addition to the obvious safety considerations, keeping the towfish away from 

 the props and wake, a task more difficult in shallow waters where towing depth 

 is limited, is desirable from a transducer performance standpoint. 



9. Bow deployment is advantageous for vessel maneuvering and precise 

 positioning of the fish relative to the object being surveyed. Midships 

 deployment minimizes undesirable vertical heaving of the towfish in seas 

 causing the boat to pitch; a boom extension may aid in getting the fish closer 

 to the structure being scanned while maintaining a safe clearance for the 

 hull. However, a disadvantage with midships deployment is that it may be dif- 

 ficult to avoid propeller noise that "blanks out" the record from the near- 

 side transducer facing the hull. 



10. Another option particularly well- suited to surveys of coastal 

 structures, such as jetties and breakwaters, is to deploy the towfish from a 

 truck-mounted crane, where the boom of the crane is extended out over the 

 water and the truck is driven onto the structure itself. Other platform pos- 

 sibilities for coastal structure inspection include helicopters and remotely 

 operated vehicle (ROV's). 



Operating Personnel 



11. Generally, it would be preferable to have one person working full- 

 time at the control unit, observing the display and making adjustments as 

 needed (note that with the modern digital image-correcting systems there is 

 little need to regulate settings once a survey is underway) , and a second per- 

 son responsible for launching and positioning of the towfish. Under some con- 

 ditions, the second person could have other responsibilities unrelated to the 

 sonar equipment; with the lightweight, flexible tow cable designed for shallow 

 tow depths, it is likely that the control unit operator could handle fish 

 positioning as well. However, a minimum total boat crew of two is 

 recommended. 



