You have brought up a very good question. Most USACE survey vessels are doing single beam surveys, and many are using multibeam gear. The multibeam units are generally fixed with a device under the hull, or over the side. That poses no problems. However, some vessels are towing a towfish connected by heavy cable astern. They are usually a few feet above the bottom, but not always. These vessels will generally be running with the channel along it''s length as turning is extremely limited. NOAA does a lot of sidescan work with a towfish astern. Their surveys include areas outside channels so that charts can be undated. We use one too, but it''s used mostly to locate underwater obstructions. Most single beam surveys involve running survey lines back and forth across the channel and beyond the edge of the channel about 200 feet. Multibeam also involves running a fixed course, but along the length of the channel including edges, centerline, and a few in between. It also requires strict tolerance in regard to speed and bearings. The best and safest thing to do is call a survey vessel on Channel 13, and if no reply, Channel 16. That takes the guesswork out of maneuvering.