Here's what I ended up sending my group. Thank you, everyone who helped!
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On the docks of Oceanus, the grizzled aasimar stares at you with undisguised interest. You've been sent to speak to Hothos the Shipwright by everyone you spoke to in Sigil, each of your contacts claiming that this is the man to talk to if you want to know about ship enchantment. "Hothos is the shipwright of the heavens," one weaselly merchant confided. "He's who the angels turn to when they need a ship made. Perhaps he'll give you advice."
So two portals and several bribes later, here you are. The air smells of the sweet scent of endless ocean and the smell of fresh-planed wood and the stink of tar. Through the window you can see the docks and the worldsea that twinkles in the golden sunlight behind them. You've explained what you want, and the charming old man draws noisily on his pipe as he considers.
"So ye've got a foldling boat, and ye've got an air keel, and ye want the two combined. But ye want the keel to fold, and ye want it to not plummet if it's dispelled, and ye want help steering it, and ye're not sure the ship's big enough for ye in the first place." He raises his eyebrows. "Ye've got a few different options. Let's us take one thing at a time."
"Ye'll want to use the magical airkeel as yer central means of levitation. We can do that, and make it fold along wi' the boat.
"It's also possible to add additional levitating supports on to it. This will ensure that a single dispel magic won't disable the entire ship, as the support beams will be able to keep the ship in the air long enough for the main keel to recover. We can possibly tie them in with the rudder. If we do, it'll give the ship the equivalent of rowers; nowhere near as fast as sails, but still speedier than being becalmed if the wind dies on ye. If ye'd rather not spend the money, you can make the boat immune to one dispel magic for a small amount, effectively working a ring of counterspells into the figurehead. That's only good once before needing recharging by casting the spell yourself, though.
"If'n ye want privacy, that's doable as well. We can make the boat invisible to scrying. I've looked into the idea of enlarging a folding boat before. Unfortunately, adding a large extradimensional space into the middle is a steep 90,000 gp. For less money we can simply allow the boat to hold items even when it's folded. It's also much less expensive to actually add an extra form to the folding ship. If'n we add a sloop as a third form and extend the enchantment, you can get the use of three different sizes.
"Here's a breakdown of the costs involved. If'n one of you with magical powers wants to stay and help during the construction, I'll give you 5% off. Ye can't do all the enchanting yerself, though, as I've got me trade secrets. Each o' my ships is the finest craftsmanship, guaranteed by me word of honor and me best labor." He smiles warmly at you, relights his pipe, and lets you decide.
Summary of costs:
Fee to attach air keel and enchant keel to fold along with current ship: 7,500 gp
- OR -
Fee to attach air keel AND make ship larger by adding a 50' sloop form: 28,000 gp
AND:
Add additional levitation beams (adds stability in storms, adds forward motion as per rowers, much more difficult to "sink" with a dispel magic): 12,000 gp
Make the boat immune to one dispel magic (before needing recharging): 4,000 gp
Make the boat immune to scrying (Mordenkainen's Private Sanctum active when boat is unfolded): 30,000 gp
Add a small extradimensional enchantment to the inside of the boat, allowing it to hold items when it folds but adding no extra room: 15,000
Camoflage (continuous illusion of clouds around boat): 30,000 gp
AND:
One-time-use featherfall devices: 100 gp each.
Wand of fly: 5,625 gp.
Sailor's Wheel (slotless, +7 to craft: sailor, useful for actually sailing): 9,800 gp
Navigator's Compass (glove slot while being used, +8 knowledge: navigation; essential for steering): 6,400 gp
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Possible costs totals:
28,000 Add sloop and air keel
12,000 Add levitation beams
4,000 Counterspell one dispel magic
0 No immunity to scrying (assumes Agar will cast spell daily himself)
15,000 Let boat hold items when folded
30,000 Cloud camoflage
2,000 20 featherfall life preservers
9,800 Sailor's Wheel (needed for sailing ship)
6,400 compass (needed to go the right direction)
Less 5% Someone helps
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101,840
