What time will we refloat?

Worked Example

At 0730 UT on Friday 13th September the small Rhu to Northernness passenger ferry has run aground. What time will the vessel float again with 0.5 metres clearance under the keel and what time can we tell the passengers that the ferry will be underway again?

We first need to calculate the height of tide when the ferry grounded at 0730. We can then calculate at what height of tide it will lift off the seabed again and what time there will be an extra 0.5m depth of water.

Step 1
Fig 5.47 – Dumbarton tide tables for 13th September.

First we need to find the height of tide at 0730 UT on the 13th September and use the nearest tide tables at Dumbaton. This time does not require conversion because the Dumbarton tide tables use universal time which is equivalent to NP standard time (Fig 5.47)

We can now look up the tide table as in the other examples. Our tidal range is 2.8m which is the difference between High Water and Low water.

0323 4.0 – 0840 1.2 = 2.8m range. This is close to the mean neaps range and will therefore be using the dashed blue neaps tidal curve line.

Step 2
Fig 5.48 – Marking up the tidal range curve.

We can now mark the tidal range heights for LW (1.2m) and HW (4.0m) on the tidal curve and draw a diagonal line between them (Fig 4.48).

You will notice that the tidal curve looks like it is upside down. and using a LW as our central reference point on the tidal curve graph. We are using the first LW at 0840 as our central reference on the tidal curve time bar. In this example we will fill in the HW time before the LW and HW time after high water as we are interested when the ferry grounds and when it will lift off again after low tide has passed.

Step 3
Fig 5.49 – FIll in the tidal graph tide bar working from LW.

We can fill in the rest of the boxes on the tidal curve time bar working from our central reference point at Low Water. When working to the left we take 1 hour off each time and when working to the right we are adding an hour each time (Fig 5.49)

Step 4
Fig 5.50 – We can use the tidal curve to find a height of tide at a certain time or to find the time of a particular height of tide.

We can now read the tidal curve to determine what the height of tide was when the ferry run aground at 0730 (Fig 5.50). We find our target time on the time bar, follow it vertically until ot meets the dashed ‘neaps’ curve then horizontally to our tidal range line to find our height of tide above chart datum at 0730hrs. We can see that when the ferry goes aground there is 1.6m height of tide above chart datum.

For the ferry to re-float again with a bit of a safety margin we need 1.6m + 0.5m = 2.1m height of tide. We can now use the tidal curve in reverse and look along the tidal height axes for 2.1m (Fig 5.0 ‘C’) and work a line backwards to the blue dashed neaps curve line past the LW.

The ferry will re-float agin with 0.5 clearance under the keel at 1220 UT (Fig 5.50 ‘B’).

Step 5

The time it will re-float is given in UT time but could convert it to tell our passengers that vessel wont be on its way agin until 1320 NP DST. Basically we have added an hour to tell the passengers because that is the time they will have on their wrist watches because they have presumably adjusted their watches to DST (British Summer Time).