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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? |
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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 |
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. |
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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. |
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 |
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 =-----------------------------= |
On Tue, 15 Feb 2005 05:44:51 -0700, Keith Hughes
wrote: The important point is that where the *volume* of the 8000lb of boat is greater than that of 8000lb of water (i.e. lower density), the boat floats. Density greater than water...glug, glug, glug... Exactly why it's hard to fly a steam-powered airplane and to sell a ferro-cement boat. No matter how good they are, something seems to be defying the laws of nature in a concrete canoe. G R. |
On Tue, 15 Feb 2005 09:07:50 -0500, DSK wrote:
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. An illustrative example or two. My boat is a light/medium displacement 33 footer rated for 8,800 lbs. displacement. I am guessing this is "empty". On the slings it weighs 10,000 lbs. on the nose with lots of tools, spares, full gas, no water except beer and pop, and three anchors, chain and about a thousand feet of line. Add 400 lbs. for crew and she sits on her lines and sails quite well. Fin keel ballast is about 47% of "empty" weight. Buddy of mine in a Wallstrom-designed ketch checked with Wallstrom himself who said it should displace 24,000 lbs. About 30% of that is in the cutaway forefoot full keel. Well, in the slings it weighs 36,000 lbs., which should create a new category of "liveaboard displacement". Still sails properly and is only a couple of inches south of its waterline, however. R. |
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No real difference, as you suspect. Design weight = design
displacement. Dry weight = dry displacement. On real ships, of course, attention is paid to salinity/density of water, which influences draft, buoyancy, etc. But shipbuilding has been around a long time and I wonder if a boat's weight was once measured by the actual displacement of water in dry dock. Did ship builders years ago really weigh ships? Would it have been worth the trouble? Did the process of building models, measuring actual displacement of the model, and scaling turn out to be "better" than scaling a ship's weight from the model? Probably. Today, of course, almost any marina can tell you what a lift gauge reads, FWIW. |
I thought I might repost "A drinking man's tunnage" from several years back:
Tonnage has nothing to do with the weight of your boat. It is a measure of how much wine you can carry. The word 'tun' was originally a size of a cask used to ship wine from Spain & Portugal to England. In 1347 a tax of 3 shillings per tun was imposed and this was called 'tonnage'. A ship's size became known by the number of casks it could carry, and the word tonnage started being used to describe a ship's size. It was found that if you took the length x the breadth x the depth of the hold under the deck and divided by 100 it was close to the number of casks. That is where we get the "Measurement ton" of 100 cubic feet per ton. There are several kinds of tonnage: The first two are used by the tax collector. The next two are used by designers. The fifth and sixth are used by freight salesmen and canal operators and the last one is used by the USCG for documenting boats. Gross Tonnage - is the internal volume in cubic feet of the vessel minus certain spaces above the main or tonnage deck like stacks and ventilators which are called "exemptions" . Net Registered Tonnage - is obtained by deducting from the gross tonnage the volume of space that can't be used for paying cargo or passengers, that is to say the space occupied by the engines, the crew's quarter, the stores, etc. Displacement Tonnage - is the actual weight of the water "displaced" by the ship and is usually quoted in long tons of 2240 lbs. Light Displacement Tonnage - is the weight with nothing in it. Loaded Displacement Tonnage - is the fully loaded weight to the maximum and is on her Summer draft in Salt Water.. Deadweight Tonnage - is the difference between Light and Loaded Displacement Tonnage....the Actual carrying capacity of the vessel. Panama & Suez Canal Tonnages - these are different to the international ones. There used to be a lot of variations between countries and they thought they were being conned, so they came up with their own for everyone. Simplified Measurement System - The USCG decided that all this was to much for the bureaucrats to deal with for yachts so they came up with this formula. (Now pay attention Capt. Schlemiel). Take the horizontal distance between the outboard ends of the boat not including rudders and bow sprits. Multiply that by the maximum beam outside to outside. Multiply that by the distance from the sheer line not including bulwarks or cap rails to the outside bottom of the hull not including the keel. Add the volume of the deck house/cabin top. Multiply by .5 for sailboats and .67 for power boats. Divide by 100. This will give you the "Gross Tonnage". Net tonnage is 90% of gross for sailboats and 80% for power boats. It should be obvious to those of you that have gotten this far that "tonnage" has nothing to do with anything except in the mind of some government bureaucrat. Enough to make you want a drink ain't it? -- Glenn Ashmore I'm building a 45' cutter in strip/composite. Watch my progress (or lack there of) at: http://www.rutuonline.com Shameless Commercial Division: http://www.spade-anchor-us.com |
rhys wrote:
.... Buddy of mine in a Wallstrom-designed ketch checked with Wallstrom himself who said it should displace 24,000 lbs. About 30% of that is in the cutaway forefoot full keel. Well, in the slings it weighs 36,000 lbs., which should create a new category of "liveaboard displacement". Still sails properly and is only a couple of inches south of its waterline, however. Its easy to do a little "napkin math" to figure the immersion factor. Assume a 40 foot LWL, a 12 foot Beam at the WL, multiply and take 70% (the pointy ends factor) gives about 350 square feet. Divide by 12 and its 30 cubic feet of water displaced per inch of immersion. At 64 pounds per cubic foot, and we're at a little under 2000 pounds per inch. Many 48 footers are at this figure, a Westsail 42 is about 1500 pounds per inch. My lightweight catamaran has the volume of these boats, but the immersion number is 700 pounds per inch, and three inches down would seriously affect performance. No wonder I favor lightweight ground tackle, etc. |
On Tue, 15 Feb 2005 11:45:49 -0400, Terry Spragg
wrote: Shippers in the past found out that there must be some reasonable (there's that word, again) value of vessel size and overall weight including cargo must be considered as some sort of maximum under some rule somewhere .... Terry K After numerous ship losses, Plimsoll legislated a hull mark in Parliament, the "Plimsoll Line" which accounts for salt and fresh, warm n cool water, beyond which a commercial vessel must not be loaded. Unaccounted ship losses were much reduced thereafter. Brian Whatcott Altus OK |
Brian Whatcott wrote:
After numerous ship losses, Plimsoll legislated a hull mark in Parliament, the "Plimsoll Line" which accounts for salt and fresh, warm n cool water, beyond which a commercial vessel must not be loaded. Unaccounted ship losses were much reduced thereafter. The difference between salt and freshwater displacement and waterlines makes sense to me, since the salt increases the density of the water, right? But why is there a difference between summer and winter? Thanks, Brent |
This is because weather is usually worse in winter therefore higher
freeboard required especially winter North Atlantic wrote in message oups.com... Brian Whatcott wrote: After numerous ship losses, Plimsoll legislated a hull mark in Parliament, the "Plimsoll Line" which accounts for salt and fresh, warm n cool water, beyond which a commercial vessel must not be loaded. Unaccounted ship losses were much reduced thereafter. The difference between salt and freshwater displacement and waterlines makes sense to me, since the salt increases the density of the water, right? But why is there a difference between summer and winter? Thanks, Brent |
That could be a factor but I wonder . . .
It happens that temperature has a greater effect on water density than salinity. Dominic wrote: This is because weather is usually worse in winter therefore higher freeboard required especially winter North Atlantic |
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Brian
Are you sure about this. It would seem to me that the cold water would be less dense. It expands as it freezes. JR Brian Whatcott wrote: On 15 Feb 2005 14:34:42 -0800, wrote: Brian Whatcott wrote: After numerous ship losses, Plimsoll legislated a hull mark in Parliament, the "Plimsoll Line" which accounts for salt and fresh, warm n cool water, beyond which a commercial vessel must not be loaded. Unaccounted ship losses were much reduced thereafter. The difference between salt and freshwater displacement and waterlines makes sense to me, since the salt increases the density of the water, right? But why is there a difference between summer and winter? Thanks, Brent Warm water is less dense than cold water. Water expands more with temperature than the metals, so a ship sits lower in hot water. Moreover, water's rate of expansion increases as the temperature rises. Another factor: water like oil, gets considerably less viscous when warm, so a hull might be expected to be livelier, and maybe the swell higher.... Brian Whatcott Altus OK |
I just did some research on this. Sea water density increases as it get
colder until the temperature hits 4 degrees celsius then expands until it freezes. So don't take a loaded boat too far north. JR Gilbreath wrote: Brian Are you sure about this. It would seem to me that the cold water would be less dense. It expands as it freezes. JR Brian Whatcott wrote: On 15 Feb 2005 14:34:42 -0800, wrote: Brian Whatcott wrote: After numerous ship losses, Plimsoll legislated a hull mark in Parliament, the "Plimsoll Line" which accounts for salt and fresh, warm n cool water, beyond which a commercial vessel must not be loaded. Unaccounted ship losses were much reduced thereafter. The difference between salt and freshwater displacement and waterlines makes sense to me, since the salt increases the density of the water, right? But why is there a difference between summer and winter? Thanks, Brent Warm water is less dense than cold water. Water expands more with temperature than the metals, so a ship sits lower in hot water. Moreover, water's rate of expansion increases as the temperature rises. Another factor: water like oil, gets considerably less viscous when warm, so a hull might be expected to be livelier, and maybe the swell higher.... Brian Whatcott Altus OK |
In article .com,
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. This means that my boat that weighs 8000 lbs must displace 8000 lbs of water in order to float. This is true and is a basic principle of floating objects. If she takes on 8000 lbs of water, she sinks cuz the buoyant force doesnt balance the weight. Right? I suspect that's false. Your boat may or may not have that much reserve buoyancy. Our 3.5 ton boat is rated to carry at least 5 tons. Would take somewhat more to sink her in calm water. Hull (without keel) is a bit more than a foot submerged. We have 3'+ more freeboard and the hull gets FAT up there. So, why do people make a distinction 'tween displacement and weight? Sounds better, and is more technically correct. Reminds me of a hairy old story: New engineer is told to find out how much more of something is on a barge. After hours of laborous measurements and calculations, the foreman comes by to see what the matter is and points out the numbers marked on the hull -- showing displacement. -- Jere Lull Xan-a-Deux ('73 Tanzer 28 #4 out of Tolchester, MD) Xan's Pages: http://members.dca.net/jerelull/X-Main.html Our BVI FAQs (290+ pics) http://homepage.mac.com/jerelull/BVI/ |
"JR Gilbreath" wrote in message
... I just did some research on this. Sea water density increases as it get colder until the temperature hits 4 degrees celsius then expands until it freezes. So don't take a loaded boat too far north. ----------snip------------ Winter displacement will be more affected by frozen spray than any variation due to temperature. Not to mention the required antifreeze for the crew. surfnturf |
We don't have to worry about frozen spray like you do but we still use
the antifreeze just to be on the safe side. surfnturf wrote: "JR Gilbreath" wrote in message ... I just did some research on this. Sea water density increases as it get colder until the temperature hits 4 degrees celsius then expands until it freezes. So don't take a loaded boat too far north. ----------snip------------ Winter displacement will be more affected by frozen spray than any variation due to temperature. Not to mention the required antifreeze for the crew. surfnturf |
Water is a rather unusual liquid. It gets more dense as it cools, then at 4 degreesC, it's as dense as it's going to get, and when cooled further, expands a little again. That's why water in cold lakes freezes from the surface down. When water turns to ice, it shows its other strange characteristic - it REALLY expands. That's what does for your outside copper pipes. Brian Whatcott On Tue, 15 Feb 2005 20:56:25 -0500, JR Gilbreath wrote: Brian Are you sure about this. It would seem to me that the cold water would be less dense. It expands as it freezes. JR Brian Whatcott wrote: On 15 Feb 2005 14:34:42 -0800, wrote: Brian Whatcott wrote: After numerous ship losses, Plimsoll legislated a hull mark in Parliament, the "Plimsoll Line" which accounts for salt and fresh, warm n cool water, beyond which a commercial vessel must not be loaded. Unaccounted ship losses were much reduced thereafter. The difference between salt and freshwater displacement and waterlines makes sense to me, since the salt increases the density of the water, right? But why is there a difference between summer and winter? Thanks, Brent Warm water is less dense than cold water. Water expands more with temperature than the metals, so a ship sits lower in hot water. Moreover, water's rate of expansion increases as the temperature rises. Another factor: water like oil, gets considerably less viscous when warm, so a hull might be expected to be livelier, and maybe the swell higher.... Brian Whatcott Altus OK |
As someone has said, the WNA marking is mainly related to weather and is
applied to ships below a certain length above a specific latitude. The basic idea being to allow some minor additional freeboard as a safety factor. There are a number of these draft markings, all the way to Tropical Fresh which are mainly used when loading at a specific locale and going to another, to give you a basis as to how deep you can load, as you can't be below the draft requirements at the next port. Regarding markings on the side of a barge ..... I've never seen any denoting displacement, however, reading the draft is a quick and easy way to know how much cargo (weight) has been loaded. otn |
On Wed, 16 Feb 2005 01:40:22 GMT, Brian Whatcott
wrote: Warm water is less dense than cold water. Water expands more with temperature than the metals, so a ship sits lower in hot water. Moreover, water's rate of expansion increases as the temperature rises. Another factor: water like oil, gets considerably less viscous when warm, so a hull might be expected to be livelier, and maybe the swell higher.... So the worst case scenario for a loaded ship would be a laden tanker enduring a Red Sea or a Persian Gulf cyclone? I vaguely remember that those areas are the hottest oceanic bodies on Earth. By contrast, on a calm day in zero C. Antarctic water (ice-free, however), the same laden ship would ride high(er) and dry. Interesting! R. |
rhys wrote:
On Wed, 16 Feb 2005 01:40:22 GMT, Brian Whatcott wrote: Warm water is less dense than cold water. Water expands more with temperature than the metals, so a ship sits lower in hot water. Moreover, water's rate of expansion increases as the temperature rises. Another factor: water like oil, gets considerably less viscous when warm, so a hull might be expected to be livelier, and maybe the swell higher.... So the worst case scenario for a loaded ship would be a laden tanker enduring a Red Sea or a Persian Gulf cyclone? I vaguely remember that those areas are the hottest oceanic bodies on Earth. By contrast, on a calm day in zero C. Antarctic water (ice-free, however), the same laden ship would ride high(er) and dry. Interesting! R. Actually, bending stresses would be more of a concern, than the minor variation in draft from Tropical to Winter. If you've flown in turbulent weather you've watched a wing flex ..... imagine watching a whole hull doing that. otn |
On Thu, 17 Feb 2005 06:34:58 GMT, otnmbrd
wrote: Actually, bending stresses would be more of a concern, than the minor variation in draft from Tropical to Winter. If you've flown in turbulent weather you've watched a wing flex ..... imagine watching a whole hull doing that. My father, who turns 80 tomorrow, was in the British Merchant Marine during WWII. He said that while the losses from U-Boats were devastating, he found a lot of sailors feared the kind of storm that could "bridge" a single-hulled, laden freighter between two wave crests, causing it to fail and essentially snap in half. He said the ships rushed out in the latter half of the war were worse for this sort of thing, akin to the "Liberty ships" in the States. R. |
rhys wrote:
My father, who turns 80 tomorrow, was in the British Merchant Marine during WWII. He said that while the losses from U-Boats were devastating, he found a lot of sailors feared the kind of storm that could "bridge" a single-hulled, laden freighter between two wave crests, causing it to fail and essentially snap in half. He said the ships rushed out in the latter half of the war were worse for this sort of thing, akin to the "Liberty ships" in the States. R. The Liberties were one of the first ships massed produced with all welded construction. Initially, from memory, there was a serious problem with breaking in half. Much of this was corrected/lessened by adding a riveted plate (crack arrestor) around the hull at the main deck level. Again, from memory, this was considered a "stop gap" solution that seemed to have good results and in fact you tended to see these "crack arrestor" plates on much construction until well into the 60's. otn |
On Thu, 17 Feb 2005 00:13:16 -0500, rhys wrote:
On Wed, 16 Feb 2005 01:40:22 GMT, Brian Whatcott wrote: Warm water is less dense than cold water. Water expands more with temperature than the metals, so a ship sits lower in hot water. Moreover, water's rate of expansion increases as the temperature rises. Another factor: water like oil, gets considerably less viscous when warm, so a hull might be expected to be livelier, and maybe the swell higher.... So the worst case scenario for a loaded ship would be a laden tanker enduring a Red Sea or a Persian Gulf cyclone? I vaguely remember that those areas are the hottest oceanic bodies on Earth. By contrast, on a calm day in zero C. Antarctic water (ice-free, however), the same laden ship would ride high(er) and dry. Interesting! R. You actually contributed another gotcha: a crude oil cargo runs thin and expands, so the CofG would go up a little higher while the freeboard goes down, in the Gulf. Brian Whatcott ALTUS ok |
"Brian Whatcott" wrote in message ... On Thu, 17 Feb 2005 00:13:16 -0500, rhys wrote: On Wed, 16 Feb 2005 01:40:22 GMT, Brian Whatcott wrote: Warm water is less dense than cold water. Water expands more with temperature than the metals, so a ship sits lower in hot water. Moreover, water's rate of expansion increases as the temperature rises. Another factor: water like oil, gets considerably less viscous when warm, so a hull might be expected to be livelier, and maybe the swell higher.... So the worst case scenario for a loaded ship would be a laden tanker enduring a Red Sea or a Persian Gulf cyclone? I vaguely remember that those areas are the hottest oceanic bodies on Earth. By contrast, on a calm day in zero C. Antarctic water (ice-free, however), the same laden ship would ride high(er) and dry. Interesting! R. You actually contributed another gotcha: a crude oil cargo runs thin and expands, so the CofG would go up a little higher while the freeboard goes down, in the Gulf. Brian Whatcott ALTUS ok G Not all crudes are "thin", and many cool down after loading, but these amounts (change in ullage) tend to be very small. Couple this with the fact that tanker GM's when loaded are normally VERY high, the degree of change in ride, be it comfort or submergence due to water temps in salt water, will not be noticed. otn |
In article . net,
otnmbrd wrote: rhys wrote: My father, who turns 80 tomorrow, was in the British Merchant Marine during WWII. He said that while the losses from U-Boats were devastating, he found a lot of sailors feared the kind of storm that could "bridge" a single-hulled, laden freighter between two wave crests, causing it to fail and essentially snap in half. He said the ships rushed out in the latter half of the war were worse for this sort of thing, akin to the "Liberty ships" in the States. R. The Liberties were one of the first ships massed produced with all welded construction. Initially, from memory, there was a serious problem with breaking in half. Much of this was corrected/lessened by adding a riveted plate (crack arrestor) around the hull at the main deck level. Again, from memory, this was considered a "stop gap" solution that seemed to have good results and in fact you tended to see these "crack arrestor" plates on much construction until well into the 60's. otn The Liberties that I've heard of were ferro-concrete and not particularly successful. We have a few of those grounded within our cruising grounds. -- Jere Lull Xan-a-Deux ('73 Tanzer 28 #4 out of Tolchester, MD) Xan's Pages: http://members.dca.net/jerelull/X-Main.html Our BVI FAQs (290+ pics) http://homepage.mac.com/jerelull/BVI/ |
Jere Lull wrote:
The Liberties that I've heard of were ferro-concrete and not particularly successful. We have a few of those grounded within our cruising grounds. Other than knowing there were a number of the "ferro" ships constructed, I have little info on them. I seem to remember there were one or two grounded above the Carquinez (sp?) bridge enroute to Martinez, Ca.. otn |
On Thu, 17 Feb 2005 20:02:51 GMT, otnmbrd
wrote: Again, from memory, this was considered a "stop gap" solution that seemed to have good results and in fact you tended to see these "crack arrestor" plates on much construction until well into the 60's I believe I recall seeing them years back on some of the older "LakeMax" frieghters here on Lake Ontario, which are limited to the lock size on the Welland Canal of about 750 feet.. R. |
On Fri, 18 Feb 2005 00:13:54 GMT, Brian Whatcott
wrote: You actually contributed another gotcha: a crude oil cargo runs thin and expands, so the CofG would go up a little higher while the freeboard goes down, in the Gulf. You're right: I wasn't factoring in the nature of the cargo. I wonder if the viscosity of the oil (and its tendency to slosh in tanks) would also affect stability in this scenario. I seem to recall the phrase "surface effect" or some such factor relating to liquid cargo causing ships to capsize unexpectedly, or at least before it was theoretically anticipated. R. |
rhys wrote:
You're right: I wasn't factoring in the nature of the cargo. I wonder if the viscosity of the oil (and its tendency to slosh in tanks) would also affect stability in this scenario. Tankers tend to have the majority of tanks divided into three separate tanks (i.e., 1P, 1C, 1S) and when loaded, the ullage will be very close to the top of the tank, so that any sloshing almost immediately pockets. Without going into a long discussion, this fact coupled with the large amount of stability, tends to negate the affects on stability. I seem to recall the phrase "surface effect" or some such factor relating to liquid cargo causing ships to capsize unexpectedly, or at least before it was theoretically anticipated. "Free surface effect". It's what caused the car carrier which recently sank after a collision, to roll over so quickly. Again, not normally a problem with tankers, unless they have double bottoms which are open port to stbd. otn |
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