Simple wash rule?

Discussion in 'Boat Design' started by alan craig, Aug 15, 2013.

  1. Leo Lazauskas
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    Leo Lazauskas Senior Member

    I see what you mean now.
    From what I have read, he doesn't actually address anything very specific
    to do with beaches or foreshores.
    The expressions for wave heights, wave periods and energy he finds can be
    used as inputs to coastal engineering models, but he doesn't give specific
    examples in the thesis.
     
  2. Crowsnest
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    Crowsnest Junior Member

    Hi all
    We usually test our boats at a very congested lake, where more than 1000 boats are registered.
    Of course we do not perform our tests beside the piers, but as far as we can.

    Albeit we have a speed limit of 3.5 kts when approaching, from my point of view, common sence is the best guide.
    Watching the ship wake is the best way for maximum approach speed . For minimum speed, the 3.5 criteria has resulted impractical many times for us, as manouvering below that speed on windy days, with some of our high speed boats, results at least, hard and hazardous.
    The result is that risks involved in low manouvering capacity due to low speed, overcome those which could arrive for proceeding faster.

    Common sence and good piloting once again.

    If anytime we have any problem with the traffic control tower our answer is:

    "We are proceeding at minimum manouvering speed on an approved boat".
     
  3. alan craig
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    alan craig Senior Member

    So many interesting replies, thanks people. I guess I already knew the short answer.
    Leo drew my attention to a paper which tries to get closer to a more "perfect" rule. I didn't read it all but p16 reminded me of a wash incident I experienced as a child. I was playing on a small beach backed by a sea wall, close to a UK ferry terminal. A few minutes after a ferry left the "tide" went out then the water came back and completely covered the beach up to the sea wall, no fatalities though.
     
  4. Leo Lazauskas
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    Leo Lazauskas Senior Member

    If you are interested in how much energy crosses a line parallel to one shore,
    then that is just half the wave resistance of the vessel.
    Of course the whole matter is complicated by the bathymetry, whether the
    bottom is muddy or stony, etc. etc.
     
  5. Leo Lazauskas
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    Leo Lazauskas Senior Member

    Wash has in part been responsible for the failure of some fast ferry
    companies. These companies thought their vessels would be economical
    and attractive to passengers, but they were then made to travel at
    half speed for some part of their journey because of the damaging
    effects of their wakes. See the attached paper for two examples
    from New Zealand and Hawaii.
     
    Last edited: Aug 12, 2015
  6. Crowsnest
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    Crowsnest Junior Member

    H Leo and all:
    Not needed to go as far as New Zealand.(As seen from W. Europe, of course)
    At Gibraltar straight the same problem was found on the new Fast ferries, when approaching Ceuta, some years ago.

    Ceuta-- 35º54'N 005º17'W
     
  7. Leo Lazauskas
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    Leo Lazauskas Senior Member

    Do you have any specific references to reports of these incidents?
    I know that some Incat catamarans travelled between Morocco and Gibraltar,
    but I can't find reports of any wave-wake related problems.
     
  8. latestarter
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    latestarter Senior Member

    Leo, this was an issue on the Belfast, Northern Ireland to Stranraer, Scotland route run by Stenaline.

    They ran this type of ferry, http://en.wikipedia.org/wiki/High-speed_Sea_Service

    When it first ran there were problems on Belfast Lough and Loch Ryan which meant over a considerable length of the trip it was reduced to less than 1/2 speed.
    The characteristic which caused the problem was the wake was not obvious as it approached the shore but at the last moment the water reared up and rushed across the promenade and into the gardens of shoreline properties with little warning.
    Some comments also here http://en.wikipedia.org/wiki/Loch_Ryan

    You can see a paper published by Queen's University Belfast here http://www.qub.ac.uk/waves/fastferry/reference/EnvImpact1.pdf

    They no longer run on this route. I understand, partly due to not being able to make use of their speed and fuel consumption/increase in fuel price.
     
    1 person likes this.
  9. Leo Lazauskas
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    Leo Lazauskas Senior Member

    Thanks for the references and the report.
    (One of the authors of the Loch Ryan paper used Michlet for his MSc thesis
    on wave wakes of high-speed catamaran ferries.)

    I'm seeing the phrase "reduced to half speed" more and more in relation to
    these vessels!
     
  10. jehardiman
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    jehardiman Senior Member

    These are effects of the wave length interaction with the foreshore. The real "wash" problem with most fast vessel wakes are the length (or period) of the wave, not the height. The faster the vessel, the longer the period (and length) of the wake wave. Since wave energy per unit length is proportional to wave height squared time wave length, and higher speed vessels generate more unit length for the same resistance, a high speed vessel leaves a wake wave of significantly lower height but longer period for the same energy. When this long wave shoals on the foreshore, it slows down its celerity and therefore shortens in length. Energy is conserved and can significantly increase wave height, up to 3 times the open water height and there are all sorts of resolutions, i.e. breaking, plunging, spilling, and surging of the wave face.

    For a short primer...

    http://stream2.cma.gov.cn/pub/comet/CoastalWeather/sww/comet/marine/SWW/print.htm#a22
     

  11. Leo Lazauskas
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    Leo Lazauskas Senior Member

    Yes, wave height is a very poor criterion and it has been abused by some
    unscrupulous operators/designers who claim that their vessels are "low wash"
    when really all they mean is that they generate waves of low height.

    That's why I like the approach (I mentioned earlier) of using different
    combinations of the wave height (H) and wave period (T), e.g. H*T, H^2*T,
    H*T, H*T^2 etc.
    One of those combinations can sometimes correlate better with some
    phenomena. For example, H*T^2 is a better predictor for the onset of
    turbidity than the other combinations.
     
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