Secondary Port tide height calculations

[MikeFloutier]

Hi,

I'm just working through the RYA's "Navigation Exercises" and I'm having trouble grasping the methodology for the Secondary Port tide height calculations; for intermediate HW & LW times.

I understand that you:

1. Start with the HW & LW times for the relevant Standard Port and then,

2. Extrapolate the Secondary Port difference figures and adjust accordingly.

What is confusing me is the methodology used to obtain the model answers eg. Q. 8:11 c)

The Difference figures quoted in the Almanac extracts are for MHWS, MHWN etc.

I would have thought that the extrapolation would be based upon the date as it relates to the Spring/Neap dates. In the case of this question, just one day before the Neap.

However, the model answers seem to indicate that extrapolation is being done based upon the HW/LW time position within the relevant range of the Standard port; eg. 0100-0700. So, in the case of the question above, the HW time of 0320 is said, in the answer, to give a height difference of +1.4 which is 1/3 of the way from MHWS to MHWN.

This seems to my thinking to be at odds with the fact that the date in question actually occurs the day before the Neap.

Does this make any sense? 

 

[richardbrennan]

There was an article on this in the October YM which may help.

 

[blackdogsailing]

Hi Mike,
Ignore the fact that it is 1 day before neaps. The interpolation is always based on the range of the tide for the day. Colville range at Springs is 4.3 metres and at neaps the range is 2.5 metres. On may 2nd the range is 3.6 metres. Doing the interpolation you would add 1.4 metres for Port slade. Hope that makes sense! Sorry, in haste.
Chris