Build and design your own ship out of ordinary (or modded!) Minecraft blocks, place a Ship Helm, shift + right-click, assemble, and watch your creation suddenly infuse with physics. No longer are your Minecraft builds bound to remain forever static, sitting in the same place for all eternity.
Valkyrien Skies is the library mod that adds high performance physics to Minecraft. Many add-ons in addition to Eureka are already in development, which will all use the physics that Valkyrien Skies provides in different ways.
Valkyrien Skies is in ALPHA. Make sure to back up your world. There will be bugs.