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Secondary Port Tide Heights >> for example the Lagavulin whiskey distillery in Isla

Being able to accurately calculate tide heights in anchorages is an essential useful skill. To anchor closer in and safe, to feel safer overnight, or even to venture in at all. To do this you must establish at what time of the tide it is when you arrive, and by how much the level will fall to the next (and subsequent) low waters, and rise to subsequent high waters. Only then can you select a safe depth to anchor using the sonar, and the scope of anchor to deploy.
That is of course unless you follow the crowd, err on over deep positions or squeeze alongside similar vessels.
 

                             


Depth to Anchor (minimum) (this is the sonar measurement and not charted depth)
=

Fall of tide to next (or subsequent) Low, because you are unlikely to arrive exactly at low or high water time. PLUS

Depth of keel assuming your sonar is set to the water line (as you should assume if chartering or unless you have checked using a lead line) PLUS

 Safety Margin with factors to consider such as swell, wind effect, barometric pressure, and the variation in depth of your circle of swing. These condition would have to be very favourable for you to rely on a mere foot of safely. Additionally the state of the bottom (would you be content to ground.)

Depth to Anchor (sonar reading) = Fall + Keel + Safety Margin

Calculation is more challenging for small secondary locations and especially so where big deviations exist in comparison with the (its) Standard Port tide height data. The anchorage at the Lagavulin whiskey distillery in Isla, West Scotland is one such example we were keen to visit and stay. Doubly so because the Loafraig distillery is a 1mile walk up the road, see photos. On both occasions we have visited the tide range on the day was on, or outside of the mean spring range at the Standard Port (Oban) and on cursory examination of the Standard Port data, and the depth in the harbour on our arrival, at around high water time, we would have abandoned the quest. Spock was concerned we would ground overnight, but with other experts aboard a vote to stay was carried.

We arrived at 17.40 BST, quite close to the time of high water at Oban (1752 UT, 4.2m) and with an overnight range of 3.3m (which is the same as the mean range at springs).

Since we were arriving around high water time (before correction), with a depth of only 2.8 below the water line in the anchoring area near the Lagavulin pier and with a keel of 1.8 meters the case might look hopeless. We would be high and dry in the middle of the night. However:

Lagavulin does not have its own entry in the almanac, rather a small note confirms Port Ellen as its own secondary port reference, and whose standard port is Oban. Due to the large tide flows in the west of Scotland, the narrow gaps between Islands, in this case between Isla and Jura, both tide times and heights vary considerably between our secondary port (Port Ellen) and its standard port (Oban) data.

OBAN 14th September

(Mean range at Springs 3.3m)

0546

 

3.8

1145

 

0.8

1752

 

4.2

0022

 

0.9

Consider the shift data table for Port Ellen (Lagavulin) to Oban

 

Standard Port OBAN ( ← )   REEDS NAUTICAL ALMANAC (data shown above)

Time of HW

Time of Low water

 

MHWS

MHWN

MLWN

MLWS

 

0100

 

0700

0100

 

0800

 

4.0

2.9

1.8

0.7

 

1300

 

1900

1300

 

2000

 

 

 

 

 

 

Differences at Port Ellen compared with the Standard data Oban

-0530

 

-0050

-0045

 

-0530

 

-3.1

-2.1

-1.3

-0.4

 

 

 

 

 

 

 

 

 

 

 

 

 

Lagavulin corrected height data

 

0.9

0.8

0.5

0.3

 

SIMPLE ESTIMATION
Later we will consider the calculation in detail however first examine the range at Oban (shift data table MHWS 4.0 – MLWS 0.7 = 3.3 Mean  Spring Range) and compare with the range at Port Ellen ((4.0-3.1)  -  (0.7-0.4)  =  (0.9 - 0.3) = 0.6)) So 0.6 is the maximum fall at Lagavulin if we arrived at high water. To apply our equation:

Depth to anchor (sonar) = Fall of Tide (0.6max) + Keel (1.8) + Safety Margin or:

Safety Margin = Depth to Anchor (is our 2.8 OK?) - Fall of Tide (0.6max) - Keel (1.8)
 = 0.4m at worst.     It was possible to stay.

Simple Visual Method
The high water line and the low water line of the Oban tide curve from the almanac are as follows. On these are marked the MHWS, MHWN, MLWS, and MLWN for Oban and also the corrected values for Port Ellen indicated as vertical stripes:

 

                                                                                                                          MHWN 2.9                             MHWS 4.0
0 - - - - - - -
n s 1 - - - - - - - - - 2 - - - - - - - - n 3 - - - - - - - - - 4 - + - - - - - - - 5 +high today 4.2

          

 

0 - - s - - n s - + 1 - - - - - - - n - 2 - - - - - - - - - 3 - - - - - - - - - 4  +low today 0.9
                            MLWS 0.7                             MLWN 1.8                                    

Observe that:
The high water variation at Port Ellen neap to spring is only 0.1m.
The low water variation at Port Ellen neap to spring is only 0.4m.
The Spring high to low Range at PE is only 0.6m, compared with the Spring Range at Oban of 3.3.
The Neap high to low Range at PE is only 0.1m, compared with the Neap Range at Oban of 1.1m.

In this case of Port Ellen we can see right away that the range today is only marginally different from 0.6m (a bit to the right of both red bars).

The fully worked visual method for this Lagavulin instance is shown below and is a quick and effective method. Similar triangles and tabulations are not needed.

 

FULL CALCULATION

Consider the time of high water first and use the triangle method, or arithmetic interpolation if you like however in words:
If a HW time of 1300 UT corresponds to a HW time shift of -0530 (five hours thirty minutes earlier at Lagavulin), and a HW time of 1900 UT corresponds to a HW shift of -0050 (Fifty minutes earlier at Lagavulin). Then since the HW time at Oban was 1752 UT then the HW time at Lagavulin would be approximately 1 hour earlier (47 minutes shift per hour equates to nearly exactly - 53 minutes) = 17.52-00.53 =  approx.   17.00 UT or 18.00 BST. Hence around ½ an hour after our arrival at 17.40 BST.
Note remember to do these calculations all in UT and before correcting for Summer Time.

Now consider the Height of both the high water and the following low water to determine the fall overnight. The following sketch adaption I make to the tide curve diagram (of the standard port OBAN) in the almanac and is an ‘easier to do’ triangle interpolation method.

The fall to the next low is 0.5m (from high to low and since we arrived close in time to the HW we could use the total fall) our draught is 1.8m. The sonar depth from the water line on arrival is 2.8m.

Clearance at LW = Depth (current) – Fall (to next low) – Draught = 2.8 – 0.5 – 1.8 =
=0.5m, and providing there were no significant swell, pressure or wind influences sufficient depth in our position to stay. As we did!

Note because we arrived near to high water it was unnecessary to use the Oban tide curve shape (or twelfths rule to determine the fall as would be required if we had arrived mid-tide.