initial stability of a ship

Initial stability:
Vertical of transverse metacenter : 5.920 m
Transverse metacentric radius : 5.796 m
Longitudinal transverse metacenter : 25.083 m
Longitudinal metacentric radius : 24.960 m

what is the definition of the above terms?
i found these detail in hydrostatic calculation in freeship. tried to find the definition in wiki and google but could not find.
please share your experience to help me. thanks in advance

 

Not one out of 14,200,000 google responses to "initial stability of a ship" had answers ? That is rotten luck !

 

Be careful, you're sounding a lot like me . . . what's really scary is he's probably a year away from graduation.

 

I'd like to think we are better than Google at something.

Um, https://en.wikipedia.org/wiki/Metacentric_height

 

I think what is missing in the description is that the metacenter is the theoretical point that the hull will rotate. when the hull rolls, it rolls about an axis, the height and location of this axis relative to the CG determines the stability. that axis the ship rotates or rolls about as shown on the cross section is called metacenter. the location of this metacenter changes as the ship rolls because of the shape of the hull, so at any given point, the center of axis is called a "metacenter" since its location usually changes as the hull rolls over.

Without knowing this concept, the description is very confusing to some one not familiar with the terminology.

 

I believe, and hope I'm not very much mistaken, that the boat turns around an axis through the center of gravity of the waterplane surface.
What do the experts think?
On the other hand, it must be said that what we call "metacentre" is actually a false metacentre that is more comfortable for the stability calculations. The actual boat metacentre calculated rarely.

 

Not sure what you mean by "the boat turns around an axis through the center of gravity of the waterplane surface." Generally the center of gravity is not at the waterplane surface. Are you referring to a method developed for calculating cross curves of stability using a planimeter?

 

Tansl,

check your terminology: by "turn" do you mean yaw? rotation in three axis on a ship is described as pitch, yaw and roll. If you mean yaw, like what happens when you change the heading on a ship, that is not related to roll stability. You will have different centers of rotation for the three different means of rotation in pitch, yaw and roll. I presume in this discussion we are talking about roll stability, and the metachenter location would seen through a cross section of the hull.

 

When one speaks of transversal metacentre is perfectly clear that movement of the boat to which you are referring, is not it?
In any case, rotation of the vessel, whatever the shaft to which you mean, will always occur around the center of gravity of the float.
Although my English is not correct, you should fully understand what I mean.
The center of gravity of the ship is one thing that has nothing to do with the point around which turns (heel, pitch and yaw) the boat. The center of gravity of the flotation is the point through which pass all the possible rotation axes of the boat. The ship may never turn around the false metacentre, in my opinion.
Probably I should check my terminology but my concepts are totally clear. And that's what really matters.

 

Well...sorta....are we talking about the absolute instant center axis the ship rolls about? Or how you apply 6 DoF forces and motions?

When a ship moves, it moves about its CG, however, because of cross coupling, when the ship rolls it also sways (not true in all cases but true for most "ship shaped" hulls, a true cylinderical hull with the CG at the axis does not sway no matter what the draft). This means that there is an instant center (i.e. a fictious point that does not appear move under the combined roll and sway) that is a function of draft, heave, pitch, yaw, surge, and roll. FWIW, the instant center is much closer to the CG than M, and is only loosely related to the waterplane as a function of hull shape.

For a discussion of Metacenter, GM, and what it means see the following post

http://www.boatdesign.net/forums/boat-building/calculation-gm-8852.html#post60405

 

I searched metacenter in wikI but it has nothin about metacentric radius and all. I thought experience is better than Google. I'm not year away from graduation. I've just completed my 12th. Just interested . So learning myself. Thanks for those who gave suggestion.

 

Thank you petros for valuable suggestions and for your time

 

@jehardiman

i think what tansl meant is, that any roll is happening around an axis and this axis is at the waterplane...
for the roll now it would be the centerline if the boat is completely upright - again your example with the completely cylindrical hull, it will not sway and not move in any way when it rolls and it will roll around its centerline along the waterplanearea...
the axis for the pitch would be transversal at LCF - if i am not completely mistaken...

 

capt vimes: that's exactly what I say.
And I add something else, attempting to clarify the phenomenon: talking about "rolling" for example, if the boat had an initial heel bigger than 0 degrees, the next move of rolling will occur spinning around an axis passing through TCF, which at that moment will not be in the center line.
Cheers.

Clarification: I always talk about a boat that floats freely. In the event of grounding, as we all know, the case would be very different. But in any case, the rotational axis can pass through the false metacentre.

 

Absolutely not, there is no constant axis, it is all a function of cross coupling. You and TANSL are imagining a piece of paper stuck to a board with a pin, but a vessel moves in 3 space as it rolls.