How To Keep The Fridge Cool?

Highlandman

We usually use the short Channel crossings or the two-hour Irish Sea crossing but this time we're going to be using the Hull to Zeebrugge ferry route later this month for the first time and it's just occurred to me that the fridge may defrost during the
14 hour crossing. The problem is compounded because we arrive on the continent on Monday 20th, which is Whitsun and many of the shops will be closed.

Does anyone have any good ideas for keeping a small supply of food cool on the ferry crossing?