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In response to recent boardings, Earnslaw manager Angus Small said the ship had security measures to combat unlawful entry of the ship.
"It's a safety thing, especially when you have drunk people going on there and sometimes jumping off into the water," Mr Small said.
"Also, it's a 100-year-old boat made mostly out of wood, so it would be an unfortunate individual who lights a cigarette and then burns down the Earnslaw."
He said there were security sensors in and around the ship, with a siren to deter trespassers and alert security as well as CCTV cameras and an alarm system for the inside of the ship.
Police recently posted photos from the CCTV cameras on their Facebook page, asking for help from the public in identifying the trespassers.
One of the pictures shows a man trying to enter the locked section of the ship early on Sunday morning, still in St Patrick's Day regalia.
Mr Small said the number of intruders was not increasing, but when incidents such as this occurred he wanted to remind people it was against the law.
"If people want to go on the Earnslaw, by all means we welcome it, but they can pay for it and go on one of the scheduled excursions like everybody else."
The maximum penalty for unlawfully being on a ship is thought be the same as that for unlawfully being on premises - three months' jail or a $2000 fine.