Kn

hello.....i have a question to ask..

can we find KN value without GZ value

tq..

 

Of course you can, in fact you have to. KN, or more accurately the KN curve because it is dependent upon angle of heel, is just a different representation of righting arm where GZ is the righting arm with respect to Vertical Center of Gravity (VCG) and KN is the righting arm with respect to Keel (i.e. VCG = 0). The KN Curve is constant for a vessel at a particular displacement, while GZ curve will be vary depending on the VCG (or loading conditions) for same draft. You have to figure out KN (the distance between the keel origin and the vertical line of action of the buoyancy) for the desired draft in either case, but once the KN curve is derived for a particular draft, GZ can be easily calculated for any loading condition using the formula GZ = KN - VCG*Sin(heel) for any heel angle.

 

seem complicated....

did KN have the specific formula or the only to calculate it by using derivation???

 

You have to calculate the center of buoyancy for both GZ and KN. If you can't do that...<shrug>


Edit...make sure you understand that GZ is not related to GM in any way, and H on the above figure is not related to GM...different concepts

 

yes i understand that GZ either KN is not related to GM, but according to your attachment there are triangle method to calculate KN, but how this could being done if KH is unknow???

 

First off:
The KN shown in the picture of Jehardiman is a total different KN value as I generally use in stability calculations!

In my end of the world, we use the value KN as the distance between keel up to the "false Meta centre". When a ship is under heel, its meta centre does not stay on the centre line of the ship. This makes stability calculations quite difficult. For simplification, we therefore place a false meta centre at the cross point of the ships centre line and the line between BM (with M being the true meta centre!) See below picture from Wiki explaining it quite well:

 

Second off:
Continuing in the presumption that the value KN which you want to know is the false meta centre height and not the value KN as Jehardiman uses:

In stability calculations it is generally GZ which we are trying to calculate using KN as a variable! Not the other way round!! So yes there are ways of calculating KN. Mathematically KN can be derived from BM at heel, however you will need a stability program for the hull shapes other than rectangular.....
For your purpose, I believe that looking into the use of scribanti's formula may be sufficient!

Thirdly:
Taking into mind the other thread (Intact stability) you have started with similar questions, I am assuming that you are trying to grasp the basics behind stability calculations. I think it’s wise to group your questions into one thread instead of spreading them out over multiple threads. You will go crazy by the multitude of answers you receive, which may not be in line with each other!

 

Hold on here Tackwise. The false metacenter is not a good concept to espouse because the wall sided assumption is not valid for large angles of heel, which is the whole reason for determining GZ. Metacenter is only a concept to determine the resistance to rolling motion (stiffness), not the resistance to capsize (stability)

Otherwise the values are used identicaly. Calling your "KN" KNphi we can show Nphi = H therefore KNphi = KG+GH and my "KN" = (KG+GH) sin theta

so all agree

GZ = (KNphi -KG) sin theta = GH sin theta = KN-KG sin theta ==> KN = KNphi sin theta which is obvious from the figures. Either "KN" is only a function of B at heel theta Because wether the line of action is measured perpendicular from the origin or where crosses the centerline is line is irrelevent as long as the point of reference is clear. KN in either case is a constant with KG as a variable loading conditiong, giving a GZ curve for that loading....all depends on who you look at the stability problem...trying to get a design to have a given GZ or determining if the cargo loaded provides sufficient GZ.

 

:?:...you caught me, sorry for being this confusing...is your method and jehardiman method are different???...

think i understand your diagram that "false meta center" depend on heel angle....but why looking at scribanti's formula may be sufficient???

 

Jehardiman,
I may need to recheck my books, but in my understanding the false meta centre is not directly related to the vertical wall sided assumption? Meaning you can also use it for not vertically walled ships.... (you are however indeed correct in that it is not applicable for large heels)

The formula of scribanti on the other hand does indeed assume wall sided ships. Which makes it unusable for other hull shapes, and makes it unusable for larger heel angles.

Concerning the rest of your reply, you are indeed correct, although I did not go a far as to check your formula's

 

Azri,

In essence, no Jehardiman and I use the same methods, we merely use different names for variables.

My suggestion that you look into the formula of scribanti, was under the assumption that you are looking into stability for your Naval architecture education. In order to understand the essence of stability you must be able to reason it out yourself, which also means being able to calculate it by hand! Complex shapes cannot be calculated by hand, but need computer calculations!

I am therefore assuming your teachers will not directly load you with complex shapes, but will let you work out the stability of relative easy shapes. Generally the formula of scribanti can be used for vertical wall sided hull shapes, such as nearly all cargo ships. Note however what Yehardiman said, it is NOT aplicable at large angles, and other hull shapes!

I stand by my suggestion which I made on your other thread, that you first try to work out the wedge theory yourself on a rectangular shape. The formula of scribanti for instance is also related to the wedge theory.By calculating the wedge out yourself, you will understand why the wedge formula works the way it works, and also why the formula of scribanti works the way it does, and why they do not work at large angles!!!

 

you guys rock,,,,thank for being highly helpful to me.....

you all the best teacher and friend to since i register in this forum,

 

tackwise,

you are absolutely right, like i have mention in my other thread this is my first time, before this i'm just depend on software, and again you are right something teacher only load a student with simple shape because it easy to teach...


thank you again,

azri.....

 

hello again....

you are right scribanti formula are related to wedge theory, now i understand how it come, correct me if i,m wrong the thoery behind this two method of calculation is applied in large angle of heel and capable for rectangular shape of vessel and barges, coz this type of shape always have positive satbility mean the M is always above the G....

what concern me is, after looking at some example of scrinbanti formula i notice that the distance of false meta center "N" to G are equal to BG...

like you say before the complex shape cannot being calculated by hand, so what your opinion regarding to my boat shape...

 

tackwise,

i already applied scribanti formula in my stability calculation, but still fail

correct me if i'm wrong,

NB=BM[(1+0.5)*tan^2 (0)] {sribanti formula}

so, my concern it "TAN", if i apply TAN in my 90 degree of heel the result is "math error"...

plz explain to me more....