3.2 The flow situation including lift

Leaving the running track sideways drift and lift forces arise.

The fin is an obstacle in the water flow. But the fin resistance is the source of the lift forces which enable the sailor to sail upwind. Therefore resistance has to be minimized and lift has to be maximized.

When the board is gathering speed from a standstill a strong vortex arises as the water flows around the trailing edge. There are no lift forces.

 

Figure 9: Arising of the initial vortices

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board is gathering speed

α angle of attack

Sv   frontal barrier point

Sh   rear barrier point

With increasing speed the initial vortices stay behind the fin and a smooth flow is passing the trailing edge.

As soon as some speed is gathered ideally a stable flow pattern comes into being around the fin and lift forces arise.

 

Figure 10: Arising of lift force

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The fin is at a constant speed and the board is in full planing. The rear barrier point has moved close to the trailing edge and the initial vortices have left the fin. There are just some trailing vortices in the turbulent wake.

Behind the trailing edge, the speed of the windward flow is the same as the speed of the leeward flow. As the distance on the windward side of the fin is greater than on the leeward side, the water particles have to accelerate faster: the streamlines are condensed and a negative pressure (suction) arises. On the leeward side, the flow is slowing down, the streamlines are widened and pressure arises. A pressure differential is created which generates the lift to drive the board along. The lift force is the sum of suction and pressure.

When you start from a standstill position, the angle of attack might be too big. The result can either be no flow at all around the fin or a delayed arising of a stable flow pattern. If you are sailing at a constant speed and then abruptly reduce your speed and increase the angle of attack the flow will also break down.

The fin needs a considerable water flow over it before it will start to lift. Therefore as you get going you should bear further off the wind to get right up to speed with a small angle of attack. The initial vortices will leave the fin quickly, the board will accelerate and soon be on the plane. The flow pattern around the fin is stable now and you may steer your board on the sailing course you wish.

The frontal barrier point is not identical with the leading edge any more, it has moved leeward. From the hydrodynamic point of view, it is the true leading edge now: it splits the flow into windward suction and leeward pressure.

Behind the fin, the speed of the windward flow is the same as the speed of the leeward flow. As the distance on the windward side of the fin is greater than on the leeward side, the water particles have to accelerate faster, the streamlines are condensed and a negative pressure (suction) arises. On the leeward side, the flow is slowing down, the streamlines are widened and pressure arises. A pressure differential is created which generates the lift to drive the board along. The lift force is the sum of suction and pressure. It increases with growing speed.

 

Figure 11: Lift force and distribution of positive and negative pressure

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α    angle of attack

Sv  frontal barrier point

Sh  rear barrier point

The graph demonstrates the predominance of the suction over the pressure. Together they create the lift force of the fin.

Figure 12: How speed is influencing the lift force

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This graph shows an ideal fin. It can be derived that with increasing speed a bigger angle of attack may be chosen and that a bigger lift force is created.

Furthermore, one can learn that an angle of attack of about 15° is delivering the maximum of lift. In practical reality, the fin profile will constrain all other parameters and the form of the curve will change. Flat profiles are delivering lift with a certain delay and the ideal angle of attack is much smaller than 15°. Therefore, if you are beating upwind with a flat fin you should choose the maximum sailing speed possible.

Sail and fin are producing much more suction than they produce pressure. Suction may go up to nearly 70 % of the lift total, so actually the board is sucked along on its track.

Depending on speed and fin profile lift will decrease with an angle of attack of 15° at the latest. The point of detachment will then begin to move towards the leading edge and the flow on the windward side of the fin will begin to come off the surface.

From this follows that the right choice of the angle of attack is an essential part of the resulting lift force.

The thickness ratio and the thickness distribution are also of crucial importance for the creation of lift. With a fin of 11 % thickness ratio you may choose a bigger angle of attack than with a fin of 9 %. A thick fin will produce more suction and therefore, the lift force is increasing. But, at the same time, the drag will increase and the fin will be slower. If you are using a rather thick fin, you may expect a high lift force at a rather low speed, you may not expect a high maximum speed.

The lift is also influenced by the thickness distribution. Fins with the maximum thickness at 33 % or less will produce more lift than fins with a higher percentage.

In addition to the cross-sectional shape of the fin there are some other important factors for producing lift, such as the trailing edge, outline, surface roughness, rake angle, taper ratio, torsion and speed.

 

 

HORIZONTAL and VERTICAL LIFT

 

When we talked about lift in this chapter we actually meant that this force is pointing windward parallel to the water surface. To avoid misunderstandings we may call this force horizontal lift. Windsurfers mostly do not use that term, they prefer to talk about upwind performance of the fin.

A closer look at the situation around the fin will show that there is more than that to be taken into consideration. Quite often, when windsurfers are talking about lift they mean something different. They mean a force that is pointing vertically upwards, consequently changing the planing angle of the board and in total reducing the weight of the system sailor/equipment. In general this vertical force is seen to be a positive factor enabling the windsurfer to achieve a higher top speed. But this is only possible if the trim of board and rig are in harmony with this force.

This force can be called vertical lift. Both vertical and horizontal lift are depending on sailing speed, angle of attack and on the fin dimensions/parameters. They may be understood as correlated forces and can be explained with the help of two separate approaches.

1. As a consequence of the forces acting upon the fin it will flex sideways in a progressive way. The result is a force pointing at an angle upwards and sideways. It can be split up to a horizontal and a vertical partial force: horizontal and vertical lift. The vertical lift has to be balanced by the windsurfer`s body weight.


flexing-fin

 

2. Some short and wide speed fins seem not to flex at all but they are still producing vertical lift. Therefore, we need an additional explanation.

If we imagine a hypothetical fin with no flex at all positioned vertically under the board this fin should be producing horizontal lift only. But there is more to say about that.

As we all know we have to push down the windward edge of the board with our body weight against the fin. The longer the fin and the stronger the wind the more weight is needed. This is true no matter how little or much the fin is flexing. Even a non-flexing fin is producing that force. Again we have to balance a vertical force – otherwise the board would turn over. The region of the pivot would be somewhere near the fin base.


non-flexing-fin

 

Both approaches demonstrate that horizontal and vertical lift cannot be separated from each other in practice. In all speed depending competitions they may be of crucial importance.