GZ value

hey guys, i'm learning the Maxsuf, can you please help me check why all the GZ value is negative, it does not make sense. I don't know what I make it wrong. thanks so much

 

Welcome to the forum....your CG is too high in the vessel. This is obvious from the spreadsheet. Average draft is 1.5m (so a KB of ~1m) but CG is ~2.5 with the fluids.
This means that BM needs to be >~1.5m.
BM = Iwp/Vol,
Vol is 56.84m^3
Iwp = (1.5)*(56.84) = (K^2*Awp)/3
Awp= 46.07 so
K^2 = 3*1.5*56.84/46.07 = 5.55
Square root of 5.55 is 2.34.
Therefore the centroid of half the waterplane has to be 2.34m off CL when the maximum halfbredth of the vessel is 2.4m....not happening.

Edit to improve math reasoning readability and fixed my Iwp approximation (k^2 vice k^3...always check units...)

 

Thanks , jehardiman . for your explanation, but I still not really get it , is that mean the vessel dimension has a problem? how can I make the change? sorry fo my stupid question. this is my first semester doing this. my lecturer also did not really helpful.

 

Another quick way of checking this, can be done.

The Lwl = 13.117m, and max Beam = 4.816.
So, if this is a rectangle, the area = 63.17m^2.
Given the WPA = 46.066, this accounts for roughly 73% of the area if a rectangle.

BM = I/V, we can guesstimate a quick I, this way too.

LB^3/12 = 13.117x4.816^3/12 = 122.10m^4.
Given that the WPA is just 73% of the total, if we pro-rate this for the I too, we get 122.1x0.73 = 89.1m^4.

Thus BM = 86.1/56.84 = 1.57m

T will be just a bit above 1/2T ~ 0.9m.
So KM = 0.9 + 1.57 = 2.47m.

This is below the value you have for KG, ergo, -ve stability.

Something is wrong with the data output, and/or input.

 

thanks so much

 

See this discussion on GM...Calculation of GM http://www.boatdesign.net/forums/boat-building/calculation-gm-8852.html#post60405 (and please ignore the sarcasm in it). Note tha GZ = sin(theta) *GM for any stable angle. If calculated GZ is always negative, that means that GM is always negative, and the vessel was never stable.

 

thanks so much I understand it now