That's to be expected I'd say. By far the dominant force to overcome driving is air-resistance, a large canoe and racks will have a very high coefficient of drag, and will add appreciable frontal-area when driving head-on. The situation is much worse however if there is a cross-wind, as the net direction of the wind would be somewhat side on, interacting with the large (17' long) surface area of the side of the boat.