The Black Arts
Boat Speed……… a guide for the bewildered 

Always assuming that you have managed to set your sail on the spars perfectly for the wind speed (another Black Art), the diagram below is a traditional explanation of sail set, and fine as far as it goes.
It clearly shows that the closer to the mean wind that the boat points, the tighter the sail needs to be. As you turn away from the wind the sail needs to be let further out until the boom has reached approx 90 degrees to the hull, (the sail does not usually go any further forward as a boat other than a Laser, or similar, has shrouds to hold the mast up, which prevent this). 

To fully understand the diagram, a little further study is required. So here goes....

If you hold your rig vertically by the base of the mast, the sail will act rather like a flag, simply blowing in the wind. If you fix your mast vertically, and slowly move the boom towards the wind, the sail will begin to act like a sail.

Assuming that you have set the sail perfectly for your spars and the wind strength, there is just one angle to the wind where the sail is absolutely perfectly aligned to the wind, to produce maximum power.

This perfect angle is achieved by letting the sail out until the front edge, the luff, just flutters, then pulling the sail in until it just does not. As the wind is constantly changing direction, even in a ‘steady’ wind, this adjustment is almost continuous.

Currently, you will almost certainly be sailing with your rig too tight.

In very simple terms, because the sail is a curve, then the airflow will have to travel a little further as it moves around the back of the sail, when compared to the airflow as it travels across the front of the sail, causing a slight increase in the speed of the airflow. 

This creates a pressure difference between the two sides, and thus a force through the sail at 90 degrees to the surface of the sail. 

It is this force that drives our boats through the water. 

Take a careful look... 

You can see several things from these simple diagrams:-
  1. The sail is at the same perfect angle. Try to think of it as the hull moving underneath the sail
  2. You will heel more when sailing to windward, there is more sideways force 
  3. When close hauled, if you centre the boom, you lose forward drive, and the Mean Power will simply push you sideways 
  4. As you turn away from the wind, the boat will go faster. There is less heeling force and more forward force, therefore you will go faster through the water. This is true up to about a broad reach, depending on wind strength 
  5. On a run, with A rig in particular because the mast is so tall, if you sail in too strong a wind for the rig, the bow buoyancy is unable to withstand the forward leverage from the top of the rig, and you will nosedive.
The big thing to remember is:-
if the luff is fluttering, then your sail is probably too loose 
if the luff is not fluttering, then you sail is probably too tight 

These diagrams also show why, whenever you change direction, even by a modest amount, you will have to re-adjust your sail back to the perfect angle. (The sail of course, is attached to the hull by the mainsheet, so the angle of the sail to the wind changes with each course change)

It is this fine adjustment that allows the most successful skippers to sail so fast.

It does take a good bit of practice

Remember, this is a simplified version of events. If you also take into account sideways drift, boat speed and it's effect on apparent wind, the efficiency of your under hull appendages etc etc. then I suggest that you apply to a University to study for a PhD. Its about that standard!