How Do Small Waves Capsize Ships?

on the open ocean you could find that a relatively small wave has the power to capsize even the most modern high-tech vessels the problem comes from how a ship moves if we force The Vessel over and then let it go it's going to roll from Port to Star but just like a pendulum with each roll of course friction with the water whether from the hull itself or from bilge Keels is going to reduce the amount it rolls until eventually the ship will settle upright as that roll reduces however notice how the time it takes to get from one side to the other remains constant we can see it clearly if I plot a graph of a ship's roll angle against time it starts off large and reduces but the time between the Peaks is the same this is a ship's natural role period as an example a 250 meter long vessel could have a natural world period of 30 seconds though that will change depending on how she's loaded the actual roll period of a vessel doesn't normally matter too much apart from in some rather specific circumstances say we have waves on the beam the extreme of any role is going to be when the ship is halfway between a peak in a trough as the wave then moves across the ship is going to roll through the upright until it reaches the extreme roll in the other direction the roll period due to the wave is just the frequency that you encounter the wave on the graph it looks consistent with each role being identical but if that wave frequency is close to the ship's resonant frequency we have a problem each successive role is going to be bigger than the last on our graph the rolling due to the wave constructively interferes with the ship's natural role period we call this synchronous rolling and it can very quickly lead to a ship capsizing even when the height of the wave looks completely benign once it starts you might only have a matter of seconds to try and break the link you could alter the ship's course to change the frequency that you encounter the waves or if you have them you could deploy stabilizers to interrupt your ship's natural role period the only thing that isn't going to work is adjusting your speed with the waves on your beam the encounter period will be the same regardless of how fast you go of course synchronous rolling due to waves on the beam is only one scenario the other arguably more dangerous one is parametric rolling before we get to that though I just want to thank surfshark for sponsoring this video as a Seafarer I've always encountered geographical issues online connecting to a ship's Network would route my traffic through satellites and down to the service provider's location making it impossible to access things locked down to my home country now however I can use surfshark which lets me choose my own virtual location by accessing the internet through one of their servers located in over a hundred countries for example I can select a server based in the UK and any site or streaming service that I visit would see my location as the UK letting me access everything as I would from home and it gets even better without a VPN the connection goes from my device to the ship's Network through the satellite Network through the ground station to the internet service provider and finally to the website I want to visit technically anyone on that chain could access my information as it passes through with surfsharkvpn however I get a client up application on my device that encrypts my data for its Passage through the chain stopping everyone else from harvesting my information sign up to surfshark using the link in the description with the code navigation and you will get an extra three months for free of course they also offer a 30-day money-back guarantee so there's no risk to try it out anyway we just covered synchronous rolling and we're about to cover the far more dangerous parametric rolling this happens when the waves are either right ahead right to stern ore from a couple of points on either of your quarters I find parametric rolling more dangerous because it's far less intuitive to understand after all no one would really believe that waves on the bow could cause a ship to roll let's take this ship which is powering into head C at this point there's a Crest near the bow and a Crest near the stern now the underwater profile at the bowenstone are quite pointy meaning there isn't much volume underwater this means that the ship is going to sit lower in the water to generate enough buoyancy to remain afloat as the ship sits lower the average position of the buoyancy the center of buoyancy is going to rise up a bit relative to the hull conversely when the situation is reversed and the peak is in the center of the vessel the center of buoyancy is going to sit lower because the ship is far boxier at that point and there's more buoyancy generated lower down in the hull remember a ship's stability is all about where the center of buoyancy and center of gravity are located her upright position occurs when they're in line with no rotational writing Force generated ideally both will be on the center line but in reality at least one of them is likely to be a little bit off and the ship will actually sit with an angle of maybe 0.1 of a degree by moving the center of buoyancy up and down you might get that to increase to 0.2 of a degree or something it's not much but it's enough to initiate a little role in the ship in time with the wave in counter frequency if that role frequency then happens to match the ship's resonant frequency it could get larger and larger as each successive wave passes through this is per parametric rolling of course we've assumed that the ship was almost perfectly upright and that the waves were coming from right ahead in reality you're likely to have additional rolling forces generated by less than perfect initial stability you're also likely to have waves striking at a bitter an angle moving the center of buoyancy from side to side as well as up and down no matter how the center of buoyancy moves if you're cycling it with a frequency that's close to the ship's resonant frequency even a small roll can quickly accelerate to capsize a ship just like with synchronous rolling you need to take immediate action to break the link between the ship's natural role period and the encounter frequency of the Waves this could be an alteration of course or speed to change the encounter frequency or again you could deploy stabilizers to interrupt the natural role frequency the thing with parametric rolling of course is that because it's all to do with a moving center of buoyancy it's much harder to realize what is powering Your Role so that you can take action to counter it it's also much more of a risk on bigger ships with large overhangs like modern massive container ships and car carriers this is because the flares at the bow and Stern have a disproportionate effect on raising the center of buoyancy when they encounter waves consider the bow of an old ocean liner to generate fairly even buoyancy across its entire height compare that to a nice flared bat which will generate more buoyancy the higher up you go of course with advances in technology come advances in education so as long as you know what causes parametric and synchronous rolling and you know what to do to counter them modern ships are no less safe than their older counterparts foreign [Music]

Channel: Casual Navigation

Views: 273,330

Keywords: casual navigator, marine, shipping, casual navigation, maritime explaination, merchant navy, sailing, marine animation, parametric rolling, synchronous rolling

Id: S9CHCocE6uI

Length: 6min 39sec (399 seconds)

Published: Fri Jun 23 2023