Enter the boat's length and displacement, the chain grade and where you'll anchor. The calculator gives Mantus's recommended chain size and checks its working load against the ABYC anchor loads. It also works out how much rode the scope needs.
Chain size
Mantus publishes a chain sizing table by boat length and displacement. When the two point at different columns, the calculator takes the larger one.
| Boat | Displacement up to | BBB or proof coil | G43 high test |
|---|---|---|---|
| up to 20 ft | 5,000 lb | 1/4 in | 1/4 in |
| 20 to 30 ft | 11,000 lb | 5/16 in | 1/4 in |
| 30 to 35 ft | 15,000 lb | 5/16 in | 5/16 in |
| 35 to 40 ft | 20,000 lb | 3/8 in | 5/16 in |
| 40 to 45 ft | 40,000 lb | 3/8 in | 3/8 in |
| 45 to 50 ft | 50,000 lb | 7/16 in | 3/8 in |
| 50 to 60 ft | 60,000 lb | 7/16 in | 7/16 in |
| 60 to 70 ft | 70,000 lb | 1/2 in | 7/16 in |
| 70 to 90 ft | 110,000 lb | 5/8 in | 1/2 in |
Loads and chain strength
Mantus reproduces the ABYC horizontal working load table, which ABYC developed for anchors, rode and cleats. For a 30 ft boat it gives 700 lb at 30 knots and 1,400 lb at 42 knots. Mantus sizes rode so its working load covers the storm figure, on the view that sustained winds over 40 knots are common for cruisers at anchor.
For chain strength, Mantus takes the working load as a quarter of the breaking strength for every grade, so they compare like for like. 5/16 in G43 high test comes out at 2,925 lb and 5/16 in BBB at 1,900 lb. For G70 and 316 stainless chain Mantus gives strengths but no size table, so the calculator shows the smallest size whose working load covers the 42-knot load.
A snubber or a nylon section in the rode cuts shock loads by a factor of 3, Mantus says. It still sizes the chain for the full load, in case the snubber fails.
Scope
Rode needed = scope × (depth + bow roller height). Mantus's holding figures by scope: 55% of the anchor's maximum at 4:1, 70% at 6:1, 80% at 8:1 and 85% at 10:1. Use the depth at high water.
On the survey
Read the size and grade stamped on the chain, and check the windlass gypsy matches it: Mantus notes a gypsy for 5/16 in high test won't take 5/16 in BBB, whose links are shorter. Range the chain out where you can and look for rust and worn links, especially at the anchor end. Note the anchor's make and weight, and on a multihull compare it with Rocna's advice to go up to 50% larger.
Citing it in a report
"Ground tackle: 25 kg anchor on about 60 m of chain marked 5/16 in G43, gypsy matched. Chain galvanising worn with rust over the first 10 m. Recommend the chain is end-for-ended or renewed." Give the anchor, the chain size and grade, the length and its condition.