Decide
The chart shows a sounding of 1.5 m. The tide table gives a height of 1.2 m. What is the depth?
The lesson in plain words
- Add the two
- The chart gives the sonda, the depth below chart datum.
- The tide table gives the height of tide above the same datum.
- The depth of water at that moment is the sounding plus the height of tide.
- The IH table states this rule itself.
- Drying heights
- Some banks and bars dry at low water.
- The chart marks them with an underlined figure, the sonda a descobrir (drying height), measured above chart datum.
- Over such a bank the depth is the height of tide minus the drying height.
- If the result is zero or less, the bank is dry.
- Clearance under the keel
- You need the depth to exceed your draught plus a safety margin.
- The margin covers swell, the boat's squat and errors in the prediction.
- The calmer the water and the softer the bottom, the smaller the margin can be.
- On an exposed bar with swell, leave much more.
- Plan for the worst moment
- For a passage over shallow water, check the depth at the low water of the day you will be there.
- If it is not enough, you must time the crossing near high water.
- Working out the exact height at a given time is a Patrão Local skill: at Marinheiro level, cross near high water and keep a wide margin.
Key data
- Depth
- Charted sounding + height of tide
- Over a drying bank
- Height of tide − drying height
- Water you need
- Draught + safety margin
- Worst case
- The day's low water
Key points
- Depth = sounding + height of tide, both from the same chart datum.
- Underlined figures are drying heights: subtract them from the height of tide.
- Need: draught plus a margin for swell, squat and prediction errors.
- If low water leaves too little, cross near high water.
Check what you have read
A bank has a drying height of 0.6 m and the height of tide is 2.0 m. What depth is over the bank?
True or false?
Decide each statement before you see the answer.
If depth equals my draught, I can pass.
Need: draught plus a margin for swell, squat and prediction errors.
An underlined 0.6 on the chart is 0.6 m of water.
The charted sounding is the depth I will find.
Underlined figures are drying heights: subtract them from the height of tide.
If low water leaves too little, cross near high water.
Practice case & exam keys
Situation
Your motorboat draws 0.9 m and you want at least 0.5 m under the keel. To reach a beach you must cross a sandbank with a drying height of 0.6 m. Today's table gives low water 0.8 m and high water 3.6 m. Can you cross at low water? And at high water?
Show the answer
You need 0.9 + 0.5 = 1.4 m. At low water the depth over the bank is 0.8 − 0.6 = 0.2 m: you would ground. At high water it is 3.6 − 0.6 = 3.0 m, enough. Cross near high water and plan your return before the tide has fallen too far. If you need the exact time when 1.4 m is available, that calculation is taught at Patrão Local.
Exam keys
- Depth = charted sounding + height of tide.
- Over a drying bank: depth = height of tide − drying height.
- Water needed = draught + safety margin.