Stability Unit, Part 1: Introduction to Stability

in this presentation I'm going to explain the basics of stability stability is the bestest ability to remain upright under a variety of conditions and it's basically determined by the interaction of two forces you have the force of gravity and the force of buoyancy force of gravity is the force pulls all things down to the earth so the force of gravity is pulling down on the vessel the force of buoyancy is the force created from the vessel displacing the water so when you put the vessel in the water it pushes some of that water out of the way and in the water pushes back by trying to refill that space so it exerts a force on the bottom of the vessel upward force so those two forces the force of gravity and the force of buoyancy are working against one another so one's pushing up one's pulling now okay when talking about the vessel and talking about how these two forces interact on a vessel we need to keep in mind two reference points so first reference point we'll talk about is the centre of gravity so we'll say that this is a set of the vessel centre gravity we're going to label it G so G is the centre of gravity you can think of the centre of gravity as sort of a balance point of the vessel it's the point where the vessels weight and all of its contents through which it's the point through which the vessels weigh in all of its contact contents interact okay so it's better to think it's it might be easier to think of it as the balance point so for example if I wanted to find the center of gravity of this marker I would find out where this marker balance is okay so that's that's the center of gravity so now another reference point that we have to keep in mind is what we call the center of buoyancy so we'll label on this vessel B so B is the center of buoyancy so the center of buoyancy is the point where the upward force of buoyancy it's the point through which the upward force buoyancy acts it's kind of the balance point of that force so now one thing I'll explain about the center of gravity is that if you don't move any of the way in or on the vessel that point will stay static the center of gravity will not move so the center of gravity of this marker isn't going to move unless I change the weight in the marker okay so the balance point won't shift okay however the center of buoyancy is a function of the shape of the water being displaced now remember how I said the force of buoyancy is created by the displacement of the water and so depending on how that water is displaced in the shape of that water and can determine where this balance point is okay so center of buoyancy center gravity so now we're going to assume that this vessel is symmetrical the port side is the same shape as the starboard side that being said the center of buoyancy but will be Ray belong here right along the center line and we'll say that being that the vessel has the same weight port starboard that it will be directly underneath our center of gravity so in a sense this vessel is going to be sitting at equilibrium so these two forces have balanced each other out and the vessel is sitting in a static position okay now a couple things I want to explain that being said being that the force of or the center of buoyancy is a function of the shape of the water that's being displaced if we were to start to tip this vessel using by an external force if we're not shifting any of the weight on board say for example a gust of wind came and started tipping the vessel it's what we call healing healing is the tipping of the vessel caused by an external force if it were caused by a shift in the center of gravity we call that a list if it's leaning due to shift in the center gravity but we're going to say that the vessel is heeling will say that the wind is starting to tip that vessel now remember the force or the center of gravity has not changed its location that's static because we haven't changed any of the weight in the vessel now because we tipped that vessel we've actually changed the shape of the water to be displaced so that being said the center of buoyancy is going to change its location under this circumstance we've tipped the vessel to the starboard side through force of healing so we'll say it was wind and you'll notice here that on this side the port side there's less water being displaced on this side so that being said the force of buoyancy is going to be more on this side than on the port side so that means the center of buoyancy has shifted okay so in this case we'll say that the center of buoyancy is shifted here now notice they're not directly on top of Earth or they're not directly in line with one another one thing I didn't have to point out too is that the force of gravity pulls perpendicular to the surface of the earth the force of buoyancy pushes up perpendicular to the surface of the water okay now a couple of the things I want to point out I want to explain to you another reference point that we use in naval architecture first draw my fill in my reference lines okay so this is green dotted line is showing the centerline of the vessel upon which the center of gravity is located so this other line that I'm going to draw is a line going up from the center of buoyancy perpendicular to the surface of water okay just like that okay this line is kind of it's basically showing the the the direction of force of buoyancy on the vessel so buoyancy is pushing up in this direction perpendicular to the surface of the water so now you have gravity pulling down perpendicular to the surface of the earth okay so we have this centre line here we have the force of buoyancy now those two lines when the vessel is heeling like that intersect right here at this point that we call the metacenter free label fm it's called a meta Center okay the meta Center is a theoretical reference point that they use in naval architecture to try to determine the vessel stability there is no particular piece of the vessel that corresponds to the meta Center it's sort of an abstract reference point and hence the name meta with the prefix meta kind of signifies it it's abstract so it's just a reference point where these two lines intersect okay now there's a distance that we also want to remember that's GM we call that distance the meta centric height okay now the metacentric metacentric height is this distance right here okay it's a theoretical distance but by determining the metacentric height of a vessel we can kind of determine what the best is ability to turn up right or the vessels stability and I'll explain that here by explaining another distance first I want to point out though notice that these two are right on top of one another so they're interacting with one another in a balanced way it's keeping the vessels sitting upright it's in a equilibrium however when that wind started blowing the vessel over causing it to heal the center of buoyancy shifted center gravity stayed here now notice they're not directly on top of one another anymore okay so that being said there is a rotational force that's trying to turn the vessel back up right so what I'm going to draw out here is another line and I'm going to draw a perpendicular to the direction of gravity but also perpendicular to the direction of buoyancy and it's drawing it over from gravity over to this line corresponding to the force of the buoyancy and where they intersect I'm going to call that Z okay now that distance right there is what we refer to is the righting arm okay now remember I said these two points of force Center gravity and center of buoyancy are offset that being said there's a rotational force being created now the righting arm is sort of the measurement of that rotational force so GZ GZ it will call that the righting arm it's another measurement you want oh you'll want to remember that you can think of the righting arm is sort of a wrench if we imagine that there's a bolt right at the center of gravity fastened to the vessel and this is a big wrench and buoyancy is basically the hand that's grabbing the end of that wrench and it's cranking that wrench around turning the vessel upright that's basically what the righting arm is okay so that being said the righting arm is what wants to turn the vector the righting arm is is the two forces interacting such that they're turning the vessel back up front so again think of it as big wrench now the righting arm changes lengths when the vessel tips so for example the righting arm here is zero okay it's sitting at equilibrium so there isn't really a writing arrow pushing one way or the other so it's sitting static now when we start to tip that vessel the righting arm starts growing as buoyancy starts to shift as buoyancy starts to shift this arm starts to grow and it will grow to a point where it reaches a maximum and then it will start to shrink okay so that means what this means is is that when you start to tip that vessel the rotational force to tip it back starts to increase but then it reaches a point where its maximum and then you keep tipping the vessel and it starts decreasing that form that writing force starts decreasing so if you tip the vessel too far what happens is I'm going to label here our center gravity and here's our center of buoyancy now remember the center of gravity again hasn't changed said our buoyancy has because it's a function of the shape of the water being displaced okay now if I draw these reference lines in we already have our brain center line here but I'm going to draw this slide marking the force of buoyancy perpendicular to the surface of the water and it's pushing up like this and again we have gravity pulling straight out of the earth now notice how these two points are these points of force are interacting they're no longer working to write the vessel they're working to tip the vessel over so you have their offset so we're going to draw our metacentric height which suddenly is down here or metacentric height just went negative because it dropped below the center of gravity now that being said our righting arm has also gone negative so instead of GZ it's zg okay now when you get a negative righting arm like that what we mean by that is you have gravity pulling down here buoyancy pushing up over there so it's creating a writing force or it's creating a force turning the vessel or a rotational force turning the vessel in the opposite direction so here we have buoyancy we have buoyancy pushing up gravity pulling down and it's giving you a net rotational force going that way so counterclockwise turning the vessel up right now here we got buoyancy pushing up gravity pulling down and it's suddenly going the opposite direction in a clockwise direction so that's trying to tip the vessel over that's a negative righting arm so if we were to want to graph this this would give us what we would call a stability curve stability curves can be used to determine at what point what angles of heel a vessel has a writing force to turn it back up right it gives you an idea of the stability of the vessel it can also give you an idea of what kind of conditions or types of waters you would want to take that vessel out on give you an idea of how you want to handle that vessel okay so I'm going to explain stability curve a I'm going to explain a stability curve by drawing a vertical access I'm going to draw a horizontal axis okay so what we're going to do is we're going to create a graphic representation of a vessels ability to turn back up right okay so where the two axes inter intersect we really was zero okay so the vertical axis is going to be a righting arm length we'll call that G Z that's a writer okay this is a righting arm length so we'll just say it's one foot two foot to be anything any other unit of measurement but we'll just say its feet okay so now this is the positive section of the graph this is the negative section of the graph here's 0 so 0 1 2 3 0 negative 1 negative 2 negative 3 now the horizontal axis here that is what we're going to call our angle of heel so basically the angle of feel is the angle at which the boat starts to tip okay so this is zero degrees because we're just sitting straight up and down and then we tip this could be in theory 20 degrees so it's ship it's tipped over 20 degrees and then here this could be 120 degrees we'll just say in just in theory okay so that would be 120 degrees okay if we want to want to graph the length of the righting arm at various degrees of heal this would give us our stability curve okay so here we are at zero at the very beginning there's no writing on the vessel sitting at equilibrium now as we start to tip that writing our grows so we were put here the different degrees of our angles of heel we can say 20 degrees I'll say 20 40 60 degrees and 80 degrees okay if we were to plot that so 20 degrees we could say we got a 1 foot righting arm and as you keep tipping the bow that righting arm starts to plot to be bigger and bigger but then you're going to reach a point where the righting arm reaches its maximum and then as you start to tip the boat further increasing the angle of heel the righting arm here starts to decrease keeps going until it gets back to 0 okay so what that means is is we've tipped that boat buoyancy is shifted out offset from gravity this distance is increased and then as we tip farther they start to come together again and where they end up over top of one another is back to zero now at that point there isn't a righting arm that's trying to push the boat either way so it could be a balance point but it's usually a pretty precarious balance point it could fall one way or the other now when you go beyond that I have to move my letters out of the way okay so this is our right angle of he'll play them out onto the screen okay so when we go beyond that point then we get a negative righting arm if we were to plot their length or the righting arm four degrees of heel so we were to draw a curve connecting all of these plots this gives us our stability curve okay so here you see the righting arm is growing growing growing that means the vessel is there's a rotational force that's trying trying to turn the vessel back up right and then it starts shrinking there's still a force there it still wants to go back up right but then it reaches that zero point and then it goes negative when you're in the negative that's where you're at this point that means that the force of buoyancy and the force of gravity are working to tip you over now in some vessels they might go negative the righting arm might turn negative SI 75 or 80 degrees of heal which means if your vessel starts to tip to that point and go a little bit beyond then chances are she's going to roll all the way over okay some vessels especially a lot of sailing vessels can go beyond 90 degrees before they get a negative righting moment some vessels can go all the way down to 110 degrees and they can tip all the way over and as long as you don't have any water entering the vessel then that vessel will come back up so you can tip her all the way over to 110 degrees and then if the wind stops or whatever it is it's tipping her she'll pop back up now if they tip over you have a hatch burner report open then water can flooded what that does is it changes the location of the center of gravity and it can do it in such a way that would give it a negative writing moment and then the vessel would tip over so that's basically how stability works that's how buoyancy and gravity interact to determine stability and this is how we can determine a vessel stability two characteristics or how we can look at a vessels stability characteristics so this can be important and getting to know getting to know a vessel and what her capabilities are going to be from a stability standpoint when you take her out in the water you

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Length: 22min 24sec (1344 seconds)

Published: Mon Jul 01 2013