Although this discussion is toned down for the masses, it still makes me smile cuz for me it's simple - water, wind and I :-)
Jim Drake – 2005
Part I – Forces and Moments
Kevin is back again on Figure 2. It shows the kinds of forces that nature confronts him with. His natural skills allow him to sense and react to these forces without really thinking about them. Most windsurfers are the same way but some, like myself and those to whom this paper is aimed, find that an analytical understanding is fun in itself and often helpful on the water.
The main forces can be grouped roughly into five categories:
- Hydrodynamic lift acting upward on the bottom of the board,
- Gravitational weight acting downward through the center-of-gravity (c.g.) of the combined mass of the board, rig and sailor,
- Hydrodynamic side force acting sideways almost entirely on the fin (or fins in the case of a board with a centerboard) in reaction to sideways component of the
- Aerodynamic wind force acting on the sail at its center-of-pressure (c.p.) and
- Hydrodynamic drag on the board balancing the forward component of the wind on the sail.
The sideways component of the aerodynamic force on the sail is called heel; the forward component is referred to as drive. A useful method for the analysis of forces on a body (in this case the body is the combination of board, rig and sailor) is that if the forces and moments acting on that body in two orthogonal (perpendicular) planes sum to zero, the body is moving at constant velocity. This is called “steady state” and is by far the easiest case to analyze.
The next step in the analysis is to make an estimate of the total weight and the breakdown between major components. An example of this is Figure 5. This is helpful in estimating the c.g. position needed on Figure 4 but crucial, as we will see, on the figure to follow.
Note the dramatic difference between the Formula windsurfer (the one pictured in Figures 1,2 and 4) and another very fast and exciting boat, the Hobie 14. The equipment – so called dead weight – of the Hobie 14 is seven times that for the Formula. (And that doesn’t even include the trailer for the Hobie 14.) The leverage this gives the sailor is the main reason why windsurfers have held and now again hold the world speed record under sail.
-- below the windsurfer’s planing speed of about 6 knots -- the Hobie has the advantage. Its long slender hulls part the water gently and leave very little wake. This reduces energy dissipated in the wake, minimizing what is known as “wave making drag”. However, its hulls are not designed to plane and thus, as speed increases, “wetted area” stays the same and friction drag increases dramatically.
Figure 9 completes the three dimensional description of the steady state forces by viewing them on the horizontal plane of the combined board-rig-sailor body (omitting the sailor and rig for clarity). The interesting feature of this perspective is that all the horizontal forces interact at or behind the tail of the board. This feature is also unique to windsurfers as compared to all other sailing craft.
any three of the properties are known, the other three are determined, though some cases are ambiguous and others are impossible.}
- The optimum lift coefficient, (CL)opt , is that associated with a well designed (and rigged) sail that is trimmed properly for the direction of sail and strength of the wind. It can vary up to over 1.0 for downwind sailing. It is proportional to the angle of trim -- β -- in this case 6.8°
- Dynamic pressure is the kind of pressure you feel on your hand when you stick it out a car window when traveling at high speed. The pressure increases as speed increases by the square of speed.
ANOTHER good article on this is - The Science of Speed:
and still ANOTHER !! speed windsurfing, modeling and numerics