Kneeboarding At 3 G's

[Man O' War]

Mark, so good to meet you here! Glad you had a good experience today and are so positive on the whole thing. May you be blessed with the Maldives' best so you can give it a real workout. Remember us when you come back.

I'll tell you how I did the glassing over the Reynolds wrap to get the hull. Forgive the non-technical language.

I shaped the bottom of the blank up to the rail line and left the top of the blank just as it was, skin and all. I ran sheets of regular aluminum foil (12") across the bottom side to side, overlapping each other by about 6" and hanging off the rails by about the same. I didn't tape down the overlaps. I did tape down the foil ends with duct tape on the deck side, stretching the foil tight as I went. The foil can't shift while you're glassing.

Some folks like to spread a thin layer of Vaseline or some other release agent over the foil.

The rest was easy. I laminated 7 full-length layers of 4 oz. right over the foil, trimming along the rail line and tail after each one. Then I did 5 partial lengths of 4 oz. The first layer started 4" from the nose and went to the tail. Each succeeding layer was 4" farther back than the previous, and all ran to the tail. That totals 12 layers of thickness in the back 46" and 7-ll layers in the front 20".

After a couple of days of curing, the hull felt stiff enough to remove. I went around the rails with my fingers, working it loose, and the whole thing suddenly popped off. The foil was still clinging to the shell. I had the almost mystical experience of peeling the foil off the hull and gazing on it for the first time, looking at my hand through it. Amazing vehicles.

Just for your info, after the rails were poured and shaped, the deck got the same number of layers as the bottom, and I staggered the last five the same way.

All told, the back 4' of the board got 24 layers and the front 1.5' got 14-22, depending. I later added 8 more, rail to rail, in the knee/stress area, the middle 3' of the board.

Does that help?

[Flexman]

.

[Man O' War]

Good stuff, Dale. ksusa has helped bring together some unbelievable riders. What if all of them had the vehicle and the chance to charge through these gears! What would they be feeling and what would we be seeing, e.g., on a big day at Blacks. There's probably a 6th gear out there... One of the beautiful things about flex is the way it pushes out vehicle limits, like afterburners or steroids. Look at the difference between vaulting with a cane pole and a glass pole. And you can't always be sure how it's going to react. The unpredictability helps keep things fresh.

[MTBarrels]

MTB - Thanks for satisfying our curiosity about "C II" ...<snipped>...What about the glass in the tail? Does your foam run all the way back? Also, fin depth and placement? ...<snipped>...

I think your questions about the foam, fin dimensions/planform/placement, (and flex) are best answered by the 2 pics (Cypselurus II is the second from the left in the first pic):





...keeping in mind (for scaling purposes in the second pic) that the overall length is 68 inches.

...<snipped>...After ten years you obviously know your board top to bottom, have its strengths and limitations dialed in, and cherish it. This spoon found a good home.

Actually, the "father" of this board is "Cetor" -- a board that I built in 1973. I liked it so much that it was my principal board between 1973 and 1994, (and I probably rode it 95%, or more, of the time during that period) and I made a mold off the bottom of it. That mold has produced four offspring. Three of these are shown in the pic above (from left to right: Cetor, Cypselurus II, Black November, Red October).

Cetor, Cypselurus II, and Black November have similar planforms, essentially identical fins and fin placement, and virtually identical bottom shape. Hence the differences--although present--in how they feel and handle are relatively small. So I figure it's probably more like 32 years that I've had to become familiar with their quirks, strengths, deficiencies, etc.. (Red October, however--which has a substantially different planform and fins--is a whole different critter.)

FWIW, in case anyone is interested, I've include the specs for each of the boards (in the order: Cetor -> Red October, as in the pic).

Vintage: 1973, 1994, 1996, 1997
Purpose: gen. purpose, small wave, gen. purpose, large lined-up walls
Length: 64", 68", 65-3/4", 69-1/2"
Max Width: 22-1/4", 24", 21-3/8", 20"
Nose: 19-3/4", 20-1/4", 18-3/8", 15-1/2"
Tail: 18", 21", 17", 16-1/2"
Nose rocker: 4-3/4", 4-5/8", 4-5/8", 5-1/2"

(An aside: Clark quit making sheet foam well before 1994, so I had to use old scrap pieces of foam that I had around to make Cypselurus, Black November, and Red October. I saved the "newest" pices for Red October which, combined with it's substantially less use, accounts for the minimal yellowing of the foam. The worst (oldest) pieces were used for Black November since the bottom sheet is PVC foam and hence I had intended to paint the board anyhow (black, with white and gold bands/pin lines)--but I never got around to it. Cypselurus I and II got built of the remaining scrap pieces.

mtb

[MTBarrels]

...<snipped>... i'm using that concept (posted in the other spoon thread by John i think???) describing Mr. Greenough's "submarine dives under waves". basically i am starting my duck dive at least 10 yards before i would with my fish. i working on getting deeper under the water where its calm & i can hear the wave breaking above me...<snipped>...

Another thing you can do if the lip impacts in front of you (while duck diving) is to keep your eyes open under water. My experience has been that if the depth of the curtain of air bubbles that descends ends above you, you won't experience much turbulence; if it gets down to your depth, or deeper, you should be prepared to hold on tighter to your board.

[red]

I love the boards.
I can see they will be fast in the water. What about chop in big waves?

Why are they made fom fibreglass rather than, say ply/aluminium/closed cell foam?

Is the central section with the knee wells there for structural purposes?

An observation: Could the rail line in the last 18" be reverse curved (like high performance skis) to yield a more uniform curve under load?

[W.G. Facenda]

I saw Greenough surf about 5 times at the ranch( Big Razors and good size Cojo) all super impressive .The time at Cojo he was on a mat in the morning and windsurfing in the afternoon going soooo fast and smooth. Later he sailed by my boat at about 25 knots and waved to a fellow kneerider.
The time I saw him at very big Rincon was insane! It was south wind and super gnarly with very few takers. My friend and I were in the parking lot leaving and Greenough pulled up with a few guys in his old highway patrol black and white cruiser so we follwed them down and he looked like a car guy getting ready for a car race or something , real into his equipment and all. he just jumped in like it was nothing and every once in a while you could see him duck diving big giant mounds of whitewater. It was real big so you couldn't really see the outside waves ,since the inside ones blocked them.I remember very clearly watching him blazing in total control across big ,very difficult to ride ,basically unridable rincon. I don't think to this day ,I have ever seen anyone ride big, bumpy, surf that well and that fast! He did it on a 5ft carbon fiber spoon 23 yrs ago. The funny thing is I believe he aimed for that kind of surf.....

[Man O' War]

MTB - What a shock. I didn't know a quiver like that existed anywhere on the planet. You've redefined the word "custom." You sure knew what you wanted.

Like many others, I'm sure, I've looked at those knee skis (hydrofoils) with their knee wells and wondered if that could be done. You line them and your shins with velcro, and you're ready to ride upside down.

Keep up the innovating. You've pushed the envelope so far on this one, you're out there alone.

[hart]

I love looking at kneeboards that are not only well designed, but well made..

and these surfboards look both



MTB you are practising what you preach..and I for one, look forward to more

hart

[MTBarrels]

red, MOW, headwax, hart - Thanks for the kind words.

red (paraphrased):

1. How do they work in chop?

They are pretty stiff, apart from the rail flex in the rear. Also fairly wide tails, hence reduced angle-of-attack compared with a board with a narrower tail. Plus hard rails everywhere. All elements that make for a hard ride in chop. I spent a good part of my first year of kneeboarding trying various approaches to reduce the effects of chop while maintaining speed capability. Eventually I came to the conclusion that the best way (for me) was to not go out in chop--rattles my brain too much. So now I lean towards bodyboarding when it's choppy and am planning on eventually trying one of Dale's mats as a better solution to chop. On the other hand, perhaps one fringe benefit of avoiding chop (and attempted airs) is that my knees are still in good shape.

2. Why fiberglass? Why not plywood, aluminum, closed cell foam, etc.?

Just seems the easiest material to work with for me, and has a good balance of the properties that I am looking for. The bottom of Black November is a sandwich with a 1/4" thick core of PVC foam and glassed top and bottom. As it turned out, even that thin a sheet of foam, when glassed, is stiffer than I would like. So "one of these days" I hope to get around to thinning out the foam in the tail area of Black November to the point where it will flex more. Also foam and aluminum are typically structurally isotropic--i.e. they have essentially equal strength in all directions. On the other hand, by using uni-directional glass (90/10 strand distribution), I am better able to match the strength of the construction to match the stresses on the board (e.g. very few boards break from front-to-back, versus from side-to-side).

3. Does the central foam with the kneewells have a structural purpose?

Yes. In fact, in my early boards, it consisted of a raised ridge running down the length of the board (except close to the tail), with fore-aft running channels on each side and between each rail (for kneeling). The purpose of the strip was to suppress longitudinal bending of the board and ending up with unwanted rocker when supporting the weight of the rider on it.

The kneewells came about from a couple of problems that I had. I found that when riding large waves (at least "large" for me), my take-offs tended to become "later" and "later" as the size increased. The later the take-off, the more I tended to be too far forward when getting to my knees (impatience to get going, I suspect). The kneewells help a lot by ensuring that I end up on the board where I want to be.

The other thing was that when dropping down the face of a decent sized wave with chop on it, each time I'd get bounced up off the board from hitting chop the friction of the water on the board would slow the board down, but I'd continue at the same speed. The end result was that when I'd reconnect with the board, I'd be a bit farther forward. On some waves with lots of chop and bounces, I'd end up WAY forward on the board. This made bottom turning awkward and sometimes problematic. The kneewells also eliminated that problem.

The only drawback I've personally found with them (your mileage may vary) is that once in a while on a free-fall take off you'll end up a bit forward and land back on the board with your knees on the area just forward of the kneewell (rather than in the kneewell). To spread out the area of impact (thus minimizing the point loading) I made that area flat on Black November. When I built Red October, I experimented with kneewells made of bodyboard foam instead.

However, I have to admit that I'm not very enthused about the latter approach and will probably eventually replace the soft foam section with the usual hard foam/glass equivalent. This is relatively easy to do on this board as the kneewell assembly bolts onto the hull proper (and can be adjusted forward or backward, as desired (you can see the adjustment holes in the pic I posted).

4. Could the rear 18" of glass near the rails be given reverse rocker so as to make the flex in that area more uniform under loading?

(I hope I got your question right)

Yes, it could. However, it's not clear to me how much loading there is on that area--and whatever there is, probably varies from side to side depending on whether you're going right or left.

The highest pressures on the hull (assuming one is not going so slow that buoyancy dominates) are where the water first makes contact with the hull (since this is where the water is primarily given downward momentum by the inclination of the hull relative to the surface of the water/wave face). This high pressure area is what gives rise to the spray that is thrown forward from this point of initial contact.

[An aside: FWIW, I have verified this conceptual "picture" by drilling holes vertically through a hull at various points along the longitudinal axis, then bonding straws into them and trimming off the ends to be flush with the bottom and the deck. The relative pressure on the bottom of the hull at the location of each hole is reflected in how high the jet of water rises above the deck at each straw when underway. At speed, it was readily apparent that the height at the first (most forward) straw within the wetted area was substantially (e.g. 4X) greater than at the rearmost straw]

When going across the face of a wave, the wetted area on the bottom of the board is longer on the side of the board towards the face of the wave, and shorter on the shore side of the board. Hence in the tail area, the shore side rear glass area will be "seeing" higher pressures than the wave side (although this is somewhat compensated for by the increased static pressure associated with buoyancy on the wave side). Hence the loadings on the tail area would be different from side to side. To compensate, the pre-load reflex should be different (if one wants the two sides to be deflected by the same amount). But then if one goes in the opposite direction, the differences between the two sides would be exacerbated instead of removed.

Since the pressure is highest near the initial contact of the hull with the water, most of the support of the weight of the rider and the board occurs in that area. Moving further downstream along the hull and the support by the pressure of the water falls off. But because of the increasing moment arm relative to the rider's center of gravity, it plays an increasingly important role in how the board trims out. Too much down reflex promotes an increased pearling tendency as the speed increases. In addition, altering the reflex can either enhance or reduce the hydrodynamic planing efficiency (by altering the aspect ratio of the wetted area, or altering the magnitude of the wetted area, and by altering the induced drag due to the altered angle-of-attack). But these latter effects are beyond my ability to estimate at the present time, so I'm unable to estimate if one would gain or lose (as a function of the degree of downward reflex), let alone quantify it.

...<snipped>...MTB you are practising what you preach..and I for one, look forward to more

hart

I can probably dig out some pre-1973 boards if the readers of this board wouldn't be bored by them. However, I can guarantee that the workmanship is of lesser quality since this was my primary period of learning how to work with fiberglass, foam, and resin. Moreover, I'm also pretty sure that the first thoughts that will come to the viewer's mind is likely to be something on the order of: "What could he possibly have been thinking when he built that??" since many of them clearly indicate moving down the wrong path.

[Man O' War]

mtb - an endorsement from bruce -- you're this far from heaven.

i don't think i'm the only one who'd like to see your early boards, failures or not. it would be a learning experience.

one question: what really is "unidirectional" cloth, how does it work?

[surfhorn]

By all means - we would love to see the earliest attempts you have; we all had to start somewhere. Watching evolution is way cool.

My ggod friend who lives in South Lake Tahoe has the first KB I made, which I call the "Bloody Sardine". He threatens to donate it to the Santa Cruz Surfing Museum if I don't keep him stocked with new wetsuits!

[MTBarrels]

mtb - an endorsement from bruce -- you're this far from heaven.

i don't think i'm the only one who'd like to see your early boards, failures or not. it would be a learning experience.

one question: what really is "unidirectional" cloth, how does it work?

Bi-directional cloth has an equal number of fibers running length-wise (down the roll) and crossways (across the roll). Hence the strength (and stiffness) is essentially equal in both directions. True unidirectional cloth has just enough fibers woven cross-wise to keep the lengthwise fibers in place while doing the lay-up. A typical ratio for the latter is about 90 percent of the glass fibers running along the direction of the roll, and 10 percent of the fibers running across the width of the roll.

There are also weaves with intermediate percentages. For example, some of my early boards were done with a tight weave aircraft cloth with 60 percent of the fibers running length-wise and 40 percent cross-wise.

In 90/10 cloth, the lengthwise running fibers are virtually straight--i.e. they do not weave up and down over the cross-wise running fibers. Instead, only the the cross-wise fibers weave over and under the length-wise fibers. Hence the longitudinal fibers are more resistant to enlongation when under tension since all the load is carried by the glass fibers and not by the resin as well (and for the same reason, it is more resistant to buckling when in compression). These properties can be used to taylor the strength of a board. But they can also be used to customize the flex of multiple-layer laminates by choosing the orientations of the length-wise fibers in each layer of 90/10 cloth, and intermixing those with layers of bidirectional 50/50 cloth when appropriate. In short, a laminate will bend easier around an axis paralleling the 90 percent fibers, but be much stiffer for bends paralleling the crosswise fibers.

Since boards nearly always break transverse to their longitudinal axis, it always seemed to me that something like a 60/40 weave would make more sense for a typical surfboard. However, I do not recommend the particular 60/40 weave that I initially used (Type 143, I think) since its tight weave made wetting out the cloth and foam without entraining small bubbles in the weave of the cloth, or between the cloth and the foam, a significant challenge.

As far as my earlier boards, when I get a chance I'll try to get some pics of some of them and post the pics here. Of course you realize that by doing so, I'm risking that after seeing them, Bruce may decide to retract his "endorsement"

[hart]

umm..

MTB

you are bolder than me..


coz I won't be posting any pics of what I shaped in the mid seventies ..



hart