[black-sapphire]

Hi there,
basically a side scan works by sending out high frequency pings and it measures the time taken for the signal to bounce back from the seabed, wreck, rock or whatever it hits. the towed array fish will have a built in depth gauge so it knows
what depth its at. basic ones will show a series of spikes on a monitor. more sophisicated models will have multiple transmitters and receivers set on a series of angled banks. the number of signals sent and received can be hundreds of times more than the basic fish finders widely available. due to the number of signals being pinged, the interval between each one is shorter so this means they all hit the object much closer together so they can give a very good image of the contours of that object which in turn means the "picture" they create will be much more difined and look like a picture instead of a spike.
Sonar sets on the hull of a vessel have a much more difficult time in making accurate pictures. the main reasons for this is that the sonar can end up being quite a distance from the object, water salinity and temperature at different depths poses more resistance for the signals and the angle/range ratio headache comes into play. if the set angle is to broad, the signals will travel too far out before going down. if shallow this dosen't really matter, in fact it will give good results. for deeper surveys the signals will disapate and become too weak to return. if the angle is set tighter the signals will reach depths much greater but the spread of the picture will be shorter. the search lanes will be much narrower, which means the the boat will have to do more sweeps closer together, which isn't very efficent. finally the machinery noise from the boat also can confuse it a little bit. Once a towed side scan fish is setup properly with correct angle ratios and deployed at the correct depths it will almost always be more effective than hull mounted sets, simply because they can get closer. and finally as the name suggests there will be a "deaf" zone directly beneath the fish. again though this can be adjusted for specific needs by setting the sonar unit up in different ways.
hope this helps a little.
safe diving jk
Forgot to add... the answer to your question on how hull sets can somtimes spot stuff towed array can't is because the hull mounted sonar set can be mounted in the centre line of the vessel and thus have no "deaf" zone, in my opinion though towed array is still much more effective.