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

General Discussion
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Here’s yet another diagram. (Couldn’t resist.) It’s a topic we touched on earlier.

You’ve got to watch the general statements about lift and drag for concave versus vee. As you can see, for the case of conserved tail width, the forces aren’t really all that different for both lift and drag for both the concave and vee, which is not what you hear, or read a lot. There are other differences, and one of them is the constrainted flow associated with a concave, which actually creates more drag. Which isn’t always a bad thing, in fact in surfing it’s a good thing up to a point of course.

Sorry to tricle this stuff out. Diagrams are nice, but they take time.

kc

[center]

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…trickle trickle…

Taking ‘force developed’ a bit further.

In the figure below are three strips, basically longitudinal sections through a flat bottom, a concave and a vee. I’ve also included suggestions as to the directions of the forces developed for a particular scenario. (See my last few posts in the thread Creative Drag:… actually I’ve left out a lot of descriptive detail, which is contained in the Creative Drag thread, and maybe elsewhere too.) I’ve only indicated the extremes - roof of concave, rails, area of apex of vee - perhaps you can fill in the rest – grading one into the other.

It’s all pretty crude, but it should provide a sense of what I wanted to point out.

In a nutshell, I’m inclined to believe that concaves offer a quality difference in the ride, as well as other points already mentioned in a prior post (fin area, etc.) and that quality difference is one of stability, or at least the sense of stability by the rider. The vee offering a more particularly more stable ride when up ‘on a rail’.

The propulsive forces developed will primarily depend on the surface area interacting with the flow, and the characteristics of that interaction. Just carving a contour doesn’t guarantee you anything – for example, what is being preserved, total surface area, or transverse (cross-sectional) width, and are the lateral walls, for the concave and vee, similar in the way they are developed relative to the average bottom plane.

With respect to contours in general, I see it as a matter of choice, loose a little lift in one direction, gain a little in another.

This is not to minimize how contours might impact certain maneuvers like pumping. The constrained flow of the concave is likely to enhance the pumping effect.

This whole approach introduces questions about a lot of other design elements, for example a role played by rocker. Here however, the approach has been in very general terms.

Shaping is wholistic. Analyzing individual elements is always risky.

kc

ps

This was done pretty quickly … Hopefully I got it right, but I reserve the right to correct any of my nonsense.

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Quote:

(snip) Fin configuration will dictate much when it comes to how much tail rocker is foiled into a board. (snip)

Rich,

Lots of great info packed into a small post – thanks!

Can you go into some detail about the relationship between fin config and tail rocker?

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Yo etmo,

With a relaxed tail rocker the board becomes more point and shot. You change directions when the wave backs off and fire down the line with speed because a relaxed tail rocker affords trim speed. Because a flatter tail rocker is more engaged in the wave face while the board is under trim the fins require less engagement and less drive than with a board with a more accelerated tail rocker. When you have more arc in the tail rocker the board will not trim a fast when the tail rocker is engaged with the wave face. It’ll will be more powerful when turning so it stands to reason than greater fin area will be needed to propel the board. Modern power surfing is accomplished by complimenting fin drive with tail rocker and bottom configuration. This is an oversimplification but it addresses you question as directly as I can conjure. Remember that simplifying surfboard dynamics is just about impossible.

Nahalo, Rich