Choosing fins for a wakesurf board?

[IDBoating]

There is an incredible array of fins/skegs/rudders available for wakesurf boards. Our IS 4Skim came with a single rather short fin, which is great for spins, but it supports up to three fins and there are some seriously huge options out there.

Going larger on fins will hurt spin-ability, so there must be some other advantage. How and why do you choose a certain fin size, shape, and quantity? We're hoping to expand our range of abilities this season and I'm wondering if we should get a few more fins to play around with.

Thanks!

[Timmy!]

As an engineer, you would think that the larger fins hurt it but sometimes they can actually help it. If you want to do 720's, the larger fins may impede that but for 360's, the large fins are fine. Watch some of James Walker's, DJ Jamez's (slayshtank rider) or Chase Hazen. They all do spins with the larger fins.

Hopefully Surfdad or Nickypoo see's this thread because they can explain the mechanics behind the larger fins.

[IDBoating]

As an engineer, you would think that the larger fins hurt it but sometimes they can actually help it. If you want to do 720's, the larger fins may impede that but for 360's, the large fins are fine. Watch some of James Walker's, DJ Jamez's (slayshtank rider) or Chase Hazen. They all do spins with the larger fins.

Yeah, but those guys are surfing gods. For we mere mortals I would think larger fins would be an impediment. But that's why I'm asking.

So... what are the larger fins GOOD for? Why do you choose them on purpose? My thought is that they are better at keeping the board from sliding sideways, and thus permit the board to track higher up the wave face. That might set you up for a big swoop down the face to gain speed for air moves and such. Just a theory. Do multiple fins increase that behavior? Or are multiple fins used for some other purpose? More fins lets you use smaller fins to achieve some sort of compromise between the two extremes?

[Timmy!]

Larger fins create more lift(ie - hydroplane) and drive(more speed). Maybe the longer fins would also provide more "traction" so to speak which would give you more control over the spin?

[IDBoating]

Larger fins create more lift(ie - hydroplane) and drive(more speed). Maybe the longer fins would also provide more "traction" so to speak which would give you more control over the spin?

If by traction you mean directional control, I get that.

Not sure about lift, though. The fins stick straight down in the water. I don't see how they would contribute to lift, at least not directly. Can someone explain?

As for drive, they don't have much cross-sectional area along the longitudinal axis. But I could understand it if the "drive" comes from our earlier exchange regarding the ability to climb up the wave face without slipping sideways. In that case, you're gaining altitude (potential energy) which could then be traded for speed (kinetic energy) as you swoop down the face again. That might feel like more "push".

This is a good discussion, let's keep it going!

[Timmy!]

Actually, they don't stick straight down. They have cant which is a slight angle. Look that term up for further explanations.

[IDBoating]

If the board is turned upside down, does "angle" mean the fins are not sticking 90 degrees straight up (i.e. perpendicular to the board)? Or does "angle" mean they are not in line with the length of the board (think "not straight")? I wouldn't think the former would have any effect on speed, but the latter might because it would present a larger cross-section to the water.

EDIT: I looked it up. In the context of surfboard fins, "cant" means "not at a 90 degree angle". "Toe" means "not straight" relative to the length of the board.

[Surfdad]

Fin theory abounds and it would be fun to go into all the nuances over time. To address the main question, I like to think of larger fins as affecting control, by increasing drag.

Most of us think of drag as a negative, but if you've ever ridden your wakesurf board finless and felt wildly out-of-control, you know that having that stability via the fins and drag can be a good thing.

You've got the cant and toe defintions down perfectly.

If we looked at the extremes, a board with no fins, it WOULD spin like a top, but the trade-off would be stopping the spin. Typically finless riders crouch to initiate the spin and then can stop it by standing up, or paddling some in the wave. So WA is correct, less fin area, is typically associated with easier spin initiation.

More fin area, typically depth or fin unit count, helps reduce that lateral sliding wildly out of control that a finless board has.

Interestingly enough, most of the tricks we do behind the boat involve causing the fins to fail. Lipslides and the like. Or like Timmy points out, extending surface rotations requires a pretty tight pattern. Short or no finned boards can almost spin in place, deeper finned boards have their turns or spins drawn out more and so the diameter of that 360 is bigger.

Reducing that lateral side slip would help the board go forward, increasing drive. It's that increase in drag laterally that provides a function of control for the rider, but also inhibits certain things - like spinning in place.

Increasing that fin area isn't the ONLY way to provide control, but it's an effective way to do it.

[IDBoating]

Interesting way to think about it: Fins intentionally creating drag. I had always thought of them as giving directional stability while trying to have as little drag as possible. They'd deliver the directional stability by wanting to carve through the water lengthwise, while resisting sideways motion (think "path of least resistance"). You could achieve that with minimal drag using a properly shaped fin.

Add more fins and the resistance to sideways motion increases. Since you can increase the lateral surface area without really changing the longitudinal cross section, you could work this relationship pretty far. Really big fins would yield extreme directional stability without a 1:1 increase in longitudinal drag.

All of this makes sense. What I wasn't sure about was WHY you'd want to do this. Forgive me, but am I correct that the main reason is to resist sideways slippage and therefore promote faster forward motion? Some tricks benefit from velocity (air moves) while others suffer (spins). Do you choose a fin setup based on the tradeoff between speed and manuverability?

Also: Seems like a multifin setup would be preferred over a single larger fin. Looking "across" the board, multiple fins would sort of "shadow" each other. You might get 2-3x the forward stability without a 2-3x increase in resistance to spin. What has been your experience?

[Surfdad]

Do you choose a fin setup based on the tradeoff between speed and manuverability?

That's definately part of it and also release. You can move the fin pod forward on the board and increase the boards ability to spin. So it's not a simple matter of depth. A single fin at the tail of moderate depth will feel less manuverable than twin fins moved up away from the tail. Also when doing rotations in an aerial, the further forward a fin is, the quicker it will release allowing the rider to start his rotation earlier compared to say a single fin at the back center.

Also as Timmy! pointed out, the cant of rail fins help grab some of the upward flow of the wake, creating lift and easing hydroplaning. That can allow smaller boards, which are also more manuverable than bigger boards. Also, I've shaped finless boards that are significantly faster down the line than finned boards, it's a very different style of riding to control them though. They tend to like to carve and without the carving, start to slide. HOWEVER, while carving, they track those turns remarkably well.

I'd think that mostly it's a matter of rider preference, there are folks that just like the way a set of 4" deep fins ride and those that prefer a single 1" deep fin. BUT you're right when folks start swapping fins in and out you'll hear them say - that set made the board too stiff, or that really slowed the board down. That has to explain the myriad variations of fin sizes and shape, they attempt to give the rider a feel they like and address the way they ride a particular board.

That said, it's also possible to shape a board, itself, to mimic just about any fin set, without actually using the fins; with rail shape, tail shape, board length, etc.

[IDBoating]

Excellent data, thank you very much for taking the time to share it.

Are fins specific to board makers? For example, must I buy fins from Inland Surfer for my 4Skim? Or are fins interchangable to some degree?

[skippabcool]

Excellent data, thank you very much for taking the time to share it.

Are fins specific to board makers? For example, must I buy fins from Inland Surfer for my 4Skim? Or are fins interchangable to some degree?

I think it depends on what fins were used. fcs, futures, etc..

[Surfdad]

Inland currently does have a proprietary fin system that uses mini-tuttle boxes on their surf style boards, but I am not sure about their skims. A number of different removable fin systems are used by different manufacturers such as FCS, Futures, etc.

If I am not mistaken the 4Skim uses wakeboard style fins, with a 1/4 x 20 bolt running through that deck, I am just not that familiar with that style of fin and would have to defer to those more knowledgeable.