Knowing the hull shape and geometry is one of the most important factors for predicting a boat's performance. Hull shape describes not only length of waterline while sailing, but also displacement, wetted surface, and the volume distribution along the hull from which hydrodynamic forces of lift and drag can be calculated. It is the only way to address properly these calculations through the Lines Processing Program (LPP) that calculates all relevant hydrostatics data used by the Velocity Prediction Program (VPP).

A hull measurement is performed to create an offset (OFF) file describing the body plan of the hull together with appendages, by using an ORC-approved hull measurement machine or any available measurement instrument capable to produce a list of the points in the co-ordinate system defined as follows:

  • X axis – longitudinal with 0 at stem and positive towards the stern
  • Y axis – transverse with 0 at the centerline and positive outwards
  • Z axis – vertical with 0 at the waterline and positive upwards
Measurement co-ordinate system   Resulting hull lines in OFF file

Hull is measured by taking half width (Y) and height (Z) of station points with equal distance from stem  (X) from the deepest point to the sheerline as follows:

  • Approximately 20 stations up to a maximum of 180 are taken from either side, spaced with maximum distance of 5 % LOA and 2.5 % of LOA within the forward 15% of the hull.
  • Two stations where freeboard measurements are taken are at the same distance from the stem on port and starboard sides:
    (1) the forward freeboard station is normally placed approximately 0.5 m from the stem, but it may also be placed in other recognized distance from stem
    (2) the aft freeboard station is normally placed at the intersection of the hull and the transom at the sheerline, but it may also be placed in other recognized distance from stem
  • Stations are also taken at the edges of any appendage, at the maximum draft and at any significant change of  profile in the longitudinal direction.

Once a hull is completely measured, an OFF file is generated using ORC-approved software by the Rating Authority or if needed by the ORC Chief Measurer. Such an OFF file can then be used for any boat of the same production model.

Therefore, no any new hull measurements are required for boats built from the same moulds for hull, keel and rudder. Any changes in keel and/or rudder can be measured separately using the same method and then modifying the original OFF file.

There are more than 12.500 offset files existing in the ORC database, including most standard production boats. Thus, new hull measurements are required only for a completely new production boat or for custom builds.

In lieu of hull measurement, OFF files can also be obtained by the designer in form of 3D surface files that include the hull and all appendages. These would then need to be verified by measurements that compare the actual boat with the hull lines provided. Offset file data is proprietary information owned by the boat designer and protected under copyright law, and is thus available only to authorized ORC Rating Authorities. They can not be disclosed or sent to any third party without expressed permission from the designer.


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