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  1. #1
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    الصورة الرمزية Mohamed Gaber Aly
    SURVEY MANAGER - DUTCH DREDGING
    تاريخ التسجيل
    Nov 2009
    الدولة
    ALEX
    المشاركات
    740
    معدل تقييم المستوى
    6089

    افتراضي Squat and Settlement Calibration Test

    As a vessel's velocity increases, it generally settles or squats into a lower profile in the water, causing an
    error in depth measurement that must be corrected--Figure 9-18

    A squat test should be performed at least annually to determine the relation between boat speed and transducer height above or below the static sounding reference plane

    Report results of this calibration test using a standard field book

    Squat correction tables/curves should be permanently posted aboard the vessel--see example in Table 9-

    Without squat correction, channels may be actually dredged deeper than the drawings indicate

    RTK DGPS systems which provide direct (absolute) antenna-transducer elevation eliminate the need for the squat
    correction, as the antenna height will record the squat in real-time

    However, if the RTK DGPS system is set up to provide only the antenna height and is not configured to resolve the transducer elevation, then the squat correction must still be applied



    Settlement - At low speeds the effect of moving the hull through the water causes a
    local depression in the water surface around the hull. The effect of increasing speed
    on vessels with planing hulls is to cause them to lift out of the water.





    Squat - Changes in vessel trim as it moves through the water. Little appreciable
    affect on transducer depth if transducer is located near amidships.



    Figure 9-18. Squat and settlement effects on vessel draft (from NOAA)

    a. Conventional differential leveling techniques are utilized to measure the required calibration
    constants under normal loading (fuel/personnel) conditions

    A level is set up on a pier or bulkhead with the boat in a static position in calm water, and elevations are taken at a point on the boat directly over the transducer, i.e., amidships (see Figure 9-19)

    With a stadia board or level rod held at this point, the boat is driven past the instrument at various speeds, and elevation differences are noted at each speed

    In moving bodies of water (wind and/or current), this procedure must be run both up and down current to obtain the
    mean speed/squat

    Boat velocities and observed rod readings are recorded on the form

    A subtraction of rod readings after due correction for tide differences gives the squat corrections at each velocit

    Figure 9-19. Squat and settlement test procedure with differential leveling


    b. Corrections are added to the soundings to refer them to a static state

    Squat corrections are therefore considered positive quantities as the transducer depresses (squats) deeper into the water at increased speeds

    In this case, a positive squat is added to the raw observed/recorded depth A negative squat may occur with high-speed planing, surface effect, or hovering type vessels

    For these types of survey vessels, a squat test is especially critical and must be performed more frequently

  2. #2
    Status
    Offline
    الصورة الرمزية Mohamed Hashem
    عضو فضى 80%
    تاريخ التسجيل
    Apr 2009
    الدولة
    ALex, EGYPT
    المشاركات
    1,576
    معدل تقييم المستوى
    7072

    افتراضي

    جزاك الله خيرا
    قُلْ إِنَّ صَلاَتِي وَنُسُكِي وَمَحْيَايَ وَمَمَاتِي لِلّهِ رَبِّ الْعَالَمِينَ

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