Does anyone have any docs or know a rule-of-thumb for estimating lightweight displacement tonnage from DWT? All I can find on SSW's website is DWT and the only DWT-LDT matrix I can find (vesselvalues.com) is behind a paywall.
Those 2 measurements represent different physical objects.
Are you trying to figure based upon a specific load out?
If not then both these figures can be obtained from vessel tonnage certificates.
LDT is the weight of the vessel as it comes from the shipyard & doesn't include any consumables (fuel, lube, water, galley stores, etc.) & is listed as gross tons on a tonnage certificate.
DWT includes all cargo & consumables minus LDT & is listed as net tonnage on a cert.
Displacement & tonnage can be very confusing as you'll encounter domestic (US) & international (ITC) certs.
My vessel is registered on the International Tonnage Certificate at 2998 gross tons which means that there is 299,800 cu. ft. of enclosed space on the vessel & it displaces 2998 tons of H2O (we don't have a domestic cert because we're a SOLAS boat.)
The net tonnage for the vessel is 1164 & this means that out of the 299,800 cu. ft. of total enclosed space, 116,400 cu. ft. is actual cargo space (to confuse things more, I can load another 1,500 or so tons on deck & this doesn't get included in the net tonnage or DWT on the cert because it's not enclosed space but you can bet it gets included on my stability calcs.)
US tonnage certs indicate much lower gross tonnage because naval architects add removable "tonnage hatches" in all internal living spaces & at least one hatch leading from an internal to an external space (usually behind the main superstructure & leading to the deck) these hatches can be un-bolted & removed to create a continuous space which is open to the outdoors.
Basically they are saying that since all this space can conceivably be opened to the deck, it doesn't exist & should not be included in the gross tonnage (try going through the Panama or Suez canal or getting pulled out of the water at a dry dock using a Domestic Tonnage Certificate - not...)
They do this to allow guys with 100 ton licenses to run a vessel that is by all rights a 400 to 500 ton boat.
Give me some vessel specifics & I may be more helpful...