To most people, sailing, from a distance, looks fairly relaxing; up close it tends to look like hard work. For an aspiring physicist, or armchair mathematician, however, it can be downright worrying.

The two (related) things which you can do in sail-powered craft - including on windsurfing boards, and most spectacularly in iceboats - which still disturb me slightly are sailing faster than the wind, and sailing in the opposite direction to the wind.

### How it works!

(See Conservation laws? below for a discussion of the situations where you can't do it, and an explanation of how all the energy and momentum stuff works.)

#### Trick #1: Curved sails and sideways planes

Spoons! [more to come...]

#### Trick #2: Keels, blades and other hidden tricks

[more to come...]

### Conservation laws?

Two of the most important concepts in physics are conservation of energy and conservation of momentum. Most people who have heard of these things seem to find the idea of what sailing craft do "feels wrong".

Specifically, it tends to seem like a craft should be able to go at a velocity anywhere between zero, the velocity of the surface, and v0, the wind velocity. Indeed, we can easily make up some situations where this is at least partly true:

1. Firstly, suppose the boat can only move in one dimension. [more to come...]
2. Next, suppose you can't interact with the water/ice. [more to come...]
3. Next, suppose the only interaction with the water is drag opposed to the direction of motion. [more to come...]

[more to come...]