Traditional materials such as wood, steel, aluminum, bronze, iron, lead, lexan, Plexiglas, polyester, fiberglass, balsa cored fiberglass panels, foam cored fiberglass panels, paint and epoxies may be used. Carbon fiber and kevlar, shall not be used. Hull, deck, and house must be made from wood or fiberglass. Further material restrictions for individual items are located in their respective paragraph.

Titanium will not be used on Thunderbirds after January 26,1994. Titanium installed on Thunderbirds prior to January 29, 1994 may remain.

The use of materials not listed above as acceptable must be approved by the I.T.C.A. Measurement Committee.

The selection of wood is at the option of the builder except as listed below but all woods must weigh in excess of 20 lb. per cubic foot (320 kg/cu. m) at 15% moisture content. All plywood is to be of Marine or Exterior type. It is expected that the yacht shall be constructed in accordance with materials and techniques normally considered sound for yacht construction.

• 3.2.1 Planking, bulkheads and inner transom are to be 1/2" (12.7 mm.) plywood as specified in the official plans.

• 3.2.2 Cabin sole, cockpit floor and decks are to be 3/8" (9.53 mm.) plywood or equivalent suitable material.

• 3.2.3 Cabin top is to be constructed with two (2) layers of 1/4" (6.35 mm.) plywood as specified in the official plans.

• 3.2.4 Cabin sides and cabin face are to be not less than 3/4" (19.0 mm.) thick.

• 3.2.5 Wood keel and false keel are to be of the minimum total thickness shown in the plans and shall not exceed these dimensions by more than 1/4" (6.35 mm.).

• 3.2.6 Stringers, sheer clamp and sheer batten shall not be smaller in cross section than shown in the official plans.

• 3.2.7 The chine shall be shaped from wood of the dimensions shown in the official plans and shall not be less than 3"(76.2 mm.) in width inside the planking after shaping.

• 3.2.8 Floor timbers are to be not less than 2" (50.8 mm.) in thickness.

• 3.2.9 A yacht's official registration number, (Thunderbird Hull No.) shall be carved or burned into the stem, forward of Bulkhead #1. Minimum number size: 2" (50.8 mm.) high and 3/1 6" (4.7 mm.) deep.

Fiberglass construction, with or without cores, may be substituted for the construction of the hull and/or deck and cabin prescribed in the plans, provided that they meet the following specifications.

• 3.3.1 Weight. The basic hull with Bulkheads numbers 2, 4, and 5, the outboard motor well, chain plate supports and keel support structure shall weigh between 825 and 1,000 lb. (374.5 to 454kg.). The hull thickness shall not be less than 3/16" (4.8 mm.) of hand lay-up fiberglass reinforced plastic at any location. The deck, cabin and cockpit shall weigh between 380 and 480 lb. (172.5 and 218kg.) trimmed and ready to install. Deck thickness shall not be less than 3/16" (4.8 mm) of hand lay-up fiberglass reinforced plastic in any horizontal unreinforced areas. The deck weight shall include all hatch covers.

• 3.3.2 Center of Gravity. The center of gravity of the hull shall be 13' 8" +- 5"(4.17 m +-127 mm) from the stem head measured parallel to the waterline, and 2' 4"+- 4"(0.71 m +- 102 mm) below the sheer.

• 3.3.3 Radius of Gyration. The hull shall have a radius of gyration of 6' 11" +- 8" (2.11 m +- 203 mm) determined by measuring the period of oscillation when swung on a pivot at or near the sheer. vertically above the center of gravity, and using the formula:

  k= (aT² h-h²)^1/2

  where T is the period of one complete oscillation in seconds: h is the distance of the center of gravity below the pivot; and a = 0.248 if h is measured in meters and 0.81 if h is measured in feet.

• 3.3.4 Certification. An affidavit shall be provided by the builder attesting to the weight, center of gravity location, period of oscillation and radius of gyration of each hull and the weight of each deck. The measurement of the first three hulls and first three decks produced by a builder shall be performed by the builder in the presence of and to the satisfaction of a fleet measurer. A builder shall inform the fleet measurer at least three days in advance of the time and place at which he will measure subsequent hulls and decks. The measurer or his representative shall have the right to be present at the measurement. The builder and measurer shall sign the affidavit and forward the original to the International Measurer.

  NOTE: A builder may choose to install additional components into the hull before removing it from the mold. In this case, the weight, center of gravity and radius of gyration shall be determined with these components installed, and the builder's affidavit shall include a list of these additional components. their individual weights and their positions measured from the stemhead parallel to the waterline and below the sheer.

• 3.3.5 Shape. The hull shall be measured, after the deck has been permanently mounted, per the specifications of Section 4.1. When three hulls from a mold have been measured, a builder may apply to the International Measurer for a waiver of the measurements of Section 4.1 for subsequent hulls from that mold. In this case, the beam at Bulkheads No.2 and 4 shall be checked for compliance after the deck has been permanently mounted, unless a complete measurement is requested by the International Measurer.

• 3.3.6 The hull shall bear external datum reference marks on the centerline and on both sheers coinciding exactly with the aft edge of Bulkhead #2 projected to the outside of the hull. The datum reference marks shall be permanent and detectable. (It is recommended that corresponding reference marks be placed inside the hull to aid the builder.)

• 3.3.7 A yacht's official registration number (Thunderbird Hull No.) shall be permanently recorded on the vessel, with raised or recessed numerals on the shell inside near the stem at a visible location.

• 4.1.1 L.O.A. 25' 9-3/4" (7.868 m.) minimum. 25' 11-3/4" (7.918 m.) maximum.

• 4.1.2 Cross sections shall be checked by official template at Bulkheads #2, #4 and #6. Allowable tolerances shall be tight against template to 1/2" (12.7 mm.) minus.

• 4.1.3 Deck crown at Bulkhead #2 shall be l"(25.4 mm.)  to 7" (178 mm.) maximum.

• 4.1.4 Chine radius shall be 1-1/4" (31.8 mm.) maximum measured at Bulkheads #4 and #6.

• 4.2.1 Chainplates shall be not less than dimensions shown on the plans, except that thickness may be reduced to 1/8" (3.2mm) minimum, provided the material is corrosion resistant steel. Turnbuckle pins have created elongated holes in 1/8" (3.2mm) chainplates on some existing boats, hence 3/16" (4.8mm) chainplates are recommended. Locations are listed below.

• 4.2.2 Forward lower shrouds (optional) shall be located not more than 22" (559 mm.) forward of Bulkhead #2 and not more than 4-1/2" (114 mm.) inboard from the sheer.

• 4.2.3 Upper shrouds shall be snug against the forward side of Bulkhead #2 and not more than 4-l/2" (114 mm.) inboard from the sheer.

• 4.2.4 After lower shrouds shall be located 19" (483 mm.) to 22" (559 mm.) aft of Bulkhead #2 and not more than 4-1/2" (114 mm.) inboard from the sheer.

• 4.3.1 Mast hole length shall be centered on aft edge of Bulkhead #2, +- 1/2" (12.7 mm.).

• 4.3.2 Hole shall be maximum 5" (127 mm.) wide and maximum 6" (152 mm.) long.