Section 4(b) in The Merchant Shipping (Prevention of Collisions at Sea) Regulations, 1975
(b)The lights and shapes on a vessel engage in dredging or underwater operations to indicate the obstructed side and/or the side on which it is safe to pass, as prescribed in Rule 27(d)(i) and (II), shall be placed at the maximum practial horizontal distance, but in no case less than 2 metres, from the lights or shapes prescribed in Rule 27(b)(i) and (ii). In no case shall the upper of these lights or shapes beat a greater height than the lower of the three lights or shapes prescribed in Rule 27(b)(i) and (ii)
