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Our group is 5th level: one wizard, one barbarian and and one warlock (old_one/tome). We got one ring of swimming only, and desperately need to find a sunken ship under the moonsea. The barb can swim pretty well, but wiz and lock are helpless without the ring. Lock can see in darkness, so he will be doing most of the searching for the shipwreck.

So we thought of using a leaded canoe upside down for air, and then doing the following
(I'll use the acronym LTH to refer to the spell):

  • Wiz dons the ring, then he and barb dive from the shore and carry the canoe. Canoe should be full of air. Use it for breathing sporadically.

  • Wiz and barb (plus canoe) reach a reasonable distance under the sea. Wiz pops head and spellbook in the canoe air pocket and casts LTH as a ritual, barb at his side.

  • Barb gets the ring, swims back to shore, gives ring to lock. Wiz remains in the LTH with canoe. Its dry inside, so he can breathe.

  • Lock dons ring, lock and barb swim up to the LTH. Barb goes inside and gets canoe out so lock can breathe. Refresh canoe's air.

  • Lock and barb go further away, Lock gets in air pocket uses his tome to cast LTH, barb at his side.

  • Barb gets ring and canoe, goes back to the previous LTH to bring mage over.

  • Wizard uses canoe air pocket to cast LTH. Rinse repeat.

Should it work?

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    \$\begingroup\$ I edited your question so that the link would point to a non copyright-infinging website, one that utilizes the SRD. The Stack frowns on sites that republish rulebooks without permission. :) Creative plan, BTW. I look forward to the answers. \$\endgroup\$ – keithcurtis Sep 20 '17 at 3:53
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    \$\begingroup\$ From a real world standpoint, this sounds like a ready-made recipe for lung ruptures and/or nitrogen emboli (aka "the bends"). Hint: the air under the canoe will be at local water pressure when they inhale it. \$\endgroup\$ – Zeiss Ikon Sep 20 '17 at 14:16
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    \$\begingroup\$ Be sure to ask your DM how waterproof that spellbook is before taking it underwater. \$\endgroup\$ – GMJoe Sep 21 '17 at 9:33
  • \$\begingroup\$ @Zeiss: If you are wanting to bring all of that in we shouldn't forget oxygen toxicity if going deeper than about 50m with standard air (en.wikipedia.org/wiki/Oxygen_toxicity). And of course there is Nitrogen Narcosis (en.wikipedia.org/wiki/Nitrogen_narcosis) which is often described as feeling drunk when at depths of ~40m or more... I suspect these are beyond the scope of your average roleplaying game though. :) Most people don't care that much about the technicalities of diving. :) \$\endgroup\$ – Chris Sep 21 '17 at 10:36
  • \$\begingroup\$ @Chris Yes, I wasn't sure of the depth anyway -- you can get lung ruptures in a swimming pool, and bends start at 10m if you stay down long enough breathing pressurized air. \$\endgroup\$ – Zeiss Ikon Sep 21 '17 at 11:03
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Ask Your DM. You have a number of questions here which don't have an official answer in the rulebook:

  • How hard is it to put a bunch of lead on a canoe? How much does that cost? Can you carry the canoe when it's not underwater and full of air?
  • How much air can you bring underwater, if you put it in a canoe and apply lead to balance the buoyancy?
  • Can you breathe from that air long enough to walk a distance undersea and then ritual-cast Tiny Hut?
  • How hard is it to breathe out of a small margin of air in the top of an inverted canoe?
  • How hard is it to keep the canoe stable, so that the air doesn't spill out? If the air does spill out, how hard is it to get the suddenly-extremely-heavy canoe back to an air source?
  • Are you planning to exhale into the canoe air, making it full of carbon dioxide and bad to breathe? Or are you planning to exhale over the side of the canoe, so you get less and less air in the canoe over time and it gets less and less buoyant?
  • Leomund's Tiny Hut says it stays dry "regardless of weather" but it doesn't say that it "generates air if you cast it underwater". It sounds like you're reading "underwater" as a type of weather, and you're arguing that in order to stay dry the spell must be capable of creating breathable air. Does this work?
  • Can the wizard's spellbook or warlock's tome tolerate being swum around with and used to spellcast in a canoe?
  • If you get attacked while carrying this canoe around, are you basically just screwed, or what?

We could give you our best guesses at the answers (mostly, this seems unlikely to work very well), but these are the sorts of questions where your DM has to make a ruling, and anything we say would get overruled by your DM anyway.

(We can do some math regarding breathing: humans breathe "5-8 liters" of air per minute when not moving, or "12 liters" of air for light exertion. Multiply by two people, and assume you want to travel nine minutes undersea before spending 11 minutes casting the spell, so you need at least 400 liters of air. 400 liters of air displaces 900 pounds of water, so your canoe needs to weigh at least that much to balance it out.)

When your DM makes a ruling, they will consider a number of questions, including (1) whether this is practical, (2) whether it will advance the story, and (3) whether there's some other way you could search for this shipwreck that you should be doing instead.

There are easier ways to do this. There are lots of spells that would work better than Leomund's Tiny Hut for exploring underwater. In particular, please consider buying, learning, and casting Water Breathing which is the same level as Tiny Hut. Alter Self is also good.

When your DM asked you to search for a shipwreck, the method they probably had in mind was that you'd just take a boat out on the sea and go diving periodically. (Or perhaps you'd make deals with some merfolk for the location?)

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  • \$\begingroup\$ A note to your gas consumption calculation - at a depth of ten metres the air will be at twice the pressure so will be half the volume so your 400 litres on the surface is 200 litres at 10m, or 100 litres at 30m. Of course if you are breathing back into the canoe this isn't as much of a problem (you just have the CO2 buildup to worry about) but if breathing out of the canoe then you will end up going through your air much quicker than your calculations reckon (assuming vaguely real world physics). \$\endgroup\$ – Chris Sep 21 '17 at 10:42
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No

It fails at the first step unless your canoe weighs about 1 tonne for each cubic meter of air or your PCs can provide an equivalent downward force. This is why ships (and submarines) float.

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    \$\begingroup\$ The force won't need to increase as you go deeper. If anything it will reduce as the air is compressed by the pressure. (buoyant force is equal to weight of water displaced - the weight of water displaced will go down as you go lower (due to the fact air is compressible but water isn't)) \$\endgroup\$ – Chris Sep 20 '17 at 8:30
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    \$\begingroup\$ Can you comment on the viability of the rest of it, presuming a sufficient mechanism is found instead of the canoe? That part seems relatively trivial to hang up the question on and the meat of the question is further into the process. \$\endgroup\$ – doppelgreener Sep 20 '17 at 8:44
  • \$\begingroup\$ C'mon, the only thing you had to say about it was to say "the canoe won't sink"? And, where is the proof it won't? I specifically mentioned a leaded canoe. Will I need a cubic meter of air? Or not? \$\endgroup\$ – Mindwin Sep 20 '17 at 9:13
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    \$\begingroup\$ Not that I disagree as I use a modicum of "reality" in my own games but to prevent the obvious, obligatory response... "Real world physics in a fantasy game? Pishaw! \$\endgroup\$ – Slagmoth Sep 20 '17 at 13:10
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The spell doesn't say anything about creating a stable platform in the spell as far as I can see so if it is full of air it will leave the inhabitants falling to the bottom and having to tread water in the part where the air and water meet. This will probably be not too bad until the wizard loses the ability to swim due to giving away his ring.

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No, but there's a way to make it work

As Dale stated, you have a lot of upward buoyant pressure to contend with in the form of the canoe, so you have to resolve that first before your plan can work.

Your plan to utilize lead to weigh down the canoe will certainly help, but it will make it very difficult to move. But you can further assist and mitigate the buoyancy issues by having another larger boat drop your team in the middle of the sea with a heavy anchor on your canoe and a lot of excess chain (you're in the hundreds of feet territory here). Using some pulleys, you can keep moving the canoe around as necessary for your LTH airlocks, but you will overall be limited by the length of chain you have from your anchor.

Personally, I don't like turning my tabletop experience into a physics calculation, so you guys could probably resolve the efficacy of the methodology by making it a skill challenge.

Otherwise, I see no reason why it shouldn't work.

Great idea, take inspiration!

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