Although it may seem far-fetched at first to anyone who counts the Baltic Sea or the south of France among their favourite destinations, seagrass is a wonderful thing – whether as a nursery for fish and other marine life, for coastal protection, or because of its ability to sequester large amounts of CO2. There are therefore good reasons why seagrass beds are protected and even replanted. In the Baltic Sea, seagrass grows at depths of two to seven metres, putting it well out of reach of fins and foils. Whilst the roots remain in the seabed for many years, producing new shoots, the leaves die off in late summer and float to the surface – where they sometimes drive windsurfers and foilers round the bend.
At many spots, therefore, you simply can’t get anywhere between June and November without a weed fin. Special weed fins have a flatter rake of 40 to 50 degrees, which helps to dislodge the seaweed. The reason weed fins are still not particularly popular is that the shallower rake also shifts the fin’s pressure point several centimetres further back, in some cases even beyond the tail. The consequences: the board sticks to the water more, glides less freely over choppy waves, and performance when riding upwind also suffers noticeably – at least in freeride conditions. Nevertheless, weed fins are the lesser of two evils, provided you use a suitable model in the correct size.
It is the surface area of the fin that is decisive, not its length
A 1:1 swap – for example, swapping the standard 42 fin on our Goya Carrera 127 test board for a grass fin with the same draught – makes no sense. Because the angle of the seaweed fin is flatter, it would have 45 to 55 per cent more surface area than the standard fin, depending on the model, whilst being the same length. As this larger surface area is also positioned further back, manoeuvrability is reduced, speed is noticeably curbed, and drag and sail pull increase. Our test demonstrated just how significant these differences are. As an experiment, we replaced the standard 42-series fin on our Goya Carrera 127 with a 40-series seaweed fin. Powered by the high-performance 6.9 GunRails Re-Zoom, we hit a top speed of just over 41 km/h in choppy Baltic Sea conditions. After swapping it for a 34 seaweed fin, the speedometer needle topped out at just under 48 km/h. This example shows that, in freeride and freerace, fin surface area is the key factor.
Some manufacturers attempt to minimise the other drawbacks of seagrass fins – poorer turning and difficulty in swimming against the current – by positioning the seagrass fin further forward on the base; in our experience, this is a sensible measure which, whilst not completely eliminating these drawbacks, at least mitigates them.
The 20/10 rule for seagrass fins
But how do you know which length of seagrass fin to use to replace the standard fin? After all, the fin surface areas aren’t always specified. As a guide, the 20/10 rule has proven useful for boards with a centre fin – these include freeride and freemove boards with a single fin, as well as freewave boards with a thruster set-up. In other words, the seagrass fin should have 10 to 20 per cent less draught than the standard fin. Whether you opt for the lower end of the recommendation or the upper end depends primarily on the discipline, i.e. the board you’re using. For example: are you looking for seagrass fins to replace the small 10-side fins on your waveboard? If you were to swap them 1:1 for a 10 seagrass fin, it would only have around five per cent more surface area. You can swap them almost 1:1 here, or choose a seagrass side fin that is one centimetre shorter – which corresponds to ten per cent. If you were to replace a 42 standard fin on your freerider with a 42 seagrass fin, the latter would have around 50 per cent (!) more surface area. So here you can easily reduce the length by 20 per cent and choose a seagrass fin measuring between 32 and 34 centimetres in length. On the following pages, we provide size recommendations with a range of two centimetres for the disciplines of freeride, wave and freestyle.
Size guide for freeride and freerace seagrass fins
| Standard fin (cm) | Seagrass fin (cm) |
| 26 | 20–22 |
| 28 | 21–23 |
| 30 | 23–25 |
| 32 | 25–27 |
| 34 | 26–28 |
| 36 | 27–29 |
| 38 | 28–30 |
| 40 | 30–32 |
| 42 | 32–34 |
| 44 | 33–35 |
| 46 | 35–37 |
| 48 | 36–38 |
| 50 | 38–40 |
| 52 | 39–41 |
The recommended length for seagrass fins is intended as a rough guide. If you weigh less than 75 kilos or surf on a freeride board with inboard footstraps, aim for the lower end of the recommended range. If you weigh over 85 kilo, like to apply a lot of pressure to the fin and are using large, high-performance sails, you should ideally aim for the upper end of the recommended range – this will give you sufficient counter-pressure from the fin and the corresponding performance.
Size guide for Wave seagrass fins
| Standard fin (cm) | Seagrass fin (cm) |
| 9 | 9 |
| 10 | 9–10 |
| 12 | 11–12 |
| 14 | 13–14 |
| 16 | 14–15 |
| 18 | 16–17 |
| 20 | 17–18 |
| 22 | 19–20 |
| 24 | 20–21 |
| 26 | 21–22 |
The lengths recommended in the table for seagrass fins are intended as a rough guide. If you weigh less than 75 kilos and mainly surf with sails under 5.0 square metres, you can aim for the lower end of the recommended range. Because the difference in surface area between standard and seagrass models is not as great in percentage terms for short wave fins as it is for freeride fins, seagrass fins can sometimes be used in waves at almost the same length as the standard fins. However, here too, the fin’s pressure point shifts slightly further back. Whilst this has little effect on glide, control and upwind performance, on waveboards it noticeably impairs turning manoeuvres in particular. The turning radii in bottom turns and cutbacks are limited, and tight turns become noticeably more difficult. Furthermore, the tail gains more grip, making it harder to let the tail break out during a cutback – this limits their suitability for lipslides and takas. For this reason, you should fit the seagrass fins into the fin boxes fit it about one to two centimetres further forward, to achieve similar turning characteristics. If you do make use of this minor tuning adjustment, the difference you notice compared to standard fins is minimal, which is why many wave surfers simply carry on using their seagrass fins even on trips to the seagrass-free North Sea.
Size guide for freestyle seagrass fins
| Standard fin (cm) | Seagrass fin (cm) |
| 16 | 14–15 |
| 18 | 16–17 |
| 20 | 17–18 |
| 22 | 19–20 |
| 24 | 20–21 |
If you’re doing basic tricks like the Vulcan, Spock, Flaka or other slide moves, you should aim for the upper end of the recommended range. The same applies if you weigh over 85 kilos and want to perform tricks with sails larger than five square metres. If, on the other hand, you’re surfing at the level of double sliding moves and power moves, you can choose the lower end of the recommended range. The 20/10 rule This also applies to freestyle boards, albeit for different reasons. As freestyle boards are already fitted with comparatively short fins and are ridden more aggressively on the windward edge, the loss of performance when switching to a seagrass fin – in terms of top speed, upwind performance and glide – is less significant than, for example, in freeriding. Rather, the main issue here is that the boards slide less effectively due to the shallower angles of the seagrass fins. Germany’s top freestyler Niclas Nebelung, who is involved in fin development at Maui Ultra Fins, explains: “Because of the larger surface area that weed fins naturally have, you need to go for slightly smaller weed fins, otherwise the ability to slide is restricted too much. Many standard boards are fitted with a size 20 standard fin, so an 18 weed fin is a good alternative. The Pro version is thinner and faster – but you need to be a bit more careful in shallow water, as the thin fin blade is also less robust when it comes into contact with rocks.”
Manuel Vogel
Editor surf
