Self-Trimming

Such even distribution is necessary for two reasons.  First to ensure maximum utilisation of the ship’s holds.  Were the ship to be loaded through a small hatch, the cargo would adopt a conical shape.  The shape of that cone would be dictated by the natural ‘angle of repose’ of the material concerned.  Angle of repose differs widely from commodity to commodity on this scale in just the same way as one observes the way simple substances behave such as in a domestic kitchen; quite different shapes are formed when pouring out, say, sugar, flour, lentils, etc. In most cases it will be easy to visualise the point of a cone of cargo protruding through the hatchway long before the hold is full.  We shall encounter the angle of repose problem again a little later in this section. The second reason for trimming the cargo across the whole of the hold is one of safety.  Even distribution of cargo is always important not only to ensure that the ship rides on an even keel but also to avoid uneven stresses in the structure of the ship. The first requirement of a self-trimmer, therefore, is as mentioned above, the largest hatch size commensurate with safe construction.  This allows for the loading appliance to spread the cargo evenly and you will sometimes encounter, in charter parties, a clause requiring the shipper to ensure that the cargo is ‘spout trimmed’. Self-trimming does not entirely apply to loading because grabs are almost invariably used to discharge such ships and grabs would not be able to reach right into the angle between the vertical side of the ship and the bottom of the hold.  This is overcome by constructing a sloping section at an angle of about 450 running fore and aft for the whole length of the hold.  This ‘trims’ the cargo into the square of the hatch within reach of the grab, thus minimising the amount of hand shovelling to just the last few tons.  That sloping section serves a second purpose in that it houses ballast spaces which can be filled with water in order to take the ship more deeply into the water for safety’s sake when the ship is sailing without cargo.