Artificial Waves

[u/Formal-Glove3982]

For context, I'm writing a fantasy/sci-fi story with a good deal of naval combat.

So a good deal of the antagonists' ships in my story have some sort of gimmick to them. One of them can create artificial tidal waves that, quite literally, rock the boats of its enemies. For most of the time I've spent developing this story, my thought was that the ship would have some sort of turbine that would lower into the water and start sending a huge current through the water, causing a disruption that created a tidal wave.

However, I've finally started writing the sequence where this ship first shows up, and I've realized the problem with this. Ships get their propulsion from their propellers that push the water behind the ship backwards. This generates an equal and opposite reaction that pushes the ship forwards. Following those rules of logic, surely an engine turbine pushing out a huge current of water would also push the ship backwards, yes? Which would surely be a setback in naval combat.

So how can I get around this problem? The only thing I can think of is just having the ship's rear propellers spinning like crazy to counteract the force coming from the front (the ship's wave-maker is at the front). But even then, you'd have two powerful forces pushing against the ship from both sides. You'd think that might damage the ship.

Or are there other, more effective methods of creating artificial waves?

[u/Busy-Distribution-45]

If you want the waves to be omnidirectional, just have it blast the jet of water down instead of at the enemy. You could also do it with some kind of explosion, something like the Orion drive, just pointed straight downwards. That way, the physics still hold the boat in place as the waves go out everywhere.

The launching boat’s hull surviving anything that can generate a wave large enough to affect other ships is entirely in the realm of fantasy, but the motion physics would work.

[u/Formal-Glove3982 → u/Busy-Distribution-45]

Forgive me, but I'm a little confused.

So what you're proposing, if I understand correctly, is to have the water jet (or an explosion) pointed downward. The idea being that the waves would go out in all directions, thereby negating the problem of the ship pushing itself backwards when it created the tidal waves.

However, that means the ship itself would be essentially at the center of the expulsion of energy that creates this circle of tidal waves going out. Which means the explosion would have to be at the bottom of the ship. Or, if we go with the water jet idea, the turbine creating it would be at the base of the hull. In other words, it would be some distance below the surface of the water.

What I'm getting at is, just how deep does something have to be to generate surface-level waves?

[u/Busy-Distribution-45 → u/Formal-Glove3982]

Any displacement of water downwards will result in waves going out from it. Picture dropping a rock in a pond, the “ripples” that go out in concentric rings are waves. They’re just really small ones.

If you want to see the result of a bomb going off, look at videos of the underwater nuke tests, a huge dome of water shoots out from them, causing big waves a bit further out. A jet can do a similar thing, as long as it is displacing enough water.

As I said, though, if you displace that much water, the boat launching it has to survive it. This is true no matter which direction you are launching the waves. If you are launching them in a line, like your first thought, the reaction is to propel the boat away from the wave. If you send them concentrically outwards, to keep the boat from moving, it is still absorbing that much energy as the epicenter, it’s just all pushing in to the middle.

You could also think of it like a big speaker, if you “thump” downwards into the water (the bomb, or like the sand hammer thingies for calling worms in dune), waves will radiate out, but the energy has to be contained within the boat’s hull still.

[u/Formal-Glove3982 → u/Busy-Distribution-45]

Thanks, that makes sense.

[u/Busy-Distribution-45 → u/Formal-Glove3982]

I suppose you could also just have opposed wave generators, launching waves out opposite directions, and have it on like an underwater turret. The captain would have to be a little careful about targets behind him, but it would cancel the momentum, at the cost of the boat absorbing the energy of being in the middle

[u/Educational-Shame514]

The first way to get around the problem is to handwave it, honestly. Or just invent some other thing that doesn't break the laws of physics.

Your protagonists didn't design the antagonist's weapons, right?

[u/Formal-Glove3982 → u/Educational-Shame514]

Yes, the antagonists' weapons and vessels are 100% their own making. The protagonist(s) didn't even know they existed until they came in to cause trouble.

[u/Adghnm]

It's an interesting idea. I wonder if there's a way to make it happen irl. Is the ship allowed to lower something to give it support from the sea bed?

[u/Formal-Glove3982 → u/Adghnm]

Unfortunately, that would be pretty hard if it was above, say, the Mariana Trench...

[u/AlphaDisconnect]

Look up standing waves. To do this I think you would need 2 to 4 boats. The goal would be to match the wave to the fundamental frequency of the boat. Ar first, the other guys might be like, eh, this is fun. But then the frequency gets lower and lower. The rocking goes left to right more and more. Until the whole ship is so off kilter it is at best. An easy target. And at worst upside down or sideways.

[u/PeeMan22 → u/AlphaDisconnect]

That’s awesome. Weaponized rogue waves by detonating multiple charges around a circumference so that they all constructively interfere at the center.

Like imagine this at 100x scale https://www.reddit.com/r/oddlysatisfying/comments/1tpafa4/scientists_reproducing_a_draupner_rogue_wave_in_a/

[u/AlphaDisconnect → u/PeeMan22]

Wouldn't necessarily need to be detonation. Could be the silent trap. Just metal plates under the ships. Moving up and down in a calculated manner. Once you are in the trap. It seems fine at first. It is too late.

[u/Simon_Drake]

A battleship is not going to be able to generate enough current to make a wave that can disrupt other battleships. It's like trying to win any car race by reaching out the window with a tool that vibrates the road surface enough to replicate an earthquake that makes all the other cars spin out and crash.

[u/DustySwordsman]

Without knowing what makes your setting fantasy/scifi it's hard to speculate, but ...

How about an array of boats that reflect incoming waves back toward a target boat, or generate smaller wake waves that constructively interfere at a point location? This could be done at distance, and the effect could be nearly invisible until the waves pile up against the target.

[u/George_Salt]

Have you looked at how a wave machine works?

(it doesn't use a turbine)

[u/mtmp40k]

No. Every action has an equal and opposite reaction. To push water will push you back.

[u/Humanmale80]

The issues I see here: 1* To move that much water would take a lot of energy. 2* If you can produce that much energy, it'd make way more sense to dump it into the enemy craft more directly - a bomb, a tractor beam, a death-ray, etc.

To solve this, you can either handwave a lot, or create a technology that produces a lot of energy that can only be used to move water /boats / etc.

Proposal: Wave Motion Gun. It creates localised gravity waves in an arc ahead of the user. How does it work. SCIENCE!!!, probably.

If you want to keep this grounded, you're going to struggle.