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Secondary Port height differences - By Calculator.
Since it is freshly drummed into my brain - having just done my Day Skipper theory. It is not a required part of Day Skipper but we did it anyway. Drawing things up on graph paper seems sort of time consuming when it can be done with a few taps of a simple calculator. Also, most of the figures can be worked out beforehand, and remain constant for that given Secondary Port and it's related Standard Port for the year. One word of caution here - a mistake I made readily at first. THE HIGH WATER DIFFERENCES ARE PLOTTED/CALCULATED AGAINST THE ' Mean HWS and the Mean HWN' of the Standard port - NOT - 'the MHWS and the M Low Water S.' This is why the MHWS and MHWN are side by side in the table. The 'range for the day' is HW to LW, but here we are interested in the figures for MHWS and MHWN. Since I've just done this, so have the figures at hand I've used 'Wachet' as an exanple, from the Admiralty Tide Tables for 2004 - standard port Avonmouth. ( Hopefully they will excuse me using there figures online - without throwing me into the Tower in chains! ) _____________________________________________________________________________________________ AVONMOUTH 3RD JAN 1613 ---- 10.6 HW LW 0200 -----------0800......................0300-------------0800.......................MHWS...........MHWN........MLWN........MLWS 1400------------2000......................1500-------------2000.......................13.2................9.8................3.8...............1.0 differences WATCHET -0035................- 0050....................- 0145................- 0040.......................- 1.9...............- 1.5.............+ 0.1............+ 0.1 ________________________________________________________________________________________ To set the factor for the difference for HW on 3rd Jan. - the range of spring to neap at Avonmouth, 13.2 and 9.8, needs to be Using the calculator - High Water. 13.2 - 9.8 = 3.4 This is the range for Avonmouth MHWS and MHWN The differences have a range of 0.4. ( 1.9 less 1.5 ) If it was + 1.9 to minus 1.5 it would be 3.4. We are interested in the 'range of the difference', in metres. Now - if we divide 0.4 by 3.4, we have a figure which will remain constant, and which will be the 'difference for Watchet for each m of tidal height away from MHWS.' - 0.4 divided by 3.4 = 0.1176. You can write it on the table data. Now - the HW for 3rd Jan is 10.6. - This is 2.6 less than MHWS. Establish the figure of 0.1176 for HW differences Watchet . Simply multiply it by the ammount that HW on 3rd Jan varies from MHWS Avonmouth. So it's fairly easy to figure that if we have minus 1.9 at 13.2m , our adjustment figure of 0.305 - call it 0.3 - will give us an adjusted height for HW Watchet of 'minus 1.6'. on HW Avonmouth, for a HW of 10.6. Do exactly the same for the LW adjustments - so you can set the high and low water figures into the tidal curve for Avonmouth in the normal way to establish a height for a certain time that day, or vice versa, a time for a given height, as normal. In this particular example for Watchet, the LW figures are constant, so there is mothing to work out. Times of High Water.
So, from 0200 to 0800 we have 6 hrs - and the range of difference is minus 15 mins. Using a calculator we have to change everything into minutes, then convert back to hrs/mins at the end. So - 6 hrs = 360 mins. For 0800 to 1400 the same factor applies, since it is still a difference of 15 mins over 6 hrs - but this time we are going the other way - from 0050 at 0800 to 0035 at 1400, all 'minus' of course. As above, if one figure is + 0035 and the other minus 0050, then the difference range in question would be 85 mins, 0125, though 85 mins is easier ti figure with. Since this example has all the times at 6 hr intervals, we only need the one factor figure. Where the time split is not even 6 hrs, then work out a factor for each time span. As above, these are constant and can remain with your Watchet/Avonmouth secondary port tables for the year. Above we have HW Watchet at 1613 ut ( do all this above in UT (gmt) and then convert your time, if necessary to BST ) So our tables now look like this - _____________________________________________________________________________________________ AVONMOUTH 3RD JAN 1613 ---- 10.6 HW LW 0200 -----------0800......................0300-------------0800.......................MHWS...........MHWN........MLWN........MLWS 1400------------2000......................1500-------------2000.......................13.2................9.8................3.8...............1.0 differences WATCHET -0035................- 0050....................- 0145................- 0040.......................- 1.9...............- 1.5.............+ 0.1............+ 0.1 _factor - 0.0972 0300 - 0800 - 0.216 0.1176 none ie LW time differences - 0300 - 0800 = 5 hrs. thant is 300 mins. Range of differences is minus 0145 to minus 0040, which is 65 mins. 65 divided by 300 = 0.2166 LW above is 2229 - which is 2 hrs 29 mins past 2000 = 149 mins. Be very careful when adding and subtracting hours and minutes. I think scientists have a time syatem of their own - in which the days have 100 hrs and the hrs 100 minutes, the minutes 100 seconds, or something like that - they realised navigating to Mars was just too hard in hours and minutes !! If you change everything into mins - then everything can be done on a calculator. To change it back, ie. 485 mins - divide by 60, = 8.083333. That is eight hours - the .083333 multiply by 60, - 0.083333 x 60 = 4.99998. Work it out - 8 hrs = 480 mins. The odd 5 mins is the 4.99998. - 150 mins divided by 60 = 2.5........ 2 hrs, plus 0.5 hrs, which is 0.5 x 60 = 30 mins Hopefully this has been clear to follow - it helps, at first, to plot them on a piece of graph paper to clarify in your mind what is going on, but on a boat, a calculator and a pencil is a lot simpler. After all, now I have my figures for Watchet, to find high water height at Watchet, I only have to multiply the metre difference between HW of the day and MHWS for Avonmouth, by the 0.1176 factor and I have the exact increment to adjust the height of tide difference for Watchet. It's a lot simpler than it looks. OK - at some stage I will be getting a cheapo/heapo laptop for 60 quid, sticking it on the boat with' tidecalc' on it - after all it's only about 15 pounds for the program for a year - but you do need to be able to do it reliably from the source tables. Sods law says that will be at the worst possible time. Best to be profecient both with the calculator and with the graph paper |