HW -6 and HW +6 What's the difference?

[snooks]

May be someone out there could help me

I have a few different copies of the excellent Yachtsman's Tidal Atlas by Michael Reeve-Fowkes, I've use them for some time, but never been able to fathom why there is a -6 hours HW and a +6 hours HW which one would assume would be the same....but they are not

Any ideas why not? a simple explanation would be good

 

[VicS]

Simply because the time interval between successive high tides is more than 12 hours.

Roughly there is a half hour gap between HW+6 and the next HW-6

 

[Chris_Robb]

May be someone out there could help me

I have a few different copies of the excellent Yachtsman's Tidal Atlas by Michael Reeve-Fowkes, I've use them for some time, but never been able to fathom why there is a -6 hours HW and a +6 hours HW which one would assume would be the same....but they are not

Any ideas why not? a simple explanation would be good

As the time of the tide advances every day, so the time of the -6 and plus 6 must change with it as they are 12 hours apart. You should always plot your tidal atlas using the closest relevant high tide, so once you get to High tide Plus 6 go to the next high water and work backwards to high tide minus- 6. Difference is around 40 minutes per day, so is relevant.

 

[Evadne]

Also, the tidal curves are rarely symmetrical.

 

[Chris_Robb]

you beat me to it. - by the way - look at the post about electrolytic corrosion in Plymouth - it needs your reply

 

[exfinnsailor]

Tides

Dont forget that the time for the -6 hour tide data is from -6.30 to - 5.30
and 6 hour is from +5.30 to + 6.30 so the overlap ocures in the extra 30 mins
at the beginning and end . As with all tidal predicions its a bit like the weather,
hights depend on atmospheric conditions and which way the wind is blowing ..

 

[snooks]

Thanks all, I knew there was a reason, I just didn't know what it was.

Now I do

 

[bilbobaggins]

Just you watch out - there will be an article on this on October's YM, and a near-identical one in PBO.....

Both written by one Aloysius Endeavour Snooks.....

( Who says blokes can't do two things at once? )

 

[mikemonty]

And also remember that the height varies between neaps and springs so generally as you go towards springs the -6 is a smaller range from the datum than the +6.
(I think I got that right?)

 

[snooks]

It wasn't the height I was worried about just the streams, direction and strength over a 12 (and now I know a 1/2) hour day

 

[mikemonty]

Yebbut... same principle applies to speed of stream as height - though obviously not as critical.

 

[Johndf1]

It gets more complicated. In Jersey, where I live, during Springs there is often only a twelve and a quarter hour difference between one high tide and the next. During neaps it can be up to a 13 hour difference. You can more or less pick this up from the tidal curves for St Helier.

 

[Danny]

The problem is how to cope with the extra 25 minutes or so when doing manual passage planning. The answer is, of course, to always plan passages of 12 hours duration and always start them at HW-6. Problem solved!