Small craft principal data — reference sheet

ISO 8666 does one job: it fixes what every principal dimension, mass and loading condition means, so that a figure quoted by a designer, a builder, a surveyor and a notified body all mean the same thing. It applies to craft with a hull length under 24 m. Unless a clause says otherwise, every measurement is taken with the craft at rest at the maximum load waterline, WL_ref 5.1.

The multihull trap

Almost every athwartship dimension on a catamaran is taken across the outer hulls — not across one hull. B_max and B_H both say so in as many words. A demihull beam is not a defined dimension in this standard and has no symbol, so recording it as B_H is a category error rather than a measurement dispute.

The exception is B_WL, which 5.3.4 does take hull by hull. The across-the-outer-hulls waterline beam is a separate symbol, B_WLmax, and it exists for multihulls only. Lengths go the other way again: L_H and L_WL are measured per hull, longest wins.

An open defect in the multihull B_max

The wording of B_max for multihulls is regarded within the working group as defective in the current edition. The opening sentence of the beam definitions reaches for the outermost parts of the craft, yet the multihull provision then ties the dimension to the outer hulls. Read literally, that shuts out anything mounted outboard of the hulls which ought to widen B_max — exactly the parts the general definition was written to catch.

Until the text is amended the published wording governs, and the table below reports it as written. Flag the point on any file where non-hull parts project outboard of a demihull, and expect it to surface at the next systematic review.

Symbol	How a multihull takes it	Basis
L_max	Whole craft, tip to tip	whole craft
L_H	Each hull individually; take the longest	per hull
L_WL	The longest individual hull's	per hull
B_max	Across the outer hulls	whole craft
B_H	Across the outer hulls	whole craft
B_WL	Each hull individually	per hull
B_WLmax	Across the outer hulls	whole craft
B_CB	Between the centres of buoyancy of the outer hulls	whole craft

Longitudinal5.2

Lengths are measured parallel to WL_ref and the centreline, as the distance between two vertical planes set perpendicular to the centreplane 5.2.1.

Figure 1 — L_H, L_max and L_WL · The bowsprit and boarding platform are normally fixed, so they fall inside L_max and outside L_H. L_WL stops short of both, at the flotation plane.

L_max

Maximum length5.2.2

Tip to tip. Counts the structure and anything normally fixed to it — fixed spars, bowsprits, pulpits at either end, stemhead fittings, rudders, outboard brackets, outdrives, waterjets, propulsion extending past the transom, diving and boarding platforms, rubbing strakes, permanent fenders. Movable propulsion is taken at full extension underway. Outboard motors themselves are out, as is anything that comes off without tools.

L_H

Length of the hull5.2.3

Structure only — stems, sterns, bulwarks, hull/deck joints. Out: anything detachable without damage and without touching structural integrity, which sweeps up most of what L_max counts, plus outboard motors and their brackets and plates. Fenders count only if they take hydrostatic load at rest or underway. On a multihull, measure each hull and take the longest.

L_WL

Waterline length5.2.4

Forward, where the stem cuts the flotation plane. Aft, where the hull cuts it. Stated for a designated load condition and its design trim. On a multihull it is the longest individual hull's.

Athwartship5.3

Measured between two vertical planes parallel to the centreplane, craft upright 5.3.1.

Figure 2 — B_H and B_max on a catamaran, plan view · Both arrows cross the open water between the hulls. Neither can be read as a single-hull dimension. They differ only by the outboard liferail brackets, which are permanently fixed but form no part of the hull. In most cases B_H is equal to B_max.

Figure 3 — The beam family at half-length of L_WL · B_WL is drawn on one hull because that is how 5.3.4 takes it. Everything below it spans the craft.

B_max

Maximum beam5.3.2

Outermost to outermost. Counts hull extensions, hull/deck joints, doublings, sheer planks, chain plates, rubbing strakes, permanent fenders and liferails standing proud of the side. Multihull: across the outer hulls.

B_H

Beam of hull5.3.3

Between the outermost permanently fixed parts of the hull. Counts structure, hull/deck joints and bulwarks. Excludes rubbing strakes, fenders, liferails, stanchions and similar. Multihull: across the outer hulls.

B_WL

Beam, waterline5.3.4

The widest cut of the hull surface through the flotation plane, for a stated loading condition. This is the one athwartship dimension that a multihull takes hull by hull.

B_WLmax

Maximum beam, waterline5.3.5

Multihulls only. The waterline beam across the outer hulls.

B_CB

Beam between hull centres5.3.6

Catamarans and trimarans only. The transverse distance between the centres of buoyancy of the outer hulls — not between hull centrelines, though on a symmetric demihull the two coincide.

B_T

Transom beam3.4

Widest part of the hull at the transom, at or below the sheerline, ignoring extensions, handles and fittings. Spray rails count where they work as chines or planing surface. On a rounded or pointed stern, or where the transom is under half the maximum beam, take the widest beam at or below the sheerline in the aft quarter length.

On most craft B_H and B_max come out equal or within a few millimetres of each other. A gap only opens where something that is not part of the hull stands proud of the topsides, and that is the uncommon case rather than the normal one.

Rubbing strakes: two readings

5.3.3 asks for the outermost permanently fixed parts of the hull. Two competing readings of that phrase are in circulation, and a rubbing strake sits squarely between them.

Reading A treats a strake as an attachment to the hull rather than part of it, so it falls outside B_H and is picked up by B_max only. That is the reading used throughout this sheet.

Reading B, held within the ISO/TC 188 working group, is that the discriminator between the two dimensions is hull versus craft, not fixed versus loose. Since almost every strake is permanently fixed to the hull, on this reading it belongs to the hull and sits inside B_H.

The two readings can differ by 30 mm to 60 mm on a moderate strake — enough to move a craft across a contractual beam limit, a berth width or a road transport threshold. Where the figure matters, state which reading was applied and record the strake dimension separately rather than leaving it inside a single number.

Vertical5.4

Figure 4 — D_max and T_max · D_max takes its top reference at half-length of L_WL but its bottom reference at the lowest keel point, which the sloping keel puts further aft. T_max starts from the waterline and reaches the same point here, because nothing hangs below the keel.

D_max

Maximum depth5.4.1

Sheerline at half-length of L_WL down to the lowest point of the keel. The two references need not share a station — on a sloped keel the deep point can sit well aft, which is what the clause note warns about.

D_LWL/2

Midship depth5.4.2

Both references at half-length of L_WL: sheerline to the lowest keel point at that same station. This is where it parts company with D_max.

Freeboard5.4.3

Sheerline down to the waterline, at a stated station and loading condition. F_A at the aftermost sheerline point (5.4.3.2), F_M at half-length of L_WL (5.4.3.3), F_F at the forwardmost sheerline point (5.4.3.4).

Draught5.4.4.1

From the fully loaded ready-for-use waterline down to a nominated point of the underwater body. Which point is a choice, so T alone is not determinate — the subscripted forms are.

T_max

Draught, maximum5.4.4.2

To the lowest point of the underwater body or any appendage, with centreboards down.

T_min

Draught, minimum5.4.4.3

To the lowest point of the craft or of any non-retractable appendage, whichever is deeper, with everything movable raised as far as it goes.

T_C

Draught, canoe body5.4.4.4

Canoe body only, at the centreline, excluding rudders and skegs. Where keel and hull do not separate cleanly, take the least steep tangent to the hull surface and intersect it with the centreplane.

H_a

Draught, air5.4.5

From the flotation plane in the light craft condition up to the highest point of structure or mast. Note the condition — this one is not measured at WL_ref.

—

Headroom5.4.6

Cabin or compartment floor up to the underside of the deck beam or deckhead, whichever is lower, at a nominated position. The builder may quote it elsewhere too.

Other data5.5 · 3.3

Deadrise angle5.5.1

arctan(height / width), taken athwartship at a stated position. Steps and protrusions are ignored on a straight bottom. On a concave bottom with keel, measure between keel intersection and chine. On a convex bottom, between one quarter and three quarters of B_H/2. On a concave bottom with a wing, between centreline and outer wing end.

A_S

Reference sail area5.5.2

Sails set abaft a mast at actual profile area, plus the maximum profile area of every mast, plus a reference triangle of IJ/2 forward of each mast. Where a forestay between masts stops short of the deck, its triangle counts only if sails can actually be set on it.

A'_S

Standard sail area5.5.3

Actual profile area of the largest windward sail plan the builder supplies or recommends as standard, overlaps included.

A_lV

Windage area5.5.4

Projected profile area of hull, superstructure, deckhouses, outboard motors and spars above the waterline, craft upright, at the relevant loading condition.

Volume of the craft5.5.5

V = V_H + V_S. The hull may be approximated as 0,15 L_H times the sum of six beam-times-depth products taken at 0, 20, 40, 60, 80 and 100 percent of L_H. Superstructure volume counts each part above the sheerline; a space open on no more than one side is included, and anything under 0,05 m³ is dropped.

—

Sheerline3.3

Where deck meets hull — the natural intersection on a rounded deck edge, or the top of the hull where there is no deck or a bulwark stands proud. Its upper position depends on the angle α between the hull/deck intersection and the actual deck: at α ≥ 45° the lower position applies, below 45° the upper. This feeds straight into D_max and freeboard.

Masses6

Two families sit here. The shipping masses describe a boat in transit. The rest describe a boat in use, and they build on one another rather than standing alone.

m_N

Mass, net shipping6.1

Everything permanent and loose the builder delivers with the craft. No shipping materials.

m_G

Mass, gross shipping6.2

m_N plus the shipping materials — cradles, supports, fastenings, covers.

m_LC

Mass, light craft6.3

The unladen craft. In: structure including ballast keel, boards and rudders; removable ballast meant to be carried underway; internal structure and accommodation; permanent engines and fuel systems, or the heaviest recommended outboard even if a lighter one is fitted; permanent internal equipment, batteries, fixed electronics, fixed fire gear, mattresses and curtains; permanent deck fittings, anchors, warps, fenders; and for a sailing craft the rig with upwind sails aboard and rigged but not hoisted. Out: loose gear, tools, spares, extra sails, personal safety and life-saving gear, provisions, bilge, waste and potable water, fuel, portable tanks, personal effects, liferafts, dinghies and persons.

m_P

Performance test mass6.4

Permanently attached standard equipment, plus the loose gear needed to operate safely, plus the crew needed to run the boat, plus personal safety gear for everyone aboard. Fuel between 25 and 50 percent of permanent tank capacity, or one portable tank per engine at least half full at the start of each trial. Fresh water, waste water, provisions and loose galley gear are out.

m_T

Mass on a trailer6.5

Only established for craft advertised as trailerable, so the owner can see what headroom is left against the trailer's capacity. Includes tank contents, unlike m_LC. The builder or dealer must list what is in and state the total, and may declare exclusions — but not structure or equipment needed to operate safely.

m_ML

Maximum load6.6

What the craft carries on top of the light craft condition: crew limit at 75 kg each and their personal effects; stores, cargo, dry provisions and consumable liquids; permanent tanks at 95 percent of capacity, with ballast water at 100; portable consumable tanks at 95 percent; a dinghy and its outboard if carried; liferafts beyond the required minimum; and non-edible stores outside the builder's standard list. Allow at least 20 kg per person for effects on a habitable craft. Annex ZA reads this as the manufacturer's maximum recommended load for the builder's plate.

m_LDC

Loaded displacement3.6

Mass of water displaced, appendages included, in the fully loaded ready-for-use condition of 7.3.

Loading conditions7

Eight conditions, listed here light to heavy rather than in clause order. Each one is defined by what it adds to the one before, so the ladder is the definition.

7.4

Empty craft

Structure, fixed ballast, accommodation, permanent engines and fuel, residual working fluids, permanent internal and external equipment, rig stowed. Boards and drop keels raised unless they lock down and the owner's manual says so. No optional equipment outside the basic outfit.

7.5

Light craft

Empty craft plus standard equipment plus removable ballast carried afloat. Heaviest recommended outboard over 3 kW in the working position. Batteries in place — one per engine over 7 kW within 1 m if there is no stowage. Upwind sails aboard, rigged, not hoisted. Variable elements symmetrical about the centreline. Mass in this condition is m_LC.

7.6

Minimum operating

Light craft plus a crew mass on the centreline by the main control position — 75 kg up to L_H 8 m, 150 kg from 8 to 16 m, 225 kg from 16 to 24 m — plus non-edible stores outside the standard list. Main storage tanks excluded; so is symmetrical water ballast meant for asymmetric trimming.

7.2

Ready for use

Fully equipped for the intended use, fuel and fresh water tanks filled, bait tanks and live wells to their designated limit. Liquids at full usable tank volume. Outboards and batteries at the highest rating the craft is meant to run.

7.3

Fully loaded ready for use

Ready for use plus persons at 75 kg each in their normal seating positions, plus personal and basic equipment of (L_H − 2,5)² kg with a floor of 10 kg, plus liferaft and dinghy if carried. This is the condition m_LDC is taken in, and the builder must declare both the mass and the resulting draught.

7.8

Maximum load condition

Light craft plus m_ML, plus an allowance for optional equipment and fittings outside the basic outfit, arranged to produce the design trim with the crew where they would normally sit underway. The waterline here is WL_ref.

7.7

Loaded arrival

Maximum load condition less 85 percent of fixed and portable tank capacity for fuel, oils and drinking water, and less 90 percent of edible stores — but carrying the worst combination of optional fittings for stability. A boat at the end of a passage, not the start.

7.1

Test

Equipped to m_P, for determining manoeuvring speed and maximum powering.

Sailing or non-sailing3.10 · 3.11

The split is arithmetic, not a matter of appearance. Compare the reference sail area against the loaded displacement:

A_S ≥ 0,07 × (m_LDC)^2/3 → sailing boat
A_S < 0,07 × (m_LDC)^2/3 → non-sailing boat

Annex ZA points these definitions at Annex I, Part A, 5.4.2 of Directive 2013/53/EU — the emergency steering requirement. A motorsailer landing near the threshold changes which requirement applies to it.

Tolerances8

Read 8.1 before using this table

These tolerances apply to published data — figures used as a specification in brochures and marketing material. Clause 8.1 says in terms that they do not apply to the technical documentation used to assess or certify a craft. Quoting a brochure tolerance at a reviewer is not an argument.

Published data	Tolerance %
Linear dimensions, rigid craft	±1
Linear dimensions, inflatable boats	±2,5
Sail areas	±5
Displacement	±10
Volumes	±5
Masses	±5
Speed, in the test condition	±5

Where a figure is critical — maximum beam, or maximum mass on the trailer — the plus tolerance does not apply. Preliminary figures must be labelled as such (preliminary, approximate, estimate, varies) and then run to ±5 % on dimensions and ±15 % on masses, displacement and volumes 8.2.

Reference lengths 8.3

How the standard reads its own length thresholds. Worth knowing when a boat sits on one.

Written as	Means
from 2,5 m	≥ 2 500 mm
longer than 6 m	> 6 000 mm
to 12 m	≤ 12 000 mm
less than 6 m	< 6 000 mm
up to 24 m	< 24 000 mm

Where it bites under the RCDAnnex ZA

EN ISO 8666:2020 is harmonised under Directive 2013/53/EU, so compliance carries a presumption of conformity for the parts of Annex I it covers, for as long as the reference stays in the Official Journal.

3(10)

Hull length5.2.1, 5.2.3, 8.3

The Directive's definition of hull length defers to the harmonised standard. L_H is where the 24 m ceiling and the 2,5, 6 and 12 m thresholds are decided.

I.A.2.2(d)
I.A.3.6

Builder's plate and maximum recommended load6.6

m_ML is what the Directive means by the manufacturer's recommended maximum load, read together with EN ISO 14946.

I.A.3.1–3.3

Structure, stability, freeboard, buoyancy3–8

Feeds EN ISO 12215 for scantlings and EN ISO 12217 for stability, buoyancy and design category.

I.A.3.4

Flooding5.2.3, 5.3.2, 5.4.3.3

Supplies L_H, B_max and F_M to the cockpit volume coefficient in EN ISO 12216.

I.A.5.4.2

Emergency steering3.8, 3.9

Turns on whether the craft is a sailing or non-sailing boat.

This sheet is a reference aid prepared by Blue Peter Marine. It is not the standard and carries no authority. Every definition here is a condensed restatement in our own words; where a figure matters, work from ISO 8666:2020 itself and quote the clause. Clause numbers throughout follow the 2020 third edition — they were renumbered from the 2016 edition, so a 2016 cross-reference will not line up. Figures are original drawings, schematic and not to scale.