Up to the job

Andrew Blyth and Tom Nighy examine RCD structural requirements.

The ability of a boat to float, and float upright, is one of the most basic requirements for any safe craft. In previous articles we have discussed the stability and buoyancy requirements (ISO 12217), and the supporting requirements relating to watertight and quick-draining cockpits (ISO 11812), and windows, portlights, hatches, deadlights and doors (ISO 12216). However, it is also essential that the boat’s structure is strong enough to withstand the worst conditions expected for its intended (designed) use. This is expressed in Essential Safety Requirement 3.1 of the Recreational Craft Directive, which reads:

‘The choice and combination of materials and its construction shall ensure that the craft is strong enough in all respects. Special attention shall be paid to Design Category ... and the manufacturer’s maximum recommended load...’

This second fundamental requirement can be addressed in a number of ways:

• compliance with harmonised standard ISO 12215 – Hull construction and scantlings.
• using other published scantling determination methods — eg classification society rules.
• direct calculation using basic engineering principles.
• trials and/or testing — eg a drop test.
• documented empirical knowledge derived from a satisfactory service history.
• comparison with a similar boat with a service history known to be satisfactory.

The last two methods are not explicitly described in any of the official documents, but are referred to in the RSG Guidelines and the British Marine Federation RCD Workshop Manual.
The RCD also specifically requires adequate strength of strong points for towing, anchoring or mooring, and this is addressed by ISO 15084.

Documentation
In all these cases it is essential that the approach adopted is properly documented in the boat’s Technical File, in order to satisfy Notified Bodies for craft being built under RCD Type Approval Procedures and in preparation for any challenge by another party. In general, calculation methods are not used for boats under about 6m (20ft) in length, and practical (test or experience) methods are most suitable for boats up to about 9m (30ft) in length.

Where previous service history is relied upon, then this must be supported with information about numbers of craft built and year of construction, the scantlings employed, the Design Category, any changes in production methods, and records of any structural problems encountered. A one-line statement in the file stating ‘scantlings proven by experience to be satisfactory’ is not sufficient!

Practical tests or trials are also acceptable. These may either take the form of the drop-test often used for RIBs, or deliberate attempts to test a prototype to destruction — eg by operation beyond the intended loading or sea conditions. In either case it is important that the design of the boat tested, and the nature of the tests undertaken be fully documented, perhaps using photographic or video evidence. The aim in both cases should be to show that the boat tested subsequently shows no signs of damage. This approach is not generally suitable for FRP sandwich construction, since delamination from the core material may not be detectable.

The temptation may be to skimp on properly documenting a design, but this will almost certainly cause considerable grief if a subsequent challenge to the CE marking should occur.

ISO 12215 Small craft – Hull construction and scantlings
When complete, this standard is expected to supersede most Classification Society Rules for small craft, which is not all that surprising as it has been developed from all existing published scantling rules. Although work on this started in 1990, only four of its nine parts have as yet been published. This is a measure of the magnitude of the task! Leading classification societies have played a major part in its development, attempting to overcome the previous considerable disparities in their scantling requirements.
Materials
The first three parts of ISO 12215 give requirements for different types of hull construction materials:
Part 1 — Thermosetting resins, glassfibre reinforcement, reference laminate specifies requirements for reinforcement fibres and laminating resins (in both liquid and cured states), and specifies minimum mechanical properties of a ‘reference laminate’, this being a chopped strand mat laminate with a glass content not exceeding 30 per cent. These requirements invoke test methods defined in numerous other ISO standards. Fortunately these really only concern suppliers of materials, and most boatbuilders need only quote them on their orders for raw materials.
Part 2 — Core materials for sandwich construction, embedded materials sets standards for end-grain balsa, expanded foam and other sandwich core materials, as well as for embedded inserts.
Part 3 — Steel, aluminium alloys, wood and other materials specifies the general properties required of these materials and, in particular, describes the documentation required to be provided by the manufacturer of the raw material.
Production
Part 4 — Workshop and Manufacturing deals with workshop conditions, materials storage and handling and fabrication processes for boat production in glass-reinforced plastic, steel, aluminium alloy and wood, the qualifications of production personnel, and final inspection and quality assurance.
Design
None of the following parts of the Standard have yet been finalised. Parts 5 and 6 are available in draft form, but Parts 7, 8 and 9 are at an early stage of drafting.
Part 5 — Design pressures, allowable stresses, scantling determination (which is aimed at monohull boats) is not yet published but has just been released for the final vote. This is technically the most challenging part of the Standard, as it attempts to quantify the loads that the hull has to withstand, and the maximum acceptable stresses resulting from these loads, according to the location on the hull, material type, construction method and structural design

The standard provides four routes to assessment:
Route 1 Using the body of the standard (virtually essential for Design Categories A & B).
Route 2 Using Annex A.1 for both power and sailing boats of up to 12m in length for Design Categories C and D.
Route 3 Using Annex A.2 for lightweight sailing boats of up to 9m (30ft) in length and for Design Categories C and D.
Route 4 Using the drop test in Annex B for single skin FRP boats of up to 6m (20ft).
The full technical method (route 1) is contained in four clauses of the standard:

Clause 6 gives formulae that enable the design pressures for hull bottom, topsides, decks, bulkheads and superstructures to be derived, the main inputs being length, draft and design category, with the addition of beam and speed in the case of power boats. Power boats are assigned a dynamic load factor, which is limited according to whether the boat is a displacement cruiser, a sportsboat or a rescue or racing craft that must operate at speed in all conditions.

Clause 7 describes how to correctly derive the dimensions of structural panels and stiffeners, in particular in relation to top-hat style FRP stiffeners.

Clause 8 gives equations to determine the thickness of the skin (or plating) for single-skin and sandwich FRP, steel, aluminium alloy or wood construction.

Clause 9 gives similar equations for the scantlings of stiffeners, including limits on their proportions to avoid buckling under load.

Tackling this approach calls for a clear mind and a scientific calculator. It is not for those who are frightened by formulae, and is probably best given to professional designers and engineers!

Annex A.1 (route 2) gives a substantially simpler way for assessing the majority of smaller boats intended for coastal or sheltered waters — using graphs instead of equations to derive the required thickness of a ‘reference laminate’ (chopped strand laminate with less than 30 per cent glass content). This is simply obtained by multiplying the requisite coefficient (from the graphs) by the unsupported width of the panel. Annex A.3 enables this thickness to be modified where the construction is in other forms of FRP, or in steel, aluminium alloy or wood.

Similarly graphs give the required bending properties (section modulus) of stiffeners. Annex G enables these figures to be translated into a variety of stiffener shapes giving the required strength.

Annex A.2 (route 3), recognising that Annex A.1 may often give conservative figures for small lightweight boats, provides an even simpler route for sailing boats under 9m (30ft) in length. One formula using boat mass, panel length, breadth and curvature gives the required thickness of ‘reference laminate’, which can again be modified for other materials using Annex A.3. On the basis that such boats will be regularly inspected, there is no minimum skin thickness for sandwich construction.

Annex B (route 4) is a simple drop test, intended for boats of up to 6m (20ft) length of single skin FRP construction but not sandwich construction. The height from which the boat must be dropped is given as graphs against boat length for varying ratios of maximum loaded speed to the square-root of waterline length. These heights vary from 0.7m (2.3ft) up to 2.5m (8.2ft), and represent the impacts that would be experienced when running at speed in critical wave conditions. It would appear that this test is intended for planing rather than displacement boats, although this is not explicitly stated.

The standard limits this method is to single skin FRP boats, but the authors can see no obvious reason why this test could not also be used for other non-sandwich materials.

Other Annexes give more detail of calculation methods for design in different materials, including the analysis of complex laminate build-ups, and giving the geometric properties of different styles of stiffeners.

Part 6 – Details of design and construction gives detailed requirements for both FRP and metal (steel or aluminium alloy) construction. It offers design guidance rather than detailed calculation methods. Amongst other aspects it covers:

• l structural continuity and detail design.
• principles of arranging suitable stiffening.
• local reinforcement — eg in way of keels, engines, or highly-loaded fittings.
• thickness of transoms in way of outboards or sterndrives.
• docking, chocking or trailer loads.
• fitting of appendages such as keels, shaft brackets.
• hull-to-deck joints.
• welding and rivetting of metal structures.

This part is undergoing validation, and should be published fairly soon.

The following parts of ISO 12215 are under development:
Part 7 – Scantling determination for multihulls.
Part 8 – Rudder stocks and bearings.
Part 9 – Appendages and rig attachments.

ISO 15084 Small craft — Anchoring, mooring and towing — Strong points
This standard was developed to address Essential Safety Requirement 3.9: ‘All craft, taking into account their design category and their characteristics shall be fitted with one or more strong points or other means capable of safely accepting anchoring, mooring and towing loads’.

The position and minimum number of strong points required increase with LH and are:

• where LH is less than 6m (20ft) one point forward for anchoring, mooring or being towed. And note this might be simply a stem eye on dinghies.
• where LH is more than 6m (20ft) one point forward plus an additional point aft, for mooring or towing.
• where LH is more than 12m (39ft) two mooring points forward and two aft.
• where LH is more than 18m (59ft) as for over 12m (39ft) plus one port side and one starboard side mooring point.
  The standard specifies formulae for the loads that these strong points should withstand without sign of permanent deformation or structural failure:
• for forward strong points for anchoring or being towed load (kN) = f (4.3LC - 5.4).
• for forward strong points for mooring load (kN) = f (3.5LC - 4.3).
• for aft strong points load (kN) = f (3.0LC- 3.8) where f = 1.0 for Categories A and B, 0.9 for Category C, 0.75 for Category D and LC = (LH + LWL) / 2 (in metres)

In any case, the load is not required to exceed the fully-loaded mass of the boat, but should be at least 125 per cent of the breaking strength of any rope or chain that is specified or supplied.

That’s all folks!
We trust that you have found this series of articles introducing the major standards associated with the Recreational Craft Directive helpful. We have tried to give the reader a general overview and introduction. However, if you are building boats then you really need access to a lot more detail, including all those standards that affect your type of boat. The British Marine Federation offer a CD ROM containing over £2,500 worth of ISO/CEN standards, plus the text of the Directive, the European Commission Comments and Interpretation, the RSG Guide and the BMF Workshop Manual including 11 in-depth case studies of different types of boats. All this for £500+VAT (£250+VAT to Federation members). The French Federation Industries Nautique (FIN) offer a similar CD ROM. We suggest you check with your own national industry body first.

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