The Steam Packet is continuing investigations into potential new berthing ports.
The company is looking for an alternative back-up to the harbours it currently uses in case it doesn't have access for any reason.
It's been considering a port in Holyhead in North Wales - last month saw a successful trial dock there of the Ben-my-Chree.
Overnight on the 29th and 30th of June Manannan will be trialled - this won't affect any passenger services.
The company says it's 'prudent' to have an alternative should access to the landing stage in Liverpool or Heysham be restricted and to ensure contingencies are in place to maintain the vital sea links for passengers and freight customers.
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