rodndtube.com's (OLD) MyPaipoBoards Forums. Reading but no posting on the OLD forums.

[baidarka]

Poobah,

Under the scornful eye of my physics professor, I once constructed a boomerang-centric board. In my ignorance, I was hoping to create ‘The Leashless Wonder,’ a board that would always return should it get separated from its rider. Alas, the day came for a trial run. The board, after being let go, was never to be seen again. I believe it to be adrift somewhere between Oregon and New Zealand. Perhaps it is worth a second attempt.

No scallops for now, but they are just a Dremel away. I hope to finish gluing up my fin today. I’m also keeping an eye open for large diameter plastic pipe scraps at local building sites.

Happy Returns!

-Seamus

[RoyStewart]

Hello Tumak,
Thanks for your encouragement. I have just made my first visit to the wonderful lamaroo site. Fascinating. I like your hurricane and mysterious turtle. I look forward to applying for membership of Poohbah's exclusive club on behalf of my new paipo which is on the drawing board.
Buddhism is a good idea. As far as keeping my cool goes, it is easier on the end of a computer than in the flesh!
regards, Roy.

Here's some more info to add to the soup.

Regarding flexibility have you noticed the parallel profile which our
boards have? This profile allows the boards to have a springloaded
flexibility, much like a longbow. Regular boards with a tapered profile
resist flexibility due to a truss-like structure and design. A lot has been
said over the years about flexibility but it didn't come to much because no
one seemed to realize that the power of a flexible board has to be
harnessed. If a board flexes with an upright fin setup it achieves not much,
the upright fin just slides up and down and the power is lost. If, on the
other hand, the board drives a horizontal fin surface up and down (e.g. a
tunnel), then the power is directly transmitted into forwards motion, just
like when a dolphin wags its tail.
I built a 14'9" called the x-15 which was only 2 inches thick and used
this principle (pictures on my site). The acceleration was phenomenal, any
subtle pumping motion or chop bouncing would make the board leap forwards
like a live thing.
This is one of the two main advantages of the parallel profile
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www.woodensurfboardbuilder.com

[MTbarrels]

Roy

I'm having a problem with understanding what you mean by the term "slip". Would you please elaborate on that (or give a precise definition)?
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MTbarrels

[RoyStewart]

Hello Empty Barrels,
I assume that you are talking about my use of the term slip when trying to describe tunnel foil behaviour. What I mean by 'slip' is what happens when you push on a fin sideways and it then moves sideways through the water. This is what happens if your fin spins out, but it can also happen to lesser degrees and in a controlled way.
Tunnel fins resist this sideways slippage (sideslip) under pressure when moving fast, but have very low resistance to it when going slowly. At critical low speed and sideways force combinations the tunnel fin will drift or slip sideways almost as if there is no fin there at all. When you release the pressure a bit or pick up speed some more it will suddenly 'snap' back into tunnel mode and zap off forwards again as if it has a life of its own. This makes them very entertaining to ride. For example if you are cruising on the shoulder and pull a cutback you can drift the fin out, spinning the board around, and then recover easily and keep surfing.
Unlike a conventional fin which doesn't seem to resist spinning out a lot better when going fast than when going slowly, the tunnel's resistance to sideways force (any force at right angles to its path) increases exponentially as you go faster
Please let me know if I didn't give you the info you are after

Roy
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www.woodensurfboardbuilder.com

[Poobah]

Roy,
it might help to define the term "pure tunnel fin." When you use that term you mean a tunnel fin with no vertical fin in tandem. Right? It's not tubular like a nacelle, but more like a half pipe. Some folks might jump to the conclusion that pure tunnel fin means completely round or tubular tunnel fins. Have you considered another name? Getting away from "tunnel fin" and "hydrofoil fin" entirely. Maybe something quasi Latin, like "Orbulator" or perhaps "Orbiplane" (orb-i-plane) ???

[Poobah]

I'm also curious about some things you may have already tested, like fillets only on the inside of the tunnel. Or varying the length with a cutaway near each base. Flatter sides more like a railroad tunnel?

And back to the side slip thing...do you have to be careful about angling the shoulder too much on a takeoff?

[RoyStewart]

Ok Poohbah,
Orbulator it is, at least around here. We now refer to the 50% pure tunnel fin as Semi-Orbulator and the one with a single as hybrid semi- orbulator. This can easily be customised for other additions e.g. hybrid semi- orbulator/square/mental projection. And so on.

About that angling to the shoulder query re. semi orbulator it is definitely the case that you will be able to sideslip down the face if you aim way down the line with zero speed. Whether or not this would be in the category of involuntary/ignominious defeat or voluntary/heroic victory depends on the exact variables. Certainly it is almost impossible to sideslip a good hybrid semi-orbulator.

Roy
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www.woodensurfboardbuilder.com

[RoyStewart]

Poohbah (Grand)
THere are various possible orbulator configurations. Remember the 'turbo tunnel' fin. They are actually, technically speaking, to be referred to as hybrid mini-orbulator/dysfunctional.
Square tunnel fins are probably a good idea for those who have cube shaped heads.
You can play with the Orbulator planshape e.g. cutaways no problem except that the whole idea is to contain the water flow inside the orbulator tube so that it can only escape by squirting out the back. Projections and cutaways are going to reduce the effective power of the device.
Technically a fillet is lumpusunnecessary/slowdownus. but they can be used as a last resort for fin attachment.
Roy
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www.woodensurfboardbuilder.com

[MTbarrels]

Roy,

Actually, what I had in mind was "slip" in regard to gliding flight. But you clarified what you mean by the term--it's essentially a measure of the component of the motion of the fin/wing/object/whatever that is pependicular to the free stream velocity vector.

The next question is: How do you know that a gliding annular wing has zero angle of attack? I know that I sure can't visually detect a small inclination--say a degree or less--of the axis of an annular cylinder in flight from orientation of the free stream velocity vector.

MTB
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MTbarrels

[RoyStewart]

Hello Empty Barrels,
Wings which generate lift using Bernouilli's effect are able to operate at a positive angle of attack and are designed to do so, particularly for low speed flight.
Wings which don't use Bernouilli's effect include some high speed wings (which are symmetrically foiled, that is on both sides), and annular wings.
High speed wings can't operate at a significant angle of attack. An annular wing will not generate lift using Bernouilli's effect, and is not designed to fly at a positive angle of attack. I suppose that it is possible that some annular wings might be able to fly with a slight angle of attack but this would be inefficient, and will be self corrected by the wing which always tends to return to a zero angle of attack. It will depend also upon the ratio between the thickness of the wing and the length of the wing (I mean the proportion of the actual spar, not the overall height of the tunnel). Any wing section which is extremely thin and is not foiled at all will have to have a zero angle of attack, but if the wing is thicker and has some curve foiled into it, it will be able to accept some variation in the angle of water or air flow without stalling. In spite of this possibility, the annular wing will always tend to return to a zero angle of attack.
This tendency to return to a zero angle of attack is why the annular wing has so much drive as a surfboard fin. A board with a an annular wing fin is able to turn, but will constantly try to return to a position where the water is flowing directly through and past the fin.
When visualizing angle of attack with an annular wing it should be remembered that because the wing is circular, we are not just referring to angles in the vertical plane, but also in the horizontal and all everything in between.
Regards, Roy
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www.woodensurfboardbuilder.com

[Poobah]

Dead link removed.

[Nels]

Poobah,
I'm not going to ask how you found that one. It's either faintly pornographic or somewhat elegant, like much of French culture.

In the spirit of anarchy, here is a link to a Latin/Euro invention which has a wing configuration elsewhere, reminding me of serious private communications with other people who have postulated that modern bodyboards would be faster if ridden upside down and backwards...

http://www.everwave.biz/

Please don't let this digression kill the thread though. Although in another life I spent a decade in aerospace work, it wasn't in wing/lift areas and this whole thing has been fascinating.

[Poobah]

Sponge riders might be the best market for semi orbulators. Something that bolts through the deck like a traditional bodyboard fin. Perhaps with the addition of a flat base plate (longer than the foil) that adds some stiffness to the tail of the bodyboard. Kill two bodyboard problems with one accessory.

Roger Wayland has used pipes and gutter tubes on handboards. That's an area that seems ripe for more experimentation. How about a semi orbulator for bodysurfing that straps onto your torso?

[Poobah]

Dead link removed.

[RoyStewart]

Those traditional torso fins and oblique orbulators must have taken a bit of finding. MTbarrels and I have been discussing hydro and aero dynamics. Nothing much has changed for practical purposes regarding orbulators, except to say that an asymetrically foiled orbulator fin (not foiled on the inside) will effectively have to be set up at a very slight angle of lift in relation to the bottom of the board in order to to achieve exactly the same effect as a symmetrically foiled one. Paipo Jim was partially correct (and partially incorrect).
This is cutting a long story short.
Roy
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