Hi Ross, greetings to all modelers.
I took this topic from a discussion born on Ron's Build Blog (first link at the end of the text).

You did well to clarify the use of the correct term and the translation. Thank you.
I always have the doubt that the terms I use are incorrect and misleading, especially the technical ones.

I am addressing Ross, because I am answering a question of his but the discussion involves everyone, no one excluded.

Since we have invaded Ron's "build blog", I am reporting my last message here, otherwise the meaning of what I say cannot be understood.

"""""""""""""""""""""""""""""""""...However, in this regard I want to clarify something that many people are confused about. Let's say they don't have clear ideas.
In scale naval models we can more or less easily obtain a "weight stability" (in Italian manuals this term is used when the center of gravity is below the center of the hull).
In reality (with a few exceptions) almost all ships and boats (passenger ships, ferries, motorboats, oil tankers, ocean liners, cruisers, aircraft carriers, destroyers, frigates, corvettes, auxiliary ships, icebreakers, container ships, but also ancient galleons and vessels, as well as naos, caravels, carracks, triremes, liburmes, etc. etc.) only had "stability of form" (in Italian manuals this term is used when the center of gravity is above the center of the hull).
It follows that the righting thrust exists when the metacentric height is positive. In other words, the ship is able to right itself only within a certain angle of heel, not beyond.
Ships with "stability of form" beyond a certain angle will no longer be able to right themselves and will capsize.
Most ships, even in adverse weather conditions, do not suffer dangerous inclinations (except for a few cases in where tragedies occur)..."""""""""""""""""""""""""""""

and that was Ross's question [A useful question for everyone]:

"""""""""""""""""""You used the term
CENTRE OF THE HULL
Possibly lost in translation, does this refer to
CENTRE OF BUOYANCY OF THE HULL?
It is best ABOVE the centre of gravity.
CENTRE OF BUOYANCY will shift as the hull heels over.
Does centre of hull and centre of buoyancy refer to the same thing?"""""""""""""""""""""""""""""""""""""""

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Yes Ross I was imprecise, I double-checked the terminology in English.
Fortunately the capital letters indicate the same terms in both English and Italian.

So, to avoid misunderstandings:

G: Center of gravity (in Italian "centro di gravità" or "baricentro" is the point where the weight force acts).

B or CB: Center of Buoyancy. ["The center through which the buoyant force acts. The CB moves as the underwater shape changes as the hull moves up and down (or rolls sideways) in the water. If the hull is floating free to trim or roll, the CB will always be directly under the center of gravity."]

M: Metacenter. ["the theoretical point around which the boat rolls or trims."]

GM: Metacentric height [It is the distance between the center of gravity of a ship and its metacentre. A larger metacentric height implies greater initial stability against overturning.]


So when I wrote "center of the hull" I meant B or CB (in Italian "centro di carena" or "centro di spinta di archimede".)

For the rest I confirm what I have already written, if something does not convince you just tell me. In this topic we can digress calmly.

In essence, I repeat, with very few exceptions, ships have a stability (which we in Italy call "stability of form" in my opinion in a very misleading way, but this is how it is written in all the manuals) in which G [Centre of gravity] is above B or CB [Centre of Buoyancy].
In this condition, common to almost all ships, (see image no. 1) the position of M [Metacenter] is important.
As long as the metacentric height is positive there will be a righting moment.
In our scale naval models (not having the real needs of a real ship) we can obtain a stability of weight (in which G [Centre of gravity] is below B or CB [Centre of Buoyancy]).
In this case the ship never capsizes, the righting thrust is always present with any angle of inclination (it will always return to the initial position).
Many modelers do not know that in real ships, not only is this condition not present but the metacentric height is not excessively high so as not to create a righting moment that is too abrupt (which would create problems for the crew and passengers, for example seasickness); in Italian we would say that we would have a ship that is "too hard". So a compromise is sought even at the expense of greater transverse stability.

If you are still not convinced, I will use your own manuals to translate the ones in Italian and illustrate the situation better.
If you want I can go into more detail and clarify the issue of transverse stability.


https://model-boats.com/forum/147178#147766