Kcasey Why does Vee help a Twin fin to put it on a rail?

General Discussion
1

Hi Kcasey,

Supposedly Vee helps Twins to bank them on a rail to turn. Flat supposedly is harder to put a twin on rail.

But how? What is the explanation?

i tried… but cant figure it out when comparing Vee to flat and Concave…

Would you or anybody be able to explain clearly?

I know that vee puts the board deeper in the water. Less planing.

Would then more rail be buried from forward on, which helps to put the twin on rail? Ie, suck the rail more into the water? Which with enough rail curvature helps to turn more easily as a result? Or does it have to do with other elements?

Wouter

2

my guess is that it would act as a tiny trailer but i’m no expert

3

Nice.

With respect to turning, at the moment three things come to mind regarding bottom contours in general, but vees and concaves in particular.

  1. Contours contribute to ‘fin’ – the lateral walls of contours tend to increase ‘fin.’
  2. Contours contribute to ‘rocker’ - contours tend to increase rocker in general, but also in a more rider controllable way, again it’s the lateral walls that allow for this.
  3. Contours have their moments – physical moments that is, this is a bit more involved, but diagrams will likely suffice.

But as you have touched on already, there’s obviously more.

Sounds like I’m about to go off on a bunch of tangents, but I’ll try to keep it short. Comparing the different fin setups, will be easier if, at least, the above three points are touched on.

I’ll finish this weekend, hopefully with some diagrams.

I don’t want to hyjack the thread, but this does seem like a perfect opportunity to explore Kendall’s comments in the bonzer theory thread regarding tweaking apparently different design elements or combinations of elements to get a similar effects -or what I referred to as convergence of function, at least as it might relate to concaves and fins. But I will try to control myself.

kc

4

Kcsey,

Dont hold back.

And what exactly about those ‘lateral walls’?

Just for arguments sake, lets keep the rail rocker the same for all 3 possibilities, and just think about why Vee would put your board on rail earlier then flat or concave.

Wouter

5

More significantly than what’s already been mentioned, I believe, is that vee lets you add rocker to the tail rail and keep the stringer rocker flatter. It’s the added rocker along the rail in the aft section that increases turnability. This holds true for both panel and rolled vee.

Vee also helps feed water into the fins, helping them do their magic. So fin cant/toe/foil/placement should be taken into account when you design… do you want it to drive? turn easily? create lift? control?

6

the question is not why it turns better, but why it is easier to put it on rail?

7

8

Rocker in the tail rail, as stated by NJ… above, performes both functions you mention. If the question is asked at the workshop, I’ll cover it in detail then.

9

Thrailkill,

How many wars did you see in your time shaping?

btw, i cant come to hawaii right now for detailed explanations, so could you elaborate on it please? Why it is easier to put it on rail?

Thanks Thrailkill

Wouter

10

Lockers…

People now mail me on my private address saying it is sooo easy to figure out, but i havent.

How does the water flow when you have a Vee, and how does the water behave at the Vee peak, how come my torque is more effective?

Hopefully someone can explain it instead of acknowledging it, i already got those ackonwledgements. I want to understand the principles… the hydrodynamics…

Thanks for helping out.

Wouter

11

In order to keep me sufficiently restrained, I would like to get a bit of language out of the way.

Take a peek at figure 1. It’s my understanding of what you want to use as a method of comparison i.e. the ‘rail rocker’ comparison. Mind you this isn’t going to change the arguement, just the language.

I have to admit thinking in terms of the ‘miniumum rocker’ language - for me it reflects the factor rocker, if any. But I’m not trying to make a case either way - it really doesn’t matter - I just want to be able to communicate.

Anyway, I’ll just assume as much - that you wish to use the rail rocker language for now.

kc

Added in edit…

Also, just to be (disturbingly?) clear… the following figure suggests the kind of rotation suggested by the phrase ‘on a rail’

By the way, maybe you can see the problem, at least in terms of ‘visual thinking’. The rail rocker comparison doesn’t necessarily put the waterline at the same level in both cases, but the suggestion from looking at the vee is that it might. The other minimum rocker comparison method doesn’t seem to force the issue as much…

Okay, it’s just semantics, but aside from eating and shitting semantics play a pretty import role in our existence.

Added in another edit…

Thought I’d throw this up too.

12

From a PM…

When you turn, the forces on the inside rail tend to want to “right” the board. These forces are larger on a board with concaves, if all else is equal, simply because the angle at which the water leaves the rail is different.

13

Wouter,

You get the short answer. Primary cause, rocker in the tail rail. Secondary cause, a more rapid disengagement of the opposite side fin, which reduces the tendancy of the board to track. If the fins are toed-in, that also adds to the speed of initiation, but your trade off is a slightly reduced speed. All these elements conspire to make it easier to initiate a direction change, which when done abrubptly ‘‘puts you up on the rail.’’ Is this helpful?

14

I believe you are right about the planing forces that are developed for both concaves and vees, or any combination of. The total lift component which is directed up and through the deck of board is reduced, but you get an addition lateral contribution, and because of the increased surface area, you do get more drag – which is not something that’s usually associated with contours.

Constraints

If by some action a surfer changes the bottom presentation of his board to the flow, the flow will be impacted, as he performs the maneuver. One aspect of why vees go up on a rail easier than concaves is because they tend not to constrict the flow like concaves, they let it escape easier.

The trapping or additional constraint on the flow, means that the flow must be forced in other directions, or subjected to additional forces, to get out of the way. That is, additional forces must be applied to move the fluid in addition to the center-to-rail direction, but back, or maybe even forward in a direction, at least parallel to the nose-to-tail direction. You could also argue that some may be being pushed vertically, down with respect to the deck of the board, but in the end, you got to do more pushing –i.e. apply more force – all else being equal. The same true when comparing a vee to a flat bottom board, just less so than when compared to a concave.

But you’ve got to be cautious here, and this relates back to your point about the forces developed during planing, which is very much a function of wetted surface area. In the above paragraph I stated ‘all else being equal’, but things can potentially change if you go far enough up on a rail. If you want to take the thread in this direction (no pun) please let me know. Also, there is also another very important matter to consider, and that is what the surfer is doing. Going up on a rail generally requires that he shift his weight, and by doing so he changes the moments, or torques to which his board is subjected to -this relates to my next point.

Stable States

Once the vee is up on one of its lateral walls, it enters into what you might call a relative stable state. This really isn’t so for a concave or a flat bottom, which don’t have this additional stable state because of design. The existence of the addition stable state is likely to be perceived by the rider as ‘ease’.

The behavior of vee’s is part perception, that is what the rider is likely to attribute to some effect, or how he is likely to explain some effect - and, man does that open up a can of worms.

kc

PS

In the end, the approach I’ve taken makes my concern for terminology moot. Which is good, as I didn’t want to redraw the diagrams I had started - let alone finish them.

15

Yes

especially the rapid disengagement of opposite fin [out of water earlier] was new to me, never thought it would have such an influence

thanks

16

Contouring the tail for a vee bottom and contouring the tail for a single concave leaves the bottom rocker with a much different affect on performance. The vee tends to put th e board down in the water and engage the board while the concave tends to make the board run high and light. A vee bottom tail can have pretty much the same rocker as the rail but a single concave will be hard pressed to have the same rocker as the rail. The single will almost always have less rocker than the rail. Old school boards with flat panel vee transition well from rail to rail with very little tail rocker. So generally speaking more rocker has to be worked into the rail on a single concave than a vee bottom if it’s going to transition from rail to rail well. Fin configuration will dictate much when it comes to how much tail rocker is foiled into a board. As far as how much whetted surface is engaged during turning is concerned that will depend on how steep the turn is. As the board is laid over more and more in the turn the depth of the concave, whether from single or double will determine how much bite the rail will have. The tail arc of the rocker that’s engaged in the water will determine how quickly the board will tend to come up the face of the wave or hack off the top. The classic hack is done by disengaging fins entirely and spinning the board in a nearly full stop position. Tail rocker and bottom configuration play a huge roll in affecting this, what one might call, ultimate transition maneuver. One the other side of transition there is that of propulsion which is created by how the fins and board foil compliment each other while they embrace laminar flow and drive with it.

There’s a hell of a lot more going on that what I’ve stated above.

No Worries, Rich

17

Vee allows the board to rock from side to side - or rail to rail - easier. The bottom of a board wants to stay flat against the water’s surface. Vee breaks a single flat into two, so the board has to be angled to be flat against the water. The transition from rail to rail is easier because you’re not fighting as hard to overpower the flat bottom. Your leverage to change direction is increased because when you’re on one side of the Vee the opposite rail is higher, giving you a fulcrum effect.

.

18
Quote:
Hi Kcasey,

Supposedly Vee helps Twins to bank them on a rail to turn.

Would you or anybody be able to explain clearly?

Wouter

Aloha Wouter

The reason a vee bottom is easier to tilt up on the rail is really quite simple.

If you consider that rocker describes the lengthwise arc in the bottom, Vee describes a side to side or crosswise arc in the bottom.

And… just as more rocker allows a board to be tilted easier lengthwise, so does Vee allow a board to be tilted easier crosswise.

Even without forward motion the effect is noticeable. Just like comparing a deep Vee cigarette type boat hull with a catamaran at dock. Regardless of the amount of lengthwise rocker in boat hulls or surfboards the Vee bottom ones will tilt side to side much easier then a catamaran. The “outrigger effect” of the catamaran which increases stability, is greatly reduced in a Vee bottom and because of this, allows more movement crosswise with less effort.

Widths matter of course, so that has to be taken into consideration but even with the same widths, the Vee bottom will tilt more easily from side to side. Since traditional Twin Fins were generally wider and shorter and that extra width made them harder to tilt up on the rail if one was standing in about the same position as they had on a narrow tailed single fin. The Twin required the rider to stand out more near the rail to leverage the board up on the rail. If the Vee was increased the amount of effort to tilt the board was less. And one didn’t need to stand as close to the rail. Though they still might and as such jump performance to another level.

Vee impacts the water first as it is lower then the rails. This creates a fulcrum point to tilt around. Consequentially, more force is accumulated at the Vee allowing the rails to be more free. Additionally, the greater angle from the center line to rails allows the water to release out toward and off the rails more easily. A concave (catamaran) by comparison resists the water flowing out to and off the rails making it more difficult to tilt the board onto the rail.

As Bill T mentioned, releasing the outer fin is important. Twin Fins have much larger fins then do Tri Fins and getting that big outer fin out of the water can sometimes be a problem. A Vee bottom has already lifted (or shortened) the outer fin somewhat.

Hopefully, I have hit on some points that will help.

19

lovely, it has revealed more of how to think about it.

thanks everyone for the input

20
Quote:
Wouter,

You get the short answer. Primary cause, rocker in the tail rail. Secondary cause, a more rapid disengagement of the opposite side fin…

Who needs to disengage the opposite side fin?

:wink: