Calculations of Tide Heights - Depth to anchor

It can be important to get this right!

 Dielette Granville Hamble Spit Northney

Tide tables assume a standard atmospheric pressure of 1013 millibars.
Atmospheric pressure
creates a down force on the sea. A cubic metre of air at sea level weighs about one kilogram. A change in pressure of one millibar will change sea level by roughly one centimetre.  This means that a pressure of 1040 mb, not unusually high, could give a sea level lower by nearly 0.3 metres than at standard pressure.  Tide heights are also affected by prolonged strong winds and storm surges.  So do not trust tide height calculations to less than 0.5 metres, maybe more!  Tidal streams are similarly not as precise as the tables indicate.

A tide curve fro your area, found in any almanac  is used to find the time and heights of the tide for any state of the tide.  The curve is a diagram for converting time to height or height to time. Find the time of High Water in the tide tables, add the hour for Summer Time if necessary (non shaded area).

Below the curve enter the time of High Water in the HW box, then count +/- hours each way.  Use the LOCAL Standard Port for tide heights; if your location is a Secondary Port look here  Secondary_Ports

Enter the HW height for today along the top line, and LW on the bottom line. Then draw the intersects as shown.

Calculation of depth to anchor or clearance at Low Water (see also anchoring)

USE THE LOCAL STANDARD PORT eg for a mooring at Dartmouth use Dartmouth tide tables. Find the time of HW and the LW and HW heights.  Find the RANGE and whether it is Springs or Neaps - the box at the top right of the tide curve has this information.

Find the height of tide H from the curve at the time you arrive at the anchorage (this may be before or after High Water).

Find the FALL of tide to Low Water = H - LW  hts.

The Depth required = Fall + Draught of boat + clearance required under the keel.

Length of chain required will be based on MAXIMUM Depth at HIGH WATER.  For chain use 4 x max depth, for warp (rope) use 6 x max depth.

Draw a diagram for clarity:

Example of errors - predicted tide height and actual observation at Sheerness during a storm surge 8 November 2007:

There are several web pages with tide height predictions for 6 days ahead. Beyond that you have to pay or use an almanac:
UK
http://easytide.ukho.gov.uk/EasyTide/EasyTide/index.aspx

France
http://www.shom.fr/ann_marees/cgi-bin/predit_ext/choixp?opt=0&

http://www.acblack.com/media/Cherbourgtidecard2009.pdf
(12 months, can be used with the Yachtsman's Manual of Tides:
The Yachtsman's Manual of Tides: The Theory and Practice of Navigating in Tidal Waters

Netherlands
This program can be downloaded free from the Dutch Hydrographic Office and predicts tide heights for longer periods (6MB file, 4MB manual):
http://www.hydro.nl/pgs/en/software_en.htm

posted 15th April 2011