a rudderless sailboat having a hydrodynamic symmetry fore and aft which can be sailed equally well in either direction . the craft is steered by tilting the mast and keel . in an alternative embodiment the sailboat also has no keel . the hulls are rigid and have a v - shaped bottom . while sailing forward , the sail can be rotated past the angle of zero angle - of - attack to that angle - of - attack where the wind is aback on the sail . this stops the boat and causes it to sail back in the general direction from which it came . this sailboat maneuver is called a tack aback .