Monofins are synonymous with modern day competitive freediving as they've been on the feet of all recent world title dives. Monofins are simpy a single fin whereby both foot-pockets are attached to a single blade, typically fibreglass but sometimes carbon and plastic. With correct technique, Monofin propulsion far exceeds that provided by stereo-fins (bi-fins) but you forgo almost all agility underwater, and ability to turn. Monofins are designed to swim straight and fast so if you're looking for a more leisurely applicable freediving fin we strongly recommend bi-fins.
Monofins demand great technique of both fining and streamline, it's not as simple as doing your best dolphin kick. We strongly recommend familiarising yourself with your monofin in the shallow end of a pool before taking it in the open ocean. It's worth taking extra care when entering the water and most of us put our fin on when we're already in the pool.
Team Diver and Adreno Instructor Brett Davison. Wetsuit here.
Monofin Foot Angle
Foot Angle refers to the angle between the blade and your feet. The idea behind foot angle is that the typical human foot does not point perfectly straight (picture a ballerina on their toes, in that case the foot is straight when compared with the leg therefore has 0°angle). A blade angle is used to make the fin-blade linear with the rest of your body. Some people require little angle, some require lots - all we know for sure is that anywhere between 5° and 25° are suitable. Higher angles are most ideal for a gliding technique, whereby the diver spends most of their time in streamline, not kicking. If you have a low amplitude kick with moderate to high frequency then a lower angle will be more ideal.
Much like angle, there is no simple answer here and only experience can assure you the ideal stiffness for your given technique, power and size. If you're familiar with bi-fins then you'll be surprised as to how much stiffer a monofin is. To be very general, a soft blade will be better for smaller divers, divers who are optimising efficiency in their high-frequency kick, divers that are developing their ankle and leg strength. A medium blade is the best starting point as it's the centre-point. A stiff blade will give the most propulsion but requires the most strength, easier for beginners in the sense that bad technique has a lesser impact on a hard blade than a soft and stiff is most ideal for the kick-glide style of fining.
Alesia Zecchini climbing the rope. Wetsuit here.
We typically find fibreglass to be the most desired monofin blade material, as opposed to the more expensive carbon-fibre blades. This is the case today but in 5 years it might be a new material so for now we'll talk about glass and carbon fibre. Fibreglass is inherently more 'flexy' due to it's immense elasticity and elongation potential, compared with carbon it can elongate something like 6x more. Carbon fibre boasts insane compression resistance properties and is typically used in applications demanding rigidity. Because a monofin isn't designed to stay straight but is designed to whip, and it requires flex to do so.
The options are numerous layers of fibreglass, or a couple of carbon layers, and carbon falls short only in the sense that if it's made for good whip then it won't have good propulsion. Fibreglass on the other hand, provides good whip because of it's inherent flex, but when laid thick it has ample rigidity for the strongest kick.
We have seen an emergence of f/glass blades with one or two external carbon laminates with the goal of increasing whip without negating propulsion. Although these have not concreted themselves at the top of the market, they're making serious waves.