Design Categories

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The Categories

The RCD has four design categories A, B, C and D and each (individual) craft must be placed in one (and only one) of these.  Note that because a manufacturer certifies a new craft for one category (eg B), it is not necessary for used craft of the same type to be placed in the same category for Post Construction Assessment (PCA).  In fact this may be difficult if the manufacturer refuses to disclose the required information, for example stability curves.

Lets first try to understand the four categories.  Each category relates to the scope of use but in fact is determined by the significant wave height and wind force (Beaufort Scale) such scope is expected to encounter.

Design Category Scope Wind Strength Wave Height Comment
A Ocean 8 + < 8m Designed for extended voyages where conditions may exceed wind force 8 (Beaufort scale) and significant wave heights of 4 m and above but excluding abnormal conditions, and vessels largely self-sufficient*
B Offshore < 8 < 4m Designed for offshore voyages where conditions up to, and including, wind force 8 and significant wave heights up to, and including, 4 m may be experienced*
C Inshore < 6 < 2m Designed for voyages in coastal waters, large bays, estuaries, lakes and rivers where conditions up to, and including, wind force 6 and significant wave heights up to, and including, 2 m may be experienced*
D Sheltered < 4 < 0.3m Designed for voyages on sheltered coastal waters, small bays, small lakes, rivers and canals when conditions up to, and including, wind force 4 and significant wave heights up to, and including, 0,3 m may be experienced, with occasional waves of 0,5 m maximum height, for example from passing vessels*

*definitions taken from 2008 RSG Guidelines issued to Notified Bodies.

Certification and Design Category

It's perhaps obvious that the hurdles for Category A are greater than for the B, C and D and that this logic applies all through.  What is less obvious however, is that the type of data required to support compliance may differ too.  Of these differences, the largest is the requirement with Catagory A and B for one or more stability curves to be available, whereas for C and D alternative physical tests will suffice.  This can be a significant issue as these curves, termed GZ curves (this relates to the righting arm) require a computer model to derive, which itself requires that a virtual 3D model of the craft to be created, based on the lines plan (a lines plan shows waterlines, buttock lines and a body plan, not a profile plan showing cabin layout or sailplan which will not suffice).  Even assuming the builder/designer is still in business, they may not be willing to provide this information.  This is particularly the case for older (<1995) sailing yachts.

We can build a model from lines plans, or indeed measure the physical hull to develop a lines plan but all this adds considerable expense and time.

This is one of the main reasons so many craft have been placed in Category C even though they may in other respects be Catagory A or B material.

Design Categories in Practice

You can probably guess where this is going.  In an environment where few took notice of Catgeory and were largely interested in whether the CE Marking had taken place (ie insurers, customs officials, brokers, in fact everyone), it is unsurprising that most craft certified since 1998 have been placed in Catagory C.

Builders have a vested interest in aiming for the higher Catagories as this gives them the opportunity to brag that their Catagory B craft is more seaworthy (and thus valuable) than their competitors Catagory C craft.  This may be true but  the fact is that most people who need a seaworthy craft are more likely to be swayed by opinion and experience than a CE Category.

So, should you strive for Catagory B rather than C,  just because the manufacturer has?  If you can, by getting the required data from the manufacturer, then Yes.  If it looks like the authorities are actually going to start to take notice of Design Catagories, then Yes.  If the answer to both is no then why bother?  Ultimately as importer it is your choice....

Clear as mud?

Update : Autumn 2010 : We are now aware that an insurer has refused to place a Category C craft on risk for offshore use in Norway.  This is in spite of the fact that the craft, a Grand Banks Trawler, is of a design known to be suitable for offshore passage making.  This is the first case to our knowledge where an insurer has restricted use based on the Design Category.  Though not yet a policy within the industry as a whole, it is likely that this will occur over time.

Written By: Rowland Smith

Rowland Smith is a Naval Architect and founder of Gablemarine.  His industry experience includes Lloyds Register, British Shipbuilders Hydrodynamics Ltd, Cammell Lairds, BP Shipping Ltd & Conoco.  He has a degree in Naval Architecture, a Diploma in Marketing (CIM) and an MBA (Cranfield School of Management).  He has also held Director level positions in a number of technology and engineering companies, including CEproof, which provides RCD compliance software to builders. 

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