The sailor wants to stick to his chosen course suffering as little sideways drift as possible. He wants to maintain a maximum speed and his manoeuvres should be supported by the fin.
With the help of the fin the sailor may choose his course on the water and maintain it. As board and rig underwent a constant technological development in the past fins had to be adapted and improved continuously. 25 years ago simple fins were used together with simple sail cuts, soft booms, heavy masts and boards. Today a high performance fin is a high tech product on the basis of hydrodynamic and material knowledge.
The sideways drift is caused by the wind force. The lift of the fin does not just compensate this force, it even enables the sailor to sail upwind “against the wind”.
If a fin generates a good lift its performance is said to be good. In combination with a high speed potential excellent all-round properties can be the result. Depending on the situation either lift or speed will be given preference.
Whatever course you sail, you will always suffer sideways drift and gain lift at the same time. The only exception is being on the run, when the board is following exactly the wind direction. Drift is becoming an increasing problem the closer you sail to the wind. Without lift no sailing course would be possible except to run with the wind.
Sailing downwind on a broad reach drift does not disturb too much. But as soon as you are sailing across the wind on a beam reach you will feel a bad fin and an insufficient trim of board and sail. As the sideways drift will increase you will not find your maximum speed and it will be difficult to maintain the course. Sailing across the wind is the most popular course therefore a bad fin will be a constant nuisance.
The closer your track is to the wind the more sideways drift you are suffering. At low speed you also suffer more drift and the lift is also reduced. Therefore you should sail at high speed when you are beating upwind to reduce the drift and gain maximum lift. But with increasing speed the closest track possible to the wind is limited by the smaller possible angle of attack.
Figure 1: Sideways drift and lift (simplified graph of wind and water forces influencing the windsurfer)
When you get going from a standstill the windward forces on the sail will determine the drift angle at that moment. While you are accelerating the drift angle will change and will determine the hydrodynamic forces of board and fin. Eventually the board will be planing and there will be equilibrium between air and water forces. All the hydrodynamic side forces of the board are minimized now and the fin is taking over by producing lift. The drag force is working against the propelling force of the sail. The board resistance is not taken into consideration.
The ideal upwind speed will be a compromise of sailing speed and chosen angle of attack. The sailor with maximum upwind speed will be first at the windward buoy. A fin with a full shape allows a closer track to the wind but it reduces maximum speed drastically. Stiffer and longer fins also mostly give a good upwind performance, but there are a few restricting factors I will talk about later.
Figure 2: Upwind speed
αopt optimum course angle
Vopt optimum speed
Vus upwind speed
Upwind speed is the windward speed, also labelled as optimum upwind performance. It is represented by the windward distance covered within a certain time period. It should be as high as possible for all wind strengths.
The possible course angle depends on the possible sailing speed. It is not ideal to choose the closest possible track to the wind. With a slightly wider course angle you are able to gain more sailing speed and therefore a better upwind speed.
The higher the wind speed the higher the possible upwind speed. But of course there is a ceiling in each individual case, determined by various factors like sailor, wave conditions, fin design, rig and board.
Fins with a flatter profile permit a higher sailing speed but they suffer more sideways drift and generate less lift. The danger of spin-out is also growing. If you reduce the length and area of the fin you will be able to sail faster but lift will decrease.
The material of the fin blade may also become a limiting factor. Carbon fibres make a fin more responsive and therefore they improve the general performance.
If manoeuverability is what you want, you should choose smaller and softer fins, more or less curved backwards.
There is always interdependence between length and design of fin, size of board and sail, wind force, wave conditions and the individual standard of the sailor. The sailor may draw several conclusions:
- There is no ideal fin for all conditions. Even the best fin has to be a compromise to be able to cope with the changing requirements.
- With increasing standard of the sailor, the choice of fin is getting more important to meet all demands of wind, board, rig, wave, course and personal sailing style.
- For optimum performance and fun under various conditions, the ambitious sailor will need several fins of his individual choice.
- A “good” fin will work unnoticed by the sailor, it will be easy to control and will match with the chosen requirements.