How can I invoke the constructor of a Scala abstract type?

I'm trying to figure out how to invoke a constructor for a Scala abstract type:

class Journey(val length: Int)
class PlaneJourney(length: Int) extends Journey(length)
class BoatJourney(length: Int) extends Journey(length)

class Port[J <: Journey] {
  def startJourney: J = {
    new J(23) // error: class type required but J found
  }
}

Is this even feasible? I'm familiar with Scala manifests but I'm not clear how they could help here. Likewise I can't figure out how to do the same with a companion object's apply() constructor:

object Journey { def apply() = new Journey(0) }
object PlaneJourney { def apply() = new PlaneJourney(0) }
object BoatJourney { def apply() = new BoatJourney(0) }

class Port[J <: Journey] {
  def startJourney: J = {
    J() // error: not found: value J
  }
}

Any thoughts gratefully received!