Post by ferryfast admin on Mar 26, 2006 10:19:30 GMT -5
Why boats float, and then they don't
Sunday, Mar. 26, 2006. 01:00 AM
BILL TAYLOR
FEATURE WRITER
www.thestar.com/
If you were playing hooky the day your science teacher explained buoyancy, you may not understand how a construction of almost 9,000 tonnes of steel floats in the first place, let alone what might cause it to sink.
Buoyancy is an upward force on an object immersed in a fluid (or gas, in the case of a hot-air balloon). The deeper an object descends into a fluid, the greater the pressure. That means the pressure on the bottom of the object is greater than the pressure on the top, since the bottom is deeper in the fluid than the top. This net, upward, difference is the buoyant force on the object.
It's a feeling every swimmer is familiar with. When you walk into the ocean up to your waist, you're not likely to feel the effects of buoyancy. But walk out until the water reaches your neck and suddenly your feet start floating off the bottom. That's because the pressure on your bottom half has overcome the effects of gravity on your top half.
In the case of a ship, its air-filled hull displaces enough water below the surface to overcome the downward pressure of gravity, allowing it to float.
Until water somehow finds a way back in...
That's basically what happened to the Queen of the North, the BC Ferries ship that foundered in the early hours of last Wednesday off the British Columbia coast, apparently after getting off-course and hitting submerged rocks. All but two of the approximately 100 passengers and crew had been accounted for as of Friday.
The car ferry was the flagship of the BC Ferries fleet; carried Prince Charles and Princess Diana from Nanaimo to Vancouver to open the Expo '86 world fair. It sank in less than an hour, survivors and rescuers said, taking 16 vehicles with it.
One witness recalled hearing the cars and trucks crashing together in the hull like roughly handled toys as the ship went down.
Fisherman James Bolton, who helped save 13 people, said the 125-metre-long ship bent as it went under. "The lights were still on until about halfway down. It sort of popped back up and then went straight down."
"The ship actually tilted to the side, levelled out... sunk down to the sixth deck, came back up like the Titanic, dipped and then it went under," said passenger Lawrence Papineau.
Other passengers reported that it settled slowly for an hour or so and then appeared to break in two, with the bow rearing up as the ship went under.
Although there's no time frame, BC Ferries CEO David Hahn says submersibles and possibly divers will eventually go down to the wreck, which could be lying in water 400 metres deep.
Until the sunken vessel can be examined, the loss of the Titanic in 1912, with the death of more than 1,500 people, is as good an example as any of the process of sinking. (The following information is taken from several well-documented websites.)
When the Titanic — more than twice the length of the Queen of the North — hit the iceberg, the ship didn't go down slowly and smoothly but rather in a series of steps.
The sheer size of a ship like the Titanic or the Queen of the North means that it has to take on enough water to overcome inertia, causing it to start to sink. Then buoyancy takes over once more and the sinking stops until the water reaches critical mass again, and the ship goes down some more.
As more and more water enters, it becomes rather like slowly pushing a book off a table. At first, the overhang isn't enough to do anything. Then as the book reaches its balancing point, it teeters gently. When the tipping point is reached, it goes down abruptly.
Once a ship has taken in enough water to overcome its buoyancy, it's game over.
Sunday, Mar. 26, 2006. 01:00 AM
BILL TAYLOR
FEATURE WRITER
www.thestar.com/
If you were playing hooky the day your science teacher explained buoyancy, you may not understand how a construction of almost 9,000 tonnes of steel floats in the first place, let alone what might cause it to sink.
Buoyancy is an upward force on an object immersed in a fluid (or gas, in the case of a hot-air balloon). The deeper an object descends into a fluid, the greater the pressure. That means the pressure on the bottom of the object is greater than the pressure on the top, since the bottom is deeper in the fluid than the top. This net, upward, difference is the buoyant force on the object.
It's a feeling every swimmer is familiar with. When you walk into the ocean up to your waist, you're not likely to feel the effects of buoyancy. But walk out until the water reaches your neck and suddenly your feet start floating off the bottom. That's because the pressure on your bottom half has overcome the effects of gravity on your top half.
In the case of a ship, its air-filled hull displaces enough water below the surface to overcome the downward pressure of gravity, allowing it to float.
Until water somehow finds a way back in...
That's basically what happened to the Queen of the North, the BC Ferries ship that foundered in the early hours of last Wednesday off the British Columbia coast, apparently after getting off-course and hitting submerged rocks. All but two of the approximately 100 passengers and crew had been accounted for as of Friday.
The car ferry was the flagship of the BC Ferries fleet; carried Prince Charles and Princess Diana from Nanaimo to Vancouver to open the Expo '86 world fair. It sank in less than an hour, survivors and rescuers said, taking 16 vehicles with it.
One witness recalled hearing the cars and trucks crashing together in the hull like roughly handled toys as the ship went down.
Fisherman James Bolton, who helped save 13 people, said the 125-metre-long ship bent as it went under. "The lights were still on until about halfway down. It sort of popped back up and then went straight down."
"The ship actually tilted to the side, levelled out... sunk down to the sixth deck, came back up like the Titanic, dipped and then it went under," said passenger Lawrence Papineau.
Other passengers reported that it settled slowly for an hour or so and then appeared to break in two, with the bow rearing up as the ship went under.
Although there's no time frame, BC Ferries CEO David Hahn says submersibles and possibly divers will eventually go down to the wreck, which could be lying in water 400 metres deep.
Until the sunken vessel can be examined, the loss of the Titanic in 1912, with the death of more than 1,500 people, is as good an example as any of the process of sinking. (The following information is taken from several well-documented websites.)
When the Titanic — more than twice the length of the Queen of the North — hit the iceberg, the ship didn't go down slowly and smoothly but rather in a series of steps.
The sheer size of a ship like the Titanic or the Queen of the North means that it has to take on enough water to overcome inertia, causing it to start to sink. Then buoyancy takes over once more and the sinking stops until the water reaches critical mass again, and the ship goes down some more.
As more and more water enters, it becomes rather like slowly pushing a book off a table. At first, the overhang isn't enough to do anything. Then as the book reaches its balancing point, it teeters gently. When the tipping point is reached, it goes down abruptly.
Once a ship has taken in enough water to overcome its buoyancy, it's game over.