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[Poobah]

Ahoy,
I get a lot of e-mails with that subject title, but (sigh) it's never really about paipo design. A while ago I was at San Onofre talking to Terry Hendricks about ease of take off. I kind of had a headache so I don't remember exactly what he said, but I do remember that the square of the length of the board was part of some equation. Hopefully Terry will eventually stop by this forum and comment on the importance of length. Let's give him the specific example of comparing the ease of take off of two boards with similar bouyancy, but one is 4 feet long and the other is 5 feet long. Rod, please give Terry a nudge for us.

Poobah

[doc]

This is more fun than watching 'ask this old house' - trust me on this one......

Okay, you got a board 5' long and another one 4' long. Other than that, lets assume that you scaled up the 4' board exactly, by 125% in every respect except thickness, to make the 5' board.

Okay, the thing is that you increased all the dimensions by 1.25, but in doing that you increased the area by 1.56 ( 1.25 x 1.25 or 1.25 squared ) . There are plusses and minuses to this. The plus is that you've got the same weight ( mass) that it has to lift when it's planing, so the load per unit area - square inch, square centimeter, square cubit, whatever you like - is about 0.64 what it was , that is 1/1.56.

So, you can be going slower and get away with it. Takeoffs are easier and don't have to be on the ohmyjeez part of the wave, you can take off closer to the shoulder.

This is how longboards work and why you see lots of them out when the waves are blah. They have lots of area.

That's the plus. The minuses include the part where more surface area means more surface area causing drag, slowing down the top speed and turns and.... this is why short boards are a mite more popular than giant tongue depressors when it gets good and why a paipo is, potentially, the fastest thing in the waves.

hope that's of use

doc..........

[Solo]

Doc,

That criteria is true for solid/rigid prone craft, but there`s more going on:

An 18 ounce surf mat, adjusted low on air and supple as a trashbag, almost ephemeral, having much less planing area, can frequently drop into and trim across unbroken swells a good distance outside the nearest surfers.

I have a mat surfing friend from Australia who shares his home break with the only other person who`s able to sit so far outside and still catch waves. That guy surfs on a 10' plus x 8\"-9\" thick rescue board...

[doc]

Heee.... I had a feeling you'd be along, amigo, so I purposely left mats outside the discussion.

Thing is, modern paipo design has, I think, dropped the ball. Along with the newer as-stiff-as-they-can-make-em bodyboards. Flexibility, as found in mats, the original plywood paipos and flexible kneeboards, conforms to the wave surface, which means a helluva lot less drag. You're not pushing water, so you go fast!!

[Solo]

You mean to say that I`ve been set up? Ha ha! Excellent points, Doc!

I`ve ridden a variety of surf things... with and without fins... prone, knee, sitting, standing. It`s been especially interesting because of the bottom line: under the right circumstances, even the most crude plywood paipos can slice into and hold high through the steepest walls and tubes.

So the primary reason I began experimenting with stabilizers for various (finless) prone craft was not to keep them from sliding out, but to make them more directional, increasing their drive/projection through longer turns and trims.

The triplanes I`ve ended up building share some of the handling characteristics of George Greenough`s kneeboards, as well as our modern generation of surf mats: they can be trimmed extremely high across fast walls/tubes... but not necessarily by penetrating the wave`s surface with the rail line.

It was 30+ years ago that George told me the highest velocity on a wave could be achieved by riding across the upper wall of a wave in front of the rapidly unfolding lip... just like a gliding seabird. But NOT necessarily through a combination of hard, carving turns! To say the least, I was curious and a bit puzzled...

Considering that the context of those days was the shortboard revolution, and who was saying all this... I could hardly believe what I was hearing! George simply called it "fall line" surfing: instead of trying to force imaginative personal tracks all over a wave, pumping arcs up and down... his technique was far more subtle, utilizing a wave`s natural (linear) lines of energy to full advantage... minimum effort = maximum effect. He said fall line surfing offered the most obvious advantages to prone riders.

Of course, there were design problems to reconcile with solid, hard boards and such an ideal: to maximize velocity, the vehicle in question not only had be positioned high on the wave face, but also had to lay over nearly flat to the (vertical) wall`s surface. And any conventional surfing equipment in that type of extended high, flat trim soon washes out and slides down the face. George`s solution was simple: attach long, shallow runners on the front half of the board. The result was no more unwanted side-slipping in steep sections.

But having a shape that can sustain a long, high line, flat to the wave face, was not enough. Part of the goal was to create one design for a wide range of waves and conditions, and the slightest chop moving up the face would instantly catch a lowered shoreward rail. So the answer was obvious: chines... raised, beveled rails which significantly reduced the problem of catching outside edges, which also added extra side-to-side leverage for improved roll through carving turns.

Yet the uninterrupted angle between the chines and the central planing area created a lot of unwanted drag from the water bending around. In fact, the induced drag from that bevel is exactly what sucks modern bodyboards tight to the wave face.

How best to control and/or eliminate the drag? An obvious solution was two sharp strakes running the length of the board, dividing the chines from the running surface, about 1/4" tall. Water passing over the strakes from the central planing area was cleanly sheared off... also reducing the active surface area. The strakes efficiently separated the suction of the chine surface from the lift of the central flat/concave planing area.

Thus, by smoothly transitioning between carved turns and high trim
lines, such a surfcraft could finally achieve the goal of elevated, flat trim lines with a minimum of adverse design compromise. The whole idea is to develop a high level of balance between maximum speed and control... and one that FEELS good, too!

Prone riders can apply similar performance ideals in their personal design quests, either with or without stabilizing fins. Surfers need to know there are functional alternatives to a quiver of different shapes for different waves and different conditions. Have fun!

Solo

[doc]

Hee hee - lets just say I'm deferring to the expert in this area. Put it another way, I know something of paipos, a fair amount concerning kneeboards, but when it comes to mats I _know_ I am gonna be in your shade, so I'm not gonna set myself up for a fall.

Interesting - I'd reached the same conclusions concerning where the highest speed was available. Though I kinda approached it differently;

First off, how do you get speed? All other things being equal, it's all about power. Given that very few surfboards, and to my knowledge exactly zero paipos, have room to mount a 6-71 diesel engine on 'em, then you have to figure on the power of the wave.

Okay - so to maximise speed, you have to maximise the power. Where is the most power available on a wave? Well, for that I thought about just what is the power available on a wave.

Aha - gravity. You're going downhill on the wave, as it were. The steeper the wave, the faster you go, which indicates more power available. It also indicates that I never really got over my exposure to Engineering Mechanics II - horrible thing to do to a kid. The available power varies as sin A where angle A is the angle of the wave face with the horizontal. The higher you are on the wave face, being as they are curved, the more vertical, the more power available - so that's where you want to go to go fast.

I hadn't thought about bevelled edges and drag as a component of what makes a bodyboard (and, parenthetically, mats taking a high line??) work - interesting. The induced drag, then, is kinda like tip drag on an aircraft wing? And the strakes then act somewhat like the winglets/vertical tips you'll see on the latest mark of 747s and the like? In addition to giving a certain amount of directional stability?

This kinda has me thinking in terms of a flexible paipo or kneeboard with the rear half's outline shape parallel with the centerline and , if you will, a kind of downward folded edge along there. I had seen something like this, I believe, but I hadn't really considered it. Interesting.....

[Solo]

Doc...

To one degree or another... gravity, inertia and centrifical force are at work while surfing. But I think there`s also something else going on, other lines of energy running through a wave that can be tapped into... if you ride certain types of wavecraft. I`ve never felt it`s effects on anything but shapes which incorporate subtle, rounded bottom surfaces and soft, egg rails. It seems to impel single fins and finless designs the most forcefully. In contrast, ubiquitous flat bottoms with low, tucked edges and multi-fins are virtually unaffected, depending at least as much on the rider, as on the wave, for generating speed.

Long, walled-up waves are the best, but this effect is in no way limited to smooth, powerful and hollow. Standing surfers who ride "transitional displacement hulls" (see Swaylocks' archives) speak in reverent tones of the same phenomenon, a sensation of being "pulled" forward and through critical sections, with a minimum of rider effort/gyrations. Although it takes a while to anticipate and take advantage of this peculiar handling characteristic, the effort is worthwhile, especially in fast, sectioning waves.

An intriguing experiment is to buy a plank of bodyboard foam. I`ve commonly purchased mine as 3" x 24" x 108", Dow 220 Ethafoam will work fine. Cut to the estimated length (about 42"-44"), width(about 24")... leave it flat w/square template and rails. I`d take a sharp serrated knife (a 8"-12" bread knife works good), and Dragonskin or coarse, wet and dry 36 grit sandpaper/sanding block to the beach.

Find an uncrowded beach, paddle out in some good waves and catch as many as you can, carefully noting the blank`s feel and handling characteristics. A few of the first things noticed will be the flexibility, tendancy to slide, overall speed and the front corners trying to catch/dig into the wave face. So go in and trim a little off the front corner template, retaining the square rail contour... at this stage, don`t worry about perfection of form or texture of surface.

Keep riding and slowly whittling away. Work on a blanket, collecting your dust and foam scraps, put in a duffle bag or back pack. If windy, I`ve put up a small tent. Be patient, as this project can take weeks. Once the template begins to feel better, next contour the rails, beginning with the bottom`s edges. Again, I recommend making very slight changes, one at a time. If the evolving shape feels just too flexible, it can be stiffened by cutting lengthwise and inserting PVC or ABS tubing. I`ve commonly used two, widely spaced 1/2" or 3/4" dia. tubes, with the ends sealed. Then glue (commercial grade contact adhesive) the blank back together, let it fully cure for 2-3 days before using. A piece of 1/4" plywood or laminated fiberglass can also be glued to the top of the blank for stiffening.

If a stiffer, harder foam is desired, planks of closed-cell, higher density polyurethane foam can also be purchased, commonly sold as architectural/sign foam. The cell structure is much finer than Dow Ethafoam, and it can be successfully glassed, too. All the sign foam I`ve used has been beige/tan. It`s made in many different densities and free sample packs are available for evaluation.

If a person faithfully follows this process, and carefully records each separate change and it`s effects (if it seems like a painstaking hassle, that`s because it is!), the knowledge gained is absolutely priceless. I am not exaggerating. Along the way, you`ll discover many new things... one of the most important will be learning to associate sensations and effects with specific contours in relation to certain positions on different waves. The possibilities for seriously fun alternatives to mainstream surfing equipment are much greater than the media and the manufacturers would have us believe.

In the end, you`ll have a much greater capacity for experiencing all the joys of surfing.

[doc]

You know, you're kinda preaching to the choir there. Carefully noting what a little change does may be painstaking, but it's about the only way you're gonna learn anything. It's the subtle bits, y'know? And intelligent testing.

Just changing stuff and giving it a try and then selling it as new - agh. Gives you an idea why so many shapers tend to be clueless copyists.

Rounded curves...there's some Coanda effect at work in there maybe..... have to think about that further.

Now- I wonder if some of that urethane coating Bud McCray has been working with would be useful for something like this, just so that a turbulent flow layer isn't messing you up - that and mebbe gluing in a couple of fishing rods as you've mentioned elsewhere.

In the meantime, it's 8 degrees F outside - warmed up since last night. Might just see if the hot wire cutter I have in the cellar is still working.... but the actual experimenting is gonna have to wait a bit. Kinda ruins things if you're shivering too much to hold onto the pencil......

[Solo]

Doc...

"Now- I wonder if some of that urethane coating Bud McCray has been working with would be useful for something like this, just so that a turbulent flow layer isn't messing you up - that and mebbe gluing in a couple of fishing rods as you've mentioned elsewhere."

Fishing rods work o.k., and laminate winding your own tubes will often do the job even better. Lexan (polycarbonate) is better than Sintra (PVC sheet)... and it can be glued inside the flex foam on edge (ala surfboard stringer), cut to the right bottom arc, and will allow rocker torque, but not increased rocker. Solid, extruded fiberglass rods are great. I`ve used a lot of them because they`re not as likely to fatigue/break like hollow fish rods, they`re easy to sand down and taper for improved flex, and they`re heavier... better for bigger, choppier conditions. The ABS and PVC tubes are cheap, readily available (plumbing supplies), quite strong and have reasonable flex properties.

Smooth coating an evolving blank is a good idea but not necessary in the early stages of discovery, especially if careful tooling, sandling or higher density architectural foam is used. The idea is to have a reasonably functional material (and not necessarily permanent) that can be shaped/modified "on-the-spot", to paddle, surf, can be quickly altered, and then immediately re-surfed. Structurally, that`s difficult to do with surfboards and kneeboards, but not always with paipos.

A cheap, smooth seal coat that abrades easily is acrylic latex house paint, but it takes overnight to dry thoroughly. Several thin, smooth coats of commercial grade contact cement will also work. I`ve shrink-wrapped blanks with polyolefin for a cheap, quick, super high gloss finish. Wet sand the deck with 220 grit for nonskid/wax base. The shrink wrap was plenty durable for my short-term purposes. A minor downside was that it wouldn`t conform to contours like concaves... it basically wants to pull tight around curves, and just lay straight/flat.

On some projects, I`ve taken files, wet and dry sandpaper and screwdrivers out in the water to modify fin/board details between rides.
The closer you can bring actual surfing to hands-on changes in design, the better. In general, the smaller the surfcraft, the quicker/easier that process becomes.

[Poobah]

What kind of glue when putting a lexan strake into a slice cut into the bottom of the booger foam?

[Solo]

Poobah,

The adhesive I`ve used most with flexible PE foams has been commerical/exterior grade contact cement, i.e. neoprene (not water-based) "formica glue". A good one is DAP Weldwood Contact Cement. Barge Cement is also an outstanding product.

The Lexan I`ve used has been 3/16" thick. I wiped it with lacquer thinner or MEK, then thoroughly roughed it with 36 grit. I usually applied the contact cement in light coats, drying and joining as per directions. I also roughed the foam's surface, applying 2-3 light coats of contact cement, really working it down into the open foam cells with a bristle brush.

You have to be careful when bonding two dissimilar materials as Lexan and polyethylene foam. There can be a lot of point loading, especially along glue lines. Balancing the effects of structural stress are very tricky with soft/flexible surfcraft.

I`ve also used a lot of 1/8" to 1/2" (smooth finish) Volara and Minicell foam laminates over the much rougher polyethylene cores, and finally vinyl sheeting/vinyl resin for design permanence.

[doc]

Hmm- now you've got me thinking about something that could be

tested/adapted for hull shape and flex

then, serve as a male mold for glass/foam/glass flex boards. Glass it heavy and ugly on one side, then that glass would be popped off and become a female mold for vaccuum-bagged glass.

Thinking about it, the latex paint you've mentioned would serve as an adequate barrier coat to use with PVA or waxes to keep things from getting kinda stuck. The paint itself isn't especially tough stuff, so if a little stuck to the glass it'd still release and it'd also allow polyester resin to be used for the female mold, no matter what kind of foam you'd used.

But, that's a spring project - a mite brisk still for laminating just now.

I am rather tickled with this thread, not least because when I get an idea I look back and see that no, you've pretty much covered that question.

Though here's one that hasn't been covered, I don't think; have you played with extruded glass rods laid in on a line that's not paralell with the centerline of the board? Or bands of directional-strength cloth to kinda tweak the flex pattern?

[Poobah]

Perhaps something perforated. Maybe if you routed or bored a sheet a lexan to look like a garden lattice pattern.

[doc]

uhm...that'd be an awful lot of work for something that'd behave about the same as slightly thinner lexan without holes. Plus, lexan sheet can be kinda funny that way - when you rout it it tends towards cracking and other no-fun stuff.

If you want to use mesh, it could be a simple, low stretch type as used in things like catamaran trampolines. That's pretty cheap, vinyl covered so it'd take glue well and hold up adequately. seewww.sailrite.com for more on that......

whaddya know, some of the boat canvas business is relevant for surfcraft.............

[Solo]

(Doc): "Though here's one that hasn't been covered, I don't think; have you played with extruded glass rods laid in on a line that's not paralell with the centerline of the board? Or bands of directional-strength cloth to kinda tweak the flex pattern?"

Yes to both. Nearly all the basic materials and features in present day bodyboards were experimented with over 25 years ago, including other things that no one will ever see because they're gone. The reality is that such mainstream equipment is largely driven by cost, i.e. means of mass-production, distribution, marketing, etc. A lot of really good ideas never see the light of day.

In contrast, with the internet and all the materials available, there never has been a better time than now for individuals to bring their personal surfcraft dreams to life.