docs: def diff_speed(sw_dens=1.028, dens_gcm3=1.053, seal_length=300, seal_girth=200,
          Cd=0.09):
      '''Calculate terminal velocity of animal with a body size
  
      Args
      ----
      sw_dens: float
          Density of seawater (g/cm^3)
      dens_gcm3: float
          Density of animal (g/cm^3)
      seal_length: float
          Length of animal (cm)
      seal_girth: float
          Girth of animal (cm)
      Cd: float
          Drag coefficient of object in fluid, unitless
  
      Returns
      -------
      Vt: float
          Terminal velocity of animal with given body dimensions (m/s).
  
      References
      ----------
      Biuw, M., 2003. Blubber and buoyancy: monitoring the body condition of
      free-ranging seals using simple dive characteristics. Journal of
      Experimental Biology 206, 3405–3423. doi:10.1242/jeb.00583
  
      Vogel, S., 1994. Life in Moving Fluids: The Physical Biology of Flow.
      Princeton University Press.
      '''
      import numpy
  
      surf, vol = surf_vol(seal_length, seal_girth)
  
      Fb = buoyant_force(dens_gcm3, vol, sw_dens)
  
      x = 2 * (Fb/(Cd * sw_dens * (surf*1000)))
  
      if x >= 0:
          Vt = numpy.sqrt(x)
      else:
          Vt = -numpy.sqrt(-x)
  
      return Vt