In imperial world “a pint of pure water has weight a pound and a quarter” i.e. pure water (PW )has SG = 1.25 lb/pint = 10 lb/UK gallon. Where weight = 1.002004 * mass, there the PW has density = 10.02004 lb/UK gallon = 0.999763245 T/m3.
In mass system standard water is not PW, but fresh water (FW) of density = unit mass per unit volume = unit density = 1 kg/L = 1 T/m3 has:
- density = density of PW/0.999763245 = 10.02241286 lb/UK gallon;
- SG = density of FW/1.002004 = 10.0236811 lb/UK gallon.
As Convention on Load Lines uses mass system so naval architects use the same system and in FW a vessel of extreme volume = Ve =224 UK gallons and Vi =Ve – volume of appendages = 223.7760002 UK gallons for draft = X in FW has:
- extreme mass = extreme displacement = De = 1.002241286 LT in metric naval architecture,
- mass = displacement = Di = 1.001239046 LT = 0.999 * LT in imperial naval architecture (Rules of American Bureau of Shipping).
This way imperial naval architects match displacement to imperial standard hydrometer of reading = SG60F = 1/0.999 = 1.001001 in FW. If one ASSUMES SG60F = 1.001001 means SG = 1.001001 T/m3, then he has to ASSUME that the vessel in question for draft = X has in FW weight = SG60F * (Di / unit density of FW) = 1.002241286 LT.
Other way the same vessel for the same draft = X in FW:
- a) had and has in FW extreme mass = De = 1.002241286 LT in metric world;
- b) had and has in FW = weight = 1.002241286 LT in imperial world, while in fact it has extreme mass = De.
2 different worlds have 2 different standard hydrometers:
- imperial standard hydrometer has reading = SG60F = 1 in pure water at 60F;
- metric standard hydrometer has reading = density = 0.999 T/m3 in pure water at 60F.
In case of “imperial ” readings of imperial hydrometer (load line hydrometer) are true for draft survey purposes and readings of metric hydrometer are true for load line purposes. In case of “metric ” vessels reading of density hydrometer is true for both purposes. The only problem is that nowadays it is not easy to recognize “imperial” vessels and “metric” vessels.
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