First of all, the Royal Navy didn't use Elco boats, they were strictly American. The only Elcos in the area were the US boats in the Med. The British used their own Vospers.
Radar: US PT boats were equiped with radar starting sometime in 1944. It was used mainly to help locate big ships, which were their main targets.
Late in the war US boats had four depth charges mounted in place of the aft torpedo tubes. Their purpose was to discourage enemy destroyers from following them when they were spotted; yes, a depth charge going off under a destroyer's hull is just as bad as a mine. I don't know if they ever actually used them, but they had them just in case. If they tried to use one on a U-boat it would have to be just after it dived-they had no sound gear at all.
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