  We study analytically and numerical the growth rate of a crystal surface
growing by several screw dislocations. To describe several spiral steps we use
the revised level set method for spirals by the authors (Journal of Scientific
Computing 62, 831-874, 2015). We carefully compare our simulation results on
the growth rates with predictions in a classical paper by Burton et al. (Philos
Trans R Soc Lond Ser A Math Phys Sci 243,299-358, 1951). Then, we propose
improved estimates on the growth rate with several different configurations,
which are in agreement with our numerical simulations. The influence of
distribution of screw dislocations in a group on a line to the growth rate, and
the growth rate by a group including different rotational orientations of
spirals are also studied in this paper.
