The mechanics of turning a surfboard

General Discussion
1

Imagine you had a board with a parallel outline, the proverbial barndoor, although a fairly narrow door (let’s say 8’x21" wide just for good meassure). Put your rear foot a the tail and the other foot a good bit further forward, lean on the back foot and the tail sinks. Now do the same thing while leaning it slightly on edge and it starts to turn. The downward force in the tail is larger then the center, put it on an edge and you get a sideways component that is larger in the tail. (this is how snowboard turn in deep powder)

However, there is nothing to stop you from spinning once you return the board back to flat. It keeps spinning. You either need to have it on an edge at all times to control it or keeps spinning (This is how snowboards need to be controled). So you add a fin to stabilize it. You note that the fin is perpendicular to the rotating motion when flat, causing stability, however the more you lay it on an edge, the less attacking surface it has against the rotating motion. When the board is perpendicular to the water, the fin is paralell to the motion. Good. Also the surface of attack of the fin is at an angle forcing the tail into the water. Good.

Now narrow up the tail. The force you apply with your back foot is the same, however the area is reduced thus the force pusing back is less. It’s easier to push it around. Good. It sinks in the wave face creating more restriction. Bad. It adds some control. Good. Conventional wisdom of surf design says a curvier outline turns better. But isn’t it just about redusing the restricting force (tail area) when pushing the tail around?

Now add a vee to the tail. Put it on an edge. The center of the board now has a higher angle of attack then the tail. The higher angle of attack in the center is restricting more than the lower attack angle in the tail because the attack angle causes there to be more upward force then sideways. So the tail will slide better while the center is more restricted. Conventional surfboard design says vee alters the rocker at the rail, however does the rocker at the rail at 30’ radius make a significant change from the center 40’ when the turning radius of the board is actually 6’ or less?

Now add rails and foil. Make the center thick with thick round rails, tail thinner with hard down rails. The round rails grab water, the tail release water with little drag. Thus the tail slides while the center is more restricted. The center volume is larger than the tail which causes more bouyancy in the center thus a restricting force, however the effect of this is propably very small at planing speeds.

Now add rocker. The angle of attack toward the rotating motion of the tail now points backward, reducing restriction at the tail and redirecting the waterflow towards the tail end of the board. Does this create (some) drive?

Now add twin fins, thrusters, etc. angled fins to the mix.

…

Did I get anything right?

Where does concaves fit in the mix?

2

Hi there, Haavard! I know long time passed since you wrote this post, but I can not let it go without congratulate you for such simple, compact and precise explanation. This is an idea(would say cientific theory) most of surfers, boardriders, and few( I mean a lot) shapers didn´t get yet. Most still sustain their surfboard design on hardedge “more bite” and softedge “less bite”, meaning this “bite” the oposite to sideslip/release. The same way with thickness, people evaluate it independent of motion surface caracteristics(glide/planning). Where it looses almost every bouyancy action, as you well note in your text.

About the rocker many see it as a feature dealing with how steep waves a `board can handle(because of wave face shape), forgeting turning radius follow up(less oposition to turning motion/more turning drive)(???). Some use it for turning/carve capability adding rocker to the tail/kicktail, but only thinking about it as a easyer lean back feature.

I have some “dubious” ideas on certain features and here I would like to ask for your precious “cientific” knowledje.

Taking your last question on concaves, I really don´t see it much as a “water channel compresser… througt the fins”. But doing it job much as a intensifier of the angle of attack/bottom/vee and the rail/foil turn function. Any how, for me, what a concave makes is to lean the surfboard bottom face before the surfboard is leaning itself, taking your words it is raising the angle of attack and restricting more water than a flat regular board would. DO YOU AGREE? (making the board to turn quicker and harder).

Another question goes on the paralelism people do on surfboard rails and airplanes wings leading edge, on wich I don´t total agree. Only because interaction object/fluid is completly dissimilar.

Others features I  am not completly clarify are the ones around the fins. I know, or I think I know, what each different design fins placement do. My problem goes on the mechanics of the thing, mainly the “toe-in” and “cant”. Also the fins placement relativ to the outline, is always making me think a lot, because I feel we are dealing with a not so linear subject.

I would be very gratful on your reply, Haavard!

Thank you very much.         Miguel

3

  You fail to take into account the size and speed of the waves, causing the surfboard to plane at totally different speeds.

  There are waves that you can ride a 7' x 16" wide board with a 10" tail.  There are waves were such a board will never even catch a wave!

  There are waves that you can ride a 6"x 22" wide board with a 18" tail.  There are waves were such a board will NEVER turn under any control.

4

Thank you LeeD for replying. I am not getting your idea, I understand what you say(and agree with it), but not getting what you mean. Also is possible my Inglish fail and I`m not making good sense.

What I can tell you is:  While studying a certain feature you have to keep the others on the very same levels as possible. For example you dont compare two different boards, taking one under some wave conditions and the other surfboard under different wave or wave conditions.

But what I really am after is the mechanics and dinamics of fins placement.

THank you once more.    Miguel

5

the answer is the question.

 

are you experienced ?

 

or have you ever been experienced ?

 

herb

6

It’s all in the hips.

7

I´m not getting it Herb! Or… Well… Course I have some ideas already made, the thing(fins placement theory on turnig ability) is so complex and with so many factors… I was, I am expecting someone would have it clearly defined. The same way Haavard have it for the rails shape, width, rocker when talking about “…turning a surfboard”. A lot of variables on game but he explaines it simple and bright.

Thank you for watching and share.      Miguel

8

don't think it too much...................

rather .............just flow with it.

surfing is not a fighting science.

it's a dance lesson.

herb

9

Thank you for your words Herb, words of wisdom…  as I said before I have the “thing” more or less locked for my likes. So, just trying to get someone else “rules” for making some comparisons, for getting other people thoughts(knowhow) and much probably to learn something else.

Thank you all.        Miguel