5. HYDROGRAPHY 



177 



taneous direct vertical measurements of the 

 depth ; and (2) by bar check. When good 

 results can be obtained by bar check, it is 

 the more accurate method and should be 

 used. Regardless of the method used, the 

 procedure is generally as follows : 



(a) The daily check of adjustments should 

 be made and the instrument should be op- 

 erated for about 30 minutes before the test 

 is made. 



(b) The vessel must be stopped, the sea 

 calm, and the current slack. If vertical casts 

 are to be made, the bottom should be level 

 and comparatively hard. 



(c) The echo sounder initial should be set 

 to compensate for the draft of the trans- 

 ducer, uncorrected for settlement and squat. 



(d) When simultaneous comparisons are 

 used, at least 5 comparisons shall be re- 

 corded. If a bar check is used, comparisons 

 should be made at a depth of about two 

 fathoms. 



(e) Sheave or leadline corrections shall be 

 applied to vertical measurements and tem- 

 perature and salinity corrections to the echo 

 soundings. The difference between the cor- 

 rected depths is the instrumental error. 



The instrument error of an echo sounder 

 shall be determined at the beginning of a 

 season, whenever the instrument is repaired 

 or changed by replacement of tubes or parts, 

 and whenever instability of operation is 

 suspected. The error should be redetermined 

 at intervals of about one month during the 

 season. 



The ship and each launch to be used in 

 sounding operations should make this test and 

 each depth recorder to be used during the 

 season should be tested. The information 

 thus obtained should disclose instrumental 

 errors such as those mentioned in 5-105. 

 It must not be assumed that the character- 

 istics of the echo sounders determined by 

 this test will remain constant over a long 

 period of time. Similar tests or vertical 

 cast comparisons shall be made whenever a 

 change in the instrumental error is suspected. 



5-108 Settlement and squat. — Although 

 an echo-sounding instrument correctly regis- 

 ters the depth from a vessel when stopped, 



there is no assurance that the correct depth 

 will be registered when the vessel is under- 

 way. This is because a point on the vessel 

 may experience a vertical displacement when 

 the vessel is underway, relative to its posi- 

 tion 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 war- 

 rant compensation, 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 displacement 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, there- 

 fore, 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 ordinarily 

 used in surveying, squat manifests itself in 

 a lowering of the vessel's stem and a rise 

 of the bow. 



The major factors which influence settle- 

 ment 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, set- 

 tlement may be quite appreciable 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, in- 

 cluding 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 



