Why is it necessary to be able to estimate the height of water of the tide?

A quick reminder: the height of water of the tide, that which is indicated on the tide directories and which is called H, indicates the height of the water above the hydrographic zero, that is to say above the level of the probes (S) indicated on the marine charts. The sum of the height of water H and the probe S gives you the depth P, so P = H + S

Profondeur Hauteur d'eau sonde

The tide directory only gives you the water height at high tide and low tide with more or less 6 hours difference, it is often useful to know the value of the water height at any time of a rising or falling tide and especially in the following cases:

  • I want to know if there will be enough water at low tide compared to the boat at anchor. Example: I arrive at anchor in a bay at 3:30 pm, I measure 4.50 m with the depth sounder and my draft is 1.50 m. At the next low tide, will I have enough water not to run aground?
  • I want to know at what time it will be possible to enter a port or to use a channel. Example: I want to enter a harbour that has a weir to maintain a constant level in the harbour. If my boat has a draught of 1.50 m, at what time will I have the necessary depth to cross this threshold?
  • I want to know how deep it will be at a given time. Example : I have planned a passage on a shoal around 2:00 pm, what will be the depth on this shoal at this time and will it allow me to pass ?

Estimate the water height by calculation

The rule of twelfths, which is taught at the offshore license, allows an estimation of the water height at any time between two tides. This method of calculation is quite methodical and mastering it allows any skipper to know the height of water whatever the conditions. To learn it, we advise you to prepare the offshore licence or to register to the online course on tides proposed by Picksea on its platform learn-a-naviguer.com

Estimate the water height with an abacus or a slide rule

There are simpler ways of estimating the height of water at any time thanks to abacuses drawn by oneself or coming from models proposed in particular by the SHOM. The indicated learning platform explains their use. In addition, there is the tide calculator from Flash-Tide Instruments (a brand owned by Nautic Training), which also allows you to determine the height of water very easily. It should be noted that the abacuses or slide rules use the rule of twelfths mentioned above as a basis. Tutorials on the Flah-Tide calculator are available on PickseaVideo.

Estimate the water height with an application

Finally, there are many applications that give you the water level in real time or forecast it according to an expected time of arrival. Among the most famous applications, we can mention Tide Info available in all environments (windows, android, mac, etc ...)

There are also tide calculation tools available on most navigation software (notably the NV-Charts tool available with marine chart packs) and on the SHOMwebsite which provides tide information worldwide.