With respect to the forward looking sonars -- I have had one since 1996 (both on Insatiable I and II) and have used them extensively in our cruising. Ours are the Interphase "Probe" model which sweeps a beam from straight ahead at the surface to straight down towards the bottom. The beam width is nominally 12 degrees. We spend lots of time in poorly charted waters, some of which are pretty opaque, and I find the Probe to be useful enough that I bought one for I-2 before we hauled her for survey
at the time of purchase!
On the longest range it looks 1200 feet ahead of the boat. We have actually seen a few things out around 800 feet, things like a wall of coral
sticking nearly straight up from the floor of a lagoon
in ~200 foot depth
. In shallower waters the range is typically 3-4 times the depth
of the water
. So far this sounds pretty good, eh?
But, consider the physics of sonar: The instrument produces a sound pulse in the transducer, focused into a beam around 12 degrees wide. The pulse propagates out at the speed of sound (around 1480 m/s), and if it hits a suitable object, some of the sound energy is reflected back to the transducer. Meanwhile, the transducer has been patiently waiting about to see if it hears anything. It can't produce the next pulse of the sweep pattern until the appropriate time interval for a return at the maximum range has elapsed. On the longest range this delay is on the order of a second or so. Thus even in the "fast sweep" mode, it takes around 15 seconds for it to complete the sequence of straight ahead to straight down when attempting to detect distant floating objects. One can improve this somewhat by limiting the sweep pattern to a smaller arc
Now, add to this the narrow beam width. If the boat is yawing a bit as it passes through the seas, the beam is directed away from the COG some of the time. This means that there is a chance that when the "straight ahead" pulse from a sweep is generated, the boat may not be aimed at a floating object that is in fact on the course line... and so it isn't seen. This reduces one's confidence considerably!
Further, consider that if there is a sea running, the surface layer includes waves, foam, turbulence etc, and any of these can generate a false signal reflected back to the transducer. This means that in typical sea conditions, there is a lot of "noise" in the return when looking ahead at shallow depths... the depths of interest in avoiding the dreaded container. The operational result of this is that the alarm
features of the Probe don't work very well at all. If you reduce the sensitivity enough to eliminate false alarms, it won't see much of anything at all! So, to be of any use in container avoidance, one would have to have an observer watching the display 24/7... not likely on our boat! (Or yours either, I reckon).
To sum it up, the forward looking sonar is a very useful tool for the cruiser who gets away from home waters, but it isn't a realistic means of avoiding small floating objects on a long term basis.
And, to add our little bit of data: we've now covered on the order of 125,000 miles in our cruising (between SF and the South Pacific) and have yet to see a floating container. I agree that there is a statistical possibility of striking one, but my personal evaluation says that there are much better things to worry about in our area of interest.
Oh... our boat (built of strip-plank composite) has not one but two crash bulkheads forward and one aft, features not often found in production boats. The degree of protection provided perhaps does not equal that of a steel hull
, but it does help us to relax a bit at sea.