Why the Mass Inventory Is Never as Simple as a Parts List
A ship deck equipment inventory that records item names and counts but not center of gravity coordinates is an administrative document. A ship deck equipment inventory that captures Mass, Amount, x_cog, y_cog, and z_cog per line item in ship coordinate axes is a naval architecture input — the raw data that feeds the stability calculation when you need to verify that a vessel's KG hasn't drifted outside the approved stability booklet limits after a fit-out change.
The difference matters most during the grey period between a major refit and the next inclining experiment: equipment added, moved, or removed in frames 180 through 230 at elevation 24000 shifts the vertical and longitudinal center of gravity in ways that are not trivial to estimate without per-item COG records. A 2-meter longitudinal shift in LCG across a vessel of significant displacement doesn't just affect trim — it affects the met center height calculation that underpins the entire stability picture.
The Lightship Classification and Why Item Category Determines Everything
The Part of Inventory classification — Category A (non-lightship, stays onboard), Category B (non-lightship, to be offloaded), Category C (lightship item not fitted on final location) — is not administrative tidiness. It's the sorting key that separates the items counted in the lightship mass from the items counted in the deadweight.
A Category B item that should be offloaded before departure but remains onboard because someone didn't check the off-load manifest adds mass and a COG offset that wasn't in the stability calculation for that departure condition. The inventory record is what the stability engineer checks against the loading instrument to verify that what's supposed to be off the ship is actually off the ship.
Category C items — lightship items not yet on their final location — are tracked separately because their COG coordinates as surveyed during an inclining experiment may not reflect their installation position. If a piece of machinery is logged as in lightship condition but is sitting in frame 190 mid-ship rather than frame 210 at the engine room bulkhead, the moment arm error goes directly into the lightship VCG and LCG.
Total Mass as a calculated field (Mass × Amount) is the output that aggregates into deck-level and frame-section totals. Mass in kilograms per item multiplied by quantity gives the section mass contribution, which then feeds into the longitudinal weight distribution curve — the foundation of trim and stability analysis by section.
Meshgrid Positioning When Exact Coordinates Are Not Available
The "Using the meshgrid" boolean and the Meshgrid text field in format A3, G7, and so on handle the practical field reality that not every deck item has surveyed COG coordinates in ship axes. For items without precise coordinate data, the meshgrid assigns a position within a defined grid overlay of the deck plan, which gives the stability calculation an estimated COG contribution that's better than nothing and clearly flagged as estimated rather than measured.
The x_cog, y_cog, and z_cog fields in ship coordinate axes — typically LCG measured from midship or aft perpendicular, TCG from centerline, VCG from keel — are the fields that carry the engineering weight of the record. When these are entered from the structural drawings or from direct measurement during a survey walk of frames 180 to 230, the record is an engineering document. When they're estimated via meshgrid, the meshgrid flag tells the stability engineer which entries to treat with appropriate uncertainty.
The picture fields per item, with two optional additional images, create the visual audit trail that ties a database record to a physical object on a specific deck frame — relevant when the inventory is audited against the actual vessel configuration during a class survey or flag state inspection.