  In the quest to develop viable designs for third-generation optical
interferometric gravitational-wave detectors (e.g., LIGO-III and EURO), one
strategy is to monitor the relative momentum or speed of the test-mass mirrors,
rather than monitoring their relative position. A previous paper analyzed a
straightforward but impractical design for a {\it speed-meter interferometer}
that accomplishes this. This paper describes some practical variants of
speed-meter interferometers. Like the original interferometric speed meter,
these designs {\it in principle} can beat the gravitational-wave standard
quantum limit (SQL) by an arbitrarily large amount, over an arbitrarily wide
range of frequencies. These variants essentially consist of a Michelson
interferometer plus an extra "sloshing" cavity that sends the signal back into
the interferometer with opposite phase shift, thereby cancelling the position
information and leaving a net phase shift proportional to the relative
velocity. {\it In practice}, the sensitivity of these variants will be limited
by the maximum light power $W_{\rm circ}$ circulating in the arm cavities that
the mirrors can support and by the leakage of vacuum into the optical train at
dissipation points. In the absence of dissipation and with a squeezed vacuum of
power squeeze factor ~ 0.1 inserted into the output port so as to keep the
circulating power down, the SQL can be beat by a factor 10 in power at all
frequencies below some chosen $f_{\rm opt}\simeq 100$ Hz, with $W_{\rm
circ}\simeq 800$ kW. Estimates are given of the amount by which vacuum leakage
at dissipation points will debilitate this sensitivity; these losses are 10% or
less over most of the frequency range of interest.
