My experience has been that you need to be on the surface, and have visual contact with at least
3 ships(IIRC, 3 merchant ships). Radar contact only won't do it, neither will visual contact on two ships and radar on one or more. This is a major failure on the devs part tha has never been corrected.
I'm trying to remember the convoy incident exactly, but failing to do so. But, it went something like this:
While on the surface, I am closing on a approaching convoy. Radar contact on a large convoy with 4 escorts. Try to send a contact report, no joy. Continue on intercept course with convoy, get visual contact on one destroyer. Try to send again, nothing. Now I get visual contact on two destroyers and one merchant. Nothing. The report would not send until I had visual contact on three merchant ships. It's possible it sent when I had visual on two merchants and two destroyer, but I doubt it. The number 3 sticks in my mind.
As this was happening I imagined to conversation going something like this.
USS Gar: USS Gar to COMSUBPAC. We have radar contact with a major Japanese convoy. We are proceeding to intercept at 4 engine speed.
COMSUBPAC: Um, negative Gar, break off contact. You're trying to sink an island.
USS Gar: Say again COMSUBPAC.
COMSUBPAC: Gar. You. Are. Trying. To. Sink. An. Island.
USS Gar: An island?
COMSUBPAC: An Island!
USS Gar: Well, the island is making 9 knots and is headed North-Northwest.
COMSUBPAC: AN ISLAND!
USS Gar: We can see it now. Your island has several smokestacks, is making 9 knots and is still headed North-Northwest.
COMSUBPAC: Well. We don't believe you! Nyah Nyah!
USS Gar: Visibility is improving. The island is being towed by a destroyer, has several smokestacks, is making 9 knots and is still headed North-Northwest.
COMSUBPAC: Not listening! Not listening!
USS Gar: Were almost there. The island has a destroyer as escort, is being towed by a destroyer, has several smokestacks, is making 9 knots and is still headed North-Northwest.
COMSUBPAC: ........
USS Gar: Got it! The island has 3 merchies docked, a destroyer as escort, is being towed by a destroyer, has several smokestacks, is making 9 knots and is still headed North-Northwest.
COMSUBPAC: OH! Why didn't you say so in the first place. That's a
CONVOY! Sink the bastards.
USS Gar: