Model of a Severn trow, the Alma, by A G Field. Trows were popular because their flat bottoms allowed them to rest on mudflats and riverbanks. Their masts and sails could be rigged or lowered to allow them under bridges.They enabled goods to move in and out of Bristol via rivers and canals, as far as Birmingham, Oxford, and the north of England. Rivers and canals were the motorways of their time, and often the fastest way of getting between major cities in the UK.. During the 1800s there were 700 trows on the Severn and the Alma,built in 1854, was one of the last two to use sails. Scale 1:48