The Design

Maherajah Water Skis

The Design

Definition of Composite Construction:

Composite construction is a generic term used to describe any structure involving multiple dissimilar materials.

In structural engineering, composite construction exists when two different materials are bound together so strongly that they act together as a single unit from a structural point of view. When this occurs, it is called composite action.

Therefore, the use of wood in multi-lam construction does not exclude it from the realm of composites but rather, it was an early definition of it.

Why Carbon/Fiber Composite Construction is a Compromise

Composite construction is actually, by its very nature, a compromise.  In engineering, all designs and material selections are based on compromise.

If carbon fiber by itself was the end all, be all, one could just carve a ski from a block of it and you would have the stiffest piece of material you could imagine.  Of course, it would sink and it would be incredibly heavy and expensive.

So the engineers strategically place the carbon fibers in what is called a matrix imbedded in a plastic (typically epoxy resin) that holds everything together.  In the case of most beams which a ski is, there are layers of carbon fibers on the top surface and on the bottom surface separated by a layer of foam or honeycomb as a shear resistance layer (these days typically CNC-machined PVC foam is used).

The end result is a very stiff and light structure.  Engineers characterize this construction method as having a high strength to weight ratio.

But as with all material selections used in a design compromise, carbon fiber is not without its disadvantages.

Disadvantages of Carbon Fiber Composites:

  • Limited useful life
  • Brittleness—(i.e., rather than just developing a crack like in wood structures, it shatters like glass)
  • Exposure of epoxy matrix to the elements—no UV resistance
  • Sometimes light weight is a liability rather than an advantage

Although carbon fiber has many significant benefits over other materials, there are also trade-offs one must weigh against.  First, solid carbon fiber will not yield.  Under load, carbon fiber bends but will not remain permanently deformed.  Instead, once the ultimate strength of the material is exceeded, carbon fiber will fail suddenly and catastrophically.  In the design process it is critical that the engineer understand and account for this behavior, particularly in terms of design safety factors.

Carbon fiber composites are also significantly more expensive than traditional materials.  Working with carbon fiber requires a high skill level and many intricate processes to produce high quality building materials (for example, solid carbon sheets, carbon fiber sandwich laminates, carbon tubes, etc).

Our assertion is that the structural advances in stiffness in both flexural bending and in torsion and the associated performance improvements experienced by the top skiers riding carbon fiber composite skis has everything to do with the strategic placement of unidirectional carbon fiber layers and little to do with the foam or honeycomb shear layer core.

We further assert that the associated light weight realized using this method of construction was a coincidental feature simply from using a foam core (which is inexpensive), and a minimal amount of carbon fiber encapsulation of this core (just enough carbon fiber material to provide the necessary strength but limited to control cost).  Then, faced with this light weight, the marketers simply said it was intended to be that way.  The fact that skier performance has improved significantly with these skis over the years we feel is due to the stiffness and not the reduced weight and in fact, the light weight simply became associated with better performance.  In fact, we further assert that the disadvantages of the low weight were successfully offset by the benefits of improved stiffness.

Therefore, we chose to fabricate our modern, predominantly-wood, composite skis utilizing carbon fiber where beneficial but also including wood for both its inherent beauty, its generationally long life, its ability to be perpetually re-finished, and its ease of custom manufacture, i.e., no expensive molds have to be made.

The resulting construction method, though a bit more labor intensive than the purely carbon fiber composites, provides the ultimate in profile, size, and rocker variability on a custom basis.

Ski Cores

In the snow ski world, manufacturers have explored many different materials for ski building over the past 50 years.  While some materials have enjoyed a fair bit of success, the bulk of the market remains relatively unchanged.  The core is the single most important part of a ski because it defines a ski’s character and flex.  The most popular skis of the past 40 years have all used wood cores, and that continues to be the gold standard for snow ski construction.

There are very few other pieces of modern sporting equipment that still rely on a natural material like wood.  Everything else is plastic, metal, or textile based.  Can you imagine if snowshoes or tennis racquets still had wooden frames?  So why continue to use wood despite its high cost?  Because it’s the perfect material for the job.  It has a combination of properties that you simply can’t find in anything else.

The most important property imparted by the wood core is its ability to store energy.  As the ski is weighted (picture a tightly coiled spring) the core loads up.  As the skier exits the turn, that energy is released, a feeling often described as “pop.”  Because of this, a wood-core ski feels more responsive than its foam-core brethren.

There’s a small problem with wood cores, however. They’re expensive.  Thus, for low-end skis where performance isn’t a top priority, the industry went looking for an alternative to wood cores.  The answer was foam either polyurethane or PVC.  Many manufacturers injection mold a PU foam into the shape of the core while others cut the PVC foam core using a CNC router.  Foam cores are cheap, but they have a number of downsides. They’re generally less strong than a wood core, they lose camber and stiffness more quickly over time, and they don’t have the same “pop” and rebound energy that we can obtain from wood.

Construction Theory:

If we examine the construction characteristics of the typical modern carbon fiber composite ski, we see what is essentially a squashed/flattened tube.  Engineers have long recognized the structural benefits of the basic tube for both flexural and torsional stiffness as seen in both race car and aircraft construction.

But also known for its torsional stiffness are I-Beams wherein the structural elements (the flanges) are separated and held in position by an integral web (the vertical member).  In this construction, all three of the elements contribute to the torsional rigidity of the structure.

Thus, a typical carbon fiber composite ski requires that the carbon fiber material is wrapped completely around the foam core because the core has virtually no torsional resistance whatsoever.  The core actually only serves to provide a convenient lightweight internal jig over which the true structural material is wrapped and cured in a mold that actually provides the final shape.

In the case of our composite construction, we are using a core material that actually has torsional stiffness—multi-laminated wood of specific types.  But we incorporate the carbon fiber layers as an “endo-skeleton” as opposed to the carbon fiber ski for which the carbon forms an “exo-skeleton.”

In our construction, one could think of it as having wood added to the carbon fiber layers and then the final ski shape is carved from the resulting sandwich construction.

Modern Snow Ski Construction

For a parallel benchmark it is instructive to quickly review the construction materials used in the latest technology snow skis.  Here we have a sport for which the manufacturers have the luxury of being able to spend significantly more R&D money to refine the construction methods and designs over a time span similar to that of the maturity level of water skiing.  Take particular note of the fact that even with all the other “high tech” materials available, both softwoods and dense hardwoods are still the engineers’ materials of choice for the core materials in preference to foam, or honeycomb—even after 50 years plus of development.

Addressing the Asymmetry of the On-Side vs. the Off-Side Turns:

This is where the custom builder’s art and this construction method really comes into its own.  For years there has been the on-going challenge of how to address the issue of on-side vs. off-side turns.  Obviously this issue is derived from the fact that the skier rides with one foot ahead of the other.  Additionally, some skiers ride with left foot forward and others ride with right foot forward.  In either event, the effect is an asymmetry in how the ski handles on one side vs. the other side.

We have seen attempts made in profile shape to address this.  Most notably, O’Brien under the guidance of Bob LaPoint actually briefly produced a ski they called the “Radius-Right” that has an asymmetrical profile.

Interestingly, if we accept the premise that the ability to achieve the best turn for one’s on-side or off-side is directly dependent upon the ski profile shape, we can see that even with over eight decades of ski development behind it, today the best skis are still but a compromise between the two turns.  Given the manufacturing need for a symmetrical profile, it not only means that the profile is not optimum for the traditionally more difficult off-side turn but this compromise extends to diminish what is optimum for the on-side turn also.

The most obvious undesirable feature for this solution is that a high volume manufacturer will require twice the tooling (a left mold and right mold) for each size ski to provide this product offering.

Not so with the custom build methodology.  The worst case for custom build would be double the number of templates which are very inexpensive.  Interestingly, if we make the templates out of a flexible material such as Masonite, thus precluding the need for a tip bent template, we can simply flip it over for each version and thus no additional tooling is required at all.

(Author’s note:  I actually had a ski I used in the tournaments in the early 70’s that I built, that for whatever reason, provided me with an amazing off-side turn that was far better than my on-side turn.  Fortunately, we still have that ski for evaluation and comparison purposes.)

Mounting Inserts

In the old days, binding hardware and fins were simply attached to a ski using wood screws.  To avoid the issue of binding screws pulling out during use, we install threaded inserts into the ski.  The idea being that a binding screw can thread into them perfectly and the insert will distribute the forces of skiing across a much wider area, reducing the chance of the screw pulling through. They also make for quick, easy adjustments of binding-mount locations without having to drill new holes or deteriorate the structure of the ski.

One of the selling points of the new carbon-fiber composite skis is that they are light in weight. And this lighter weight purportedly makes them more responsive. Perhaps some real data would be instructional. Let’s take a look at just how much lighter they really are:

a. Blank: 3.63 lbs
b. Front Boot: 2.60 lbs
c. Rear Boot: 2.30 lbs
d. Fin: 0.50 lbs

• Goode, 9100, 67”, carbon-fiber, 2012, w/plate mount bindings: 9.03 lbs

• Maherajah LaPoint, 66”, all wood, 1973, plate mount full bindings 10.20 lbs

• Kidder Red Line, 66”, graphite (carbon fiber), 1985, plate mount full bindings 12.75 lbs

• Jobe, 64”, honeycomb/fiberglass, 1975, plate mount full bindings 11.50 lbs

• Connelly Kevlar, 67”, 1985, plate mount bindings 11.84 lbs

• Composite Structures, EP X2, 67”, 1973, foam/fiberglass, plate mount bindings 9.50 lbs

• Maherajah 3.5 Classic, standard full bindings 9.38 lbs

• Maherajah 360 Spruce, plate mount bindings 9.38 lbs

• Maherajah Stiff Ski, 66”, blank 5.75 lbs

• Western Wood, fiberglass, “Funnel Tunnel”, 66” blank 7.44 lbs

Question: What are the two things the following names all have in common?
Stradivarius, Bosendorfer, Steinway, Guarneri, Black Widow Bows, Gibson, Fender, Gretsch, K2, Rossignol

Answer: First, their products represent the best of the best in their respective fields, and second, they are all predominantly made from various wood species combined with other specific material choices to optimize the overall performance.

Wood: The ultimate sustainable resource

Weight Comparisons of Various Skis Over the Years

One of the selling points of the new carbon-fiber composite skis is that they are light in weight. And this lighter weight purportedly makes them more responsive. Perhaps some real data would be instructional. Let’s take a look at just how much lighter they really are:

a. Blank: 3.63 lbs
b. Front Boot: 2.60 lbs
c. Rear Boot: 2.30 lbs
d. Fin: 0.50 lbs

• Goode, 9100, 67”, carbon-fiber, 2012, w/plate mount bindings: 9.03 lbs

• Maherajah LaPoint, 66”, all wood, 1973, plate mount full bindings 10.20 lbs

• Kidder Red Line, 66”, graphite (carbon fiber), 1985, plate mount full bindings 12.75 lbs

• Jobe, 64”, honeycomb/fiberglass, 1975, plate mount full bindings 11.50 lbs

• Connelly Kevlar, 67”, 1985, plate mount bindings 11.84 lbs

• Composite Structures, EP X2, 67”, 1973, foam/fiberglass, plate mount bindings 9.50 lbs

• Maherajah 3.5 Classic, standard full bindings 9.38 lbs

• Maherajah 360 Spruce, plate mount bindings 9.38 lbs

• Maherajah Stiff Ski, 66”, blank 5.75 lbs

• Western Wood, fiberglass, “Funnel Tunnel”, 66” blank 7.44 lbs

Observations:
For 50 years of history and development, overall slalom ski weights have decreased by less than a pound on average. Consider a comparison of the Goode 9100 at 9.03 lbs vs. the Maherajah 3.5 Classic at 9.38 lbs. This is bordering on the insignificant.

This further reinforces the observation that the light weights of current carbon composite construction methods are an associated by-product rather than a design priority.

Therefore the term “responsiveness” couldn’t refer to overall weight and its effect on acceleration since the overall weight improvement of the moving system (ski and skier) amounts to less than 0.5%—well within normal variations of the system due to the human body being the predominant and least consistent contributor.

Perhaps “responsiveness” refers to polar moment of inertia—the ability of the body to rotate the ski about the vertical (standing) axis. But even then, given that the weight of the bindings predominates, and they are located near the center of rotation, the contribution to an increase of polar moment of inertia is also insignificant.