  This paper describes the measurement of the W boson mass, M_W, and decay
width, Gamma_W, from the direct reconstruction of the invariant mass of its
decay products in W pair events collected at a mean centre-of-mass energy of
sqrt{s} = 172.12 GeV with the OPAL detector at LEP. Measurements of the W pair
production cross-section, the W decay branching fractions and properties of the
W decay final states are also described. A total of 120 candidate W^+W^- events
has been selected for an integrated luminosity of 10.36 pb^-1. The W^+W^-
production cross-section is measured to be sigma_WW = 12.3 +/- 1.3(stat.) +/-
0.3(syst.) pb, consistent with the Standard Model expectation. The W^+W^- ->
qq(bar) l nu and W^+W^- -> qq(bar)qq(bar) final states are used to obtain a
direct measurement of Gamma_W = 1.30^{+0.62}_{-0.55}(stat.) +/- 0.18(syst.)
GeV. Assuming the Standard Model relation between M_W and Gamma_W, the W boson
mass is measured to be M_W = 80.32 +/- 0.30(stat.) +/- 0.09(syst.) GeV. The
event properties of the fully-hadronic decays of W^+W^- events are compared to
those of the semi-leptonic decays. At the current level of precision there is
no evidence for effects of colour reconnection in the observables studied.
Combining data recorded by OPAL at sqrt{s} ~ 161-172 GeV, the W boson branching
fraction to hadrons is determined to be 69.8^{+3.0}_{-3.2}(stat.) +/-
0.7(syst.)%, consistent with the prediction of the Standard Model. The combined
mass measurement from direct reconstruction and from the W^+W^- production
cross-sections measured at sqrt{s} ~ 161 and sqrt{s} ~ 172 GeV is M_W = 80.35
+/- 0.24(stat.) +/- 0.07(syst.) GeV.
