Worked tide example
Anchoring: How to Work Out the Depth at Low Water and How Much Chain
Depth at low water = depth now − (height of tide now − height at low water). Anchoring in 5.5 m at a 4.6 m high water with a 1.0 m low to come leaves only 1.9 m at low water. Anchor where there is at least draught + margin + fall, and let out at least four times the maximum depth in chain.
The question
You arrive at an anchorage at high water, 1400 (4.6 m). LW is at 2000 (1.0 m). Your echo sounder (reading below the waterline) shows 5.5 m. Your draught is 1.7 m. Is it deep enough to stay overnight, and how much chain do you need?
| Item | Value |
|---|---|
| Depth now | 5.5 m below the waterline |
| Height of tide now (HW) | 4.6 m |
| Next low water | 2000, 1.0 m |
| Draught | 1.7 m |
| Clearance wanted at LW | 1.0 m |
Working, step by step
- Fall of tide to LW = 4.6 m − 1.0 m = 3.6 m.
- Depth at LW = 5.5 m − 3.6 m = 1.9 m. Under the keel that is only 0.2 m: too little.
- Minimum depth to anchor in now = draught + clearance + fall = 1.7 m + 1.0 m + 3.6 m = 6.3 m.
- Move to 7.0 m. At LW you will have 3.4 m, which is 1.7 m under the keel.
- Chain: at least 4 × the maximum depth = 4 × 7.0 m = 28 m. Use more in strong wind or on a poor-holding bottom.
Answer
5.5 m is too shallow. Anchor in at least 6.3 m, for example 7.0 m, and let out about 28 m of chain.
The tides in this example are made up for practice. For real passages, use official predictions such as ADMIRALTY EasyTide or a nautical almanac, and the port's own tidal curve.
Try it yourself
Put your own numbers into the free tide calculator, which uses the same Rule of Twelfths and works out under-keel clearance.
Learn the method
More worked tide examples
- Height of tide at a given time
- Time the tide reaches a height
- Depth from charted depth
- Under-keel clearance
- Crossing a drying height
- Bridge clearance
- Springs or neaps from the range
- Tidal stream rate from a diamond
- Secondary port: time of HW
- Secondary port: height of HW