This paper describes the measurement of the W boson mass, MWMW , and decay width, ΓWΓW , 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 s√=172.12s=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−W+W− events has been selected for an integrated luminosity of 10.36 pb −1−1 . The W+W−W+W− production cross-section is measured to be σWW=12.3±1.3(stat.)±0.3(syst.)σWW=12.3±1.3(stat.)±0.3(syst.) pb, consistent with the Standard Model expectation. The W+W−→qq¯ℓν¯ℓW+W−→qq¯ℓν¯ℓ and W+W−→qq¯¯¯qq¯¯¯W+W−→qq¯qq¯ final states are used to obtain a direct measurement of ΓW=1.30+0.70−0.55(stat.)±0.18(syst.)ΓW=1.30−0.55+0.70(stat.)±0.18(syst.) GeV. Assuming the Standard Model relation between MWMW and ΓWΓW , the W boson mass is measured to be MW=80.32±0.30(stat.)±0.09(syst.)MW=80.32±0.30(stat.)±0.09(syst.) GeV. The event properties of the fully-hadronic decays of W+W−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 s√∼161s∼161 –172 GeV, the W boson branching fraction to hadrons is determined to be 69.8+3.0−3.2(stat.)±0.7(syst.)69.8−3.2+3.0(stat.)±0.7(syst.) %, consistent with the prediction of the Standard Model. The combined mass measurement from direct reconstruction and from the W+W−W+W− production cross-sections measured at s√∼161s∼161 and s√∼172s∼172 GeV is MW=80.35±0.24(stat.)±0.07(syst.)MW=80.35±0.24(stat.)±0.07(syst.) GeV.