المساعد الشخصي الرقمي

مشاهدة النسخة كاملة : Squat and Settlement Calibration Test



Mohamed Gaber Aly
05-20-2010, 05:56 PM
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



http://www14.0zz0.com/2010/05/20/15/784562425.jpg


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.


http://www2.0zz0.com/2010/05/20/15/806458512.jpg





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)
http://www13.0zz0.com/2010/05/20/15/963441470.jpg


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

http://www5.0zz0.com/2010/05/20/15/397424000.jpg
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

Mohamed Hashem
05-22-2010, 10:13 PM
جزاك الله خيرا