The plan was hatched in the depths of winter: to spend a Sunday afternoon in May on a boat to mark our daughter’s birthday. Tate has a special smile that only comes out during boat trips, and a love of nautical adventures, so what better way to celebrate? At that point of peak greyness and coldness, it was impossible to imagine the days being longer, or warm enough that we wouldn’t need a million layers to keep out the chill. But gradually the days grew longer and brighter, and then the day of our voyage arrived. We drove out to the coast, had lunch of fish and chips (plus a few oysters), and then headed to the quay to board the X-Pilot, a small boat.

This was not the sort of trip we do in the Mediterranean, where we spend lazy hours diving and swimming from a boat, or looking for sea turtles. Instead we chugged out into the rather murkier waters of the Thames estuary, heading into Britain’s nautical past, and towards some unusual structures: the Red Sands Maunsell Forts. Soon their distinctive forms appeared on the horizon.

These eerie, abandoned structures look like something out of a dystopian sci-fi movie or video game. As our boat passed around and between them, a light mist descended, adding to the other-worldly mood. The walkways that once connected the forts have all now collapsed, and hang down like giant ladders.

Several groups of these forts were built during the second world war as anti-aircraft batteries, to defend the docks east of London from German bombers. Placed in the middle of the Thames estuary, they could hit aircraft that were out of range of batteries on the north and south banks. After the war, during the 1960s, the forts housed pirate radio stations for several years.

Today the Maunsell Forts are giant, rusting hulks that are occasionally used as film sets. You can see why: visiting them is like entering another world. And that was why Tate wanted to go on this particular boat trip. The sites of sea battles normally look like anonymous waves, but the forts are an unusual example of a nautical battlefield you can actually see. Not everyone’s cup of tea, perhaps, but just the sort of thing Tate loves.

On our way back we passed another relic of the second world war: the wreck of the SS Richard Montgomery, which sank while bringing munitions to Britain from America. There are still 1,500 tons of explosives on board the wreck, which sits inside an exclusion zone. Only the masts of the ship protrude above the water. Removing the explosives risks setting them off, which would send a wave up the Thames that could flood central London.

After our haunting encounters with the forts and the shipwreck, we warmed up with cups of tea and a tot of whiskey, before heading back to port. And yes, we did get to see Tate’s special boat-trip smile!

kirstin