Page 513 echo sounding 553 



day, and more frequently if necessary. Immediately after an echo-sounding instru- 

 ment has been repaired, after tubes or parts have been renewed, or after the instrument 

 has been changed in any way, the instrumental error should be determined and correction 

 made for it. 



The frequency with which bar checks should be made is prescribed in 557. 



553. Settlement and Squat 



Although an echo-sounding instrument correctly registers the deptfi from a vessel 

 when stopped, there is no assurance that the correct depth will be registered when the 

 vessel is underway. This is because a point on the vessel may experience a vertical 

 displacement when the vessel is underway, relative to its position at rest. Acoustic 

 units in a ship's hull are affected by such a vertical displacement, depending on their 

 location. The magnitude of this displacement may be such as to warrant compensa- 

 tion, especially where precise soundings in shoal water are to be obtained from a vessel 

 running at moderate to high speeds. The factors accountable for this vertical dis- 

 placement are settlement and squat. 



Settlement is the general lowering in level of a moving vessel, relative to what its 

 level would be were it motionless. Settlement is due to a regional depression of the 

 surface of the water in which the ship moves. It is not an increase in displacement 

 and, therefore, cannot be determined by reference to the water in the immediate 

 vicinity of the ship. 



Squat refers to the change in trim of the vessel when underway. At speeds ordi- 

 narily used in sm-veying, squat manifests itself in a lowering of the vessel's stern and 

 a rise of the bow. 



The major factors which influence settlement and squat are hull shape, speed, 

 and depth of water under the vessel. The effect of squat on the draft of the acoustic 

 units is usually not appreciable if they are mounted amidships, or a little forward of 

 amidships, as they generally are. On the contrary, settlement may be quite appre- 

 ciable at normal sounding speeds. In depths approximately seven times the draft, 

 for a survey ship it will probably amount to about one-half foot and in extreme cases 

 may be as much as 1 foot, increasing slightly as the depth lessens. 



The combined effect of settlement and squat at various sounding speeds used, 

 shall be determined for each survey vessel, including auxiliaries and launches, used for 

 hydrographic surveying in shoal or moderate depths. A test to determine this for each 

 vessel should be made at the beginning of each season. The vessel should be carrying 

 an average load and be in average trim. This value may be assumed to be a constant 

 for the season's work. Where the result of the test shows that the combined effect of 

 settlement and squat is less than 0.2 foot, it may be neglected, but if it is more than this^ 

 an instrumental adjustment should be made to all echo-sounding instruments used in 

 shoal water to compens.ate for it. 



Where the index has been adjusted to compensate for the amount of settlement 

 and squat at normal sounding speed, it is necessary to make an arithmetical correction 

 to any soundings taken from the vessel while stationary or running at slow speed. 

 (See example in 5522.) 



Two methods may be employed to determine the combined effect of settlement 

 and squat. In both, the tests should be made at either high or low water, when the 

 tide level is varying slowly. Provision must be made to measure any tidal change 

 which does occur during the tests. 



