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

Discussion in 'Stability' started by Furkan, Nov 21, 2022.

Tags:
1. Joined: Nov 2020
Posts: 10
Likes: 1, Points: 3
Location: Turkey

FurkanJunior Member

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.

2. Joined: Aug 2002
Posts: 16,475
Likes: 1,494, Points: 123, Legacy Rep: 2031
Location: Milwaukee, WI

gonzoSenior Member

Do you mean something like a canoe?

3. Joined: Oct 2007
Posts: 3,287
Likes: 1,315, Points: 113, Legacy Rep: 37

bajansailorMarine Surveyor

Hi Furkan,
I presume that you have designed a small water craft and you now want to calculate the stability for it.
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.

4. Joined: Aug 2017
Posts: 1,491
Likes: 658, Points: 113
Location: France

DolfimanSenior Member

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 :

rwatson and bajansailor like this.
5. Joined: Aug 2002
Posts: 16,475
Likes: 1,494, Points: 123, Legacy Rep: 2031
Location: Milwaukee, WI

gonzoSenior Member

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.

rwatson likes this.
6. Joined: Aug 2004
Posts: 3,597
Likes: 974, Points: 113, Legacy Rep: 2040
Location: Port Orchard, Washington, USA

jehardimanSenior Member

bajansailor likes this.
7. Joined: Jan 2016
Posts: 3
Likes: 0, Points: 1, Legacy Rep: 10
Location: United Kingdom

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.

8. Joined: Sep 2011
Posts: 7,251
Likes: 652, Points: 123, Legacy Rep: 300
Location: Spain

TANSLSenior Member

@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.

Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.