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In my world, which is similar to that described in this answer, with suspended platforms between 5 and 40 km above the global sea, with 1 ATM at 25km altitude, with a similar gravity and scale height to Earth, so that the sea-level pressure is nearly 20 ATM, there are airships similar to that described in this answer, which are essentially wooden sailing ships that can fly with the assistance of magical keels.

The thing with these suspended platforms is that they have landscape on top of them, and those in a position that allows them to have liquid water (anything below about 27km) may have waterfalls that fall off the edge of the platform.

This makes me wonder... how far down might the water from a large, Niagra-sized or larger waterfall be dangerous to one of these square-rigged wooden flying ships and the crew on their open decks?

EDIT:

I see this as a problem of water dispersal. Air resistance will cause the water column to break apart into smaller droplets... but how far down would this large amount of water still be dangerous and not just a heavy shower?

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The question, IMO, is not one of height, but of mass flow rate

Niagara falls is pretty big - Google says 3,000+ tons of water flow over the lip every second. Granted that is for the entire falls.

Your typical wooden sailing ship has a displacement of between 250-300 tons.

The two things I would be more worried about is the weight of water that would be on the deck and the free surface effect.

To put into perspective - using the SS Normandie capsizing - that was a 20th century ship that capsized due to the weight of water being used to fight a fire. She was registered at 83,000 GRT with a 60,000 tonne displacement, yet sank due to the 6,000 tonnes of water that was onboard due to fighting the fire.

In short - it takes not a whole lot of water to generate sufficient list that goes past the point of no return and 'sinks' the ship.

If we assume for the sake of argument, that it's 10% of the displacement of the ships tonnage - that's only 20-50 tonnes in the case of your sailing ship.

The average flow rate for Horseshow falls is 2,400 M3 per second and it's 790 metres wide, which means for a 1 metre wide section of the falls, you're getting 3 cubic metres of water per second. 1 Cubic metre of water is exactly 1 tonne. Which means in a single second if 1 metre worth of ship goes under the falls, that's 3 tonnes of extra weight. Going back to our above estimates - that gives you between 7 and 10 seconds before you hit the amount of water needed to potentially 'capsize' the ship.

That said - the downward column of water would act exactly like a Downburst/Windshear event - which can be deadly even to modern airliners.

In short - Any sailing/flying vessel would probably make a point of giving any such waterfall a very wide berth.

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  • $\begingroup$ This world isn't Minecraft... water doesn't form a single, continuous stream going all the way down, it'll break up after a way, and disperse, and eventually become a heavy rain... but how far down? $\endgroup$
    – Monty Wild ♦
    Commented Oct 7, 2023 at 5:33
  • $\begingroup$ @MontyWild, Time to crack open Naiver-Stokes equations if you want better detail. For more approximate consider tall waterfalls, some with limited dispersion over hundreds of m. It would be reasonable to believe falls would be a threat to ships well in excess of 10Km. $\endgroup$
    – Gault Drakkor
    Commented Oct 11, 2023 at 1:09

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