The Merchant Shipping (Registration of Ships) Regulations 1993

6.  A scale of decimetres, or metres and decimetres, denoting a draught of water shall be marked on a ship, other than an exempted ship, on each side of its stem and its stern post, as follows:—

(a) in figures in two-decimetre intervals, if the scale is in decimetres; and

(b) in figures at each metre interval and at intervening two-decimetre intervals, if the scale is in metres and decimetres;

the capital letter ‘M’ being placed after each metre figure; the top figure of the scale showing both the metre and (except where it marks a full metre interval) the decimetre figure; the lower line of the figures, or figures and letters (as the case may be), coinciding with the draught line denoted thereby; the figures and letters being not less than one decimetre in length and being marked by being cut in and painted white or yellow on a dark ground, or in such other way as the Secretary of State approves.