THE RETURN OF THE TRI-FIN

 

Multi fins - twinsers, tri-fins, thrusters and quad fins have been used in various guises with varying success for many years. There are a wide number of reasons why you might  want to use multiple fins and I will go on to explain those in some detail later.

At Witchcraft we have worked with trifins for over 8 years and have found that for all-round waveboards they offer the best performance of all fin systems and will provide the main focus of these pages.

 

Initially tri-fins were used to increase grip on the inside rail when wave riding. This style of set up was made famous by non other than the legendary Robby Naish, where the added an extra flare to his time less style.

 

(Old Picture of Robby Naish)

 

Another tri-fin system was developed in the Columbia River Gorge to increase control in the incredibly heavy messy chop found at this mega windy inland Mecca. By setting the fins up to create a down force and increase drag, sailors found they could keep the tail of the board in contact with the water more of the time. This may sound contrary to what most boards are trying to achieve, but when you have tried to sail overpowered on a 2.5 in Hurricane force winds you want that anchor to slow you down!

 

Both Robby Naish’s boards and the Gorge boards were based on a standard sized single fin boards with extra side fins.

 

However; there is a better way to use tri-fins and this is the one that makes most sense for most people. It is using the fins to create more fin area but without the disadvantage of long leverage or a very thick slow foil. What this does is improves early planing and upwind performance in the same way a bigger fin would but combines this increase in performance with better manoeuvrability. Which makes it perfect for wave sailing and freestyle.

 

This is actually the same reason twinsers or twin fins became very fashionable in the mid ‘90s i.e. more fin area but with more manoeuvrability and planing performance.

 

Twinsers

Twinsers have seen a recent resurgence in interest largely because former World Wave Champion Kauli Seadi show cased his exceptionally tight and radical turns at the Ponta Preta, Cabo Verde World Cup Event earlier this year.

 

The reduced leverage with increased finarea of twinsers does give better manoeuvrability and/or upwind performance over a single fin, but, to our experience, the problem with them is that they lack predictability. Each fin is situated closer to the rail and on harder bottom turns it is easier to ventilate/catch air on the outside fin, causing the tail to spin out or at least create a very slippery scary turn - not very nice by itself and worse still with tonnes of water luring over you. High rockered boards, which need less backfoot pressure to turn, had less problems but just lost too much planing ability.

 

Moreover, the further apart the fins are located the stronger this ventilation effect is. By moving the fins closer together different problems start. The fins can interfere with each other the more they stand in each others shadow with turbulent water reducing all the early planing advantages and causing more spinout problems. Also with the fins placed closer together there is more drag as water passes slower between the fins as it is squeezed and slowed down between them.

 

We found they worked best putting them about 12-14cm apart and giving them a slightly flatter inside foil to reduce the inside drag effect and interference. But we never really found a truly satisfying result. Don’t be fooled into thinking that they will instantly radicalise your sailing, they require a very good sailor with a sensitive back foot and pretty clean waves.

 (Note to editor: maybe ask Keith Teboul on Kauli´s twinsers boards)

 

Quad fins

We have never gone to the extreme and tried quad fins so I could be wrong, but in my opinion they provide similar properties to those of twinsers but potentially exaggerate them which means it doesn’t make much sense in using them.

 

Tri-fins

Tri-fin systems have real advantages over twinsers, it combines the best of both single fin and twinsers, the centre fin adds control in the same way a single fin does, but you get even better manoeuvrability than with twinsers. The centre fin allows the side fins to be placed further apart so they do not interfere with each other. If one of the side fins looses grip in an especially hard turn, you only lose between 20-33% of the control (depending on the total fin area). This means you are more likely to stay in control throughout the turn when compared to a twinser which could be up to a 50% loss of grip. Another positive is the wide variety of options available, which can change the focus away from control to manoeuvrability or vice versa by varying between centre and side fins sizes. You could even go crazy and have an asymmetrical fin set up!

 

 

The main reason, I think, why tri-fins have not really taken the world by storm until now is because of the wide variety of options how to place them. There are so many ways of not quite getting them right. For a shaper a single fin is a very safe bet, there’s not much that can go wrong. The position is more or less fixed within a few cm forwards or backwards, the only thing to vary is the fin itself and with such a wide market of fins available for general consumption, it is easy to find a fin that works best for a particular board. I have found that most fin development is reactive to board development - changes in sailing styles and/or marketing purposes - rather than done in parallel with board development.

 

As said the options for the set up of tri-fin systems are almost endless, side fin positions, tow angles (the direction a fin points), cant angles (whether a fin is perpendicular to a boards rocker or not), side fin profiles and sizes and flex and profiles between the fins can all be varied.

(Pictures: Tow and cant angle)

 

For shapers it’s hard to find the best tri-fin set up for their boards. As mentioned before, in theory it’s easy to explain tri-fins advantages over single fins - more fin area, less leverage means earlier planing, better upwind, more manoeuvrability, more grip on the rail, and the board won’t ride up on the fin when overpowered. However; with so many variables, how do you to find the right set up to actually take hold of these advantages?

 

Nearly all surfboards are tri-fins because of the increased planing and manoeuvrability characteristics. There are some similarities in the how windsurfers need them to perform, but there are also differences, which include: the force of the sail, speed, manoeuvrability and the use of foot straps.

 

So what do we want our tri-fins to accomplish? Because the fins are shorter (for the same area of a single fin) an improvement in manoeuvrability is fairly easy to achieve. However; for optimum efficiency - early planning, upwind performance and least drag - we have to get all the fins working together for each other. Each fin has to do the same amount of work as the others. So each fin needs to have the same percentage of profile thickness and flex, so no assymmetric sidefins like on surfboards. This is still pretty easy to achieve. The angle of attack of the water flow passing over each fin is important too. And this is the more difficult bit. At first glance you would think that all fins need to be parallel to make them all have the same angle within the water flow, but you’d be wrong.

 

New Insights

After discussions with Tim Fitzhardinge, a Western Australian local (a prodigal son to windsurfing who came back after 20 years of surfing) we decided to do some more testing of tri fins. Tim was disappointed to find that board design had not made the same advances as sail or surfboard design had since he had last sailed in the 1980’s. As a result of his surfing experience he fitted many production wave boards with FCS style surfboard side fins.

Exocet Universal Wave with FCS fins, Picture Tim Fitzhardinge

 

With this inspiration we looked into the water flow at the tail of the board whilst planing and even during bottom turns. We found that the water left the board slightly sideways when compared to the boards direction of travel.

 

Confronted with this epiphany, I finally understood that what happens to displaced water when you jump on a motionless board - the water wants to escape from under the board in every possible direction (picture 1, all directions) - also happens when sailing along. When you add this to the direction of travel of the board you get the ‘apparent water flow’. (picture 2, water flow- V bottom)

 

The big question was by how much exactly?

 

To find out exactly what was going on the most scientific way I should of commissioned extensive research using water flow sensors, connected to a transmitter sending split second data to a quantum super computer modelling system onshore. At the same time as having a high speed digital video camera examining the sailor to get synchronic data transfer of the water flow.

 

But instead I decided to use the much more sensible, fun, but slightly less scientific method of good old trial and error – by far the most used method of R&D in the windsurfing industry. Now I had a problem…

 

I needed a dozens of boards with every fin angle and set up all on the beach to test this theory properly…

 

The 4 way fin system

A good friend came to my rescue. Dean Geraghty, a renowned surfboard shaper from Cape Town, brought to my attention a fantastic new fin system he had developed for surfboards, the 4-way fin system (www.4wfs.com). I thought it might be too weak for windsurfing and too complicated for most end users, but for me as a shaper it was a godsend. With this system I was able to play with the angles without having to make dozens and dozens of boards! Just a little tweak here or there with a few screws and I could turn the fin degree by degree and sail out again. Shapers heaven!

 

I was surprised at how noticeable the differences were. By starting at the extremes of the tow angles and narrowing it down from there, it was fairly easy to find the “sweet spot”.

 

I found the tow angle was by far the most important angle to get right.

If you add also the sideways drift to the theoretical waterflow you can see that the angle of attack to each fin becomes the same.

The cant angles were of lesser importance, some people believe that it can give the board more lift or hold the board better in a hard turn. I think it is an inefficient way of creating lift (if it does give lift at all and not pull you down!), better to leave the vertical forces to to the board itself  and the horizontal forces to the fins. If they needed to work as a hydrofoil they would be designed as a hydrofoil.

 

(Picture Tim Fitzhardinge)

We found that a cant angle of close to 90° i.e. perpendicular to the board and parallel with the centre fin works best.

 

Additional influence to board design

It was even possible to notice the difference between boards with V, flat, mono or double concave or inverted V hulls. This also helped to better understand what different bottom shapes do to the water flow under a board and their influence on performance. (Pictures water flow V, double concave and inverted V)

 

 

I wrote an article, published in Windsurf in may (?) ’05, on the exciting developments we had made to rail shapes – available from http://www.witchcraft.nu/download/The%20Evolution%20of%20the%20Rails.swf. In this article I highlighted the transition point between water entrance and water release point on the rail. Due to these new insights we could now move this transition point considerably further forward along the rail, adding to planing, top speed, manoeuvrability, responsiveness and grip.

Picture: Rail shape changes

 

 

Drag

Looking down the board from the point where the waterflows come together somewhere around the nose we can see the full leading edge of each fin. What you see from this point of view - the fins - is what creates the main drag. It looks like there would be more than that of a single fin, but as these fins are not only shorter but also thinner this drag is not noticeably different to that of a single fin with the same total area as the added up trifin area.

When the fins are not set up correctly, you would see more of the fins and there would be more drag.

The main reason why tri-fins still cause somewhat more drag, even when they are set up correctly, is the turbulence created at the base and tip of each fin. The turbulance at the base can be avoided with a good flowing fit and can actually have a lower drag than the open slots of a US box. The theoretical increase in vortex drag created at the tips is unavoidable, however in practice it appears to not be of any particular relevance and the benefits of added light wind and up wind performance of the tri fin will give more speed when you need it most!

 

Conclusion

So what did we find with the optimised tri-fin system?

 

We found so many advantages including:

 

 

All these performance benefits helped on perfect side shore days, but actually made the most impact to those less than ideal on-shore and/or light wind days! We found it also helped significantly in spots where you need extra drive because of rips or fluky winds.

 

Tri-fins also had some hidden secrets, for example many freestylers use super short fins for better slide. But these fins generally lack drive, upwind performance, pop and control. A tri-fin set up with the same fin length has a lot more fin area so early planing, upwind performance, control, drive and pop are all improved and has similar slide advantages. Having watched all the World Tours top pros time and again drifting downwind out of the competition zone at World Cup events in Fuerteventura, this extra drive is not to be sniffed at.

 

Contrary to belief, when compared to twinsers, tri-fin boards are not hard to sail. Even less experienced sailors find a tri-fin set-up helps early planing, give greater control and are easier to jybe.

 

Since 1998, the first year we started making Trifin boards about 10% of all boards we made were Trifins, gaining experience with them, over the years this percentage has gone up to 98% in the last few years. Even the most conservativ thinkers have been converted.

 

To cover all conditions and sailors, we have developed 4 different sidefin sizes from 11.5cm to 14.5 and 4 centrefin sizes from 14.0cm to 18.5, which can all be mixed between eachother. We´ve found that for allround perfromance it´s best to have the centrefin slightly bigger than the sidefins. For example 12.5+15.5, but for a more skaty style this could also be 13.5+14.0 or for more control 11.5+17.0. For different windstrengths it´s best to change the sidefins rather than the centrefin as when it´s light you could have a more skaty board and for strong winds more control. But you´ll find hardly any at all need to change fins to cover practically all conditions.

 

Trifins are not the most important detail in board design but they do help in creating the highest performing wave board for allround conditions with a big range of use.