SHIPBOARD MEASUREMENTS

A ship’s size and capacity can be described in two ways--linear dimensions or tonnages. Each is completely different yet interrelated.

A ship’s measurement is expressed in feet and inches--linear dimensions. A ship is a three dimensional structure having length, width, and depth.

A Ship’s Dimensions

Draft - The depth of a ship in the water. This vertical distance is measured from the bottom of the ship to the surface of the water. Draft marks are cut into or welded on the surface of a ship’s plating. They are placed forward and aft on both sides of the hull and also amidships. At the midships draft we will also find the authorized Load Line markings which designate maximum drafts allowed for vessels under various conditions.

Freeboard - The vertical distance from the water line to the top of the weather deck on the side.

FREEBOARD

Freeboard is the distance between the waterline and the freeboard deck at mid length. The freeboard deck is the uppermost continuous deck which has means of closing all openings. Rules allow different freeboards for different ships in relation to their construction and cargo they carry. There are two types of ship;

Type A -which covers vessels designed to carry only liquid cargoes.

Type B-Which covers all other types of ship,

For type A ships cargo tanks must only have small openings which can be effectively sealed

Type B ships must have sufficient bulkheads and sealing arrangements for openings, but such openings e.g. hatches can be large

The freeboard allowed will be smaller for the type A ship compared to the type B ship of similar length because of the type of cargo carried and means of access for water. Type B ships classed as B-60 may have their freeboard reduced by 60% of that required for a normal B-100 ship provided that its method of construction approaches that of the type A ship. This type exists with OBO's.