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IMCA Rules

30/31st October, 1999

Authority. lnternational Yacht Racing Union. 60 Knightsbridge. London SW1 X 7JX Date of International Status. 1st September 1972

1. General

  • The International Moth is a single-handed development class boat. The Intention of these class rules is to give the designer and builder the fullest liberty in design and construction, within these rules to develop and produce faster boats.
  • The official language of the class is English And in the event of a dispute over translation, the English text shall prevail.
  • These rules are complementary to the measurement form. Any interpretation shall be mane by the l.Y.R.U which may consult the International Moth Class Association. (IMCA).
  • In the event of discrepancy between the rules and the measurement form, the matter shall be referred to the l.Y.R.U.
  • In countries where there is no National Authority, or where the National Authority does not wish to administer the class. its function as stated in the rules, shall be carried Out by l.M.C.A or it's delegated representatives I National Associations).
  • Neither the l.Y.R.U nor the IMCA accept legal responsibility in respect of the rules, or any claim arising therefrom.

2. International Class Fee

  • The amount of the I.C.F. may be reviewed by the l.Y.R.U on recommendation of l.M.C.A.
  • The l.M.C.A is responsible for the collection of the l.C.F and the distribution of l.C.F receipts on the above basis.
  • The l.M.C.A may delegate its responsibility to collect the l.C.F and issue receipts to National Associations.
  • The I.C.F is payable by the Builder on each boat built whether or not it is subsequently measured and registered. Payment shall be made direct to the National Authority or its delegated Representative (National Association I. which shall issue an endorsed I.C.F receipt. The l.C.F receipt shall be delivered by the builder to owner on sale of the boat.
  • I C.F receipts shall be valid only if mane out on official forms issued by the l.Y.R.U. The purchase price in each case represents the Proportion of the I.C.F. due to the I.Y.R.U. and the IMCA. The National Authority or its delegated Representative National Association, may charge an additional sum not exceeding the administration cost of carrying out this function.

3. Registration and Measurement Certificate

  • No boat is permitted to race in the class unless it has a valid measurement certificate.
  • The owner shall be a fully paid up member of the relevant l.M.C.A National Association.
  • Each National Authority or its delegated Representative National Association) shall .issue sail numbers which shall be consecutive and the number shall be preceded by the official National letters. A National Authority or its delegated Representative snail only .issue a sail number on receipt of evidence that the l.C.F has been paid.
  • No two boats in the class registered in the same country shall have the same name.
  • The certificate is obtained as follows:
    - The owner or builder shall apply to the appropriate National Authority or its delegated Representative (National Association. for a sail number enclosing the l.C.F receipt, and at the same time submit the Proposed name(s) of the boat. The National Authority or its delegated Representative shall enter the sail number in the l.C.F receipt.
    - The owner or builder shall obtain a measurement form from the National Authority or its delegated Representative, and shall have the boat measured by a measurer officially recognised by the National Authority. The completed form shall be supplied to the owner of the boat.
    - The owner shall send the completed measurement form to his National Authority or its delegated Representative together with any registration fee that may be required. On receipt of this the National Authority or its delegated Representative may issue a certificate to the owner.
  • Change of ownership invalidates the certificate but shall not necessitate remeasurement The new owner may apply to his National Authority or its delegated Representative (National Association for a new certificate. returning the old certificate together with any re-registration fee required And stating the necessary particulars. A certificate shall then be issued to the owner.
  • It a the owners responsibility to ensure that his boat, spars. sails And equipment comply with the class rules at all times. Alterations and replacements shall be measured. when relevant, by a measurer recognised by the National Authority or its delegated Representative (National Association), who will, if necessary, endorse the measurement certificate accordingly, and shall notify the National Authority or its delegated Representative where this would apply.
  • Notwithstanding anything contained in these rules the l.Y.R.U or the National Authority or its delegated Representative (National Association) shall have the power to refuse to grant a certificate to. or withdraw a certificate from arty boat.
  • The l.M.C.A shall obtain at regular intervals from each National Authority or its delegated Representative (National Association details of sail numbers and l.C.F receipts issued.

4. Measurement

  • Only a measurer officially recognised by the National Authority or its delegated Representative shall measure a boat. its spars. sails and equipment, And shall sign the declaration form that they comply with the class rules.
  • The measurer shall report on the measurement form anything which he may consider to be unusual or to depart from the intended nature of the boat or to be against the general interest of the class and a certificate may be refused, even if the specific requirements of the class are satisfied.
  • A measurer shall not measure a boat, spars or sails or equipment owned, designed or built by himself, or in which he is an interested party or he has a vested interest.
  • New or substantially altered sails shall be measured by a measurer who shall stamp or sign and date the sail near the tack. The details shall be recorded on the certificate and the entry signed by the measurer or secretary of the National Authority or its delegated Representative (National Association) (National Association).
  • All boats. spars. sails And equipment shall comply with the current rules.
  • All boats. spars. sails and equipment shall be liable to remeasurement at the discretion of a race committee or the National Authority or its delegated Representative (National Association).

5. Identification Marks

  • The class emblem shall be a representation of a Moth and shall conform in shape and size to the pattern held by the I.Y.R.U. copies may be obtained from l.M.C.A or National Authority or its delegated Representative (National Association).
  • The sail number and national letter(s) of the boat shall be cut into, or marked indelibly on the hull, aft on the port side in figures not less than 3Omm high. These shall not be removed for the life time of the boat.
  • The sail number, national letter(s) and class emblem on the sail shall conform with I.Y.R.U Yacht Racing Rule 25. letters and numbers shall be not less than the following sizes: Height: 250mm Width: 165mm (excluding one and letter I) Thickness: 35mm Space between adjoining letters and numbers: 50m4m, (For sails measured, before 1st March 1973,the minimum height shall be 230mm and minimum width (150mm).
  • All emblems, numbers and letters shall be off a durable material and securely attached.

6. Hull

  • The overall length of the hull. excluding attached rudder fittings and stem fittings shall not exceed 3355mm measured between perpendiculars with hull level transversely and waterline horizontal. No attempt at increasing waterline length shall be made by fairings comprising part of, or attached to, rudder or stem fittings.
  • The overall beam shall not exceed 2250mm.
  • Catamaran or multihull configurations are prohibited. There' shall be no visible air gap dividing the boat longitudinally throughout its length when afloat upright, fully equipped but without crew. In the case of a boat with a flexible or hinged transom this test shall be made with the transom in the fully raised position.
  • Within the static waterplane area there shall be no hollow in the hull more than 75mm in any section closer than 2700mm from the aft perpendicular as described in rule 6(i). The reference line for this hollow shall he a stringline stretched tightly around the underside of the hull from points on the static waterline either side of the hull, and the stringline shall be at right angles to the hull. The static waterplane area shall be defined as the area bounded by the extremities of overall length and maximum waterline beam when afloat upright. fully equipped. but without crew.

7. Buoyancy

  • Boats shall have not less than two separate buoyancy tanks or bags attached firmly to the hull which together must be sufficient to float the boats own weight plus 75kg, approximately level when capsized or full of water, and which, with any one tank flooded must be sufficient to float the boats own weight, plus 10kg. If a measurer cannot ascertain compliance with this Rule by visual inspection, he shall undertake a buoyancy test to so satisfy himself.
  • The measurer shall satisfy himself that the buoyancy compartments are effective by means of flotation or air test. (As a guide, leakage should not exceed 5% of volume of tank in half an hour).

8. Spars.

  • The overall length of mast shall not exceed 6250mm.
  • Measurement Bands, not less than 15mm wide shall be marked on the spars so that they are clearly discernible when racing. The inside edges of these bands define the limits to which the sail may be set.
  • The distance between the bands shall not exceed 5185mm.

9. Sails

  • The boat shall only carry one sail. No extra sail shall be on board when racing.
  • The sails and spars shall be measured in accordance I.Y.R.U "Measurement and Calculation of Sail Area". The l.Y.R.U. Sail Measurement Instructions shall not apply. The measured and calculated area shall not exceed 8.00m2 metres area, except that, Clause 3.2.5(b) of the l.Y.R.U Measurement and Calculation of Sail Area shall not apply.
  • Only the area of that part of the spars that will not pass through a ring 9Omm internal dia, shall be included.
  • For a sail which encloses the mast, an area equivalent to the length of the luff multiplied by 50mm shall be excluded.
  • For a sail which encloses the boom an area equivalent to the length of the foot multiplied by 9Qmm shall be excluded.
  • Battens shall extend not more than 150mm from the sail. No attempt at increasing sail area shall be made by the number or size of the battens used.
  • Where the sail is set on spars, no part of the sail luff shall extend beyond the lower edge of the upper mast band, or below the upper edge of the lower mast band.
  • International Moth sails do not require a boom band to be declared, and shall be measured in accordance with l.Y.R.U Measurement and Calculation of Sail Area clause 3.3. (Clause 3.2.3 shall apply for area of main triangle). The owner shall declare the max sail foot length (BC) and shall mark this declared foot length clearly and indelibly on the foot of the sail at the tack in figures at least 10mm high. The BC measurement shall be less than the measured distance from outside the cringle to outside the clew. when the sail is stretched Out and pegged down for measurement.


10. Crew

  • There shall only be one person on board when racing. The righting moment of the helmsman's weight shall only be transferred to the sail through the hull or sheet or similar, in which case it shall be through blocks attached to the hull.

11. Prohibitions

  • Moving or detachable seats and trapezes.

12. Anchor

  • An anchor need only be carried when specifically prescribed in the sailing instructions.