PaddleWise Discussion on Surge from Ship's Wake

Date: Tue, 12 Oct 1999 22:24:52 EDT
From: Gratytshrk
Subject: [Paddlewise] question for the hydrodynamics experts...

Hi yall,
I have a question about a bizarre (and somewhat terrifying experience) i had 
today.  I was fishing in a canal in southeast louisiana today, approx 1/2 
mile wide with a very deep channel through the center, which probably planes 
out towards the sides which are surrounded by marsh and other smaller canals. 
 I was having a great day of fishing and after one spectacular cast i 
happened to look up and see a huge freighter coming down the channel.  I 
expected a large wake from the ship and it was too near to do any major 
maneuvering, so i kept my kayak near the side of the channel ( an ENOURMOUS 
mistake!!)  While the ship was about 1/8 of a mile away all the water from 
near the sides began rushing out toward the ship.  I was in about 4 ft of 
water which drained to the bottom in about 20 seconds.  The force of the 
outrushing flow was greater than any undertow i have ever encountered.  This 
is when i realized i was in serious trouble.  My kayak rushed out toward the 
ship (which was at least 1/4 mile away)  Soon a wave of returning water and 
ship wake (which was at least 2-3 feet above my head from where i was sitting 
in the trough, I would estimate it at 6-8 feet) was rushing back toward the 
bank, as i was rushed toward the wave.  The outrushing water and inrushing 
wave collided to form a huge breaker which crashed right on my face.  In my 
current state of terror i somehow managed to remain upright through the 
massive breaker and the slightly smaller wake waves which ensued.  Everything 
under my deck bungies was washed about 2 feet up on formerly dry land, then 
returned when the water level returned to normal.  I was soaked, and i am 
still picking dirt out of my teeth.  I was very lucky to be wise enough to 
have my pfd and srayskirt on.  My question is this- What force contributed to 
the outrushing water before the ship reached my location.  I theorized that 
if anything the ship should displace water would raise the water level.  Or 
could it be that the propeller acted as a vacuum of sorts that sucked water 
ahead of the ship by forcing it through the prop and displacing it in the 
form of a wake behind the ship?  I am totally fascinated by this incident.  
After the ship passed i decided to call it a day and happened to notice 
another equally large tanker coming through the canal.  I decided to hide in 
a side canal and paddled back about 1/2 mile into a canal and thought i could 
hide around a couple of bends.  I was amazed to see the same effect occuring 
as the ship approached, I rushed back into the middle of the side canal and 
again faced some serious waves although not nearly as large.  These were only 
3-4 feet and did not break.  I understand that the shape and break of the 
waves had to do with the depth of the water where i was.
Thanks for any input.
Robin Lovelock
New Orleans.

Date: Tue, 12 Oct 1999 20:56:23 -0700 From: Dave Kruger Subject: Re: [Paddlewise] question for the hydrodynamics experts... What you experienced was the "surge" portion of the ship's wake. In brief: that surge produces a minimal vertical excursion of the water's surface *when the water is very deep* (at least as deep as the "length" of the wave the surge forms in deep water). That's why you would not notice the surge if the ship passed by in deep water. OTOH, when the water is very shallow, the leading part of the wave (the trough) arrives first, and because its "ideal" form is impossible to maintain in shallow water, it "sucks" water back toward the ship, to help form *the crest* of the wave (which arrives just behind the trough). Because the water is too shallow to sustain the crest, a breaking wave is formed, similar to surf, and the wave becomes akin to what happens in a tidal bore. What happened to you is sort of a small scale version of a tsunami hitting a shallow, sloping beach. For more about waves (and beaches), chase down Willard Bascom's classic work, written for the lay person: Waves and Beaches, Anchor Books, 1980 (revised version), ISBN 0-385-14844-5. Out of print, I am sure, but in any good library's earth science section. A treasure trove of stuff useful for ocean paddlers, covering both waves on the water, and how coastal landforms are affected by waves. You'll be a better person, and have an improved sex life if you read this! - -- Dave Kruger Astoria, OR
Date: Sat, 16 Oct 1999 08:49:17 -0500 From: tfj Subject: RE: [Paddlewise] Book on Waves Thanks to whoever recommended "Waves and Beaches" by Willard Bascom. It is, as reported out of print, but I was able to get a copy at the local public library and am enjoying it already.
Date: Wed, 13 Oct 1999 14:34:14 -0700 From: Jerry Hawkins Subject: Re: [Paddlewise] question for the hydrodynamics experts... Robin, I can't explain the physics but can tell you that the same thing happened to a ship thousands of times your size (the S.S. New York if I recall) when the Titanic started out from port. The ship was ripped from its moorings and thrown out into the bay, nearly causing a collision. The same effect is used to trigger certain types of anti-shipping mines, like those the Navy put into Haiphong harbor about 30 years ago. I have not found any advice in any of the kayaking books regarding safety in a channel when large ships are around except to keep out of their path. I was wondering about this as I kayaked down the Westpoint Slough with a heavily laden ocean barge moving ahead of me on a parallel course, at roughly my own speed. Should I attempt to pass it, stay even with it, pull in directly behind at a safe distance, surf its wake, or what? jerry.
Date: Wed, 13 Oct 1999 17:44:33 -0400 From: "Robert Woodard" Subject: RE: [Paddlewise] question for the hydrodynamics experts... I haven't experienced this kayaking yet, but where I grew up in FLorida I do remember large freighters coming up the river where my dad used to take me fishing. It was as if someone pulled the plug as the water pulled way off the shore. And then as the ship went by, the water would all come rushing back in. Pretty cool as a kid, and I would have never thought about it as it applies to kayaking if you hadn't brought it up. I never thought it was related to the wake though, I always figured it had to do with the amount of water being sucked through the props. Ya learn something new every day... Woody