Too Close For Comfort - Our “Wake”-Up Call

We left Annapolis for St. Michaels on October 26, at about 11:00 am.  There was fog on the Chesapeake.  We've read that a captain should know how to handle fog if it happens, but should avoid heading out into it.  We've also read that a wise captain never follows an agenda, because that can lead to taking chances.  So we waited until we could see better.

However, when we were about half way across the Bay, the fog came back.  Our radar was working well, seas were calm, and there was very little traffic.  So we weren’t worried.  Just a bit apprehensive.

Fog is scary stuff.  Back home on the Great Lakes, many ships have sank due to fog, often from collisions, although most of that was before radar.  

Fog plays tricks on you.  You hear things but can’t tell where they are coming from.  You strain to see things through it, and usually you can’t, but you sure THINK you can see things.  That’s the scariest part.  Fog messes with your imagination.

See a boat anywhere?  When you get into fog like this, you’ll think you see several as the time passes. The trick is, seeing them when they actually are there.  It’s very tiring keeping that kind of watch.  But it's necessary.

Anyhow, things were going well when a ship appeared on our AIS.  Before I talk about the ship, let me tell you what AIS is. 

AIS stands for Automatic Identification System.  The system is another way other than radar for vessels who each have AIS to “see” what’s out there, in good weather or bad, within eyesight, or beyond eyesight.*  In some ways, AIS is even better than radar.**  Because you don’t just “see” another ship that has AIS.  And it doesn’t just “see” you.  Each of you gets data.  Usually that data includes vessel name, type, size, direction, speed, destination, course and, coolest of all, a computed time and distance your vessel and the other will either collide, nearly collide, or safely pass each other.  

When the data show either of the first two of those possibilities, the “make way” vessel is supposed to signal, early and obviously, a change of course so that the “stand on” vessel knows the two boats will pass safely.  In maritime language, the “make way” vessel is the one that is supposed to get out of the way of the “stand on” one.  This is the most important part of the maritime rules of the road (called COLREGS, which stands for “The International Regulations for Preventing Collisions at Sea”).  Signaling early and obviously means the “make way” vessel changes course in a way that shows the other vessel this is happening.  When navigating a tight channel, or when one or both vessels have “restricted maneuverability” (which means one or both vessels have difficulty changing course, or must remain on course because they require the depth) there’s little room or time to change course.  That’s when both captains should communicate by radio to confirm which way each will pass.  By the way, even though these are the right practices, if two boats collide, the Coast Guard will find both captains at fault, because when the right rules aren’t followed, the #1 rule of the road for both captains is:  DON’T HIT EACH OTHER!

*  Many boats don't have AIS.  While ships are required to have it, smaller boats aren't.   

** Many boats don't have radar either.  To avoid a collision, they can't "see" us.  So we must avoid them.

There's one other rule when it comes to avoiding a collision on the water.  It's an unwritten one.  STAY OUT OF THE WAY OF BIG SHIPS!  Because in a collision, the biggest boat "wins."  In fact, the crews of the biggest ships might not even know they hit you.  

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Now back to the story…

We clicked on our AIS and saw the ship was a Maersk container ship called Maersk Yukon.  (Maersk is the largest container and supply vessel operator in the world.)  

Our AIS data on Maersk Yukon right after she passed in front of us.

And she was HUGE.  So huge, I decided to research and crunch some numbers for fun... 

Yukon is exactly 121.5 feet longer than the Paul R. Tregurtha, the largest ship on our Great Lakes.

The Paul R. Tregurtha.  My favorite Great Lakes freighter.  We've heard her captain answer radio calls with "This is the Paul R."  My dad and I think that's cool.  And funny too.  The boat is so large and famous, it's captain gets to shorten its name. 

Not only is Yukon huge, she is more than 3 times wider than our Nordhavn is long.  Her displacement, before cargo, is 116,440 metric tons which is 127,868 short tons.  (A short ton is what we call a ton in the U.S.  It weighs 2,000 lbs).  

Given our boat displaces 42.5 short tons, the Yukon displaces 127,825.5 short tons more than our boat, before her cargo.  

How many times heavier is she than we are?  Well, her 127,868 short tons divided by our 42.5 short tons = 3,009 times heavier.   

😳

But wait...

What does she weigh loaded?  I looked it up.  The Yukon has a capacity of 10,062 TEU.  What’s a TEU?  I had to look that up too.  A TEU is “twenty foot equivalent unit.”  And what is that?  It’s about equal to a standard 20′ shipping container, which is 20 feet long x 8 feet wide x 8 feet 6 inches high.  

A TEU

And how does much does THAT weigh?  Well, a full TEU is assigned an approximate weight (a sort of average weight of a 20 foot container) of about 11.8 short tons per TEU.  So, 10,062 TEUs times 11.8 short tons per TEU means the Maersk Yukon can carry, when fully loaded, 10,062 x 11.8 = 126,048 short tons of cargo.  (Apparently Maersk significantly underreports her capacity by as much as 3,000 to 4,000 TEUs.)  

By using the figure they officially use of 10,062 TEUs, given our Nordhavn 47 weighs 42.5 tons, that means the Yukon can carry the equivalent of 2,966 Nordhavn 47’s onboard.

😳 😳

But wait again...

So what is her total displacement if fully loaded?

Well, her ship displacement of 127,868 short tons + her conservatively stated TEU capacity of 126,048 tons = 253,916 short tons (which is 507.8 million pounds).  

And this gets me to my main point.  When loaded, that ship’s total displacement is 5,947.6 times our own (her 253,916 short tons divided by our 42.5 tons).

😳😳😳

I used the words “displacement” and “displace” instead of “weight" and “weighs.”  

Why?

I’ll get to that shortly.  I promise you, it’s cool!

 

Which brings me back to the wake.

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As we were the “give way” vessel (the Yukon was crossing us to our starboard), we signaled to her early, with a course correction and reduction in our speed, to let her know that we “saw” her in the fog.  

As she was the "stand on" vessel, she made no course change or reduced her speed.  By the way, trying to reduce the speed of a ship displacing that much would have been pointless anyway.  It takes miles to stop that much momentum.  And thousands of feet to turn it.  (Remember Newton’s law about how mass and speed affects the force of objects in motion.)

__________

We still didn’t see the Yukon though.

Then, she slowly appeared through the fog, crossing our bow a couple thousand feet ahead of us.  

There she is.  Can you see her?

Getting closer.

To my dad, that seemed like plenty of distance.  And really, it was, to avoid any collision.  We knew we were safe to miss her.

However, we learned some pretty valuable things that day.  And they had to do with her wake.

—————

So what causes a boat’s wake?  Mainly, it’s the the boat’s displacement at speed.  Some speeds cause the boat to displace more water.  The more the boat dips down into the water, the more water it displaces.  That water has to go somewhere.  Some of it is pushed outward by a bow wave and some by a stern wake.  Generally, the bigger the displacement, the bigger the potential for wake. 

Why just the potential for wake?  Well, speed has a lot to do with wake size.  Wake gets bigger at a boat speeds up, but then can eventually get smaller at higher speeds (when some boats lift themselves more and more out of the water, displacing less.).  For maximum wake, the sweet spot is when the displaced water is at its greatest, but can’t rush in quickly enough behind the boat to fill that gap.  Instead, it converges behind the boat and is forced outward in a wake.  THAT’s when it can become its biggest.  (That’s what wake boats are designed to do:  strike that balance.)

                                                                ↙

This wake boat is going quickly enough and digging down deeply enough to leave a trough behind.  Wake happens when water rushes in to fill that trough, slams together, then spreads apart.

Given the Maersk Yukon displaced as much as it did, and was traveling at a quick 16.5 knots (over twice our speed), she was bound to kick up a big wake.  And, that wake would be traveling away from her at around the same speed she was moving.

But we didn’t know all these things at the time.

__________

So here’s what happened.

• The Yukon passed safely off our bow.
• We felt good about steering clear of her.
• My dad steered a bit to starboard so we would quarter any wake (meaning, we’d hit the wake at a 45% angle to our boat).
• We saw the wake in the distance. It looked fine.
• We saw it closer.  It didn’t look fine.
• It arrived.
• Ouch.

When we saw what was coming, my dad grabbed his iPhone to capture it for our photo album.  

Notice our Halloween decoration, “Stack of Bones” ("Stacky" for short) after the wake had passed?  It’s like his jaw dropped.  AND, he’s looking right where the ship went.  :)

Here I am looking at Yukon as we passed each other.

__________

We knew there would be wake, but not THAT big.  You saw where our boat hit the first wave, rose up, then fell into its trough right as the next wave arrived.  That’s when it was its worst.

Over the course of our travels, I’ve learned that the effect of waves on us depends on timing.  We’ve hit big waves and felt a lot.  Other times, we’ve felt next to nothing.  I saw how this depended on the angle of out boat when we hit each wave and what motion the previous wave had given her.  Sort of like jumping on a trampoline.  The first jump doesn’t do a lot.  But jump several times at the right time and you go higher.  With Yukon’s wake, the impact became worse after each wave until we were through it.

We never really felt scared.  Our boat has always seemed to leap out of the water in waves.  

So I decided to research why.  The more I looked, the more interesting it became.

Buoyancy

Way back in 221 BC, a Greek genius called Archimedes developed a principle of buoyancy.  He learned that an object in fluid (such as our boat on the Chesapeake) is buoyed (kept afloat) by a force equal to the weight of the fluid the object displaces.  

It's a balancing act.

And that is why the term displacement is used to describe how heavy a ship is.  A ship weighs the equivalent weight of the water it displaces.

So, the more our bow sinks into a wave (provided water doesn’t enter the boat), the more our boat will resist sinking further, and the harder she will try to rise up.  

A cubic yard of water weighs 1,686 lbs.  Think about that heaviness two ways:

1. When our bow slams into wake or a wave, she is pushing aside an incredible weight in water.
2. When our boat sinks deep into a wave, she experiences tremendous force pushing her upward again.  The exact same weight pushing her up as the water she pushed down into. (Which is also the same water weight that would otherwise fill her at that point.  But let’s not think about that…)

There are a number of other things that have to do with our boat’s handling of wake / waves like Maersk Yukon's.  They include:

• Her hull shape. 
• Her weight distribution.
• Her center of gravity.
• Her roll period (the time it takes for her to roll from side to side).
• Her speed.
• Her power (her ability (or not) to power through or over waves).  

Maybe I’ll write about those someday...  

__________

So, other than the science about buoyancy I learned, here’s what I learned about handling our boat in this kind of situation for next time.

What Did We Do Right?

• We were tracking the Yukon.  She didn’t surprise us.  
• We changed our course "obviously" to signal Yukon that we knew where she was.  
• We turned our boat to quarter Yukon’s wake.
• We were watching for the wake, so when we saw it, we had time to brace ourselves. 

What Could We Have Done Better?

• We should not have been so close to her.  We should keep a greater distance from boats this size.  Wakes lose power the further they travel as their amplitude (the difference between their peaks and troughs) decreases.  
• We should have faced her wake at a lesser angle (but not too much less).  Why?  Our boat has stabilizers — underwater fins that stick out sideways.  They have sensors which move them to counteract roll (which is side to side boat motion), but not pitch (which is up and down motion of the bow).  Letting our stabilizers handle more of the roll would have helped reduce the amount of pitching we faced.
• We should have slowed down more than we did.  Hitting such a wake at a slower speed would have reduced the violence we experienced.  
• We should have secured things inside the boat better before heading out.  We’d learned about locking and padding fragile things before heading out on rough days, or when the water turned rougher.  But this day was calm…  Well, we learned that big waves can happen on calm days too.  (In case you are wondering, all that broke was a halloween decoration, which we glued back together.)

I hope you enjoyed this story.  It was fun to research and taught me a lot.  Still, this was an adventure we’ll try not to go through again.

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For Further Reading / Watching

Man has been building boats for thousands of years.  We've learned a lot over time.  And we are still learning, about boat designs and wave habits.  Today’s boats are way more sophisticated in how seaworthy they are (able to withstand harsh conditions), how sea kindly they are (comfortable for the crew in harsh conditions), and how efficient they are. 

If you want to learn more about boat and ship design, watch this.  It's called "The Science of Ship Design."  You'll see how computers and software are making things way better.

https://youtu.be/T7bYl0jKt70

If you want to watch an amazingly seaworthy boat handle terrible conditions, watch this.  It's Archimedes principle tested to the max!  

https://youtu.be/RMv3SZIEJ6Q

And lastly, I’ve discovered I like to end my blog postings with something funny.  Here’s a hilarious clip about a boat hitting a wave, that didn't end well.

https://youtu.be/3m5qxZm_JqM