How much clearance under Keel ay low water?

Worked Example

At 1732 SP DST (zone -0100) on Monday the 27th May, A vessel decides to anchor for the night just outside November Marina, as there are no visitor berths left. November Marina is just north of Hamilton Sound in the Southern Peninsula.

The depth of water shown on the echosounder reads 8.2m from the bottom of the keel. The vessel has a draft of 2.3m . How much clearance will be under the keel at low water?

Step 1 – Mark up the tidal curve with tide table information.

Fig 5.37 – Tide Tables for Hamilton Sound on 27th May.

So we need to find the height of tide at 1732 SP DST which when consulting the tide tables realise we need to adjust to standard time for the southern peninsula. We must therefore convert 1732 SP DST to 1632 SP to work with the curves and tide tables.

Fig 5.37 shows the tide tables for Hamilton Sound on Monday 27th May. The range of tide that we are interested in between 1301 and 1914 is 5.8m-0.5m=5.3m and can see with reference to the Tidal range information box on the Tidal Curve page that we are dealing with a Spring Range and that we will work from the solid red spring tide curve line (Fig 5.0 ‘A’).

Step 2 – Enter the tide table information in the tidal curve

Fig 5.38 – The tidal curve for Hamilton Sound.

Now we can mark our date on the tidal curve (Fig 5.38) then mark down the 5.8m HW (Fig 5.0 ‘B’) and 0.5m LW (Fig 5.0 ‘C’) with a diagonal line between the levels. We can write the times of HW (Fig 5.0 ‘D’) and LW (Fig 5.0 ‘E’) form the tide tables (Fig 5.37) in the relevant boxes of the tidal curve time bar.

Step 3 – Enter the information in the tidal curve time bar

Fig 5.39 – Enter the time for HW & LW then fill in the time for each hour of tide starting from HW.

As before, we can see that the tidal curve runs from left to right increasing in time by an hour in each box when moving to the right along the time line. The central HW box is out reference box and we enter the time of Low Water

We can see that our target time of 1632hrs follows the HW time of 1301 so know we must be dealing with a falling tide and the right hand side of the curve. The Time of LW can be placed in the right hand side box but remember that when we fill in the the rest of the timeline boxes that they might not exactly tally with the +5 hours after high water box. Whenever your working with the Admiralty curves, and entering information into them, you must always be asking yourself, “ Are you entering in the right time?”, ie Standard Time, in this case SP, -0100 or ‘French’ Time.

Fill in the rest of the boxes, starting with HW+! and working to the right towards the LW box adding 1 hour as you go.

Step 4 – Reading the Curve

Fig 5.40 – To read the tidal curve we find our target time, 1632 SP, follow it up to the curve and then horizontally to our tidal heights line and again to the scale to read off the height of tide at 1632hrs SP.

From the question we know we are looking for the height of tide at our target time of of 1632 SP (-0100, French Standard Time). All we need to do is have a look at the timeline axis for this time. The difference between HW+3 at 1601 (SP, -0100, French Standard Time) and our target time of 1632 (SP, -0100, French Standard Time) is 31 minutes. So we move along three 10 minute subdivisions and can ignore the 1 min left (Fig. 5.40)

All we need to do now is draw vertical line from this timeline axis vertically until we come to solid red spring tidal curve, making sure the lines you draw are parallel to the printed lines on the curve. Where the vertical line intersects the red curve line, as before, then draw a horizontal line to intersect with our previously drawn tidal range line. Then bring another line up or down to the tidal heights scale axes and read off the height of the tide. Always have a close examination of this scale line as sometimes different curves are working on slightly different scale and number of metre subdivision.

Step 5 – Correct for time and put answer in Context

Its good to use diagrams to help visualise whats going on with the tidal height calculations. You can use the pro-forma diagrams in the appendix but should get into the habit of sketching them out yourself (Fig 5.41a)

Fig 5.41a -The expected fall of tide is the difference between the height of tide from the tidal curve and the next low water (2.2m-0.5m=1.7m)

First we need to calculate the fall of tide. To do this we need the height of tide at the time given (2.2m) calculated from the tidal curve. We also need the height of tide at low water (0.5m) which we can get from the tide tables. Remember both these numbers are related and measured from chart datum. Using the tidal curve we calculated the height of tide at 2.2m. So the tide will fall 1.7m from 1632 hrs to LW at 1914 hrs.

We can now use the fall of tide and relate it to whats actually happening with the boat and what depth the echosounder is reading. Remember the echosounder is giving us a depth down to the seabed and it has no idea where Chart Datum is. This is the reason we use the fall and rise of tide in tidal height calculations as we have to relate our calculations to the sea bed for them to be useful to us on the vessel.

Fig 5.41b – Water Depth 8.2m – fall of tide 1.7m – draft of vessel 2.3m = 4.2m clearance

Again we can use a diagram to help visualise things for us (Fig 4.51b).

So when we anchor the vessel at 1632 SP we are in 8.2 m of water depth from water surface to seabed. At the next LW the tide has fallen 1.7m which means that the water depth is 8.2m – 1.7m = 6.5m. The keel is 2.3 draft which means that the clearance between the keel and seabed is 6.5 – 2.3 = 4.2 m.

At low water there will be 4.2 metres clearance between the seabed and the keel.