Secondary Port Worked Example
Tides (UT) for the Port of Trumpton in June:
It is Monday 2nd of June and you are bound for Barty old harbour and a drying berth. Here you may attend to a spot of maintenance below the water line then follow up with a light lunch. You arrive at 1000DST at which time there is 3.1m of water below the keel. The boat draws 1.0 m. When will the boat dry out? When will the boat re-float?
General notes for solution of such problems
(i) Work out secondary port tides
(ii) Draw the range line onto the standard port tidal curve
(iii) From the curve get the height of tide at the the time of arrival.
(iv) If the height of tide is greater than the actual depth work out the drying height; else work out the fall of tide down to low water to get the depth below the keel at low water.
1. By means of graphs, arithmetic or spreadsheet get the secondary port tide values:
Plot the range of the secondary port tide onto the standard port graph:
Arrival 1000DST, (0900 - 0630 = 0230 after HW)
From graph, height of tide = 9.5m at this time
Observed depth = 4.1m; Therefore drying height = 9.5 - 4.1 = 5.4m
Time when height = 5.4m is HW + 0510 = 1140UT = 1240DST
The boat dries out at 1240DST
The boat will re-float when depth is 5.4+1 = 6.4m
This occurs at approximately. HW- 0400 on the next tide.
The spreadsheet gives 1827 for the next HW, thus at 1827 - 0400 = 1427UT = 1527DST the boat re-floats.
The second HW is at 12.7m; an increase of only 0.2m over the height of the previous HW. for this reason I did not construct a new range line for the last part of this question