33 C.F.R. § 84.06 – Shapes.

(a) Shapes shall be black and of the following sizes:
1. (i) A ball shall have a diameter of not less than 0.6 meter.
2. (ii) A cone shall have a base diameter of not less than 0.6 meters and a height equal to its diameter.
3. (iii) A diamond shape shall consist of two cones (as defined in paragraph (a)(ii) of this section) having a common base.
(b) The vertical distance between shapes shall be at least 1.5 meters.
(c) In a vessel of less than 20 meters in length shapes of lesser dimensions but commensurate with the size of the vessel may be used and the distance apart may be correspondingly reduced.