Section 6 in The Merchant Shipping (Prevention of Collisions at Sea) Regulations, 1975
6. Shapes.
(a)Shapes shall be black and of the following sizes:(i)a ball shall have diameter of not less than 0.6 met;(ii)a cone shall have a base diameter of not less than 0.6 metre and a height equal to its diameter;(iii)a cylinder shall have a diameter of at least 0.6 metre and a height of twice its diameter;(iv)a diamond shape shall consist of two cones as defined in (iii) above having a common base.(b)The vertical distance between shapes shall be at least 1.5 metre.(c)In a vessel of less than 20 metres in length shapes of lesser dimensions but commensurate with the size of the vessel may be used and the distance a part may correspondingly reduced.
