what is ships Gross tonnage and Net tonnage

What is gross tonnage and tonnage and why they are important ?

The Gross tonnage is calculated using a formula that takes into account the ship's volume in cubic meter below the main deck in the enclosed spaces above the main deck this volume is then multiplied by a coefficient which results in a dimensionless number all measurements used in the calculation of molded dimensions in the drawing that tonnage is given a different color within gross tonnage which is more or less the whole ship to indicate the difference between Net Tonnage amd Gross tonnage.

In order to minimize the daily expenses of a ship. The ship owner will keep the GT as low as possible one way of doing this is by keeping the depth small some more cargo can be placed on Deck as a consequence dangerous situations can occur as the loss of Reserve buoyancy can result in a loss of stability and in more water on Deck.

The Net tonnage is also a non dimensional number that describes the volume of the cargo space. 

The NT is derived from the GT by subtracting the volume is occupied by crew, the navigational equipment, the propulsion equipment, partly workshops and ballast.

The NT may not be less than 30% of the GT.