Why is the speed of a sail boat limited by the length of the hull?

Froude number.

See a great explanation on Wikipedia -- link below

Most non-naval architects use Hull Speed instead of Froude number (they are related). See: http://en.wikipedia.org/wiki/Hull_speed

The limit is caused by the size of the standing wave caused by the boat going through the water. This wave will eventually form the boat's wake. Once the standing wave gets reasonably close to the length of the boat, the resistance goes up considerably. The boat is always trying to go up the wave. It is hard to go up hill.

It is also possible to exceed the hull speed by getting a boat to "plane" on the surface. Small speedboats do this as do windsurfers. But most sailboats weigh too much and have too little power to accomplish this. Catamarans can also exceed the hull speed because they use to very narrowed beamed hulls. The narrow beam limits the size of the waves which allows them to exceed hull speed.

Consider the other factors same - i.e. wind direction, wind speed, weight, size of sails, viscosity of the sea water, density of water ... etc.

Generally, shape of the hull are very similar, so the longer a hull, the more hull surface is in contact with water - causing more drag. However, the physics is similar because of shapes and amount of surface area of hull.

Now consider a sky diver having a terminal velocity when falling through air. The higher the speed of the motion the more drag is created - until gravity cancels out drag. When both are at equilibrium, terminal velocity is reached. Object in motion remains in motion - at constant velocity.

In the case of sail boats, drag on hull surface is generated by movement of boat through the water. The higher the speed, the more drag the motion generates, eventually, the amount of drag cancels out the force of the wind powering the sails and the boat reaches terminal velocity.

An exception is with high power boats with jet foils. They are able to cut through the water at higher speed than regular boats, because the physics is different.

At certain speed, the foils are able to generate lift against gravity pulling at the boat. Drag above the foils is less than the drag under the foils, so a lift is generated to counter the weight. When the hull is lift completely off of the sea water so drag decreases. Jet foils act like wings under water. The lift generated by the foils cancels out the drag of the hull leaving smaller surface area of the foils.