NauticTools2Tides
Depth of water – charted depth, height of tide and water under the keel
Will my keel clear the shoal? From charted depth and height of tide NauticTools2 works out the depth of water, compares the water under the keel with your safety margin and gives the height of tide you need at least. Sliders and a cross-section let you try out what happens as the water rises or falls.
- Charted depth
- Height of tide
- Water under the keel
- Safety margin
- Recreational and commercial
- Cross-section
What is the depth of water?
The chart does not show how much water is there right now, but the charted depth (CDp): the depth below chart datum (CD). The water usually stands above chart datum, and this difference is the height of tide (HoT). Together they give the actual depth of water. Because both refer to the same chart datum, its exact definition does not matter for the calculation.
When do you need it?
- Shoals and bars: is there enough water for my vessel, and how much reserve is left?
- Drying areas: from what height of tide can a drying patch be crossed?
- Berths and anchorages: how much water will be left at low water?
- Commercial shipping: is the under keel clearance sufficient when it is set as a percentage of the draught?
The working
- Depth – depth of water
- CDp – charted depth, measured down from chart datum; drying heights are negative
- HoT – height of tide, measured up from chart datum
Depth of water
Add charted depth and height of tide.
Water under the keel
Subtract the draught from the depth of water.
Compare with the safety margin
The water under the keel must be at least as large as the safety margin (SM).
Height of tide needed
Minimum HoT = draught + SM − CDp. This value is the link to the tide tools.
Worked example
A yacht drawing 1.80 m wants to cross a patch charted as drying 1.5 m (CDp = −1.5 m) with a safety margin of 0.50 m. The height of tide is 4.06 m, as in the example under Tides BSH. Is there enough water?
Depth of water: −1.50 + 4.06 = 2.56 m
Water under the keel: 2.56 − 1.80 = 0.76 m
Compared with SM: 0.76 m ≥ 0.50 m, sufficient
Height of tide needed: 1.80 + 0.50 + 1.50 = 3.80 m
Enter the 3.80 m as the required minimum height in Tides BSH or Tides ATT, and these tools give the times during which the patch can be crossed.
What to watch out for
- Drying heights: a depth underlined on the chart is entered as a negative value, for example −1.5.
- Safety margin: wind, air pressure and swell change the water level. Squat and heel change the draught. The tool does not include any of these, so the safety margin has to cover them.
- Height of tide: the result is only as good as this value. For navigation, the official tide tables are authoritative.
- Old surveys: in shallow, changing waters the true depth can differ from the charted depth.
How it works in NauticTools2
-
Choose the type of vessel
At the top you choose Recreational or Commercial shipping. This sets the ranges of the sliders, the scale of the graphic and the vessel drawn. Recreational covers charted depths from −3 to 10 m and heights of tide from 0 to 8 m, commercial from −2 to 25 m and from 0 to 10 m.
-
Enter the values
Enter draught, safety margin, charted depth and height of tide. For commercial shipping the safety margin can be given in metres or as a percentage of the draught.
-
Try it out with the sliders
Charted depth and height of tide can also be set with the sliders. Graphic, result and working follow at once.
-
Read the graphic
The cross-section shows seabed, chart datum, water level and the vessel. With enough water the vessel is green, otherwise red.
-
Save and print
Save and Load keep a calculation, and the printout contains the inputs, result and working.
Frequently asked questions
Where do I get the height of tide?
From the tide tables or from the tools Tides BSH, Tides ATT and Simplified Harmonic Method.
How do I enter a drying height?
As a negative charted depth. A drying height of 1.5 m is entered as −1.5.
How large should the safety margin be?
That depends on the vessel, the area, the seabed and the sea state. It has to cover everything the tool does not calculate: wind, air pressure, swell, squat and heel.
Related tools
- Tides BSH – height of tide and time window for a minimum height by the German BSH tide tables
- Tides ATT – tidal predictions and crossing a bar by the Admiralty Tide Tables
- Age of Tide – springs, mean or neaps for a date
NauticTools2 is available for Windows 10 and 11 (free) and for Android (€9.99 one-off).
Windows: get it from the Microsoft Store Android: get it on Google Play