sentences:  They were built in three slightly different sub-groups, the first sub-group, the 26-group, is distinguished by pennant numbers 26xx, and the second and third sub-groups, the 27-group, are distinguished by pennant numbers 27xx.The 26-group was of wood and non-ferrous metal composite construction and the 27-group was of all-wood construction.The third sub-group is distinguished by a prominent rubbing strake around the hull and slightly larger dimensions.