 Construction of the Izumo @-@ class ships was awarded to the British shipbuilder Armstrong Whitworth of Elswick , the same shipyard that had built the earlier two Asama @-@ class armored cruisers of the " Six @-@ Six Fleet " . They were also designed by Sir Philip Watts , who took advantage of rapidly advancing boiler technology to substitute lighter Belleville boilers in lieu of the cylindrical boilers of the earlier ships and used the weight saved to increase the thickness of the protective deck and improve the hull structure . The increased number of boilers required an extra funnel , which became the primary means of distinguishing between the two classes . 
