RCD Design Category Calculation and Stability Test Report Requirements

What Are RCD Design Categories?

The Recreational Craft Directive assigns each boat a design category from A to D, reflecting the sea and wind conditions the vessel is designed to withstand:

• Category A — Ocean: wind force exceeding 8, waves exceeding 4 meters
• Category B — Offshore: wind force up to 8, waves up to 4 meters
• Category C — Inshore: wind force up to 6, waves up to 2 meters
• Category D — Sheltered: wind force up to 4, waves up to 0.3 meters

This category appears on the builder plate and in the Declaration of Conformity. It determines which stability and buoyancy standards apply.

Calculation Per ISO 8666 and ISO 12217

ISO 8666 defines the principal data required for stability assessment — hull dimensions, displacement, and CG position. ISO 12217 uses this data to calculate stability through two main approaches:

Part 1: Non-sailing boats (applies to RIBs and inflatables)

• Method 1 — Full GZ curve calculation from hull form and mass distribution
• Method 2 — Simplified assessment using down-flooding height and angle of vanishing stability
• Method 3 — Buoyancy assessment for inflatable boats with reduced stability criteria

The method selected depends on vessel type and design complexity. Open inflatables under 6 meters typically use Method 3, while cabin RIBs may require Method 1.

Stability Test Methods

Inclining Test

The physical inclining test measures the vessel's righting moment by moving known weights across the beam and recording heel angle. Results yield the metacentric height (GM) and confirm the calculated GZ curve. This test is mandatory for:

• Design Category A and B boats over 6 meters
• Any vessel where simplified methods cannot confirm compliance

Buoyancy Calculation

For inflatable boats, buoyancy calculation verifies that the tube collar and any built-in buoyancy elements provide sufficient reserve. The calculation accounts for:

• Tube volume at rated pressure
• Compartmentalization (each chamber must support the vessel independently)
• Swamped buoyancy — the vessel must remain afloat when fully flooded

Required Documentation Format

The stability test report must include:

• Vessel identification (model, HIN, build number)
• Test conditions (loading, fuel, water, crew positions)
• Measured data with uncertainty statements
• Calculated GZ curve and stability parameters
• Comparison against ISO 12217 minimum values
• Statement of compliance with the declared design category

Presenting Results in the Technical File

Stability documentation in the RCD technical file should include:

• Complete ISO 12217 calculation sheets
• Inclining test report with photographs
• Buoyancy calculation with tube volume data
• Maximum load and CG range derivation
• Any assumptions or deviations from standard methods

Results must tie directly to the builder plate values. Any inconsistency between the technical file and the marked data is a compliance failure.

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