Marine engineering tool
Enter a vessel's particulars and this works out its Equipment Number, then returns the anchors and chain cable the published tables require — with the chain, joining shackles and anchors that match, ready to quote.
The Equipment Number is a single figure classification societies calculate from a vessel's displacement, breadth, and the height and profile area of everything above its waterline. It is not a weight or a length: it is an index into the anchoring equipment table, which gives the number of anchors, the mass of each, and the length and diameter of chain cable required.
Show me how to measure chain diameter
Measuring an existing chain
The quickest way to confirm a chain size is to measure the steel rather than read a certificate. Three measurements, on a link away from any wear or distortion:
- Link diameter (d)
- The thickness of the bar, measured on the straight side of the link, clear of the stud and clear of the crown. This is the number the chain is sold by.
- Link length
- Outside length of one link, end to end. On stud link chain it runs about 6 × d.
- Link width
- Outside width across one link. About 3.6 × d.
If the three do not agree, trust the bar diameter and send us photographs with a tape or caliper in frame — worn chain measures small, and a stretched link measures long.
How this works
The Equipment Number
EN = Δ^(2/3) + 2.0(hB + Sfun) + A/10
- Δ
- moulded displacement, in t, to the Summer Load waterline
- B
- moulded breadth, in m
- h
- effective height, in m, from the Summer Load waterline to the top of the uppermost house; h = a + sum(hi), counting only tiers of breadth greater than B/4
- a
- vertical distance at hull side, in m, from the Summer Load waterline amidships to the upper deck
- Sfun
- effective front projected area of the funnel, in m^2 (AFS - Sshield). Taken as zero when the funnel breadth is B/4 or less at all elevations
- A
- side projected area, in m^2, of hull, superstructures, houses and funnels above the Summer Load waterline, within the equipment length and of breadth greater than B/4
The funnel term is zero on most vessels — it only counts when the funnel is wider than a quarter of the breadth.
IACS UR A1 Rev.7 A1.2.1
One table, every society
The anchoring equipment table is an IACS Unified Requirement, which every member society adopts. Lloyd's Register, DNV, ABS, Bureau Veritas, ClassNK, the Korean Register, the China Classification Society, the Indian Register of Shipping and RINA all size anchors and chain from the same figures. So choosing a society here does not change the result — it only decides whose certificate comes with the steel.
- IACS UR A1 Table 1 — Ships of unrestricted service, EN 205 and above.
IACS UR A1, Rev.7 2020 / Corr.1 2021 - IACS Rec. 10 Table 1 — Ships not covered by UR A1, 50 <= EN < 205. Grades 2 and 3 share a single diameter column.
IACS Recommendation No. 10, Rev.4 2020
Small vessels
UR A1 starts at an Equipment Number of 205. Below that, down to 50, the figures come from IACS Recommendation No. 10 — a recommendation rather than a binding requirement, which matters for the small workboats, fishing vessels and tugs that fall in this range.
The same table, seventy years ago
This table is also printed in our own Keating catalogue, alongside the equipment letter-codes the societies used at the time. Its chain lengths and anchor masses agree with the current rules on every row they share. Two things did change: the catalogue requires three anchors where the rules now ask for two, and it offers no Grade 3 below an Equipment Number of 840. If you are working on an older vessel, that is the kind of difference worth knowing about.
Questions
Does the classification society change the answer?
No. The anchoring equipment table is an IACS Unified Requirement, so Lloyd's Register, DNV, ABS, Bureau Veritas, ClassNK and the other members all use the same figures. The society only determines which certificate is issued with the chain and shackles.
Is this displacement or deadweight?
Displacement — moulded displacement to the Summer Load waterline, in tonnes. Using deadweight or gross tonnage will give an Equipment Number that is wrong, usually badly. The figure is in the stability book or on the capacity plan.
Can I fit a lighter anchor than the table says?
Only if it is approved as high holding power, in which case each anchor may be 75% of the table mass, or as super high holding power, which allows 50% but is limited to restricted service. The reduction comes from full-scale testing of that anchor design, so it is not something you can elect to apply.
Is the chain length per anchor or in total?
The table gives the total, and it is divided in approximately equal parts between the two bower anchors. The result above shows both figures, and the per-anchor length in shots.
What is the difference between Grade 1, 2 and 3?
The steel, not the size. A Grade 2 and a Grade 3 chain of the same diameter are physically the same and take the same joining shackle, but the Grade 3 is stronger — its proof load is the breaking load of Grade 2. Going up a grade buys roughly 40% more breaking load in the same section.
My vessel is not classed. What should I use?
The calculator still works, and it will tell you it is using the IACS table without confirming the vessel as classed. But an unclassed or small commercial vessel may be governed by flag-state regulations or by whatever standard the builder chose, so measure the chain actually fitted and treat any certificate that came with it as the better authority.
Does this cover mooring chain?
No. Anchor chain cable is Grades 1, 2 and 3. Offshore mooring chain is Grades R3 to R5, under a different requirement, and the accessories are not interchangeable — approval for one grade is not approval for another.