Lightweight Setup

It is to be noted that only one lightship definition is allowed for within a Calc project and this is then associated with all of the different deadweight loading conditions that are subsequently created. Hence you can only accept and not modify the system assigned title of Lightweight Definition. Calc will not prevent a user from including more than one Lightweight node within the Application tree, however the internal files and the Edit command will only operate on one single Lightweight file, namely the first one that is created.

The preferred description is in the form of itemized lengthwise segments of distributed material that can overlap if necessary and when summed together given a more meaningful and accurate description of the longitudinal breakdown and corresponding center of gravity position. Otherwise longitudinal strength calculations become inaccurate and of little value.

A user can, if required, group the various input items into different categories by name, for example into hull steelwork, outfit steelwork, superstructure/deckhouses, outfit systems, machinery, , and can carry the refinement to whatsoever level that he has some data for and/or has confidence in. The input data does not need to be sequential and several different items can be simultaneously distributed over overlapping lengths of the hull,

For each weight item, you specifies:

• A Name to simply help in identification, and Weight (tonnes or tons).

• Its centroid, defined by the LCG, TCG, and VCG, which are measured relative to the vessel's origin (units: meters or feet). you must provide an entry for each LCG position, however the TCG and VCG items can be given zero values. (Clearly an item that is symmetrically disposed about the vessel's centreline will have a TCG of 0.0.).

• A value of free surface moment, FSM, if necessary, (units of: tonnes-meters or tons-feet).

• The Aft ext and Fwd ext, which are the extents of the item over which its weight should be spread, measured relative to the vessel's origin (units: meters or feet). The lightship weight distribution is made up of trapezoidal items, the number being dependent upon the detail required. The LCG must be within the range if the given extents. If the LCG of the item is within the mid third of the defined extent of the item, then the distribution will be trapezoidal, otherwise it will be triangular over the defined extent. Large local weights could be distributed over a relatively small length, which would be consistent with the physics of real objects. Note that if the range is zero, then the weight will be assumed to be distributed over the entire length of the ship.

• A summary of the current total lightweight, the corresponding centre of gravity position, and the FSM, is displayed at the bottom of the dialog. This summary is automatically updated following each complete row of data entry.

The ShipWeight program from BAS Engineering (http://www.bas.no) is useful for producing a good lightweight estimate and longitudinal weight distribution. This program can output a file that can in turn be read into Calc by clicking the Import ShipWeight button.

Clicking the Export ShipWeight button, enables you to write a ShipWeight type file, which can in turn be re-imported into Calc. The ShipWeight file is a simple TAB delimited file that can be read into, for example, MS Excel.

A Lloyd’s Register type distribution can be generated, by clicking the LR Distribution button. This will bring up a dialog, where you can enter the parameters for constructing this, ie weight, centre of gravity, block coefficient, and parallel midbody. A number of weight items, identified by the prefix, LR, will be entered in the table. These will approximate the LR distribution. The number of these items can also be specified by you. you also has the option of deleting all existing items, or just adding to those already defined.