Willie:
ROR, ANNEX I, section 6
6. Shapes
(a) Shapes shall be black and of the following sizes:
(i) a ball shall have a diameter of not less than 0.6 meter;
(ii) a cone shall have a base diameter of not less than 0.6 meter
and a height equal to its diameter;
(iii) a cylinder shall have a diameter of at least 0.6 meter and a
height of twice its diameter;
(iv) a diamond shape shall consist of two cones as defined in (ii)
above having a common base.
(b) The vertical distance between shapes shall be at least 1.5
meter.
(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.
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Imho, in a vessel of less than 20 m, in the event you were inspected, I believe that the inspector would be so astonished that you actually -had- the shapes to display, that there would be no criticism of what size they might be. FWIW: .6 m = 23.61 inches. On a 35 ft boat, that is rather large. If you had shapes that were half-size, I believe that this would be appropriate. Note, that the shape size is not specifically defined, for vessels of less than 20 meters length.