sophiecentaur said:
That makes a lot of sense in practice but the accuracy depends on knowing absolutely everything about the structure and contents of the ship. No one would be in a position to challenge your answers but would it matter?
As a matter of interest, does the inclining test result agree well with the calculations that surveyors do? There is a philosophy that tells us to be pessimistic in design ratings and that works well except when a spot of corruption affects construction methods and materials.
It's interesting that most disasters where structures are involved can always be put down to commercial (and even criminal) interests, rather than the good old Engineers and Surveyors. Good regulation is the key.
You can easily understand that it is not possible to know
exactly what is the weight and the G coordinates ( referred to an Oxyz ideal reference frame, have a look at my sketch), of every steel plate, every section frame, every engine, motor, pump, compressor, pipe, electrical device, crane, rope, and so on, including furniture, that are assembled together to make a ship, whichever kind of ship : passenger, general cargo, bulk carrier, tanker, even navy ships!
So, concerning commercial ships ( but navy ship too are subject to similar rules), it is compulsory that , at the end of construction ( or almost the end...I cannot enter into details here!) , a ship undergoes an inclining test. This test is simple to describe in line of principle, but not so easy to carry out, believe me! I have directed a lot of tests like this, and it takes first of all an accurate survey of the ship in construction , in order to unload as many unnecessary items as possible, f.i. welding machines that are still on board...and other! The masses that create moments are moved from port to starboard several times, and the relevant angles of inclination are measured, in several ways : a test like that can require a whole day !
The result of the inclining test, imposed by international safety rules ( e.g. , first of all, the rules issued by the IMO = International Maritime Organisation) are
sufficiently reliable, to be the basis of all subsequent stability calculations that are to be carried out in every given loading condition : the empty ship weight, and the coordinates of its center of gravity, are the basis for calculations of the ship’s stability future condition of loading, and these conditions are to be in compliance with the international safety rules , first of all SOLAS ( safety of life at sea) rules, which are in a continuous evolution and improvement , since the first London convention , which followed the TITANIC sinking. Nowadays , we are no longer in the conditions of the TITANIC , there are mountains of rules issued by periodical conferences at the IMO. IF you wish , look for its site, to get a simple idea ! You will be astonished against the quantity of rules to be applied in this context ! The shipping industry is a very complicated world , I hope you trust me.
It's interesting that most disasters where structures are involved can always be put down to commercial (and even criminal) interests, rather than the good old Engineers and Surveyors. Good regulation is the key.
Well, dear Sophie, don’t be so extreme! It is out of doubt that disasters at sea occur , but not always for commercial or criminal interests. A ship cannot depart a harbour with more goods on board than allowed by its Load Line marks, that establish the maximum allowed draft (Plimsoll marks, never heard?) : no Port Authority will let she go. Good regulation are there, and of course must be respected. But disasters sometimes occur for unforeseen circumstances, f.e. technical failure, and of course human errors, which are always behind the corner! I have been nominated by italian courts as an expert in some ships disasters, and have learned a great lesson from my experiences : big disasters are often due to a lot of concurrent causes, some of which may be considered not significant , if taken alone ! In any case, the heavy work of surveyors isn’t to be underestimated .
One of the guy, Bob 012345, asked what “reading the drafts” means; Bob, have a look at the attached sketch :
You have to imagine that the still water surface ideally cuts the ship in two parts : only the portion under the surface displaces water, of course. The Archimede’s principle says that the weight of the whole ship ( or a floating body in general) equals the weight of the amount of water displaced, for it is in equilibrium under its weight and hydrostatic buoyancy.
On both sides of a ship, draft marks are welded, fore, middle and aft. The mean draft allows you to enter the ship’s hydrostatic curves, nowadays replaced by computer calculations, which give you the immersed volume, so S = dgV ( d= density of water) , and other geometrical characteristics of the ship. But the final weight and position of G, when the ship is loaded, cannot be read from geometrical characteristics, they depend strictly on the amount of goods loaded , and their position on board.
Excuse me for my bad English.
