I have a question about the number of thwarts needed to control the shape and provide the required support for a 16 to 18 foot wood and canvas canoe. This length of canoe is ussually designed with three thwarts. However, I have seen pictures of these canoes (Chestnuts and Atkinson Travelers to name two) with either the bow or stern thwart removed; presumably for more room. How much strength is lost by removing a thwart? Will the shape of the canoe change without all the thwarts (assuming the seats are bolted in)?