Sounding the Depth, Sea Marks and Bridge Lights

Posted by PITHOCRATES - December 11th, 2013

Technology 101

It’s Important to know both the Depth of the Water beneath you and the Hidden Dangers below the Surface

On November 10, 1975, the Great Lakes bulk ore carrier S.S. Edmund Fitzgerald sank in a powerful Lake Superior storm.  Waves of 35 feet crashed green water across her deck.  But time and again she bobbed back up from under the waves.  Until she began to lose her buoyancy.  No one knows for sure what happened but the Fitzgerald was taking on water prior to her sinking.  One theory said that she bottomed out on Six Fathom Shoal off of Caribou Island.  As she fell into the trough between two huge waves.

A fathom is 6 feet.  So six fathoms would be 36 feet.  Though the water over Six Fathom Shoal could be as shallow as 26 feet.  Which is pretty deep.  But is dangerously shallow for a ship like the Fitzgerald.  For she had a draft of 25 feet.  At best she had 11 feet (36-25) of clearance between the shoal and her hull.  Or in the worst case, 1 foot (26-25).  With the gale force winds pushing the waves as high as 35 feet that would put the trough approximately 17.5 feet (35/2) below the ‘calm’ surface level of the lake.  Which would bring the top of the shoal above the hull of the Fitzgerald.  Thus making a strong case that she bottomed out and fractured her hull and began to take on water.

The theory continues that as she took on water she settled deeper and deeper into the water.  Growing heavier.  And less buoyant.  Until a wall of water swept across her that was too great for her to shake off.  Sending her to the bottom of Lake Superior so quickly that the propeller was still spinning when the bow hit bottom.  Causing the hull to break.  With the torque of the spinning shaft rotating the stern over until she rested hull-up on the bottom.  This is only one theory of many.  People still debate the ultimate cause of her sinking.  But this theory shows the importance of knowing the depth of the water beneath you.  And the great danger of unseen objects below the surface of the water.

Ships use Sea Marks to guide them Safely through Navigable Channels

Those mariners who first crossed the oceans were some of the bravest ever to live.  For if a ship sank in the middle of the ocean chances are people never saw those sailors again.  For there’s nothing to sustain life in the middle of the ocean.  Everything you ate or drank you brought with you.  And crossed at the greatest speed possible to get to your destination before your supplies ran out.  Which was easy to do in the deep waters of the middle of the ocean.  But very dangerous when the water grew shallower.  As you approached land.  Especially for the first time.

If a ship struck a submerged object it could break up the hull and sink the ship.  Especially if you hit it at speed.  This is why they had lookouts high up in the crow’s nest looking for land.  Or indications that the water was growing more shallow.  And they would ‘heave the lead’.  Big burly men (leadsmen) would throw a lead weight on a rope as far out in front of the ship as possible.  Once the lead hit bottom they’d pull it up.  Counting the knots in the rope spaced at 6-foot intervals.  Or fathoms.  Sounding the depth of the water beneath them.  As the sea bottom raced up to the water’s surface they furled their sails to catch less wind.  And slow down.  As they approached land they would approach only so far.  And safely anchor in a safe depth of water near a promising location for a harbor.  Some sailors would then board a dinghy and row into the shallow waters.  Sounding the depth.  And making a chart.  Looking for a safe channel to navigate.  And a place suitable to build a deep-water dock.  Deep enough to sail in to and moor the large sailing vessels that would sail to and from these new lands.

Of course, we could do none of this during the night.  It may be safe to sail in the middle of the ocean at night but it is very dangerous in the shallow waters around land.  At least, for the first time.  After they built a harbor they may build a lighthouse.  A tall building with a beacon.  To guide ships to the new harbor in the dark.  And even add a fog horn to guide ships in when fog obscures the light.  This would bring ships towards the harbor.  But they needed other navigational aids to guide them through a safe channel to the dock.  As time passed we made our navigational aids more advanced.  As well as our ships.  Today a ship can enter a harbor or river in the black of night safely.  Thanks to sea marks.

If Ships wander just Inches off their Course the Results can be Catastrophic

Landmarks are navigational aids on land.  Such as a lighthouse.  While a sea mark is a navigational aid in the water.  Typically a buoy.  A buoyant vessel that floats in the water.  But held in place.  Typically with a chain running from the bottom of the buoy to an anchorage driven into the bottom of the water channel.  Holding it in place to mark the edge of the navigable channel.  In North America we use the colors green and red to mark the channel.  With the “3R” rule “Red Right Returning.”  Meaning a ship returning from a larger body of water to a smaller body of water (and ultimately to a dock) would see red on their right (starboard).  And green on their left (port).  If you’re leaving dock and heading to open water the colors would be the reverse.

As ships move up river the safe channel narrows.  And there are bridges to contend with.  Which compounds the problem of shallow waters.  Fixed bridges will have red lights on piers rising out of the water.  And a green light over the center of the safe channel.  A vertical lift span bridge or a double leaf (lift) bascule bridge will have red lights at either end.  And red lights over the center of the channel when these bridges are closed.  When the center span on a lift bridge is open there will be a green light marking the center of the channel on the lifted center span.  Showing the center of the channel and the safe height of passage.  When the bascule bridge is open there will be a green light on the tip of each open leaf.  Showing the outer edges of the safe channel.

Ships are massive.  And massive things moving have great momentum (mass multiplied by velocity).  The bigger they are and the faster they go the harder it is to stop them.  Or to turn them.  Which means if they wander out of that safe channel they will probably hit something.  And cause great damage.  Either to the ship.  Or to the fixed structures along the waterway.  Like on an Alabaman night.  When a river barge made a wrong turn in poor visibility and entered an un-navigable channel.  Striking a rail bridge.  Pushing the bridge out of alignment.  But not enough to break the welded rail.  Which left the railroad block signal green.  Indicating the track was clear ahead.  The river pilot thought that one of the barges had only run aground.  And was oblivious to what he did.  And when Amtrak’s Sunset Limited sped through and hit that kink in the track it derailed.  Killing 47 people.  About twice the loss of life when the Fitzgerald sank.  Showing the importance of navigation charts, sea marks and bridge lights.  For if ships wander just inches off their course the results can be catastrophic.


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