We see a wide range of fin shapes and a couple of finbox mounting systems on boats at the moment, so what might the differences mean?
Fins provide direction to the boat and also resistance during turning. Their shape and size will impact your rowing, even if this seems subtle at first.
I will look at 3 main shapes which can be labelled as follows:
1) The Swept Back fin
2) The Golf Club fin
3) The Upright fin
... and 2 mounting systems
a) The 'US Box'
b) The 'Swift Box'
I use all the fins shown here - they all have pros and cons. Life is a compromise, decide what your priorities are and then develop the boat and fin combination that works best for you.
Fin location is also important, as can be seen in the Beach Sprint (towards the bow) and Offshore (towards the stern) options on the Swift C1X solo.
The following words are my thoughts and feelings after many hours of using them. Let me know if you have any other experiences or thoughts - the sport is rapidly evolving and we will no doubt see fin designs changing to suit different water conditions and boat shapes.
1) The Swept Back Fin
This is the classic 'wave fin' shape seen on surf and windsurfing boards. In windsurfing this shape is known to grip the waves when turning tightly on the wave face and when moving at reasonably slow speeds. I have found similar performance in coastal boats - with the fin gripping well even in low speed. This can make the turns wider rather than tight.
The swept back leading edge works reasonably well in shedding weed, although it can collect at the root of the fin which creates drag and dramatically slows the boat down. The best way to clear it is to stop, reverse and turn.
2) The Golf Club Fin
This fin was seen on early windsurfers but fell out of fashion. It gives good directional control through its large area. It also sheds weed reasonably well as it is swept back from the root in a similar way to the Swept Back fin.
The large area of the fin makes it very directional although it does seem possible to 'kick' the fin around by putting rapid pressure on one foot at the catch - which seems to cavitate the fin, allowing it to break away and spin the boat.
3) The Upright Fin
This shape of fin was used in slalom and speed windsurfing boards. It gives good directional control for its length and area when moving at speed. When the speed is reduced it becomes much 'looser', allowing you to easily kick it around the turns. Once the speed increases after the turn, then the directional control returns.
I have been using different length fins of this shape on my solo and double boats - slightly longer versions for the double.
The leading edge is very upright, hence it is the most prone of the 3 shapes to weeding up.
Fin Mounts
a) US Box
Most boat manufacturers seem to be using variants of the 'US Box' which allows the fitment of a wide range of fin type. The sizing is universal and so you can easily replace a damaged fin or try new shapes.
The fin fits in a 'T' slot and uses at bolt / threaded washer to locate and tighten it. This allows the fin to be moved longitudinally forwards and backwards to bow or stern. Often the available adjustment is small but it will marginally change the turning ability. Windsurfers often fill the exposed slot with a plastic strip to reduce drag but this may be less important at coastal rowing boats speeds.
The safest fin design for your boat will have the bolt/washer located at the bow end of the fin. In this configuration many of the fins are designed to shear upon impact with rocks, etc hence protecting the fin box and boat.
b) Swift Box
This box seems to be standard on all Swift boats. It has a location tag at the bow end of the fin and a grub screw tightens the stern end. The fin has one longitudinal position. We have not found any other fins manufactured to fit this box design, hence you will need to buy replacements from Swift.
In the event of front impact, the location tag can shear off within the box. In the past we have replaced this with a small self-tapping screw to avoid a major removal of the whole box.
Summary
Our sport of coastal rowing is still in its infancy - with new race formats appearing and a wide range of recreational activities that are opening up due to the stability of the boat hulls. We can expect the understanding of fin performance to evolve too, leading to new variants in shape, materials, stiffness and mounting. Hopefully the thoughts I have outlined will add to the debate!
In the meantime please experiment with different fin designs and see what works best for your boat / water conditions / type of rowing.
If you have any thoughts on fin design and its impact on boat performance, please let us know.