The designers intended that the navigation continue upstream along the Boyne to Trim where it could connect with the Royal Canal. The section from Navan to Trim was never built, and the Boyne Navigation remains disconnected from other inland waterways in Ireland. The Boyne Navigation Company began work on the lower section of the navigation from the sea lock at Oldbridge to Slane in 1748 and was completed in the 1760s. The upper section from Slane to Navan was completed in 1800. The main cargo on the navigation was grain and flour between the mills on the river and the port of Drogheda and coal in the other direction. At four places along the route the towpath switches from one side of the river to the other. Where this happened the horse would step onto the barge while it was poled across to the other side.