What size anchor chain should I use for mooring my boat?

For a 40ft cruising boat weighing about 10 tonnes, suitably rated 10mm anchor chain is a practical starting point; the final choice depends on displacement, windage, exposure and chain grade. A permanent swinging mooring or buoy connection needs a separate specification, with chain size governed by wear, corrosion and the mooring arrangement.

Check displacement and windlass fit

Start with the boat's loaded displacement and the chain specification for the windlass gypsy, the pocketed wheel that grips each link. Length alone is a poor guide. A heavy cruising yacht and a light motorboat of equal length impose different loads, while a tall wheelhouse increases windage.

For ordinary cruising anchoring, use these ranges as starting points for discussion with the boatbuilder or chain supplier, rather than a final sizing chart:

• Small boats up to roughly 7m: 6mm chain is common, often combined with nylon rope.
• Boats around 8 to 10m: 8mm chain is common, subject to loaded weight and exposure.
• Cruising boats around 11 to 13m: 10mm chain is a common choice; a 40ft, 10-tonne boat needs chain with adequate certified strength.
• Heavier boats around 13 to 15m: compare rated 10mm and 12mm options against the boat's requirements and handling equipment.

Read the windlass manual and any markings on the gypsy before ordering. Two chains labelled 10mm can have different link dimensions. DIN 766 and ISO chain are not automatically interchangeable at every size; a mismatch can make the chain climb, jump or jam. Keep hands clear of the gypsy and isolate the windlass before checking its pockets.

Specify chain strength

Ask for calibrated, hot-dip galvanised anchor chain with a declared grade, minimum breaking load and working load limit. Grade 30 and Grade 40 are common specifications, but diameter alone does not establish capacity. Do not compare the chain's breaking load directly with the boat's weight when choosing a size: anchoring loads come from wind, waves and shock loading.

Higher-strength chain can sometimes reduce weight in the bow, but downsizing also changes the gypsy requirement and the strength needed from joining hardware. Treat it as a complete system change.

Check the anchor shackle, swivel if fitted, bow roller, chain stopper and securing points. An unmarked connector can undermine otherwise suitable chain. Inspect existing links where they rub together and where the chain regularly passes over the roller. Deep pitting or reduced steel diameter matters more than surface discolouration.