The flat boat was introduced a little later than the others. It was a rough strong boat with a perfectly flat bottom, and perpendicular sides, and covered throughout the whole length.[16] Being constructed to float with the current, they did not usually return after descending the river, though as early as the year 1789 they were in use for traveling up as well as down stream.[17] Burnet describes the flat boat as being "made of green oak plank, fastened by wooden pins to a frame of timber, and caulked with tow, or any other pliant substance that could be procured. Boats similarly constructed on the northern waters were then called arks, but on the western waters, they were denominated Kentucky boats. The materials of which they were composed were found to be of great utility in the construction of temporary buildings for safety, and for protection from the inclemency of the weather, after they had arrived at their destination,"[18] These boats were much used by emigrating families to transport themselves down the Ohio.[19] In the year 1794, four keel boats, carrying probably not more than twenty tons each, were supposed to be sufficient for the trade between Cincinnati and Pittsburg.[20] The boats were advertised as having "cover made proof against rifle or musket balls, and convenient port holes for firing out of. Each of the boats are armed with six pieces carrying a pound ball, also a number of good muskets, and amply supplied with plenty of ammunition." "Tables accurately calculated for the rates of freightage, for passengers, and carriage of letters to and from Cincinnati to Pittsburg ... may be seen on board each boat, and at the printing office in Cincinnati."[21] Previous to the year 1817 the number of keel boats on the Ohio had increased to about one hundred and fifty, of about thirty tons each, which made the voyage from Pittsburg to Louisville and back in two months, or about three such trips in the year.[22]