 The boats used to transport coal were short and narrow , and each contained a row of boxes used for carrying coal . Each box had a base of two halves , hinged and held closed with chains . These boxes would be lifted out of the boats , positioned by crane over a bunker or cart and emptied by releasing the chains on the base . This design helped keep the canal competitive , as it increased the speed with which loading and unloading of the boats could be performed . 
