a similar stage of constructionmeans the stage at which— (a) construction identifiable with a particular ship begins, and (b) assembly of that ship has commenced, comprising at least 50 tonnes or one per cent. of the estimated mass of all structural material, whichever is the less;
existing shipmeans a ship which is not a new ship;
lengthmeans 96 per cent. of the total length on a waterline at 85 per cent. of the least moulded depth measured from the top of the keel, or the length from the fore-side of the stem to the axis of the rudder stock on that waterline, if that be greater.
new shipmeans a ship the keel of which is laid, or which is at a similar stage of construction, on or after 1st July 1998;