Question about tides and sea caves

I used my friend Google, but he likes to use more jargon than I know what to do with. Here's what I'm working with...

I'm writing a YA fantasy novel set in medieval times (roughly the 1390s). My MC lives on an island known for its sea caves, and a fun way the island punishes its criminals is by chaining them to an anchor in the caves and letting high tide take care of them (I still need to research precisely how they would chain them down).

The idea is that my MC ends up chained in the caves herself at the end of the novel. Currently, I'm having an issue with the tides. I'm thinking I need a semi-diurnal tide that goes from high tide to low tide in about six hours to make the scene work. Does that make sense?

I've looked at different measurements of high/low tides and I think I just need to make sure she's unchained and out of there in a few hours. That way, she's not submerged in cold water for too long and her friends can assist her in an escape before the cave is completely flooded.

So is this plausible? I'm not sure I want to invest in the scene more if it's going to fall apart under scrutiny.

Nekko, she doesn't need the full six hours to escape. There just needs to be enough time for her friends to figure out that she's even been put there and give them time to regroup and find a way to get her. I'd say she could be in and out in a couple of hours easily.

Drachen, would it be better for there to be an entrance by land where they lower her into the cave? Or is that even less plausible? I'm modeling this cave after one I saw in a documentary, so I know it is possible for sea caves to be friendly to visitors--until a freak wave comes and kills them all, of course.

King Neptune said:

Where it it set? In the British Isles and along the west coast od Nprway there are enough small islands to have a fast or a slow tide.

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Technically, it is in a fictional kingdom on a fictional island, but I do picture it in the middle of the Irish Sea between Ireland and England.

Netz said:

I love that King Neptune has stepped into this thread.

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Lol, that was my first thought when I saw he'd replied.

Nekko said:

In a fictional world, almost anything goes as long as you've laid out the 'facts' believably.

In the real world, the tidal change along the east coast of Ireland isn't very dramatic, ranging from a change of 1-3 meters (or 3-9 feet). Now, 9 feet is certainly high enough to drown a person, but I suspect that the tide doesn't come rushing in, so your storyline would work fine.

For dramatic effect, you could have your coastline have narrow channels as Drachen said, where the tide rushes in with force (think of having one of those hose nozzles that can be adjusted to force the water out in a tiny stream. The same amount of water that normally travels through the hose is trying to push its way out of a narrower opening, so it comes out under higher pressure)
This would increase the pressure on her friends to get in and out quickly.

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As long as it doesn't sound totally implausible, I think I'll start nailing down the details and researching some of the difficult parts.

Thanks for the advice everybody (though if anybody has anything to add, let me know).

King Neptune said:

If the island is of suitable size, then think of it up the west coast of Scotland and just say that the tides are irregular, take different periods depending on how high they will be. because the channels between islands are too narrow for the water to rise all that fast. Unless the tides are a big part of the story, then you can say that she was put there at any point in the cycle, and give her an extra hour, if that is needed.

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Oops, just saw this after I posted my last reply. Thanks, that's very helpful!

blacbird said:

A major point about sea caves is that they are formed by wave erosion. Waves get funneled into them with a great deal of violence, and they are very dangerous places to be in for that reason alone. People have been killed by being trapped in sea caves and bashed against the rocks by waves. The tides along a coastline with sea caves wouldn't just be gentle rises and falls in sea level.

caw

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Thanks, I had worried about that. I think I have a way around it, but we'll see how it comes together.

Drachen Jager said:

If it's a fictional world, how many moons does it have and what size are they?

You could get away with a lot if you had multiple large moons.

Of course, realistically, you couldn't have multiple Luna-sized moons, but hey, "magic" right?

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I should add a few dozen extra moons just to make this one scene work. A different moon for every month in the year would fit nicely.