edit
Looking through my Reeds this is for London Bridge, Page 193 of the 2025 almanac, or Lowestoft, P194, or Leith page 195,
 
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As I said in post #2 For your 2300 difference you interpolate between 2100 and 0300

Using my Braye example then that difference would be 0040 + (0050 -0040)*2/6 = +0043
 
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Galwad said:
A question on 2ndry ports.
Based on pbo almanac has 2 columns for HW..0300 and 1500
0900 and 2100.
Click to expand...

Yes, you'll find the times are generally aligned to springs and neaps but you're not really extrapolating for 2300, you are interpolating between the 2100 and 0300 columns as they will be presented as

0300 ... 0900
1500 ... 2100

If you imagine this repeated, you'd have

0300 ... 0900
1500 ... 2100
0300 ... 0900

So 2300 is 2/6ths on the way "back" from your 2100 to 0300.
 
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