I thought military radar would be able to see through the waves to a certain degree - obviously wrong there!
Wouldn't something semi metaliic stand out very strongly on radar, even in a horrible State 8 sea?
Unfortunately, radio waves are too attenuated by the medium of water to allow useful penetration from a RADAR detection point of view.
Semi metallic? I'm afraid it's the 64,000 dollar question. There are too many variables to come up with an answer anyone would feel comfortable with.
When we were out hunting Red October and its ilk, we had all manner of detection ranges given to us from a variety of sources. But the best way to catch the b****r was to literally test the water. We would drop in a bathythermal buoy. As the thermistor head descended it gave us a trace of the thermal profile of the water, locally. From this, once we had consumed our dairy cream sponges, we could establish a sound velocity profile.
If you want an accurate idea of the sort of track spacing to employ in "this" body of water, you would ideally lob something of comparable RADAR reflectivity and/or visual reflectivity into the water and do a RADAR and visual search for it. Of course, most maritime patrol aircraft don't carry conveniently-sized (for the launchers) objects in a variety of materials to experiment with in this way. Flotation devices on sonobuoys are not in dayglo colours.
One of our crew members had experimented with modifying a sonobuoy with a RADAR reflector. His idea had been that if he could convince the manufacturers to incorporate such a thing, then Nimrod Dry sensor operators (the above-water team) could draw some benefit from them (i.e. practicing RADAR homings) as well as we Wet sensor operators (the acoustics team) could derive from the data they were primarily designed to yield. Unfortunately, he was collared by the harbourmaster and almost sparked a live Search and Rescue incident when, in response to, "What the b****y hell are you doing?" he declared there was a buoy in the water! (NOTE: The phonetic impact of this ONLY works with the British pronunciation of "buoy".)
Track spacing in any search is one of those "what do you think" decisions. Might a droppable device, based on a sonobuoy, with a dayglo flotation device (as opposed to military green) help lookouts establish a realistic detection range? If the RADAR could pick it up, then you have a measured visual range, as opposed to a "guestimate". Calm seas are one thing. But in this case, the searchers are on a hiding to nothing. A measured visual range established like this also gives the rest of the world (including Prime Ministers, Presidents) an honest idea of what they are truly up against. Cost of such a device (plus delivery of course). Still peanuts by comparison with the overall operational cost!!