Sailing the Menai Strait with Charts, ChatGPT, and a Healthy Dose of Caution

Sailing down the Menai Strait takes meticulous planning as here, timings aren’t just important, they’re critical. This narrow, winding stretch of water has claimed more than a dozen wrecks, from the training ship HMS Conway to Pwll Fanog, a medieval cargo vessel.

After chatting with the harbour master at Deganwy, we knew the trip would be “interesting.” Pete reached for his Admiralty charts, I consulted AI, and between us we plotted a plan. The most notorious part of the passage, The Swellies, lies between Menai Bridge and Britannia Bridge. Our final challenge was to reach Caernarfon harbour entrance before it closed at 11:50 am.

Asking AI for a Route Plan

I asked ChatGPT to create a route plan from Deganwy Marina to Caernarfon Marina. It returned a detailed set of timings and waypoints, which matched my own research — mostly.

AI’s “Ideal” Timing (summarised)

• Leave Deganwy: High Water Liverpool –3 hours

• Pass under Menai Bridge: At High Water Liverpool to catch slack water through the Swellies

• Caernarfon Bar: Arrive at half tide rising for safest entry

The hazards were correctly listed — Swellie Rock, Cribbin Rock, Stanley Rock — and the navigation advice was sound. But then came the sticking point: tide times.

The Tide Time Disagreement

For 6 August 2025, AI gave me:

• High Water Liverpool: 08:54 BST

• Slack Water Swellies: ~09:30–10:00 BST

But my Reeds Almanac clearly said 10:05 BST for High Water Liverpool.

When I challenged the discrepancy, AI suggested that Reeds might be listing UTC times, using a different Liverpool dock, or even containing a printing error. In other words… it politely told me I was wrong.

I wasn’t convinced. For an experienced sailor, this was a mild irritation. But for a beginner relying on AI alone, it could have been dangerous.

Trust, but Verify

I recalculated using my Reeds time of 10:05 BST. Pete and I agreed to slip at 06:45 — just as the sill at Deganwy dropped — and reached the Swellies at slack water (09:50–10:20). From there, the tide carried us all the way to Caernarfon Bar. We even had 30 minutes to spare before the sill closed, giving us a stress-free mooring.

Lessons from the Menai Strait

The Menai Strait lived up to its reputation: stunning scenery, exhilarating navigation, and plenty of hazards if you get it wrong.

This passage reinforced something I’ve said before — AI is best treated like an enthusiastic intern: it can be helpful, fast, and full of ideas, but it’s not the skipper. Always cross-check with official sources, your own charts, and local knowledge.

Navigating the Menai Straight

In our case, the combination of Pete’s seamanship, my research, and yes, even AI’s enthusiasm got us through safely — with time left for a celebratory cup of tea in Caernarfon.