Boat Stability

Yes… I am back to study mode, ticking off the final few bits of my Ocean Yachtmaster course. Next week I am starting a Meteorology course with some friends, so hold on to your hats for those posts people! But for now you are going to have to deal with a post on yacht stability…

yawn…

Actually I took a group of golfers out on my boat once. They had never been sailing before…

…it was a reasonably windy night, but nothing too major, but they were terrified that we were going to tip over! Wildwood is a bit tippy, but not overly so. She has got 1 tonne of lead on her keel, so I wasn’t worried about it, we did probably have a bit too much sail up but as their eyes grew ever wider I had to reassure them that I was 100% confident that we wouldn’t tip over.

IMG_6665

As I read the books on boat stability my eyes start to glaze over, they quickly get in to physics talk and it gets a bit complicated, but I am going to do my best to explain below.

Actually boat stability is a pretty important thing. In the case of the Easy Rider fishing boat, sadly eight people died when the boat was overloaded and capsized. In this case this boat became unstable when too much gear was loaded on the deck. Effectively making it top heavy and it tipped over and sunk.

So what is the story with boat stability then?

Boats have a centre of Gravity and a centre of Buoyancy. When floating upright the centre of buoyancy is midships and so is the centre of gravity. So gravity pulls the boat downwards and buoyancy pulls the boat upwards.

As the boat heels over the centre of buoyancy moves outboard away from the centre of gravity. The two forces then work against one another. The centre of gravity pulls downwards and the centre of buoyancy pulls upwards and the boat tries to right itself. Just like in my handy little drawing below:

FullSizeRender-48

So this is how the boat should act if it is nicely balanced and not overloaded. However if it has too much weight up high, or perhaps a lot of water in the bilge, then the physics start to change. The centre of gravity moves to the outside of the boat and the centre of buoyancy move in the opposite direction, and then they both work together to tip the boat over…

FullSizeRender-49(Just use your imagination with my drawings please…)

Ok are you with me so far? Now is when it starts to get a bit more complicated… lets talk about Angles of Vanishing Stability… (eeeeek!!!!)

So forget about the overloaded/water in the bilge boat, and lets go back to the nicely balanced boat scenario. Sailing along happily minding your own business, until a huge gust of wind comes along…

So at 0º of heel – the centre of gravity and centre of buoyancy are as per the first picture on the left hand side. At 30º heel (i.e. leaning over 30º) then you might look something like the boat on the right hand side of the first picture.

The centre of buoyancy and centre of gravity gradually move further and further apart, until they reach a point and then the forces start to come back together again, but then the centre of buoyancy passes under the centre of gravity and out the other side. This is bad… the forces are still working in the same way, but now they are working together to keep your boat upside down…

This is called a GZ curve (The G stands for gravity and the Z for Buoyancy – no I don’t get that either…?) The degree at which the centre of buoyancy passes below the centre of gravity is called the Angle of Vanishing Stability. The greater the area below the line, the more stable your boat will be when it is upside down.

FullSizeRender-50

I can imagine that the next time you get knocked down you are going to be thinking about this post and how insightful it was… fortunately – (as every cloud has a silver lining) if you do find yourself in this position, it is likely to be very rough and the next wave is probably going to flick you back up the right way again. (Unless you are a catamaran or trimaran)

Thankfully you don’t have to go through this in practice to figure out how stable your boat is. In fact all boats going offshore should have stability curve data.

The curve for multihulls looks a bit different to the one above. At 90º the curve drops sharply across the line – making the boats equally stable upside down as they are right side up.

If you compare the AVS angles of various boats, then you can get an idea of how stable various boat designs are. There are many factors that feature in capsize resistance including the displacement of the vessel, weight of the keel, weight and placement of cargo (heavy stuff should be stowed down low), beam of the boat, freeboard etc.

GZkurver

Thanks very much to Sten Engelstoft for that great graphic above. You can read more about boat stability on this website.

So here is what else I need to know about stability:

Stability

  • Understanding stability GZ curves applicable to varying types if yachts and power vessels – yes got that – as above
  • Awareness of the desirability for a yacht to return to upright even when inverted – Yes I think it would be very desirable for the boat to want to turn the right way back up…?!?!
  • Awareness of how instability is achieved by deck design in an inverted yacht – yes apparently the design of the coachroof can make a boat more stable upside down, also free water trapped in the cockpit can affect the boat’s stability.
  • Awareness of the requirement for stability data for power vessels leaving NZ – yes as part of the Cat 1 requirements
  • Awareness that yachts leaving NZ are required to have positive stability – i.e. the design of your boat means that it should want to turn itself back up the right way again.
  • Awareness of the motion of a dismasted yacht and that it is more prone to capsize – yes the mast provides another force to the equation called inertia. Apparently a tall heavy mast provides a surprising amount of opposition to the capsizing effect of a big wave.

Sooo…. show me your curve! Do you have the data for your boat? Got any good stories about boat stability to share? Comment below. 🙂

6 thoughts on “Boat Stability

  1. Pingback: Ocean Yachtmaster | Astrolabe Sailing

  2. You are better than I am! I could not bring myself to sign up for the boat stability class. I know the bare bones basics (don’t overload one side or the other or the boat will not right itself), but not the intricate physics and mathematics. The very idea of sitting trough classes on the subject make me want to go back to bed. I had a hard enough time with the classes I did take! 😛

    Liked by 1 person

  3. Pingback: Coastal Skipper | Astrolabe Sailing

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