Methods and comparison of calculating Tidal Heights

Tidal curve

The Tidal curve is the preferred and most accurate method of calculating tidal heights. It’s reason for accuracy is due to the fact that the curve is created specifically for the harbour in question taking in local and topographical features.

Almost all curves are based upon HW, but be aware that in a few special circumstances where HW is unreliable, then we use LW as our central datum. In this case the curve is inverted.

The setting up of the curve is fairly straight forward;

• Find the time of HW at the harbour required and place in the HW box on the curve. (fill in subsequent hourly boxes, +1,+2,+3,-1,-2,-3 etc.)
   
• Find the height of HW at the harbour required and mark on the scale on the top left of the curve.
   
• Find the height of LW at the harbour required and mark on the scale on the bottom left of the curve.
   
• Join a straight line between HW mark and LW mark.
   

The curve is now ready for use;

If we want to find out how much water we have at a certain time, look at the time required on the bottom scale, follow this up to the curve (choosing Neaps or Springs as required).  Go across the page to your line and then up or down to read off the value 

or

If we want to find the time when we will have a certain amount of water, locate the height of tide required on the left of the curve, drop down to your line, then across to the curve (choosing Neaps or Springs as required), then drop down to the time scale to read off the time.

Rule of 12ths

The rule of 12ths bases itself on a sine curve, and as such produces a generic curve for all harbours. The rule works by dividing the range (difference between high and low water) into 12ths.

e.g.

Range of 4.6m

HW     Drop in Range          Drop (m)

-1         1/12                            0.38m
-2         2/12                            0.76m
-3         3/12                            1.15m
-4         3/12                            1.15m
-5         2/12                            0.76m
-6         1/12                            0.38m
LW

The rule is reasonably accurate, but unless your good at maths, you may need a calculator to work out a 12^th of the range.

Percentage(%) Rule

The percentage rule is similar to the rule of 12ths, however, by using a percentage, we can make the maths a bit easier.

e.g.

Range of 4.6m

HW     Drop in Range          Drop (m)

-1         10%                            0.46m
-2         15%                            0.69m
-3         25%                            1.15m
-4         25%                            1.15m
-5         15%                            0.69m
-6         10%                            0.46m
LW

A Comparison

Height of tide required at 1406 UT 4^th November 2011

HW 1206 UT 4.4
LW 1839 UT 2.1
Range 2.3 Neaps

Using the curve

2 hours after HW                                                                            4.0m

Rule of 12ths

HW     Drop in Range          Drop(m)         Height

+1        1/12                            0.19m           4.4-0.19 = 4.21
+2        2/12                            0.38m          4.21-0.38 = 3.83m

Percentage (%) Rule

HW     Drop in Range          Drop(m)         Height

+1        10%                            0.23m          4.4-0.23 = 4.17
+2        15%                            0.35m          4.17-0.35=  3.82m

You can see there is very little difference between our rules, but compared with the curve, there is a difference of 0.2m. However, be practical, 0.2m is still quite a small amount and normally we would never aim to navigate with such a small margin of error. In practice we would give ourselves at least a metre and probably 2 for safe clearance.