Lessons
Under-keel clearance over the shoal patch
The water over the patch is charted depth plus height of tide, the ship in it is her deepest draught plus squat, and your clearance is the difference.
The rule
- Water over the patch = charted depth + height of tide.
- Ship in the water = deepest draught + squat.
- Squat makes her sit deeper — always add it, never subtract it.
- Use the deepest draught, not the mean: the deepest point touches first.
- Speed is the one term you still control — slow down and squat gets smaller.
Worked example
| Charted depth over the shoal patch | 15.7 m |
| Height of tide at the time of crossing | 1.85 m |
| Deepest draught on arrival | 15.62 m |
| Mean draught on arrival | 15.44 m |
| Squat expected at transit speed | 0.65 m |
- Water over the patch = charted depth + height of tide = 15.7 + 1.85 = 17.55 m
- Ship in the water = deepest draught + squat = 15.62 + 0.65 = 16.27 m
- Under-keel clearance = 17.55 − 16.27 = 1.28 m
Common mistake
Leaving the squat out of the sum.
Squat comes from her speed, not from the chart or the tide table, so nothing on the desk reminds you of it — leave it out and you report 1.93 m when she really has 1.28 m.