Why a lightship makes a poor mobile fleet decoy

Could a fleet bring a lightship to make itself appear closer to shore?

Not effectively, because lightships are too slow for fleet operations and their movement would reveal the deception.

The idea works only in theory because a lightship is designed to remain on station rather than manoeuvre with warships. It lacks the speed for rapid transit and sustained fleet operations. If the vessel moved in an attempt to shift the apparent coastline, observers would quickly recognize that the supposed fixed light was mobile. The deception would therefore impose a conspicuous slow unit on the fleet while offering little durable confusion. A successful deception must suit the operating characteristics of the platform carrying it.