One of the greatest secrets of the viking ships, next to it's flexible hull, was the way the sail was atached to the yard (cross beam). It allowed, with a fair amount of wind, to tilt the sail, so that it not just propelled the vessel forward, but lifted it out of the water... Thus gliding over the water, rather than plowing through it! This made speeds of up to 35 knots, or 40 mph, possible!!!