Every figure on a builder's plate, in a technical file and on a declaration of conformity rests on a definition. This is that set of definitions — drawn, and in plain language.
Open the reference sheetA surveyor, a builder and a notified body can measure the same boat carefully and honestly and still file three different figures. Not because anyone measured badly — because they were measuring different things. ISO 8666 exists to close that gap, and it is the standard most often assumed rather than read.
Ask for the beam of a catamaran and you will get the beam of one hull about as often as the beam of the boat. Only one of those is BH.
The standard says it plainly: on a multihull, the beam of the hull is measured across the outer hulls. A demihull beam is not a defined dimension at all. It has no symbol. Filing it as BH is not a close call about where to hold the tape — it is the wrong quantity.
The sheet sets out which dimensions span the whole craft, which are taken hull by hull, and why BWL is the odd one out.
Every symbol in the standard, each with its clause number, so it points you back to the source rather than standing in for it. Four original drawings.
Lmax · LH · LWL
What counts as the hull and what is only bolted to it. The 24 m ceiling turns on this.
Bmax · BH · BWL · BWLmax · BCB · BT
Six beam symbols, and the multihull rules that decide which of them span the whole craft.
Dmax · DLWL/2 · F · T · Tmax · Tmin · TC · Ha
Depth, freeboard and the four draughts, each with a different reference point.
β · AS · A'S · AlV · V
Deadrise, sail and windage areas, volume — and the sheerline that half the sheet depends on.
mN · mG · mLC · mP · mT · mML · mLDC
What is in and what is out of each one. The lists are where the arguments start.
All eight, light to heavy
Empty craft through to maximum load, laid out as a ladder because each is defined by what it adds.
Table 2, with the caveat
The published-data tolerances, and the clause that says they do not apply to your technical file.
Annex ZA
Where each definition bites in Annex I of Directive 2013/53/EU, and what it feeds.
The tolerance table gets quoted constantly — ±1 % on linear dimensions, ±5 % on masses. Clause 8.1 says in terms that those tolerances apply to published data, meaning brochures and marketing material, and not to the technical documentation used to assess or certify a craft. A brochure tolerance is not an answer to a reviewer.
Free to read, free to share, nothing to sign up for. Built for the desk and the boatyard alike.
Small craft principal data →Peter Jacops — Blue Peter Marine
CE INSPECTOR · IMCI AND IIMS ACCREDITED MARINE SURVEYOR · ISO/IEC 17025 AUDITOR
Chair of the South African mirror committee to ISO/TC 188 Small craft since 2000, and 28 years of survey and certification work behind the definitions above. The sheet grew out of the questions that come back most often in CE certification files — usually about a multihull beam.
The reference sheet is a working aid, not the standard, and carries no authority. Where a figure matters, work from ISO 8666:2020 itself and quote the clause.