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    Re: DR thread from Nov-Dec '04
    From: Bill B
    Date: 2005 Jan 20, 15:44 -0500

     > I'm not quite clear what Bill's "mashed potatoes" looked like but there
    > are a number of possible causes.>
    Perhaps a problem with word usage.  I think of swells as a long series of
    nicely formed "wave forms" 100 or more yards in length (across the crest).
    Picture perfect.  Next would come "waves" which may come in a shorter series
    but are maybe 75 ft or more wide, a bit choppy.  Some may be "squares", a
    wall of water.  Mashed potatoes are a form confused sea. Piles of water,
    perhaps the shape of pile of sand, 10'-12' high in this case .  (I say
    perhaps as I could only view one side).
    I generally define a "confused sea" as being the result of a change in wind
    direction causing chop between the old wave direction and forming of waves
    in a new direction, or tide vs wind. In this case the wind had been out of
    the north at 10-20 kt. for days, so I don't attribute the piles to confused
    sea as I usually define it.  Since we were near the southern shore (maybe 6
    miles off at times) I attributed it to a combination of shallow depth and
    some kind of bounce back from the shoreline.
    You are correct in that it is a sandy and relatively shallow bottom, with a
    gradual slope.  Sand bars do form near shore, causing rip currents.  We were
    probably in 70 to 170 of water or more, with trough-to-crest measurements of
    waves in the 6-9 foot range (measured from trough to crest), many
    predictably 50% higher of course.  Wind had picked up from 12-18 kt to 23-28
    kt true during the day (the later being well above the forecast, but a sunny
    day so we may have gotten a nice boost from onshore breezes when closer to
    shore).  Our course was about 266 T--headed for the Gary air show.  Chicago
    is about 288 T from Michigan City, and approx. 30 nm.
    Now that you mention it, wind from the north would want to run parallel to
    the lake's western shoreline.  If it were a billiard ball hitting the
    Chicago coastline (rail) it would indeed head almost directly toward
    Michigan City.
    If it were a set up, then the water would continue to pile up on the
    southern end as the wind velocity increased, right?  In this case, could it
    have, like water in a creek that piles up to the outside of a bend, been
    redirected as a southeast current (rather than the northeast current you
    All I can say for sure is that a 34' Catalina is usually pretty dull
    compared to my Hobie 16 in the mid-20 kt breezes, but that day day was pure
    >> Jared
    >> You bring up an interesting memory that I had put aside regarding fetch and
    >> waves (which would also affect current).  This summer (AKA the "summer that
    >> never was" on Lake Michigan) a lot of wind from the north.  Sailing out of
    >> Michigan City on the southern coast. 10-28 kt breezes out of the north for
    >> several days one extended weekend. 10 statute miles (love those Great Lakes
    >> charts) big but nicely formed wave trains and swells.  Within 2-3 miles of
    >> the dunes, mashed potatoes--huge piles of water.  Reading Trevor and
    >> thinking back, I was "set up."  What we saw was water bouncing of the
    >> southern shore in depths ranging from 40-200 feet.
    >> In this case, although fetch was several hundred miles and time was in days,
    >> because we were in near proximity to the shore (an perhaps factor in onshore
    >> breezes kicked up a notch by the the northerly breeze--exacerbated by
    >> shallow depths) it was a wild ride.
    >> While fetch related to waves/wind-induced current may indeed be measured in
    >> shore-to-ship distance for Doug on the high seas, in this case you make a
    >> strong point that what is leeward of the craft and proximity also plays a
    >> role.
    > I'm not quite clear what Bill's "mashed potatoes" looked like but there
    > are a number of possible causes.
    > Water depth was likely part of it, with the waves becoming shorter and
    > steeper as they felt bottom.
    > With a steep shoreline, there is the possibility of the waves being
    > reflected, so that two wave trains running across one another -- the
    > ordinary wind waves and their reflection. That can make for a wicked
    > sea. However, a shoreline of dunes suggests a sandy beach and beaches
    > absorb wave energy rather than reflect it, so I doubt that reflection
    > caused Bill's wild ride.
    > Refraction of waves around shoals and islands can have interesting
    > effects too but it sounds like Bill experienced his ride across too
    > large an area, with no islands and few shoals to windward.
    > So I'd guess that the the lake water set up against the Chicago
    > waterfront was escaping to the northeast in a sort of giant rip current,
    > following the beach. If so, the waves from the north would have entered
    > a "wind over tide" situation (except that it wouldn't be a tidal stream)
    > as they also entered shoal water. That would make things pretty exciting
    > for a small vessel.
    > Trevor Kenchington
    > --
    > Trevor J. Kenchington PhD                         Gadus@iStar.ca
    > Gadus Associates,                                 Office(902) 889-9250
    > R.R.#1, Musquodoboit Harbour,                     Fax   (902) 889-9251
    > Nova Scotia  B0J 2L0, CANADA                      Home  (902) 889-3555
    > Science Serving the Fisheries
    > http://home.istar.ca/~gadus

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