Ground tackle

Anchoring: how the numbers add up

Five measurements decide whether you float and whether you hold: the depth of water, your freeboard, your draught, the clearance under the keel, and the scope you let out. They are all the same picture — this one.

At the bow: the rode has to reach from the bow roller — up at deck level — all the way down to the seabed. That full height, not just the charted depth, is what your scope is measured against.

The numbers, one at a time

Depth of water: What is actually under you right now: the charted depth (measured from chart datum) plus the height of tide at this moment. The chart shows the worst case; the tide sits on top of it.
Freeboard: How far the bow roller stands above the water. It matters because you pay the rode out from up there, not from the surface — so the rode has to span depth plus freeboard before it even reaches the seabed.
Draught: How deep your keel reaches below the waterline — the number you compare with the depth to know you float.
Under-keel clearance: The water left beneath the keel: depth of water − draught − a safety margin you choose (half a metre is a common floor in calm water; more with swell). Work it at low water, when there is least.
Scope: The length of rode you lay out, measured against the rise from the seabed to the bow roller (depth + freeboard). Enough scope keeps the pull on the anchor horizontal, so it digs in instead of being lifted out.

Working scope from the bow roller

The rule of thumb is a ratio: at least 4:1 with all chain, or 6:1 for a chain-and-warp mix — more in wind or swell. The catch the rule hides is the “1”. It is not the charted depth; it is the height from the seabed to your bow roller, so it includes the height of tide and your freeboard.

A worked example. You anchor where the chart shows 3 m, with 2 m of tide on top, and your bow roller stands 1 m above the water. The rise the rode must reach is 3 + 2 + 1 = 6 m. At 4:1 all-chain that is 24 m of chain — not the 20 m you would have laid if you forgot the tide and the freeboard. And remember the tide is still rising: lay for the high-water depth, or you will be snubbing short by midnight.

Calibrating the depth sounder

Every one of these sums starts with a depth reading, and a depth reading is only honest if you know where its zero is set. The transducer measures from itself; the instrument then offsets that to display one of three things — and you must know which:

Below the keel. The display is true under-keel clearance — run aground and it reads zero. Safest for thin water, but it under-reads the actual depth by your draught.
Below the waterline. The display is the real depth of water, to compare straight with the chart — but you must subtract your draught in your head to know what is under the keel.
Below the transducer. The raw reading, offset by neither — the least useful, and the default on a set nobody has calibrated.

Find out which yours does — drop a lead line beside the boat in calm water and compare, or read the offset in the instrument’s setup — and set the offset deliberately. A sounder you trust is the difference between “3 metres” and “3 metres under what?”.

The anchor types →Which anchor for which ground, animated.The scope trainer →Slide the tide and watch 4:1 become 2.5:1.The anchoring lesson →Choosing the spot, laying it, proving it holds.