Trucks, Trains, Ships and Planes

Posted by PITHOCRATES - August 21st, 2013

Technology 101

Big Over-the-Road Tractor Trailer Trucks have Big Wheels so they can have Big Brakes

If you buy a big boat chances are you have a truck or a big SUV to pull it.  For rarely do you see a small car pulling a large boat.  Have you ever wondered why?  A small car can easily pull a large boat on a level (or a near level) surface.  That’s not the problem.  The problem is stopping once it gets moving.  For that is a lot of mass.  Creating a lot of kinetic energy (one half of the mass times velocity squared).  Which is dissipated as heat as brake shoes or pads rub against the wheels.  This is why you need a big truck or SUV to pull a boat.  So you can stop it once it gets moving.

Big trucks and big SUVs have big wheels and big brakes.  Large areas where brake pads/shoes press against a rotating wheel.  All of which is heavy duty equipment.  That can grab onto to those wheels and slow them down.  Converting that kinetic energy into heat.  This is why the big over-the-road tractor trailer trucks have big wheels.  So they can have big enough brakes to stop that huge mass once it gets moving.  Without the brakes turning white hot and melting.  Properly equipped trucks can carry great loads.  Moving freight safely across our highways and byways.  But there is a limit to what they can carry.  Too much weight spread between too few axles will pound the road apart.  Which is why the state police weighs our trucks.  To make sure they have enough axles supporting the load they’re carrying.  So they don’t break up our roads.  And that they can safely stop.

It’s a little different with trains.  All train cars have a fixed number of axles.  But you will notice the size of the cars differ.  Big oversized boxcars carry a lot of freight.  But it’s more big than heavy.  Heavy freight, on the other hand, like coal, you will see in smaller cars.  So the weight they carry doesn’t exceed the permissible weight/axle.  If you ever sat at a railroad crossing as a train passed you’ve probably noticed that the rail moves as the train travels across.  Watch a section of rail the next time you’re stopped by a train.  And you will see the rail sink a little beneath the axle as it passes over.

If a Ship is Watertight and Properly Balanced it can be covered in Green Water and Rise back to the Surface

So the various sizes of train cars (i.e., rolling stock) keeps each car from being overloaded.  Unlike a truck.  Steel haulers and gravel trains (i.e., dump trucks) have numerous axles beneath the load they’re carrying.  But these axles are retractable.  For the more wheels in contact with the road the more fuel is needed to overcome the friction between the tires and the road.  And the more tires in contact with the road the more tire wear there is.  Tires and fuel are expensive.  So truckers like to have as few tires in contact with the road as possible.  When they’re running empty they will have as many of these wheels retracted up as possible.  Something you can’t do with a train.

That said, a train’s weight is critical for the safe operation of a train.  In particular, stopping a train.  The longer a train is the more distance it takes to stop.  It is hard to overload a particular car in the string of cars (i.e., consist) but the total weight tells engineers how fast they can go.  How much they must slow down for curves.  How much distance they need to bring a train to a stop.  And how many handbrakes to set to keep the train from rolling away after the pressure bleeds out of the train line (i.e., the air brakes).  You do this right and it’s safe sailing over the rails.  Ships, on the other hand, have other concerns when it comes to weight.

Ships float.  Because of buoyancy.  The weight of the load presses down on the water while the surface of the water presses back against the ship.  But where you place that weight in a ship makes a big difference.  For a ship needs to maintain a certain amount of freeboard.  The distance between the surface of the water and the deck.  Waves toss ships up and down.  At best you just want water spray splashing onto your deck.  At worst you get solid water (i.e., green water).  If a ship is watertight and properly balanced it can be covered in green water and rise back to the surface.  But if a ship is loaded improperly and lists to one side or is low in the bow it reduces freeboard.  Increases green water.  And makes it less likely to be able to safely weather bad seas.  The SS Edmund Fitzgerald sank in 1975 while crossing Lake Superior in one of the worst storms ever.  She was taking on water.  Increasing her weight and lowering her into the water.  Losing freeboard.  Had increasing amounts of green water across her deck.  To the point that a couple of monster waves crashed over her and submerged her and she never returned to the surface.  It happened so fast that the crew was unable to give out a distress signal.  And as she disappeared below the surface her engine was still turning the propeller.  Driving her into the bottom of the lake.  Breaking the ship in two.  And the torque of the spinning propeller twisting the stern upside down.

Airplanes are the only Mode of Transportation that has two Systems to Carry their Load

One of the worst maritime disasters on the Great Lakes was the sinking of the SS Eastland.  Resulting in the largest loss of life in a shipwreck on the Great Lakes.  In total 844 passengers and crew died.  Was this in a storm like the SS Edmund Fitzgerald?  No.  The SS Eastland was tied to the dock on the Chicago River.  The passengers all went over to one side of the ship.  And the mass of people on one side of the ship caused the ship to capsize.  While tied to the dock.  On the Chicago River.  Because of this shift in weight.  Which can have catastrophic results.  As it can on airplanes.  There’s a sad YouTube video of a cargo 747 taking off.  You then see the nose go up and the plane fall out of the sky.  Probably because the weight slid backwards in the plane.  Shifting the center of gravity.  Causing the nose of the plane to pitch up.  Which disrupted the airflow over the wings.  Causing them to stall.  And with no lift the plane just fell out of the sky.

Airplanes are unique in one way.  They are the only mode of transportation that has two systems to carry their weight.  On the ground the landing gear carries the load.  In the air the wings carry the load.  Which makes taking off and landing the most dangerous parts of flying.  Because a plane has to accelerate rapidly down the runway so the wings begin producing lift.  Once they do the weight of the aircraft begins to transfer from the landing gear to the wings.  Allowing greater speeds.  However, if something goes wrong that interferes with the wings producing lift the wings will be unable to carry the weight of the plane.  And it will fall out of the sky.  Back onto the landing gear.  But once the plane leaves the runway there is nothing the landing gear can gently settle on.  And with no altitude to turn or to glide back to a runway the plane will fall out of the sky wherever it is.  Often with catastrophic results.

A plane has a lot of mass.  And a lot of velocity.  Giving it great kinetic energy.  It takes long runways to travel fast enough to transfer the weight of the aircraft from the landing gear to the wings.  And it takes a long, shallow approach to land an airplane.  So the wheels touch down gently while slowly picking up the weight of the aircraft as the wings lose lift.  And it takes a long runway to slow the plane down to a stop.  Using reverse thrusters to convert that kinetic energy into heat.  Sometimes even running out of runway before bringing the plane to a stop.  No other mode of transportation has this additional complication of travelling.  Transferring the weight from one system to another.  The most dangerous part of flying.  Yet despite this very dangerous transformation flying is the safest mode of traveling.  Because the majority of flying is up in the air in miles of emptiness.  Where if something happens a skilled pilot has time to regain control of the aircraft.  And bring it down safely.

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Archimedes’ Principle, Buoyancy, Spar Deck, Freeboard, Green Water, Bulkheads, Watertight Compartments, RMS Titanic and Edmund Fitzgerald

Posted by PITHOCRATES - January 2nd, 2013

Technology 101

(Originally published April 4th, 2012)

The Spar Deck or Weather Deck is Where you Make a Ship Watertight

Let’s do a little experiment.  Fill up your kitchen sink with some water.  (Or simply do this the next time you wash dishes).  Then get a plastic cup.  Force the cup down into the water with the open side up until it rests on the bottom of the sink.  Make sure you have a cup tall enough so the top of it is out of the water when resting on the bottom.  Now let go of the cup.  What happens?  It bobs up out of the water.  And tips over on its side.  Where water can enter the cup.  As it does it weighs down the bottom of the cup and lifts the open end out of the water.  And it floats.  Now repeat this experiment.  Only fill the plastic cup full of water.  What happens when you let go of it when it’s sitting on the bottom of the sink?  It remains sitting on the bottom of the sink.

What you’ve just demonstrated is Archimedes’ principle.  The law of buoyancy.  Which explains why things like ships float in water.  Even ships made out of steel.  And concrete.  The weight of a ship pressing down on the water creates a force pushing up on the ship.  And if the density of the ship is less than the density of the water it will float.  Where the density of the ship includes all the air within the hull.  Ships are buoyant because air is less dense than water.  If water enters the hull it will increase the density of the ship.  Making it heavier.  And less buoyant.  As water enters the hull the ship will settle lower in the water.

The spar deck or weather deck is where you make a ship watertight.  This is where the hatches are on cargo ships.  We call the distance between the surface of the water and the spar deck freeboard.  A light ship doesn’t displace much water and rides higher in the water.  That is, it has greater freeboard.  With less ship in the water there is less resistance to forward propulsion.  Allowing it to travel faster.  However, a ship riding high in the water is much more sensitive to wave action.  And more susceptible to rolling from side to side.  Increasing the chance of rolling all the way over in heavy seas.  (Interestingly, if the ship stays watertight it can still float capsized.)  So ship captains have to watch their freeboard carefully.  If the ship rides too high (like an empty cargo ship) the captain will fill ballast tanks with water to lower the ship in the water.  By decreasing freeboard the ship is less prone to wave action.  But by lowering the spar deck closer to the surface of the water bigger waves can crash over the spar deck.  Flooding the spar deck with ‘green water’.  Common in a storm with high winds creating tall waves.  As long as the spar deck is watertight the ship will stay afloat.  And the solid water that washes over the spar deck will run off the ship and back into the sea.

The Titanic and the Fitzgerald were Near Unsinkable Designs but both lost Buoyancy and Sank

Improvements in ship design have made ships safer.  Steel ships can take a lot of damage and still float.  Ships struck by torpedoes in World War II could still float even with a hole below their waterline thanks to watertight compartments.  Where bulkheads divide a ship’s hull.  Watertight walls that typically run up to the weather deck.  Access though these bulkheads is via watertight doors.  These are the doors that close when a ship begins to take on water and the captain orders, “Close watertight doors.”  This contains the water ingress to one compartment allowing the ship to remain buoyant.  If it pitches down at the bow or lists to either side they can offset this imbalance with their ballast tanks.  Emptying the tanks where the ship is taking on water.  And filling the tanks where it is not.  To level the ship and keep it seaworthy until it reaches a safe harbor to make repairs.

They considered RMS Titanic unsinkable because of these features.  But they didn’t stop her from sinking on a calm night in 1912.  Why?  Two reasons.  The first was the way she struck the iceberg.  She sideswiped the iceberg.  Which cut a gash below the waterline in five of her ‘watertight’ compartments.  Which basically removed the benefit of compartmentalization.  They could not isolate the water ingress to a single compartment.  Or two.  Or three.  Even four.  Which she might have survived and remained afloat.  But water rushing into five compartments was too much.  It pitched the bow down.  And as the bow sank water spilled over the ‘watertight’ bulkheads and began flooding the next compartment.  Even ones the iceberg didn’t slash open.  As water poured over these bulkheads and flooded compartment after compartment the bow sank deeper and deeper into the water.  Until the unsinkable sank.  The Titanic sank slowly enough to rescue everyone on the ship.  She just didn’t carry enough lifeboats.  For they thought she was unsinkable.  Because of this lack of lifeboats 1,517 died.  Of course, having enough lifeboats doesn’t guarantee everyone will survive a sinking ship.

The Edmund Fitzgerald was the biggest ore carrier on the Great Lakes during her heyday.  These ships could take an enormous amount of abuse as the storms on the Great Lakes could be treacherous.  Like the one that fell on the Fitzgerald one November night in 1975.  When 30-foot waves hammered her and her sister ship the Arthur Andersen.  No one knows for sure what happened that night but some of the clues indicate she may have bottomed out on an uncharted shoal.  For she lost her handrails indicating that the ship may have hogged (where the bow and stern bends down from the center of the ship held up by that uncharted shoal).  The handrails were steel cables under tension running around the spar deck.  If the ship hogged this would have stretched the cable until it snapped.  She had green water washing across her deck.  Lost both of her radars.  A vent.  Maybe even a hatch cover.  Whatever happened she was taking on water.  A lot of it.  More than her pumps could keep up with.  Causing a list.  And the bow to settle deeper in the water.  Waves crashed over her bow as well as the Andersen’s.  The ships disappeared under the water.  Then reemerged.  As they design ships to do.  Then two massive waves rocked the Andersen.  She was following the Fitzgerald to help her navigate by the Andersen’s radar.  So these two waves had hit the Fitzgerald first.  The Fitzgerald had by this time taken on so much water that she lost too much freeboard.  When she disappeared under these two waves she never came back up.  It happened so fast there was no distress call.  The ship was longer than the lake was deep.  So her screw was still propelling the ship forward when the bow stuck the bottom.  She had lifeboat capacity for all 29 aboard.  But the ship sank too fast to use them.  Or even for the Andersen to see her as she sailed over her as she came to a rest on the bottom.

Our Ships have never been Safer but Ship Owners and Merchants still need to Protect their Wealth with Marine Insurance

We build bigger and bigger ships.  And it’s amazing what can float considering how heavy these ships can be.  But thanks to Archimedes’ principle all we have to do to make the biggest and heaviest ships float is too keep them watertight.  Keeping them less dense than the water that makes them float.  Even if we fail here due to events beyond our control we can isolate the water rushing in by sealing watertight compartments.  And keep them afloat.  So our ships have never been safer.  In addition we have far more detailed charts.  And satellite navigation to carefully guide us to our destination.  Despite all of this ships still sink.  Proving the need for something that has changed little since 14th century Genoa.  Marine insurance.  Because accidents still happen.  And ship owners and merchants still need to protect their wealth.

www.PITHOCRATES.com

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Archimedes’ Principle, Buoyancy, Spar Deck, Freeboard, Green Water, Bulkheads, Watertight Compartments, RMS Titanic and Edmund Fitzgerald

Posted by PITHOCRATES - April 4th, 2012

Technology 101

The Spar Deck or Weather Deck is Where you Make a Ship Watertight

Let’s do a little experiment.  Fill up your kitchen sink with some water.  (Or simply do this the next time you wash dishes).  Then get a plastic cup.  Force the cup down into the water with the open side up until it rests on the bottom of the sink.  Make sure you have a cup tall enough so the top of it is out of the water when resting on the bottom.  Now let go of the cup.  What happens?  It bobs up out of the water.  And tips over on its side.  Where water can enter the cup.  As it does it weighs down the bottom of the cup and lifts the open end out of the water.  And it floats.  Now repeat this experiment.  Only fill the plastic cup full of water.  What happens when you let go of it when it’s sitting on the bottom of the sink?  It remains sitting on the bottom of the sink.

What you’ve just demonstrated is Archimedes’ principle.  The law of buoyancy.  Which explains why things like ships float in water.  Even ships made out of steel.  And concrete.  The weight of a ship pressing down on the water creates a force pushing up on the ship.  And if the density of the ship is less than the density of the water it will float.  Where the density of the ship includes all the air within the hull.  Ships are buoyant because air is less dense than water.  If water enters the hull it will increase the density of the ship.  Making it heavier.  And less buoyant.  As water enters the hull the ship will settle lower in the water.

The spar deck or weather deck is where you make a ship watertight.  This is where the hatches are on cargo ships.  We call the distance between the surface of the water and the spar deck freeboard.  A light ship doesn’t displace much water and rides higher in the water.  That is, it has greater freeboard.  With less ship in the water there is less resistance to forward propulsion.  Allowing it to travel faster.  However, a ship riding high in the water is much more sensitive to wave action.  And more susceptible to rolling from side to side.  Increasing the chance of rolling all the way over in heavy seas.  (Interestingly, if the ship stays watertight it can still float capsized.)  So ship captains have to watch their freeboard carefully.  If the ship rides too high (like an empty cargo ship) the captain will fill ballast tanks with water to lower the ship in the water.  By decreasing freeboard the ship is less prone to wave action.  But by lowering the spar deck closer to the surface of the water bigger waves can crash over the spar deck.  Flooding the spar deck with ‘green water’.  Common in a storm with high winds creating tall waves.  As long as the spar deck is watertight the ship will stay afloat.  And the solid water that washes over the spar deck will run off the ship and back into the sea.

The Titanic and the Fitzgerald were Near Unsinkable Designs but both lost Buoyancy and Sank

Improvements in ship design have made ships safer.  Steel ships can take a lot of damage and still float.  Ships struck by torpedoes in World War II could still float even with a hole below their waterline thanks to watertight compartments.  Where bulkheads divide a ship’s hull.  Watertight walls that typically run up to the weather deck.  Access though these bulkheads is via watertight doors.  These are the doors that close when a ship begins to take on water and the captain orders, “Close watertight doors.”  This contains the water ingress to one compartment allowing the ship to remain buoyant.  If it pitches down at the bow or lists to either side they can offset this imbalance with their ballast tanks.  Emptying the tanks where the ship is taking on water.  And filling the tanks where it is not.  To level the ship and keep it seaworthy until it reaches a safe harbor to make repairs.

They considered RMS Titanic unsinkable because of these features.  But they didn’t stop her from sinking on a calm night in 1912.  Why?  Two reasons.  The first was the way she struck the iceberg.  She sideswiped the iceberg.  Which cut a gash below the waterline in five of her ‘watertight’ compartments.  Which basically removed the benefit of compartmentalization.  They could not isolate the water ingress to a single compartment.  Or two.  Or three.  Even four.  Which she might have survived and remained afloat.  But water rushing into five compartments was too much.  It pitched the bow down.  And as the bow sank water spilled over the ‘watertight’ bulkheads and began flooding the next compartment.  Even ones the iceberg didn’t slash open.  As water poured over these bulkheads and flooded compartment after compartment the bow sank deeper and deeper into the water.  Until the unsinkable sank.  The Titanic sank slowly enough to rescue everyone on the ship.  She just didn’t carry enough lifeboats.  For they thought she was unsinkable.  Because of this lack of lifeboats 1,517 died.  Of course, having enough lifeboats doesn’t guarantee everyone will survive a sinking ship.

The Edmund Fitzgerald was the biggest ore carrier on the Great Lakes during her heyday.  These ships could take an enormous amount of abuse as the storms on the Great Lakes could be treacherous.  Like the one that fell on the Fitzgerald one November night in 1975.  When 30-foot waves hammered her and her sister ship the Arthur Andersen.  No one knows for sure what happened that night but some of the clues indicate she may have bottomed out on an uncharted shoal.  For she lost her handrails indicating that the ship may have hogged (where the bow and stern bends down from the center of the ship held up by that uncharted shoal).  The handrails were steel cables under tension running around the spar deck.  If the ship hogged this would have stretched the cable until it snapped.  She had green water washing across her deck.  Lost both of her radars.  A vent.  Maybe even a hatch cover.  Whatever happened she was taking on water.  A lot of it.  More than her pumps could keep up with.  Causing a list.  And the bow to settle deeper in the water.  Waves crashed over her bow as well as the Andersen’s.  The ships disappeared under the water.  Then reemerged.  As they design ships to do.  Then two massive waves rocked the Andersen.  She was following the Fitzgerald to help her navigate by the Andersen’s radar.  So these two waves had hit the Fitzgerald first.  The Fitzgerald had by this time taken on so much water that she lost too much freeboard.  When she disappeared under these two waves she never came back up.  It happened so fast there was no distress call.  The ship was longer than the lake was deep.  So her screw was still propelling the ship forward when the bow stuck the bottom.  She had lifeboat capacity for all 29 aboard.  But the ship sank too fast to use them.  Or even for the Andersen to see her as she sailed over her as she came to a rest on the bottom.

Our Ships have never been Safer but Ship Owners and Merchants still need to Protect their Wealth with Marine Insurance

We build bigger and bigger ships.  And it’s amazing what can float considering how heavy these ships can be.  But thanks to Archimedes’ principle all we have to do to make the biggest and heaviest ships float is too keep them watertight.  Keeping them less dense than the water that makes them float.  Even if we fail here due to events beyond our control we can isolate the water rushing in by sealing watertight compartments.  And keep them afloat.  So our ships have never been safer.  In addition we have far more detailed charts.  And satellite navigation to carefully guide us to our destination.  Despite all of this ships still sink.  Proving the need for something that has changed little since 14th century Genoa.  Marine insurance.  Because accidents still happen.  And ship owners and merchants still need to protect their wealth.

www.PITHOCRATES.com

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