There are tide tables and current-charts and tables on board of the ships of Waddeninzicht. In preparing your voyage you can also look at www.getij.nl . Tide -tables refer always to NAP and charts are based on LLWS ( Low Low Water Spring) which is something like the lowest possible waterlevel. From the tide-table applicable to Schiermonnikoog you can see that the difference between NAP and LLWS at Schiermonnikoog is 170 cm . This means that - for to calculate the true water level at high-water - one have to convert the water-levels in the chart.

Example

For August 20th 2005 the following is indicated:
High-Water 11.46 a.m. : a rise of + 145 cm vis à vis NAP
Low-water 5.30 a.m. : a drop of -124 cm vis à vis NAP

The gully through the flat Brakzand in between Lauwersoog and Schiermonnikoog in the chart of 2004 gives a drying up fall of 1,2 m (thus 120cm) ( the line underneath the depth figure in the chart indicates a drying fall of vis à vis LLWS)

At high-water the calculation goes as follows:
Difference with NAP- drying up fall + rise at high-water+=
170-120+145=195cm

At low-water the calculation goes as follows::
Difference with NAP- drying up fall - fall at low-water=
170-120-124=-74cm (that is how much the bottom of the gully will be sticking out of the water at low-water.)

This example shows that the rhythm of high and low water is always there and never ends. It is an aspect which the Wadden-sailor has to bear in mind. When you are not sure of your calculation you can always approach Waddeninzicht and ask them to calculate with you.