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Thermal cycles of underwater wet welds by gravity arc welding process were measured. Then the effects of cooling rate on occurrence of underbead cracks and on the mechanical properties of welds were investigated. Results obtained are summarized as follows : 1) When the shielding processes are used, the cooling rate and the hardness of underwater welds can be lowered. 2) Empirical formulas to estimate the cooling rate at 500℃ (R), the cooling times from 800℃ to 500℃ and to 300℃ (S_<500>, S_<300>) of underwater welds are given by : R=6.25×10^5αQ^<-0.95>t^β(℃/s) S_<500>=455 a R^<-1.09>(s) S_<300>=780 b R^<-1.09>(s) where Q : weld heat input (J/cm), t : thickness of base metal (mm), and α, β, a and b are constants given as follows : non-shielding : : α=1.0, β=0.17, a=1, b=1 refractory shielding : α=0.28, β=0.45, a=1, b=1 oil putty shielding : α=0.036, β=1.0, a=0.67, b=0.08 (3) Underbead cracks occurring in underwater welds of SM50A steel less than 9 mm in thickness may be prevented by holding the cooling rate at 500℃ below 35℃/s and the maximum hardness of the weld at less than Hv360 approximately. Such low cooling rate can be obtained by oil putty shielding. (4) The welded joints obtained by oil putty shielding have nearly comparable mechanical properties with those of open air welds.