Limiting KG problem

Hello everyone, i am Naval Arch. student from Indonesia. I am doing final project designing a glass bottom catamaran boat. I am trying to calculate the stability of this boat. When i do the calculation using maxsurf stability (in full loading condition), i got 1 criteria failed. Then i do the limiting KG to know the KG limit that satisfied the criteria. But the problem is the result of the Limit KG is negative. Anyone can help me with this please ? Thank you very much

 

Welcome to the Forum Yusuf.

How big (length x breadth) is this catamaran?

Your second attachment mentions a minimum GM of 6.474 m. - is this the calculated GM for your boat with 25 people on board?
If so, then your boat has pretty good stability really

Your table mentions the Lightship KG as 1.329 m, and the loaded KG (with 25 pax and crew) as 1.49 m. w1hich sounds reasonable.

Re your negative KG, that seems to suggest that it is below the keel, which is obviously wrong!

 

Hello Bajansailor, thank you for replying this. This cat has 11 m LPP and 4 m Breadth. The lightship KG is calculated by hand through LWT estimation where i calculate the contruction, engine, and all of the boat outifitting weight and its center of gravity. This boat has 20 pax plus 2 crew. And yes, GM of 6.474 m is calculated with full people on board, but the limit KG is negatif. Do you know why the limit KG number is negative ?

 

It is advisable that you carefully review the weights of the lightship and its CoG. The lightship's CoG height is usually a little below the main deck. Check that this is so.
On the other hand, I don't know what stability criteria you are considering but, in catamarans, it is normal that the maximum GZ angle criterion for normal hulls is not met. In catamarans the initial stability is so great that this criterion cannot be met.
Another issue that has nothing to do with your question: the glass bottom windows must be located on perfectly flat surfaces. It is mandatory.

 

Hello TANSL, thank you for replying. I use IMO code on intact stabilty A.749(18) Ch.3 Design Criteria applicable to all ships.

 

Hi Yusuf
As I told you before, the maximum GZ angle criterion is not applicable to multihulls. Have you indicated in Maxsurf that the boat is a catamaran? The application should take this into account and, if it does not, select, in the criteria window to apply, the criteria corresponding to catamaran.

 

Thank you for the advice, may i know which criteria is the best for catamaran with 15 knot speed ? i also have defined in maxsurf that the vessels type is catamaran.

 

In the Spanish Administration the following criterion is required:
If “alpha” is the angle at which the maximum GZ is obtained, it must be verified that the value of the area under the GZ curve, up to that angle alpha, is greater than: 0.055 + 0.001 (25 – alpha).
I suppose that is a universal criterion, not unique to the Spanish Admin. Check with your administration.
On the other hand, if your boat is less than 24 m in length, you may prefer to apply the ISO 12217-1 standard, which establishes different stability criteria that are probably easier to meet.

 

Thank you TANSL, you are really helpful. I believe i can also use HSC Code annex for multihull criteria, right ?

 

Yes, I suppose the criteria regarding the GZ will be the same for any catamaran, regardless of its speed.

 

Thank you very much, i really appreciate your help

 

You are welcome. Do not hesitate to consult any other topic that I will gladly answer, if it is within my knowledge.

 

Hello TANSL, i need to ask you again. After i calculate the stability with HSC Annex 7, i got a problem with "invalid parameter"in the second criteria. This is my first time using this criteria so i don't really familiar with this. Any chance you can help me with this ? Thank you

 

Check, in the "Analysis" menu, that the lower and upper limits of the heeling are correct.


I don't know what Htp, Ht or Hw means, but in any case, it is impossible for these numbers, added two by two or all three added together, to give the same result, unless two of them are null.
My advice is that you forget about the HSC criteria and calculate by hand, based on the curves and GZ values that Maxsurf gives you, if the formula that I have given you is met.

 

As you will note from the HSC code, there is a stability GZ criterion to be met for multihulls, despite what other are suggesting.

The full criteria to be meet for intact stability, in the HSC code, is:
1. The area of A2 up to 30deg/theta(d) shall be 0.055x30deg/theta(d) - where theta(d) is the downflood angle
2. Max GZ shall occur at an angle greater than 10deg.
3. Theta(h) not greater than 10deg from HLs lever - theta(h) is heel angle
4. Area A2 from HTL up to theta(r) is equal/greater than 0.028mrad - theta(r) roll angle from high speed turn

So this brings us to your question.

The nomenclature in the software is not the same as the HSC code.
It appears to be this:

Hpc = passenger crowding lever
Hw = HL3 = heeling due to wind
Ht = HL2 = heeling due to wind + gusting
Hpc + Ht + Hw = HTL = heeling due to wind + gusting + pass. crowiding or high speed turning

And these are levers you need to calculate based up the design of the vessel, as per HSC code.