What to choose between a Low Aspect and High Aspect Foil?

When you are new to foiling, or want to progress, it can be difficult to know which foil to choose. Low Aspect, Mid Aspect, High Aspect : what are the differences? Which foil should you choose according to your level and practice? We explain everything in detail.

The Foil has revolutionised board sports. It allows to fly over the swell, to move from one wave to another, partially or totally out of the water. It is accessible to all sizes, all levels, all conditions… Whether it is with a wing board, kite board, surf board, SUP… Here are all the criteria to take into account when deciding between a Low Aspect or High Aspect fin.

Comprendre le fonctionnement d'un foil

The Foil works like an underwater plane by generating hydrodynamic lift through the speed it acquires. Consisting of a mast, a fuselage, a front and rear fin (the stabiliser), the foil allows the rider to glide through the water. The fins are arranged parallel to the board, and can be controlled simply by foot pressure. The front fin has a curved shape at the top and a flatter shape at the bottom, which creates lift. The width of the fin, measured from the centre of the leading edge to the centre of the trailing edge, is called the “chord“. The length of the fin, measured from one end to the other, is called the “span“, the same principle as in aeronautics.

What is the ratio aspect of a foil ?

Before choosing between a high aspect ratio or a low aspect ratio foil, it is important to understand what an aspect ratio is. The aspect ratio of a foil is a mathematical ratio that describes the shape of the wing. It is the ratio of the wingspan (the distance between the two tips) to the chord (the distance between the leading edge and the trailing edge).

The higher the aspect ratio, the greater the span of the wing in relation to its chord. This means that the wing is longer and narrower. Conversely, a low aspect ratio indicates a shorter and wider wing.

There are 3 categories of aspect ratio for a foil:

Low Aspect Ratio: These foils have a lot of chord and very little span. These are good foils for beginners but they quickly saturate in speed because of their large chord.
Medium Aspect Ratio: This is a compromise between a high aspect ratio foil and a low aspect ratio foil. They correspond to our range of classic foils. These foils are accessible for beginners while keeping a very good margin of progression.
High Aspect Ratio: These high aspect ratio foils have very little chord and a large span.

How to calculate the aspect ratio of a front wing ?

There are two calculations for calculating the aspect ratio of a front foil:

The most accurate formula for calculating the aspect ratio : wingspan squared divided by the wing’s load bearing area. This formula takes into account the chord variations over the entire wing shape and allows for an accurate calculation of the aspect ratio. On the other hand, this formula can be more complex to use as it requires knowledge of the wing’s load bearing area, which can be more difficult to measure.
The simplest formula for calculating the aspect ratio is the wingspan divided by the chord of the wing. This formula is convenient if you don’t know the wing’s load-bearing area. However, it does not take into account chord variations and therefore only gives an approximation of the aspect ratio.

Choosing between a high aspect ratio foil or a low aspect ratio foil

Here are some key points to consider when choosing between a low aspect ratio foil and a high aspect ratio foil :

Speed

The speed of a foil is proportional to its drag. It is usual to think that a front wing with a high aspect ratio (high aspect ratio) is faster than a wing with a medium aspect ratio, but this is not quite true. In fact, drag is generated by different elements such as the overall aspect ratio of the wing, its twist and the speed squared. In summary:

Drag is maximised by : speed, aspect ratio, thickness, and chord of the front wing.
The more twist, the more stable the front wing is, but this reduces the speed.
The profile of the front wing also plays an important role: for example, if the masterbeam (the maximum thickness) is very far forward on the front wing, the foil will start earlier but will lose speed.

As you can see, the speed of a wing is not just about its aspect ratio, there are many other factors to consider. As a general rule of thumb we can say that with the same parameters (if we only play with aspect ratio) a wing with a high aspect ratio will be slower than a wing with a medium aspect ratio.

Maneuverability

A low aspect ratio foil will be more manoeuvrable, ideal if you want to make tight turns but still have great control. The chord offers stability in the longitudinal axis. The aspect ratio is a limiting factor for manoeuvrability. The higher the aspect ratio of a wing, the less manoeuvrable it will be.

Weight

The weight of the fin has no influence on the overall performance of the foil, but we can give you a few pointers to help you understand.
Firstly, the weight of the fin is determined by the amount of material you put into the wing. If the wing is foamed, then the weight will be less.

However, a high aspect ratio fin does not necessarily have to be very thick. Therefore, it will be made of carbon (depending on the thickness), so it will be a bit heavier than a foam wing. Low aspect and thin wings, on the other hand, require a lot of rigidity and will therefore be heavier because they are made of 100% carbon. Conversely, a wing with a moderate aspect ratio, but thick, will be foamed. However, as the wing is larger, it will be just as heavy.

Level of practice

Low aspect ratio foils are more suitable for beginner foilers as they offer better stability. However, they do not offer much room for improvement. That’s why at Sroka we have decided to design foils with a moderate aspect ratio which offer ease of learning, forgiveness but also excellent glide so that your foil can evolve.

Conditions of practice

The choice of kite depends on what you want to do. To glide on swell trains, a high aspect ratio kite will be more efficient. Indeed, in most cases, foils with a higher aspect ratio generate more glide and allow you to fly longer without having to use the wing. However, with the same surface area, profile and thickness, they will not go faster than a medium aspect.

If you are looking for control and manoeuvrability at high speed, a wing with moderate aspect ratio is more interesting. This is the case if you want to start freestyling for example.

At Sroka, our philosophy is to create products that perfectly suit your needs, while remaining versatile, rather than forcing you to adapt to the equipment. We are always open to comments and feedback on the Sroka sessions Facebook group, as we design these foils primarily for you and to share our passion.