[snip]
I think people don't understand my idea of building vehicles and "engines"
I'll make some sort of diagram...
So I read the idea and don't dislike what you're getting at as a way of building large vehicles.
However, I still think that there should be a separate Cartwright's (now I remember the correct word) and Boatbuilder's workshops for making small vehicles and essential components for larger ones, along with restrictions on the use of metal in most vehicles. Metal armour plating on a seige tower would be an excellent idea, but an entire wagon built out of metal doesn't really work.
Constructing larger vehicles such as siege towers will be of limited use until players in fortress mode can do things off the map, though when Adventurers can build these things they'd probably be very valuable indeed.
Anyways, here's how I figure large vehicle construction should work, when it's all dealt with:
My idea for vehicle construction is, you designate a Vehicle Assembly (or whatever) and use a Slipway construction for building ships.
For vehicles:
In the Vehicle Assembly Zone, you construct wagons, siege towers or whatever using Wagon Parts and other resources. The resulting structure has a "Finish" action in the Building Menu, which sends it on its merry way. There should be some kind of "Design/Build" process for large vehicles, like the Architect-Mason/Carpenter system. Perhaps Engineers design and Cartwrights build. Anyway.
Wagons require 3 Wagon Parts per 3x3 square. Put it together, and that's what you get. Otherwise, you have a 3x3 square on which to put other items. Note that weight should be an important consideration; if the wagon isn't built from strong enough Parts, your 8-level siege tower will collapse as soon as you finish it. Otherwise, you can build walls, ladders, seige engines and the like as with any other vehicle.
Note that certain siege engines - anything over 1-2 z-levels - should require disassembly for transport to the battlefield, and be put together there by the Seige Engineers. On the other hand you should be able to put a ballista on the back of a wagon, haul it there and just start plugging away, provided you bring another wagonload of ammo.
Ships, obviously, should require a more complicated system involving a slipway or drydock. Perhaps allow a ship to be two tiles wider than its Slipway rail, meaning a 3x? ship would require a one-square-wide rail, while a 2-square-wide slipway allows a 4x? ship, etc. All ships must have a Bow, a Keel and a Stern, and need a Large Oar or Rudder (more complicated Sternpiece) for steering. Oars or a mast/sail setup should be needed to move the ship around. I don't really see mechanical screws as being in-genre at all. Ships should have stability limits on their height above the waterline. Otherwise I'm not so sure.
Another thing that would be useful would be some kind of light wall structure, as currently a walled 3x3 construction only lets you have 1 square free. While thick walls are fine for dwarven architecture, being able to designate an interior with flimsy walls around the sides would make much more vehicle potential, even though flimsy walls shouldn't be able to support more than a canvas roof. Perhaps call it "plates" (for metal) or "planks" (for wood), and allow low-level building destroyers to break them down. It would also let us make windows more realistic.
Another thing that would be fun to include is some kind of mounted weapon in between siege engines and crossbows. Call it an Arbalest and make it work like a heavier but slower-firing crossbow, best deployed on a weapon mount. This would allow the "dwarven tank" idea people keep flinging around without breaking genre, since you could put a bunch of these on a wagon and tow it into battle, use it to defend a ship against sea monsters, etc. If ridable elephants get put in, putting a Howdah on top of a war-elephant with an arbalestier would be perfect. (Alternately this would make a fantastic human/goblin attack unit).
also, again for the army arc but relevent here, Barding should be available for animals as well. Dwarves don't really have much of a reputation as mounted combatants, but it should be available to buy from caravans at the very least and definitely there in Adventure Mode. Specialising it per animal may be difficult, but if animals work in particular "types" (donkeys, horses etc; elephants; giant bats and flying creatures (leather only); jabberers) it could be slightly usable anyway. And somebody should work out how the whole War Horse (ridable) and War Dog (just goes and bites the enemy) thing should work, too.
Finally, Toady has mentioned many times that he doesn't want steam engines. Vehicles really shouldn't be steam-powered, at all. Flamthrowers and
Greek Goblin Fire would work just fine, but that's more a matter for the Siege Engines Megathread, I guess.