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I am crafting a hybrid power/sailing airship that needs an exterior scene similar to swashbuckling on the main decks among the sails. Putting a mast on an airship with a big round envelope just isn’t making the visuals I need, so I imagined catamaran airship as opposed to a single envelope airship. This leaves a deck space between and beneath the pontoons with rigging, masts, and spars for my scene, all at dizzying heights. I have been considering that catamaran airships offer better stability and increased weight carrying capacity as a plus, but I am not how large (or small) the pontoons need to be to make usable masts and sails down the centerline. I assume each pontoon should be ½ the size of the equivalent standard airship envelope. This gives a lower and wider profile to the wind as well, for better or worse I don’t know. The goal is to have a good working main deck for a dozen or so pirates to parlay.

Environment

Note, this is obviously not earth. The lifting gas will be nitrogen, the air pressure is about 1,500 kPa. The climate has a constant West-to-East wind that the ship leverages for half of its journey. People/pirates work outside in environmental suits.

The Ship

This is a luxury liner for 250 passengers with all the basic amenities you may find on a 19th century steamer. Assume material weight requirements are about 1.5x of that required to build an equivalent zeppelin airliner. The ship lowers its boom or possibly even the whole mast when traveling under power to reduce drag. If you’re curious, [sailboats can use a drogue in place of a rudder][3] in emergencies. An airship drogue would be steerable itself when the ship is at a wind shear boundary and would serve as a rudder.

A bad idea, but would it work?

It’s OK if this design is ultimately a bad idea, humans have built many bad ideas. This ship is unique in my universe but plays a crucial role in the story. Right or wrong, certain people are zealots for the unique design and swear by it. Bottom line; for better of worse they built it, and here we are. It just needs to fly.

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    $\begingroup$ Catamarans (like any boat) work due to the density difference between water and air plus gravity. You're throwing both of those out. (a) Why on earth do you need a specific, engineering-quality answer? Nobody can prove you wrong and we'd need to design the whole ship to get the answer. (b) On water (dens A > dens B) a catamaran is essentially a 2D object. In your world (dens A = dens B), it must be a 3D object or it won't ever stay "upright" without counter-weights. Put the pontoons in a triangle around a central core that has the deck and sails. How far? As much as your story is wanting. $\endgroup$
    – JBH
    Sep 10, 2023 at 6:12
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    $\begingroup$ If the lifting gas is nitrogen, what is the air made of? Sulfur hexafluoride? And what is the use of the masts and sails? $\endgroup$
    – AlexP
    Sep 14, 2023 at 0:35
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    $\begingroup$ You lost me at "masted airship." This is a similar dynamic to trying to use sails to propel a submarine. Complete lack of interface between two mediums. You might want to look into the Honor Harrington series, where Weber designed spaceships to imitate the age of sails and cannons. Maybe you can find an excuse for stringing platforming targets between a pair of dirigibles. $\endgroup$ Sep 14, 2023 at 3:01
  • $\begingroup$ @RobertRapplean It’s for downwind trips (Eastward) $\endgroup$
    – Vogon Poet
    Sep 14, 2023 at 3:41
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    $\begingroup$ you don't need a sail to float downwind in a LTA craft, its literally what it will try to do anyway $\endgroup$
    – jk.
    Sep 14, 2023 at 9:29

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It would not work

Firstly, for sails to work you need a second force counteracting the wind. If not, all you can do is drift with the wind with no way of stearing. For ships this second force is the water acting on the keel. For soaring aircraft its the gravity (its "sails" are horizontal and it can only use upwards winds).

Neither works for your ship.

Additionally, your ship is in big risk of beeing flipped over by a wind gust, a heavy keel is not a good match for an airship and the catamaran design does not give you stability in the air.

How to (maybe) make it kind of work

So you need a second force. You can't push off of clouds. Idea: Drop a inverse hydrofoil shaped anchor down into the water that will provide lateral pull on the ship. Now your ship becomes kind of a kite on a line (with the other end beeing a kite under water).

You will need to balance the attachment points to not be flipped over and some way to trim and control the sea anchor.

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  • $\begingroup$ There is no water however considering that the atmosphere is 64kg/m^3, it is effectively light water. Thermal boundaries have cross currents which allow a drogue to do as you say. This is all designed into the world already, but I considered too tedious for my question. So, your sea anchor is the way I have already designed it, and the sails will likely be both top and bottom as I imagined it; or convertible as needed. A scene uses this in particular; antagonist kicks a latch and drops the boom for his escape. $\endgroup$
    – Vogon Poet
    Sep 14, 2023 at 14:22
  • $\begingroup$ I’m actually more concerned about the effects side-by-side envelopes would have on righting. I don’t know if buoyant objects like this tend to naturally have a horizontal or vertical orientation in the fluid. $\endgroup$
    – Vogon Poet
    Sep 14, 2023 at 14:30
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    $\begingroup$ Double hulls will give you virtually no righting moment in air (or under water). That only works when it sits on top of a medium that is substancially more dense than the air above. $\endgroup$
    – LazyLizard
    Sep 14, 2023 at 15:34
  • $\begingroup$ So would it more logically have two gondolas, each under a pontton, or a central gondola for ballasting? $\endgroup$
    – Vogon Poet
    Sep 14, 2023 at 15:40
  • $\begingroup$ You will need some kind of ballast, and also make sure any significant forces (like those on the sails) will be balanced, because the ballast can not be heavy enough to balance those out in an airship. Also note that a drogue will maybe give you a litte bit of maneuverability, but it will be terrible. You really want something like foils that you can manipulate to have less resistance in the direction you want to go. $\endgroup$
    – LazyLizard
    Sep 14, 2023 at 15:44

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