The Tanuki's Mud-Boat (Drawing Up the Specs)

[Tier-Group 3, 10th Floor, Reception Room]

"This dungeon can only summon libraries and their fittings. Boats are used for mobile libraries too, but we can't get them in any number, and there are other constraints, such as the fact that this dungeon can't obtain fuel. So we've decided to build a 'tanuki's mud-boat' out of dungeon structure instead."

Marie laid out the background, though the Dungeon Master and the shipwright present already knew all of it.

"Because of the constraints on obtaining materials and fuel, propulsion will be battery-powered electric drive. Beyond that, the important parts of a ship's specifications are size and speed."

Mito, the shipwright, brought up several illustrations on the screen.

"So a sailing ship really is out of the question, then."

The Dungeon Master asked.

"Sailing takes skill, and a sailing ship can't pass under bridges. It would be possible to lower the mast, but there's a danger the maneuver wouldn't be done in time and the ship would strike the bridge. Both the bridges and the ships are dungeon structures, so they themselves won't be damaged by a mere collision, but a mast-lowering mechanism can't be made out of dungeon structure, so that part would be destroyed and the crew killed or injured."

There is a constraint that dungeon structures can only be made of things with a simple composition and shape. In the case of a ship, the hull itself can be made as a dungeon structure, but the engine and the like cannot.

"So, how big will the ship's 'size' be, roughly?"

"The bigger a ship is, the more cargo it can carry, but if it's too big it's wasteful, since there's no cargo to fill it. In the otherworld's Edo period, the sengokubune, the thousand-koku ship able to carry some 150 to 200 tons, was in wide use, so about that size, or a little larger, would be best."

"That sounds reasonable enough."

"But how do you load the cargo onto the ship? Hauling rice bales on your shoulders the way they did in the Edo period would be backbreaking, I'd think."

Marie asked.

"Since bookmobiles can be summoned, we strip the library portion off a truck-type bookmobile, use it as a truck, and load it onto the ship as it is. A container ship would be more efficient, but a crane would have to be provided at each port, and the cargo would have to be packed into containers, so it seems difficult for the time being."

Mito brought an image up on the screen: a hull roughly 40 m long and 10 m wide, shaped like a small ferry with the passenger-cabin section removed.

"For now, a small RO-RO ship is best. The drawbacks are that the center of gravity becomes high, and that the opening-and-closing mechanism of the ramp doors at the bow and stern is a weak point since it isn't dungeon structure, but cargo handling becomes easy."

"What about a ship that runs right up onto the beach, like in the movies? Then it could be operated even without a port, couldn't it?"

"That sort of ship becomes inefficient because of its shape. And there aren't many sandy beaches along Minuma's shores, so it wouldn't have much chance to shine."

"And the last one was speed, was it not."

"The output the motor needs is proportional to the cube of the speed. For that reason a ship is more efficient the slower it goes, but if it's too slow it can't reach the next town within the daylight hours. Night sailing is best avoided, at least at first."

"The post-towns are spaced 10 ri, that is, 40 km, apart, but judging from the map, the distance over water will run to about 60 km at the longest. Factoring in loading and unloading time, we'd need a speed of at least 10 km/h, I should think. And if we want to allow for going on to the port one stop farther when necessary, perhaps about 15 km/h."

"Eight knots leaves ample margin. If you asked for 11 knots, 20 km/h, the battery would end up weighing as much as an elephant. It can be handled by design, but the trouble is that a high-output motor can't be obtained as things stand."

"What happens at 15 km/h?"

"Using two 150 kW motors of the kind made for buses and medium trucks gives about 15 km/h. Assuming ten hours of continuous sailing, the battery capacity comes to 3 MWh, ten buses' worth, and the battery weighs 2.5 tons."

Naturally, since an automobile isn't running all the time, it gets by with a smaller battery than a ship.

"Even so, 2.5 tons, hm."

And even that is thanks to the otherworld's semiconductor storage batteries, which perform remarkably well; the large batteries used even for emergency power hold 10 kWh in something under 10 kg. A short-range passenger car carries two or three of these, a route bus around thirty.

"Then please build the first ship to those specifications. If there are problems, we'll revise the design."