How Stabilisers Stop Ships Capsizing

You’re dining at a lovely restaurant with your charming partner; risking it all with the potentially deadly fugu delicacy and a sense of humour you learnt from your father. Which is riskier on such an occasion?

You’re inspecting the puffer fish, mostly out of curiosity, to see if there are any signs of poison possibly left in the fish. You are rest assured that the chefs are well-trained and highly skilled at removing the deadliest organs of the fish before serving up.

You’ve already blown it on that risky joke your friends guaranteed would be a hit. You don’t want that bad luck continuing in this date. As you put your fork down and reach for your drink, the glass, the table it’s sitting on, and your chair all lurch sideways causing you to spill wine all over your date’s outfit.

Graciously, your date laughs off the fact their favourite outfit is now ruined – and on the first evening of your cruise together! They begin explaining it wasn’t your fault, that the open seas can be quite rough, and the ship’s stabilisers are doing they best they can in the conditions.

Credit: Karen Bache, 2019.

You’ve always been impressed by how smart they are – especially their nautical knowledge. You ask them, while dabbing a napkin over the spilled wine on the table, how cruise ships usually remain so stable in such unforgiving weather. Your partner is delighted by your curiosity and thinks for a moment. They pull out a pen and begin drawing on a fresh napkin.

“When a ship is stable,” they begin saying, “it’s centre of gravity and centre of buoyancy are aligned vertically.”

Credit: Casual Navigation, 2019. Adapted by: Kieren Topp, 2019.

You’re amazed at the colours and gradients they were able to achieve with just a blue ball-point pen on a napkin, but you continue to gaze at them with interest while they continue.

“In the open seas, however, larger and longer waves apply a greater tipping force to the sides of the ship. When the wave hits the left side of the ship, a large force is applied which tips the ship to the right. As the wave travels along, it applies no tipping force in the middle, and then an opposite tipping force when on the other side. These forces continue to cycle through with every wave and result in the classic seasick, wobbling feeling you landlubbers experience!”

You partner throws a playful look in your direction before diving straight back into the explanation. You’re wondering how they have managed to draw the diagram in such a way that it looks like it is moving – like the napkin is some Harry Potter parchment!

“To maintain stability in the presence of these additional forces, stabilisers must provide an equal and opposite force, as you can see on this napkin diagram.”

Your partner points out the opposing forces resulting in a stable ship. The shock of this optical illusion wears off and is replaced by pure enthrallment.

Credit: Casual Navigation, 2019.

“There are two types of stabilisation: active and passive. Passive stabilisation is cheaper but not as effective. For example, many ships install these bilge keels on each edge of the ship. Their purpose is to produce turbulence and reduce the force of the wave. They only work once the ship is already rolling, so active stabilisation is required to prevent it in the first place.”

Credit: Casual Navigation, 2019.

“The most common form of stabilisation found on cruise ships – including this one, I found on my research before embarking – is the fin stabiliser. It looks like the bilge keel, except you can control the direction of the fin.”

Your partner somehow magnifies the drawing on the napkin and adds a moving fin next to the bilge keel.

Credit: Casual Navigation, 2019.

You’ve become less focused on the words and are in a trance-like state watching the diagrams dance across the napkin. Your partner notices the distant look and decides to elaborate further.

“Like the wing of an airplane, adjusting the angle of the fin allows you to control the direction of the water running past it.”

Credit: Casual Navigation, 2019.

“Operating these fins on either side of the ship allows you to control the amount of opposing force you can apply against the rolling waves. This results in a cruise ship that is much more stable, even in quite severe conditions.”

You take one last look at the napkin ship flapping it’s fins like an awkward metal whale and decide that maybe it was better the wine was splashed over your partner rather than coursing through your bloodstream – you’ve clearly had too much already. After thanking your partner for a wonderful conversation and mumbling some final apologies about their outfit, you excuse yourself and get an early night, dreaming of cruise ships with passive bilge keels and active fin stabilisers.

Credit: Casual Navigation, 2019.


Further Watching

Casual Navigation: How Stabilisers Reduce a Ship’s Roll (2019).

5 Responses to “How Stabilisers Stop Ships Capsizing”

  1. Kieren Topp says:

    Thanks for the kind words, everyone! 🙂

  2. Elizabeth Greenwood says:

    The first mention of the colours and gradients from the single blue ballpoint had me laughing out loud. This was a very nice article to read and I love the story telling.

  3. Ishra Ranatunge says:

    Man oh man. I love it when articles take a joke, make it go too far, but just far enough that you can’t help but adore it. I never would’ve been interested in cruise stabilization unless it was on those Potter-like napkins. Thanks Kieren!

  4. Sijia Yang says:

    This is an amazing story, you explained the principle very clearly. The best part is the pictures. And I am sure my partner won’t know this.

  5. Kieren Topp says:

    My sincerest apologies for not only joking about GIFs being ‘Harry Potter parchment’ napkins, but also for maintaining the joke throughout this narrative piece. 😅