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Displacement and weight
Maybe I've killed too many brain cells over the years or just forgotten
some basic stuff but my understanding of Archimedes Principle is that the buoyant force on an object is equal to the weight of the fluid it displaces. This means that my boat that weighs 8000 lbs must displace 8000 lbs of water in order to float. If she takes on 8000 lbs of water, she sinks cuz the buoyant force doesnt balance the weight. Right? So, why do people make a distinction 'tween displacement and weight? |
#2
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#3
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wrote in message oups.com... Maybe I've killed too many brain cells over the years or just forgotten some basic stuff but my understanding of Archimedes Principle is that the buoyant force on an object is equal to the weight of the fluid it displaces. This means that my boat that weighs 8000 lbs must displace 8000 lbs of water in order to float. If she takes on 8000 lbs of water, she sinks cuz the buoyant force doesnt balance the weight. Right? So, why do people make a distinction 'tween displacement and weight? This may not be what you are looking for, but...... There are a number of weights and tonnages associated with boats/ships, that need to be differentiated. For instance: Displacement Deadweight Lt. Ship Gross tonnage Net Tonnage Etc. So .... each of these "weights" (and not all are) refer to different, specific weights/measurements associated with the boat otn |
#4
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otnmbrd wrote: wrote in message oups.com... Maybe I've killed too many brain cells over the years or just forgotten some basic stuff but my understanding of Archimedes Principle is that the buoyant force on an object is equal to the weight of the fluid it displaces. This means that my boat that weighs 8000 lbs must displace 8000 lbs of water in order to float. If she takes on 8000 lbs of water, she sinks cuz the buoyant force doesnt balance the weight. Right? So, why do people make a distinction 'tween displacement and weight? This may not be what you are looking for, but...... There are a number of weights and tonnages associated with boats/ships, that need to be differentiated. For instance: Displacement Deadweight Lt. Ship Gross tonnage Net Tonnage Etc. So .... each of these "weights" (and not all are) refer to different, specific weights/measurements associated with the boat otn Hmmm...."Displaces more water than its weight..." According to Archimedes Princ. thsi means that there is a net upward force on the boat instead of a balance. With a net upward force, she would rise out of the water so I am not sure this is possible. I may be wrong, its been many years since I thought about this. |
#6
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#7
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DSK wrote: wrote: Maybe I've killed too many brain cells over the years or just forgotten some basic stuff but my understanding of Archimedes Principle is that the buoyant force on an object is equal to the weight of the fluid it displaces. Yep. ... This means that my boat that weighs 8000 lbs must displace 8000 lbs of water in order to float. Yep. ... If she takes on 8000 lbs of water, she sinks cuz the buoyant force doesnt balance the weight. Right? Not necessarily. There is a relationship between weight, displacement, and immersed volume. The "displacement" in reality is the immersed volume, and the weight of that water *will* equal the actual weight (or mass would be a better term) of the boat & all it's contents. So, if your boat weighs 8000#, and you add another 8000# (doesn't matter if it's inflooding water, bricks, or feathers) then your boat might sink *if* it does not have the hull capacity to create that new larger immersed volume, and thus displace that much more water. Capische? So, why do people make a distinction 'tween displacement and weight? Usually what people mean by "displacement" is shorthand for 'displacement when floating at the boat's designed waterline.' That could be light, with no people or stores; it could be fully loaded ie all tanks full and crew and stores, or it could be something like 'half load displacement' which is defined as 1 average sized person for each bunk, no stores, and all tankage half full. Most boat builders quote the lightest figure for displacement and leave the buyer to guess what the boat's capacity for stores etc etc really is. Fresh Breezes- Doug King I believe Doug is correct. |
#9
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Courtney Thomas wrote:
Is there any way to determine 'real' displacement as a practical matter for a typical sailor, other than loading the boat per it's intended use, and seeing what happens ? :-) Sure. A lot of Travel-Lifts have scales built in. Just have one of them hoist your boat. For is it not typically the case that... someone buys a boat, is uncertain as to what and how much loading will then ensue, but then sets about their sailing regime... That's pretty much it, and of course people raise their waterlines all the time... This might seem to indicate that...the safe thing to do is to buy heavy displacement. Comment ? Depends on what you mean by "safe." How about positive flotation? Is "heavy displacement" inclusive of a high ballast/disp ratio? The main thing that heavy displacement boats offer IMHO is a smoother ride. A regime that you could persue yourself, with relatively lo-tech & lo-budget means, is to tape a ruler to your waterline fore and aft. Bring aboard a known weight, and see how much it takes to immerse the boat per inch. The next level is to do an incline test and measure how many foot-pounds of righting moment the boat generates at varying angles of heel; do this for the boat when stripped light, and when loaded, and then you'll have some useful information that most sailors take for granted. It'll also help test how secure your stowage is FB- DSK |
#10
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Is there any way to determine 'real' displacement as a practical matter
for a typical sailor, other than loading the boat per it's intended use, and seeing what happens ? :-) Easiest way I can think of is to ask the crane operator the next time you get your boat hauled out. Boat cranes often have a load cell so can tell what the weight of your boat is and avoid overloading their crane. May not be very accurate though. Paul =-----------------------------= Renewontime A FREE email reminder service for licensed mariners http://www.renewontime.com =-----------------------------= |
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