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.
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
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
