Stud Link Anchor Chain Cable — Grades U2 and U3

The two grades that get specified, and what separates them

Grade is not a marketing tier. It is a different steel, a different heat treatment and a different test regime, and it changes the diameter you need for a given Equipment Number:

• U2 — special quality, 490–690 N/mm², yield at least 295 N/mm², impact energy 27 J at 0 °C. The workhorse grade: tugs, coasters, general cargo, dredgers and most coastal tonnage.
• U3 — extra special quality, at least 690 N/mm², yield at least 410 N/mm², impact energy 60 J at 0 °C or 35 J at −20 °C, and always heat-treated — normalised, normalised and tempered, or quenched and tempered. Its breaking load is about 1.43 times U2’s at the same diameter, so U3 gives the smallest and lightest cable for a given duty. That matters when the windlass, the hawse pipe and the chain locker are all fixed.

There is a third grade, U1, in mild steel at 300–490 N/mm². It needs the greatest diameter for a given duty and is consequently the heaviest cable, it is generally confined to inland craft and small harbour tonnage, and many owners running high-holding-power anchors will not accept it. We have left it out of the tables so U2 and U3 have the room — ask us if a low Equipment Number genuinely calls for it.

One relationship worth knowing when you read the table: an accessory is proof-tested at the proof load of the chain it serves, and each grade’s proof load is the next grade down’s breaking load. So the columns step through each other one grade apart, at every diameter.

Dimensions are proportions, not a catalogue

Every dimension of a stud link is a fixed multiple of the nominal diameter d: the link is 6d long, the pitch is 4d, the external width is 3.6d and the stud is 1.3d wide. That is what makes chain from different works interchangeable over the same cable lifter, and it is why the table below can be generated rather than copied. A length must comprise an odd number of links.

Shots, shackles and how much cable a vessel carries

Chain is made up and certified in shots — also called shackle lengths — of 27.5 m, joined by Kenter shackles. A vessel typically carries ten to twelve shots per side, so roughly 275 to 330 m of cable on each anchor. The total is set by the same Equipment Number table that sets the anchor mass and the chain diameter, so it is not a free choice.

Testing — and the certificate that is usually missing

Two different tests happen, and buyers routinely receive only one of them:

• Proof load: every single 27.5 m length, in the surveyor’s presence, with the chain free of paint and scale.
• Breaking load: a sample of at least three links from every fourth length, held at load for at least 30 seconds. The sample must have been made, welded and heat-treated with the chain it represents, and cut off in front of the surveyor.

The breaking-load certificate is batch-based, which is precisely why it is the one absent from grey-market and re-certified chain — the seller cannot show the batch the sample came from. If you are buying chain on certificate, ask for that one specifically, and check the stamped marking on the end links matches: grade, certificate number and the society’s stamp appear at both ends of every length.

Wear and replacement

Chain is ranged and gauged at drydock. A link is condemned once its mean diameter has lost 12 % — measured at the crown, where the wear concentrates. Cable is usually turned end for end long before that, so the bitter-end shots that have done little work move to the hawse. If you are replacing shots rather than the whole cable, the replacement must match the grade already on board: send us the marking stamped on the end link.