Anchoring Lab

Start with boat length: every major manufacturer (Danforth, Fortress, Lewmar, Rocna, Mantus) publishes a length-based sizing chart, and the Size tab of this tool applies all five at once. Length alone covers most recreational boats, but refine the pick with displacement and wind exposure — a heavy 30-footer needs more anchor than a light one, and a boat that anchors in exposed waters should go one size up. As a rule of thumb, anchor weight scales roughly with boat length cubed times windage, which is why charts get conservative fast past 40 feet.

Scope = rode length ÷ total depth, where total depth is water depth + expected tide rise + bow-roller height (freeboard) above the water. To work backwards: rode length = (depth + tide + freeboard) × scope ratio. Example: 20 ft of water, 3 ft tide rise, 4 ft bow height, and a 7:1 ratio gives (20 + 3 + 4) × 7 = 189 ft of rode. Forgetting tide and freeboard is the most common scoping mistake — at high tide you may have half the scope you think.

For rope or combination rode: 5:1 minimum for a calm lunch stop, 7:1 for overnight, 10:1 or more in a storm. All-chain rode can use roughly half: 4:1 calm, 5:1 moderate, 6-7:1 rough and storm, because chain's catenary (sagging curve) absorbs shock loads. Never go below 4:1 for an overnight stay — below that the pull angle on the anchor is too steep and even a good anchor can break out.

For a combination rode, use at least one-third of the total rode as chain, or a minimum of 6 ft of chain per 25 ft of water you anchor in — whichever is longer. The rest is nylon anchor rope, which should be three-strand or double-braid for its shock-absorbing stretch. Chain does three jobs: it keeps the pull on the anchor horizontal (which is what makes the anchor dig in), it resists chafe on the bottom, and its weight forms a catenary curve that smooths out gusts. All-chain rode needs about half the scope of rope rode for the same holding.

Chain excels at keeping the pull angle low and resisting abrasion, and all-chain rode lets you swing in half the scope — a big deal in crowded anchorages. Its downsides are weight in the bow and zero stretch, so a snubber is needed in gusts. Nylon rope is light, cheap, and stretches to absorb shock loads, but needs more scope and chafes on the bottom. Most coastal cruisers use a combination: enough chain to hold the pull horizontal (a third of the rode or more), the rest nylon rope for stretch.

Swing radius ≈ rode length + boat length (LOA): when the wind shifts 180°, the bow can reach anywhere within that radius of the anchor. The Swing Circle tab computes this and checks it against the distance to the nearest hazard — add at least a 20% safety margin, plus tide/current slop. In a crowded anchorage, also check your neighbors' swing circles: two boats swinging on different scope lengths can collide even if their anchors are far apart.

Displacement matters most for heavy boats and for brands that publish weight-based charts. Rocna's selection matrix combines length and displacement (a 10 m boat at 25 tonnes needs a bigger anchor than the same length at 8 tonnes), and Mantus publishes separate day/cruising/storm tables keyed to both length and displacement. This tool's Size tab uses length charts first, then applies displacement checks for Mantus and Rocna — if your boat is heavy for its length, it sizes up automatically.

Sand: almost any anchor works; scoop anchors (Rocna, Mantus) set fastest. Soft mud: fluke anchors (Fortress/Danforth) with a wide 45° fluke setting hold best. Clay/hard bottoms: scoop or plow (Lewmar Delta). Rock: plow/claw types only — fluke anchors foul and can bend. Weed/grass: sharp-tipped scoop anchors cut through; flukes skate over the top. The Bottom tab ranks anchor types for your bottom and warns you away from bad matches like flukes on rock.

More chain improves the angle of pull and adds catenary damping, which genuinely improves holding — but it is not a substitute for anchor weight and fluke area. Think of it as a multiplier, not a replacement: an undersized anchor with lots of scope still breaks out in gusts sooner than a correctly sized one. Size the anchor to your boat first, then use adequate scope — never the reverse.

Work from the bow roller: total depth = water depth + tide rise + freeboard. Multiply by your scope ratio. Worked example: anchoring in 15 ft of water with a 5 ft tide range and 3 ft of bow height, planning an overnight stay (7:1, combination rode) → (15 + 5 + 3) × 7 = 161 ft of rode, of which at least a third (~54 ft) should be chain. The Scope tab does this arithmetic for you and splits chain vs rope automatically.

Follow the standard working-rode table: up to 30 ft LOA → 1/4-inch chain with 1/2-inch nylon rope; 31-40 ft → 5/16-inch chain, 9/16-inch rope; 41-50 ft → 3/8-inch chain, 5/8-inch rope; 51-60 ft → 1/2-inch chain, 3/4-inch rope. These are minimums for winds up to 30 knots — go up a size for exposed cruising or storm anchoring. The Rode tab shows the full table plus each Fortress/Rocna/Lewmar model's recommended chain.

Wind load on your boat grows with the SQUARE of wind speed: the load at 42 knots is about twice the load at 30 knots, and a 50-knot storm loads an anchor roughly three times harder than a 30-knot breeze. That is why this tool bumps you one anchor size up for offshore or storm exposure. Practical rules: if gusts are forecast 10 knots above the current wind, treat it as the forecast, and always size for the worst expected conditions, not the current ones.

Yes, indirectly: what matters is windage, not hull type. A sailboat with a tall rig and dodger presents more windage per foot of length than a low powerboat, so it loads the anchor harder in the same wind. Multihulls are the extreme case — two hulls and a wide beam mean very high windage, and catamaran owners commonly carry anchors sized for a monohull several feet longer. If your boat is tall, heavily canvassed, or a multihull, size up one anchor model.

A kellet (anchor sentinel) is a weight slid down the rode partway; it steepens the catenary and damps swinging. A snubber is a stretchy line (usually nylon) attached to chain rode near the bow roller — it absorbs gust loads that rigid chain would otherwise transmit straight to the anchor and your windlass. Both are most valuable on all-chain rode in gusty conditions; neither replaces adequate scope or a correctly sized anchor.

The usual suspects, in order: (1) too little scope — by far the most common cause; (2) the anchor never set properly (always back down on it firmly after dropping); (3) the bottom does not match the anchor type (flukes on rock or weed); (4) an undersized anchor for the wind; (5) a wind or tide shift that yanked the anchor out and it failed to reset; (6) fouled rode or a fouled anchor. If you drag once, suspect the setup, not bad luck — recover, re-assess, and re-anchor with more scope or a different spot.

Millions of boaters do it safely every night, but only with the right setup: 7:1 scope minimum (10:1 in forecast wind), an anchor one size up from the calm-day chart, a firm set verified by backing down, an anchor alarm on your plotter or phone, and a clear swing circle with hazards charted. Someone should always be able to respond — anchor watch or a reliable drag alarm. This tool helps you PLAN the numbers; it is not a substitute for seamanship, local knowledge, or checking the forecast. Dragging in the night can end a boat — take it seriously.