Total Mass

The only thing left is to add in the engines.  But first we need to know how much mass the engines will be moving.  To do that we tally up the mass of all ship components, the mass of the hull and any additional armor the ship is carrying.  This gives us the "empty" weight of the ship (i.e. carrying no cargo).

To determine the loaded weight, we assume that when the cargo area is loaded, it will be filled with cargo that has an average density of 2 tons per cubic meter.  To determine the loaded weight, multiply the number of cargo units that the ship is designed to carry by 300 tons and add it to the ship's empty weight. 

Keep these two numbers handy as they will be used to determine the maximum acceleration the ship can achieve in it's empty and loaded configurations.  For military vessels, passenger liners and other ships that aren't designed to haul a lot of cargo, these numbers will not be very different and you can just use the loaded weight in the following steps if you desire.  For freight haulers the numbers become quite important and will vary greatly.