Regulation 11
SCHEDULE 4FAMILIES OF VESSELS
Vessels form part of the same family if they differ from the prototype only–
(a)in diameter (provided always that they meet the requirements of paragraph 9 of Schedule 1 as to wall thickness);
(b)in the length of the cylindrical component, provided that–
(i)where the prototype has one or more shell rings in addition to the ends, they all have at least one shell ring; or
(ii)where the prototype consists of two outwardly dished ends, none of them has any shell ring; or
(c)in both (a) and (b).