So, Bob, where is it you do your skiing and surfing? You seem to know quite a bit about these water sports, for one who is landlocked. yes surfers move on the board, this is what makes the board turn. If the rider stands still, the board runs straight (though this does not give points in a competition) And perhaps you did not notice that on water skiis, the riders feet are strapped down, no way to move them, and again shifting weight only results in manuvering the ski. I use these as examples because I know them(skiing since age 4, surfing since about age 8 - piloting ski boats, powerboats, sailboats since age 12) if you want to know what kind of forces occur in a system, there is nothing like sticking your face in it. BTW the forces applied to skiers and surfers are both internal and external, just like a 'self contained' boats. Gravity pulls on surfboards (at a rather constant rate), the surfer must push back, the rope of a water skiier pulls him along, he must pull back. An airboat prop pushes or pulls on the air, and the air pushes or pulls back. Newtons' second law - for every force, there is an equal, but opposet force.