How was width of ships decided

Shipbuilding is an imprecise science. There are certain ratios which work, and that is about all.The determinants of width (beam) are the purpose(s) for which the ship is intended. A warship neded a narrow beam for speed and manoeuvrability. A cargo ship or troop/horse transport obviously has to have a broad beam to fit in the intended cargo. Then the speed required comes into play – having got the width, streamlining requires a certain length to width. Then there is draught – does it have to be shallow draught to negotiate shallow waters, and even be drawn up on the beach, or is it to operate in deeper waters and anchor off the beach/bank. However the deeper the keel, the generally more stable the ship, and the more sail it can carry.Ancient warships were impelled by both oars and sail. Oars were used in action for manoeuvrability, and sails (supplemented by oars) for transit. To keep the vessel narrow and speedy, triremes had the oarsmen staggered over the side in outriggers. They were also flat bottomed to allow them to be drawn up on the beach overnight and defended from attack if a naval engagement was refused.There was also the matter of overland mobility. In the days before the Corinth Canal, both cargo and war ships were portered between the Aegean Sea and Corinth Canal/Adriatic Sea on rollers across the isthmus.Similarly the Vikings portered their ships across the low hills between the Baltic riverheads into the Black and Caspian Sea rivers, and so were able to go as far afield as Baghdad and Constantinople. The first rulers of a partly-unified Russia were Vikings.