How do you decide if your small water craft is stable?

Hello all,

My question is about stability calculations for a small watercraft.
Which calculations do you make and which rules you look for when you carry out stability calculation for your own design?

Any method to follow? Or any recommended software(with the theory of the used method)?

Thanks in advance and have a great day.

 

Do you mean something like a canoe?

 

Hi Furkan,
I presume that you have designed a small water craft and you now want to calculate the stability for it.
If so, can you post more details about the boat please?
Are you familiar with stability calculations? Do you know the basic formula BM = I / V ?
Where BM is the distance from the vertical centre of buoyancy to the metacentre, I is the second moment of area of the waterplane, and V is the volume of displacement at this waterplane.

 

At first, for small boats, say < 5-6 m, the crew weight is an important part of the total weight, and so the stability is closely dependent of your assumption for the crew weight and position.
Once a "design" crew weight and position are set, you can compute both the initial stability (i.e. the GM) and the dynamic stability (i.e. the righting moment energy within a rolling event of say 0-25 deg of heel, or less if the watercraft is open deck and the freeboard is too low). I did the exercise for a dinghy and proposed scales of stability for the two criteria, see here :
About dinghy intrinsic stability : proposition for a standard assessment https://www.boatdesign.net/threads/about-dinghy-intrinsic-stability-proposition-for-a-standard-assessment.66226/

 

Stability on a small watercraft has to include the skill of the crew, since the weight of the crew can be over 60% of the displacement.

 

USCG--Simplified Stability website https://www.dco.uscg.mil/Our-Organization/Assistant-Commandant-for-Prevention-Policy-CG-5P/Commercial-Regulations-standards-CG-5PS/Office-of-Design-and-Engineering-Standards-ENG/Naval-Architecture-Division-ENG-2/Simplified-Stability/

 

Hi Furkan,
There are two perspectives of your query. First, basic engineering checks i.e. how will you know your boat is stable enough or not? Last, which regulatory requirements you will need to comply? Regarding the first, follow @Dolfiman procedure. But, regarding the rule compliance, it depends on many things, location of operation of the vessel, owner requirement etc. But a good starting point should be as @jehardiman said. I knew there are some ISO and Norwegian standard for small vessels.

GM = (KB + BM) - KG >0.1
KB = (2T/3 ~ T/2)
BM = IT/displacement

Regarding static stability, (KB + BM), you can control during your design. For existing vessel, you will have no control over them. However, you can play with the vertical distribution of weight and ballast to make the vessel to have sufficient stability.

Wish you all the best.

 

@Furkan , a boat is stable or not if it meets certain stability criteria. To do this, the first thing you must do is choose the criteria, standard or regulation, that must be applied to your boat. Since you are in Turkey you may want to design your boat according to ISO standards for small craft. They give the necessary procedures and calculations for the stability and buoyancy assessment and categorization.
Do not forget that, in addition to stability, you must also analyze buoyancy.