Merchant Ships were not natively created. However, due to the deposits of tin in Cornwall, Britain continued to receive them throughout the fifth and sixth centuries.
Merchant Ships had simple designs, beginning with a keel that ran from one end of the ship to the other, ribs that ran from the top of the ship, down to the keel, and back up the other side to form the hull. Planks were used to seal up the sections in between. Each of them was interconnected with the planks above and below them and intersected the next plank. Sails gave the ship speeds up to 4 miles per hour, and a separate one at the bow of the ship allowed for steering.
Merchant ships were only designed for trade. They mainly held amphorae, baked clay jars, filled with wine, olive oil, and other items which were traded for items native to Britain. Chiefly this was tin, but also iron and other raw materials. Photo from http://www.freeweb.com