The Ebro Delta: A Logic Puzzle for Travelers

This week, we have a cool logic puzzle that can be easily inserted into any ocean journey! Failing to solve it has memorable (but not catastrophic) consequences. And at the end, we have a way to turn the logic puzzle into the start of an easy-to-improvise adventure!

Noted sailor, writer, and delightful liar Tristan Jones (the man for whom I was named), says the following about the area around the mouth of the Ebro river in Spain:

“Navigation thereabouts can be difficult owing to night mists, which can be expected during the winter, and also to the confusion that arises because three lighthouses, each on different sides of the delta, show simultaneously. Two of them, Port Farangar and Cap Baña, show exactly the same light (three flashes white every fifteen seconds), and unless one stays well out in the offing or is extremely careful, it is easy to be sitting smartly in the delta mud.”

Now, while Jones is not the most trustworthy source, let’s take him at his word and reconstruct the situation he describes.

It’s late at night and you’re on deck – you can’t sleep. Fog lies heavy upon the sea’s surface, so thick you cannot see the coast, though you can still see three lighthouses flashing away in the dark. You notice the helmsman is asleep at his post, an empty bottle at his feet. The fog is so thick you could be almost aground and you wouldn’t even know it. It’s up to you to determine whether you need an immediate course correction to save the ship.

You see three lights. One, which flashes white every five seconds, is off your 9 o’clock. A second, also flashing white every five seconds, is a hair to the left of your 12 o’clock. And a third, this one flashing yellow every ten seconds, is off your 1 o’clock. Is the ship in danger?

(Don’t scroll further unless you want to reveal the solution)









The answer is yes! The ship is east and slightly south of the southern white lighthouse. Unless a course change is made immediately, it will run aground. Fortunately, shifting just a few degrees to starboard will remove the immediate danger.

So what if the party’s ship runs aground? Running aground in mud is not the end of the world. You just have to wait for the tide to come in enough to float the ship off, then pole backwards far enough that you can sail away. While waiting for the tide to come in (which could take anywhere from an hour to 23 hours – it’s the GM’s call), an unfortunate situation or two could befall the ship.

The obvious answer is a combat encounter. The area’s monsters could have learned that the fog sometimes yields beached ships full of tasty sailors. When the mists roll in, the delta’s lighthouse keepers lock themselves inside and won’t come out for any reason. They know that on these nights, everything that lurks in the lightless corners of this land comes out to patrol the beach, waiting for ships to run aground. Heck, in a fantasy game, you could even have one of these monsters call up the fog as a way to trick unwary mariners.

Alternately, you can handle this as a roleplaying encounter! Wreckers – folks who supplement their income by salvaging goods from shipwrecks – may come out and offer their assistance. They may be heavily-armed, outnumber the ship’s crew, and insist upon helping – in exchange for a sizable fee. Defusing the situation will likely fall to the PCs.

As a bonus, here’s two variants of the puzzle!

The first puzzle works best if the PCs are traveling in a direction where they’re keeping the coast on their left. That could be northbound on a landmass’ eastern coast, westbound on a northern coast, southbound on a western coast, or eastbound on a southern coast. But what if the PCs are traveling with the coast on their right? Just switch the observation to this:

You see three lights. One, which flashes white every five seconds, is off your 3 o’clock. A second, also flashing white every five seconds, is a hair to the right of your 12 o’clock. And a third, this one flashing yellow every ten seconds, is off your 10 o’clock. Is the ship in danger?

Finally, you can set up an adventure where the PCs are being dispatched by sea to investigate the cause of a rash of ship disappearances along a particular route. The reason for the disappearances is the wreckers! They’re secretly changing the lights on foggy nights to cause ships to run aground. Then they’re looting the ships, and maybe even killing or enslaving the crew.

The chart that the PCs and their crew are using still looks like this:

But the wreckers swap two of the lights on foggy nights so the actual region looks like this:

The bearings the sailors shoot against those lights won’t make any sense relative to their charts. The captain shrugs and asks the PCs whether he should change course, and if so, in which direction. If the PCs manage to pass the delta safely, they’ll make it to their destination without discovering the cause of the disappeared ships. If they turn around and sail the same route from the other direction, they’ll pass the delta on another foggy night, but this time, the lights look like this:

How do they respond?

Whether the PCs run aground or become curious and investigate the lighthouses, once they come ashore, they’ll find evidence of what’s going on: wrecked ships, the lighthouses broken into, and probably some wreckers skulking about. In the wreckers’ village, the party will likely find the survivors of the shipwrecks, caged to sell into slavery at some later point. Note that the wreckers are in this nasty business purely for material gain. They may even be desperate, having lost this year’s crops to blight, and having wrecking as their only known alternative. While violence can certainly solve this problem, the PCs may be able to find another way to resolve the situation.

The images of the seacoast around the Ebro River delta are modified from data provided by the Open Sea Map project and are released under a Creative Commons Attribution-Sharealike 2.0 Generic license.


Everyone needs content for their RPG campaigns: adventure hooks, puzzles, NPCs, political machinations, combat encounters, and adventure sites. That’s what this site provides! I draw RPG content from real-life fact and folklore, then give advice on how to adapt it to your fictional campaign. I believe content that is grounded in reality (however fantastical) is richer and more vibrant, and your players will appreciate the difference.


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