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Some hull shapes are inherently unstable. The slightest deviation from pristine vertical balance will make the ship flip. However, even hull shapes that are ini

Some hull shapes are inherently unstable. The slightest deviation from pristine vertical balance will make the ship flip. However, even hull shapes that are initially stable at some angle reach their limits. All of these examples assume the deck is perfectly sealed and that water doesn’t get into the hull. Loosely related: here is a video of the German Maritime Search and Rescue Service (DGzRS) trying to 'sink' one of their (then new) smaller rescue lifeboats which has self-righting capabilities: https://www.youtube.com/watch?v=dz_N6MG5tt0 https://www.youtube.com/watch?v=dz_N6MG5tt0 (Ofcourse it was a test if it does have these capabilities, not an attempt at actually sinking it.)

jasonwatkinspdx 5y ago
Yeah, this is a fantastic blog post but is a little inaccurate in some edge cases. In solo around the world races like Vendee Globe, the boats are required to be fully buoyant and self righting no matter how they end up. The most common approach to achieving this is to rig a canting keel with a device that when the boat capsizes, lets the keel swing to one side, creating a weight imbalance that rights the boat. They're quite serious about it too: you don't get to race the boat unless you demonstrate it works that way at the pier.
- JshWright 5y ago
  It's not inaccurate though... The hull shape does reach a point of instability, at which point the hull shape changes.
  - jasonwatkinspdx 5y ago
    There are hull forms (without the canting keels I mentioned) that have positive righting moment through 180 degrees. Life rafts are universally designed this way. For boats it's just not that necessary ultimately, as capsize is pretty dang rare on keel boats as a baseline. Vendee Globe et all are hardasses about it because they know if the worst happens, there's no rescue possible on a short timeline.
dtgriscom 5y ago
Interesting. The designers can probably analyze the rate at which the boat righted to quantify its stability.
lambdasquirrel 5y ago
One cool thing to think about is the effect of tumblehome hull forms. Kind of makes you check in, that you really know what the center of buoyancy is. https://www.dco.uscg.mil/Portals/9/DCO%20Documents/5p/CG-5PC/CG-CVC/CVC3/references/Stability_Reference_Guide.pdf https://www.dco.uscg.mil/Portals/9/DCO%20Documents/5p/CG-5PC...
- mleonhard 5y ago
  What does 'tumblehome' mean? The document you linked does not contain the word.
  - _Microft 5y ago
    "Tumblehome is a term describing a hull which grows narrower above the waterline than its beam", with "beam" being its width at its widest point. https://en.wikipedia.org/wiki/Tumblehome https://en.wikipedia.org/wiki/Tumblehome

jasonwatkinspdx 5y ago

Yeah, this is a fantastic blog post but is a little inaccurate in some edge cases. In solo around the world races like Vendee Globe, the boats are required to be fully buoyant and self righting no matter how they end up. The most common approach to achieving this is to rig a canting keel with a device that when the boat capsizes, lets the keel swing to one side, creating a weight imbalance that rights the boat. They're quite serious about it too: you don't get to race the boat unless you demonstrate it works that way at the pier.

JshWright 5y ago
It's not inaccurate though... The hull shape does reach a point of instability, at which point the hull shape changes.
- jasonwatkinspdx 5y ago
  There are hull forms (without the canting keels I mentioned) that have positive righting moment through 180 degrees. Life rafts are universally designed this way. For boats it's just not that necessary ultimately, as capsize is pretty dang rare on keel boats as a baseline. Vendee Globe et all are hardasses about it because they know if the worst happens, there's no rescue possible on a short timeline.

dtgriscom 5y ago

Interesting. The designers can probably analyze the rate at which the boat righted to quantify its stability.

lambdasquirrel 5y ago

One cool thing to think about is the effect of tumblehome hull forms. Kind of makes you check in, that you really know what the center of buoyancy is. https://www.dco.uscg.mil/Portals/9/DCO%20Documents/5p/CG-5PC/CG-CVC/CVC3/references/Stability_Reference_Guide.pdf https://www.dco.uscg.mil/Portals/9/DCO%20Documents/5p/CG-5PC...

mleonhard 5y ago
What does 'tumblehome' mean? The document you linked does not contain the word.
- _Microft 5y ago
  "Tumblehome is a term describing a hull which grows narrower above the waterline than its beam", with "beam" being its width at its widest point. https://en.wikipedia.org/wiki/Tumblehome https://en.wikipedia.org/wiki/Tumblehome