You can have two kinds of stability. I believe the correct words are primary and secondary stability.
If you sit in a canoe or particularly a kayak, you will often have poor primary stability.
What this means is that all it takes a little sideways force for the boat to tip and it will easily tip more and more.
This often freaks out people new to the hobby.
But often what happens, if the hull shape if of a certain design, it that at some point in the tipping, the amount of sideways force to cause even more tipping goes way up. So, it is kinda like a “wall” of force that must be overcome for an actual tip over to occur. You can do it, but it takes signficantly more “work” than the initial tipping did.
Paddlers, once accustomed to it, can and often do use this ability to their advantage.
Yes, when you start looking at the physic of stability and hull design as it relates to performance it can get pretty interesting.
I’m not sure anyone cited Anthem of the Seas, the Carnival cruise ship that just encountered hurricane-force winds off the southeast US. It apparently swayed heavily, and lots of stuff inside the ship got smashed, but it never came close to capsizing.
I can’t find the article, but one passenger was tweeting about all the chaos on board, and she mentioned going to get her kid from the play area, and seeing another kid sitting in a toy car, rolling back and forth across the room.
I think a half-circular hull cross section, with a flat (or highly tapered) top, slightly above the waterline should do it. Normally with the cg below the CB, it’s stable. But once it starts rolling so that the edge dips below the surface, it starts losing buoyancy on that side while still keeping the same buoyancy on the opposite side. Thus the CB shifts enough laterally to cause an overturning moment.
Chronos just requested that the CoM be below the CoB, which is the case with my suggestion. Any non-symmetric object will have its centers move once it starts tipping. Still, naita is probably right that Chronos wants an example of an unstable equilibrium (like a pencil balanced on its tip). Perhaps YamatoTwinkie’s suggestion will work.
Sailboat keels aren’t usually made of lead any more, but anyway, yes, sailboats are a great example of these principles. There is one huge difference between keel boats and catamarans in this respect, though. Keel boats exhibit continuous correcting feedbacks as they heel in the wind, namely, that the greater the angle of heel, the greater the restoring moment of the keel, and also as a side benefit more of the wind pressure is bled off the tops of the sails. Whereas a cat exhibits virtually no heel at all until a critical point when one hull is entirely submerged and no longer provides any proportional correcting moment, at which point it may capsize.
Kind of ironic that novice sailors may find the heel of a keel boat in high winds to be disturbing, but it’s really the level-sailing cat that is potentially more hazardous if you don’t know what you’re doing in high winds and waves.
I’ve done a bit of sailing in respectable mid sized keel sailboats.
Most operators freaked out if the boat even leaned a bit.
Dude, its DESIGNED FOR THAT. Barring random very rough seas and some significant operator error you AIN"T flipping your damn boat over.
So instead we crept along at a few mph with a 2 degree tilt and the sail at about 60 degrees from the wind direction with hardly any tension on the sails.
Frack, just pull out the fossil fuel motor and be done with it.
IIRC there is an old sailing/marine engineering joke. Catamarans are perfectly stable. In two states. One upright. One inverted
You can have an inverted weighted keel sailboat that is somewhat stable in that position. But it does not take too much to un invert it (like the bad weather that caused it in the first place…you just have to hang on for the ride).
Don’t fishing boats have to have Plimsoll lines? Even if they don’t you’d think the crabbers would be familiar with the concept enough not to make this mistake.
I recall reading somewhere that early ironclad designers thought that a ship with a low freeboard (similar to the Monitor style) might be more stable, as the oceans waves washing over the top of the watertight hull would act as a dampening effect on rolling.
Something kept nudging the back of my memory in this thread, and I just realized what it is. COB and COM mean center of buoyancy and center of mass in this thread about Ships that appear top heavy.
In the Ask The Escort thread, COB and COM mean something else entirely.
I read a similar story, from the late 70’s? 2 boats were taken to a shipyard in Washington. They were repainted, and the line was painted 2 high. Both boats turned turtle with days of starting the season.
Hate that. May as well motor around the marina, then.
Had a go in a friend’s Sabre 402 a few months ago. 25 knots of wind with all wives and nervous passenger types left on shore, so we drove the boat pretty hard. According to my nephew who went along, the smiles on our faces were in direct proportion to the heel angle.
My brother sailed in Very Large Bulk Carriers. When loaded, the decks were awash and the only safe places above deck were on the catwalk or the bridge wings. This was, however, noting to do with being top heavy or not, and everything to do with not breaking in half.
If they were too high out of the water, the action of the swell was enough, over such a long vessel, to make it flex. Keeping it partly submerged minimised the effect.
Did that once long ago with my Hobie 18. Damn near ripped my wife’s foot off when it got caught in the tramp webbing while she sling-shotted in her trapeze around the mast.
Her ankle & foot got repaired OK eventually, but some ortho surgeons earned their pay.
Up until the downwind bow dug it was super-fun. Afterwards, not so much. One small guy can’t *quiiiite *right one of those in favorable conditions, much less in a 20 kt wind.
