What anchor scope should I use in 35 knot winds?

In 35-knot winds, you need a minimum scope of 5:1 for an all-chain rode, or 7:1 for a rope and chain combination. Always calculate this ratio using the vertical distance from your bow roller to the seabed at high water, rather than relying on the depth sounder alone.

Calculating True Anchoring Depth

Skippers frequently miscalculate scope by relying entirely on the depth gauge. Your transducer sits at the waterline or the bottom of the keel. To find the true anchoring depth, add the height of the bow roller above the water to the expected tidal rise for the anchorage.

If the sounder reads 5 metres, the bow roller sits 1.2 metres above the waterline, and the tide will rise another 1.5 metres overnight, the true anchoring depth is 7.7 metres. A 5:1 scope requires 38.5 metres of chain. Multiplying the 5-metre sounder reading by 5 yields just 25 metres of chain. That leaves the boat with a dangerously short 3.2:1 true scope.

Chain Versus Rope Rodes

Rode material dictates the required scope because chain and rope behave differently under load. At 35 knots, a 35-foot sailing yacht generates between 400kg and 500kg of windage load on the bow.

• All-chain rode: An 8mm Grade 40 chain weighs 1.4kg per metre. In light winds, this weight creates a catenary curve that pulls horizontally on the anchor. In 35 knots, the chain lifts off the seabed and pulls bar-taut during gusts. A 5:1 scope keeps the angle of pull low enough to keep a modern 16kg anchor buried. If swinging room permits, paying out to 6:1 or 7:1 provides an excellent safety margin.
• Rope and chain rode: Nylon rope lacks the weight to create catenary. To achieve a horizontal pull on the anchor shank, use a minimum scope of 7:1. Keep at least 10 to 15 metres of chain between the anchor and the rope to resist seabed chafe.

Keep hands and clothing clear of the windlass gypsy when paying out chain under load.