# 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

# Flat-Bottomed Boat, Keel, Standing Rigging, Chinese Junk, Daggerboard, Balanced Rudder, Compartment and Junk Rig

Posted by PITHOCRATES - May 16th, 2012

# Technology 101

## Typical River Transport has a Flat Bottom and a Shallow Draft with Little Freeboard

What do most of the oldest and greatest cities in the world have in common?  Madrid.  Lisbon.  Paris.  London.  Amsterdam.  Belgrade.  Vienna.  Rome.  Cairo.  Kiev.  Moscow.  Baghdad.  New Delhi.  Shanghai.  Ho Chi Minh City.  Bangkok.  Hong Kong.  São Paul.  Buenos Aires.  Santiago.  Quebec City.  Montreal.  Detroit.  Boston.  New York.  Philadelphia.  Pittsburgh.  What do these cities have in common?  Rivers.  Coastal water.  Or safe harbors on the oceans.

Why is this?  Is it because their founders liked a good view?  That’s why people today pay a premium to live on the water’s edge.  But back then it was more necessity than view.  These were times before railroads.  Even before roads connected these new cities.  Back then there was only one way to transport things.  On the water.  And rivers were the early highways that connected the cities.  Which is why we built our cities on these rivers.  To transport the food or raw materials a city produced.  And to transport to these cities the things they needed to survive and grow.  And some of the earliest river transports were flat-bottomed boats.  Like the scow.  Punt.  Sampan.  And the barge.

Rivers are calm compared to the oceans.  Which allows a different boat design.  River transport doesn’t have to be sturdy to withstand rolling waves and high winds.  Which allows the design to focus on the main purpose of a boat.  Hauling freight.  Typical river transport has a flat bottom.  A shallow draft with little freeboard (i.e., sitting very low in the water with the top deck very close to the surface of the water).  And a square bow.  This allows these boats to operate in shallow waters.  Allowing them to run up right onto a river landing or beach.  Where they can be easily loaded with their cargoes.  Or unloaded.  And their flat, rectangular shapes maximize the cargo they can carry.  Propulsion is simple.  A man can push a small boat along with a pole.  Animal power can pull larger barges.  Or, later, motors were able to power them.  Or a tugboat could pull or push them.

## The Chinese Junk had a Flat Bottom with no Keel allowing them to Carry a Lot of Cargo

These flat-bottomed boats are great for hauling freight.  But they are not very seaworthy.  Because the ocean’s waves will toss around any boat with a shallow draft and little freeboard.  Breaking it up and sending it and its cargo to the bottom of the ocean.  Which has confined these to the calm of rivers, bays and coastal waterways.  Cargoes that have to travel further than these allow are loaded onto an ocean-going vessel with a deeper draft.  And a higher freeboard.  With a keel.  That can withstand the leeward force of the wind.  So instead of being pushed sideways (or simply rolling over) the keel allows those sideway winds to fill a sail and propel a ship forward.  By sticking deeper into the water.  So as the wind tries to push the boat sideways the large amount of water in contact with the keel pushes back against that leeward force.  Allowing it to sail across the wind.

But there is a tradeoff.  The curved sections of the hull that form the keel reduces the amount of cargo a ship can carry in its hull.  Also, these ocean-going vessels have a lot of sail.  And a lot of rigging to hold it in place.  Standing rigging.  While the sails required running rigging.  To raise and lower sails depending on the wind conditions.  Which takes up space that can’t be used for cargo.  And requires a lot of sailors.  In fact, much of the upper deck is full of rigging and sailors instead of cargo.  But this was the tradeoff to sail into the rougher waters of the ocean.  You had to sacrifice revenue-earning cargo.  But there was one ship design that brought together the benefits of the flat-bottomed river scow and the ocean-going fully rigged sailing ship.  The Chinese junk.

The Chinese junk dates as far back as the 3rd century BC.  And began crossing oceans as early as the second century AD.  Long before the Europeans ventured out in their Age of Discovery.  The junk has a flat bottom with no keel.  But a high freeboard.  Which lets it carry a lot of cargo.  And operate in shallower waters than a fully rigged sailing ship.  But it could also sail in the rougher seas of the ocean.  When it did it lowered a daggerboard.  A centerboard that can lower from a watertight trunk within the hull into the water to act like a keel.  To resist those leeward forces.  Often installed forward in the hull so as not to take up valuable cargo space in the center of the ship.  Because they mount this forward the leeward forces could cause the back end of the ship to torque around the daggerboard. To counteract this force they use an oversized rudder on the stern.  To balance the resistance to those leeward forces.  Because the rudder was so large and had to deflect a lot of water it was difficult to turn.  Taking a team of men to operate it.   To help turn such a large rudder they developed ‘powered’ steering.  With a balanced rudder.  The axis the rudder turned on was just behind the leading edge of the rudder.  So when they turned the rudder the water hitting the part in front of the turning axis helped turn the rudder in the direction the crew was trying to turn it.  So the large rudder area past the turning axis could deflect the large volume of water necessary to turn the ship.

## The Chinese gave us Papermaking, Printing, the Compass and Gunpowder but the Europeans Conquered the World

So the junk could travel in the shallow waters of harbors and rivers.  And the deep water of the ocean.  It was the first ship to compartmentalize the hull.  Making it very seaworthy.  Especially if it struck bottom and punched a hole in the hull.  Because of the compartments the flooding was contained to the one compartment.  Allowing the ship to remain afloat.  A design all ships use today.  The junk also used a different sailing rig.  The junk rig.  It’s low tech.  Was inexpensive.  And required smaller crews.

A three-mast junk has three masts.  And three sails.  One sail per mast.  And the masts are free standing.  They don’t need any standing rigging to hold them in place.  Because they don’t carry heavy loads of running rigging and sailors.  The sail is stretched between a yard and a boom.  The yard is at the top.  The boom is along the bottom.  Between the yard and the boom battens give the sail strength and attach it to the mast.  Think of a batten as that stick in the bottom of a window shade.  Grabbing this batten allows you to apply an even force on that window shade when pulling it down.  If this stick wasn’t there and you pulled down on the window shade the uneven forces across the shade would tear it.  Same principle on a junk rig.  Which allows them to use less expensive sail material.  To raise this sail up the mast you pulled up the yard via a block and tackle at the top of the mast.  From the deck.  With fewer crew members.  The sail is attached to the mast near one edge.  It’s pivoted to catch and redirect wind to the stern.  Propelling the ship forward.  And the battens will bend in strong enough winds to curve the sail.  Creating lift on the other side of the sail to pull the ship forward.

The Chinese gave us papermaking, printing, the compass and gunpowder.  But it was the Europeans that used these inventions to conquer the world.  For the Chinese had no interest in civilizations outside of China.  For when you had the best, they thought, what was the point?  So the Europeans came to them.  Even took Hong Kong from them.  When it was the Chinese that could have had the technologically advanced civilization.  An army fielding muskets and cannon.  And a navy of junk warships that could have gone anywhere the Europeans could have gone.  And farther.  Into the shallow waters and up the rivers where the European warships could not go.  They could have sailed up the Thames to London.  Up the Seine to Paris.  Even into Amsterdam.  Home of the Dutch East India Company.  That took such a great interest in all those Asian goods in the first place.   That brought the British to China to compete against the Dutch.  Leading to the Opium Wars.  And the loss of Hong Kong.  Imagine how different the world would be had China embraced their technology.  Like they are today.  Perhaps we will soon see the answer to that great ‘what if’ question.

www.PITHOCRATES.com

# 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