In order to measure the deflection in long span dynamically, a new type deflection gage was designed. The gage is made up of eleven steel pipes placed in one row, ten connecting pieces (pickups) and two end pieces. The pickup contains four electric resistance strain gages and the output voltage is in proportion to the angle change between adjacent pipes. The sensitivity is 0.3 mv per 1/1, 000 radian. (0.06 degree) The accuracy of deflection over 66 ft. span is 1/50 in., and maximum measurable deflection is 10 in. in same span. Satisfactory results were obtained in experimental voyages of four wooden ships in rough sea, and the influence of hull deflection upon the bending of shafting was explained.