Measurement is a fundamental part of any rating system. In order to have a boat's rating calculated, the boat’s characteristics need to be defined by taking the measurement of all parts that affect the boat's performance.

The International Measurement System (IMS) is a set of rules that defines what and how is measured on the boat. By using IMS, the complete boat can be measured, including the hull shape, appendages, propeller, stability, rig and sails. IMS uses WS Equipment Rules of Sailing (ERS) and its definitions are amended only for items specific to offshore boats. Measurements procedures are explained in this section are for information and description only. Complete measurement rules can be found in the IMS Rule available in the rules section.

Boats shall be measured by official measurers as appointed by the National Sailing Authorities and approved by the ORC. Use of ERS makes use of IMS easy for measurers of other international classes, while for parts specific to offshore boats and measurement principles in general, ORC regularly organizes Measurers Conferences.
 

From Measurement to Rating

IMS is used by the rules described in the ORC Rating Systems to generate ORC International and ORC Club certificates. Both are fully compatible, with the only difference being that all data on an ORC International certificate has been taken from measurements defined by the IMS, while for ORC Club this data can be measured, declared by the owner (subject to verification by the Rating Authority) or obtained from any other source, including photos, drawings, and/or data from identical or similar boats. Since they have fully measured data, ORC International certificates are therefore more accurate than ORC Club.

Unless otherwise specified, measurements shall be taken and recorded to the nearest greater value as follows:

• Metric system: All measurements shall be in meters to three decimal places except that sail measurements shall be to two places of decimals. Weights shall be in kilograms to one decimal place.
• Imperial system: All measurements shall be in feet to two decimal places except that sail measurements shall be to one decimal place. Weights shall be in pounds.

Boats shall be measured dry in light ship condition without crew, which as defined in details in IMS Rule E2. Items on board include:

• Internal ballast
• Batteries
• Fixed interior accommodation, hatch covers and floor boards
• Fixed machinery, electrical and plumbing systems
• Mast, boom, spinnaker pole and any sprit
• Standing rigging and fittings
• Rudder, wheel/tiller and steering gear
• Fixed electronics, instruments, compasses, lights, antennas and masthead devices
• Pulpit, stanchions and lifelines
• Mattresses and permanently installed table and doors
• Permanently installed stoves, heaters or other electrical devises

Specifically excluded from the measurement trim are the following:

• Water and any liquid contents of any tanks or voids in the keel or other appendages
• Anchor and anchor lines
• Deck and mooring lines
• Any sails, including storm and emergency sails
• Sheets, blocks, winch handles
• All portable safety gear
• Cushions, pillows and any other bedding
• Cooking and eating utensils
• Any food or stores
• Miscellaneous portable and personal gear, books, navigational tools etc.

An easy explanation of lightship measurement trim would be that if the boat is rotated upside down, everything that falls out shall not be on board.

When IMS is used a boat is in compliance with its ORC certificate when:

• All measured, declared or recorded values shall be as close as possible to those on the certificate. Differences are allowed only if the values on the certificate give a faster rating.
• The sail area shall be smaller or equal to that printed on the certificate.
• Crew weight shall not be considered as issue of compliance with certificate, but shall not be greater than the declared or default value noted on the certificate.

All measurement data can be recorded and stored by freely available IMS Editor software.