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Paipo "Super Slicer" Hydrofoil:  The Next Generation

Terry Test Driving Somewhere in Baja
July 2003

July 2003 Comments on Super Slicer - Aug 2003 Update - March 2007 Update
Funny Video on Measuring Super Slicer's Speed. See the patent here.
R.I.P. Terry Hendricks, June 13, 2013 - a short video at the Swamis Paddleout, June 29, 2013, celebrating Terry's life.
The Swami's Paddleout announcement.

Terry Hendricks provided the following comments about the Super Slicer:

The primary design goals were:
   1. High maneuverability
   2. Paddle-in (vs tow-in)
   3. Flight elevation "autopilot"

Secondary goal:
   4. At least as fast as a state of the art conventional board with a planing hull.

It has two foils. A large (main) foil in the rear. This foil is fully-submerged (typically 10"-12" mean depth) and carries 90 to 100 percent of the combined weight of the surfer and board. It is MUCH more hydrodynamically efficient than the forward foil. The forward foil is a ventilated surface-piercing foil. It's functions are to provide some roll stability, assist in turning, and maintain the flight elevation within a narrow range of values.

Since the rear foil is by far the more efficient, maximum speed is achieved when the weight of the rider and the board is exclusively carried by this foil (hence rearward position of the rider on the board). This is contrary to a conventional board, and hence requires some effort on the part of the rider to overcome his ingrained reflexes.

The presence of the foils increases the drag when paddling the board. So it paddles slowly, and take-offs are typically late. Fortunately, it does late take-offs very well. The present board makes the transition from displacement mode to flight mode at the low speed of ~5 mph, and this change is virtually transparent to the rider (i.e. one is not aware when it occurs).

Once in flight mode (mean hull elevation ~ 6-8"), the board is unstable in roll (i.e. like a bicycle). As with a bicycle, one stays upright by making turns to balance the craft. The more experienced the rider, the smaller the turns, and the less wandering of the flight path. Turns are initiated by banking the board. It is very sensitive to this (like changing from driving a bus to a gran prix car), allowing quick and easy turning, but also seeming a bit "twitchy" for the beginning rider.

At typical surfing speeds on So CA waves (e.g., on a 4' wave), most of the drag of a conventional planing hull board is associated with the induced drag from generating the lift necessary to support the weight of the board and rider. On this hydrofoil, the primary source of drag is skin friction drag. A sharp turn, can increase the induced drag several-fold. Since the induced drag is the primary drag on a conventional board, this means that there is a greatly increased total drag during the execution of a turn--and the board slows down substantially as a consequence. On the hydrofoil board, the induced drag has the same factor of increase, but as the induced drag is a much smaller fraction of the total drag to start with, the increase in total drag is much less (and the loss of speed is much less).

Combine the ease and quickness of maneuvering (peculiar to the lack of stability and the low moment of inertia of the rider around the roll axis), and the reduced loss of speed, and you have a maneuvering demon.

A combination of direct measurements, and modeling of the hydrofoil hydrodynamics on the face of a wave, indicates that for a 4' wave the hydrofoil board should trim across the face of the wave at a speed that is about 12 percent higher than a state-of-the-art conventional board with a planing hull. That's if the main foil is carrying 90-percent of the total weight. If the hull is trimmed by a highly skilled rider so that the rear foil is carrying 100-percent of the weight, this increases to a 22-percent speed enhancement.

Whether this is true, or not, remains to be seen since the calculation involves some approximations to the flow field in the face of the wave. However, design improvements planned for the next version should change the 22-percent improvement to a 29-percent increase, so it is likely that the hydrofoil board will prove to be at least as fast as the conventional board--and probably faster.

At the present time, I do not deliberately ride the board in really hollow waves (although a bit hollower than in the pics would have been nice). A major reason for this is concern for self-preservation. The board has a lot of lengthy sharp edges (leading and trailing edges for the forward foil, trailing edges of the main foil and its struts), so I'd like for my skill level to be such that there is a reasonably high probability that the board and I won't be churning around together in a hard-breaking wave (I wear a helmet all the time, and some 'body armor' in the more hollow waves). However, I have inadvertently gotten on a few steep faces, and the board seems to go fine (observers tell me it also appears to accelerate quickly).

So far, the board seems fairly forgiving as long as you manage to hold on (motivated by the desire to stay on the side away from the hydrofoils). I've only pearled once on the board--even though I've inadvertently done a number of free-fall take-offs. It also does spectacular "S" turns (it's just up to the rider to hang on and go with the board).

Bummers? Sea weed is a major pain--both getting out and riding waves (bottom turns when you are the most susceptible). It can also be quite difficult to "get around" white water from a broken wave (you keep getting turned toward shore by the soup surging against you and the side of your craft). Ditto for getting out of the white water if you "straighten out". The greater drag while paddling also means that it takes more energy to paddle to the break, and increases the number of waves that you may have to duck dive enroute. So generally the number of waves ridden per session is down. On the other hand, there is a bonus if you duck dive properly (it glides forward as you rise toward the surface driven by your buoyancy)...but a significant penalty if you do it wrong.

It is also a very rough riding board in the presence of chop. Whereas the lift force may increase from 1g to something around 1.7g when encountering a 4" chop with a conventional board (at ~15 mph), the vertical acceleration on the hydrofoil board can momentarily increase to almost 4g. On a few occasions, I have even
been "bucked off" the board (but managed to hang on and get back onboard).

July 2003

August 26, 2003 Update

In response to comments about hydrofoil wave-riding craft...

Just as you wouldn't mix a kayak and a kneeboard together in discussing wave-riding vehicles with planing hulls, it is inappropriate to mix together all hydrofoil-based wave-riding vehicles.

The design objectives of Laird's airboards are to maximize speed and minimize the effects of surface chop. In return, one compromises maneuvering capability, requires tow-in to catch a wave, and imposes a substantial workload on the rider to maintain the flight elevation within an acceptable range.

Within this set of design objectives and compromises, the air board appears to work superbly. The reduced induced drag (typically a major source of drag for boards with planing hulls) that is obtained from the pair of hydrofoils is readily evident by the ability of these boards to move at speed across wave faces with very modest slope. Also if you compare Laird riding one of his strap-in planing hull boards in chop with him riding the airboard in the same chop, you will immediately notice the improved ride resulting from detaching the lift surfaces of the board from any contact with the sea surface.

The hydrofoil paipo (HYPO) board described in the first of your links was designed with a totally different set of objectives--in particular, outstanding maneuverability, speeds equal to, or exceeding that of state-of-the art planing hull boards (but not one of Laird's airboards), capable of being paddled into waves, and allowing the rider to concentrate on riding the wave by eliminating the need for him to control the flight elevation.

The latter is achieved by the addition of a surface-sensing forward foil (which also assists in turning, and in adding some roll stability). Since this foil is deliberately in contact with the water, the craft is short-coupled to maximize maneuverability, and the hydrodynamic efficiency of the hydrofoil requires only a small angle of attack (substantially reducing the induced drag), it automatically follows that it will be sensitive to chop.

Hence one design essentially totally isolates the rider from the effects of surface chop (airboard), while the other (HYPO) is very sensitive to chop. Similar differences with respect to maneuverability, and in how one can catch a wave. In short, two hydrofoil-based boards with totally different characteristics.

Adapting a hydrofoil to a craft does not necessarily mean that it will work to the rider's satisfaction. My first wave-riding vehicle was a copy of Gaylord Miller's hydrofoil paipo design (your link #2) that I built and "rode" in late 1960. It had very substantial stability and control problems (which are shared with the airboard design). Although it would support most of the rider out of the water, a consequence was that it was virtually never ridden as a true hydrofoil craft, but rather as a planing hull craft with a greater aspect ratio than a normal surfboard (or kneeboard).

With regard to danger...

All surfing is dangerous to some degree--and the hydrofoil-based boards have a lot more length of "sharp" edges. However, I put the "sharp" in quotation marks as that varies considerably from design to design. More specifically, the edges (both leading and trailing) on the HYPO board are substantially less sharp than the corresponding leading and trailing edges (and tips) of conventional surfboard fins. So which is more dangerous?...more dull edges--or fewer, but sharper edges? I think the jury is still out on this. BTW, the HYPO (a prototype and hence typically built heavier than a production version) weighs 12 lbs complete--hull and foils (and all foam and fiberglass--no metal).

The HYPO board definitely paddles slower than a conventional paipo board (at least when paddled right-side-up). Again, part of this is a consequence of this being a prototype (actually, it is the seventh in a series of constructed and tested designs, with another seven or so partially built, or existing only on paper). The next version will incorporate refinements to at least partially reduce the present difference in paddling speed. In any case, it handles steep take-offs more like a bodyboard than a kneeboard.

While Laird's boards have had durability problems, that has not been the case with the HYPO board. After seven months of use, the only "repairs" that has been made is the addition of a small strip of glass to the forward junction between the main struts and foil to avoid peel problems when riders started trying aerials (a stunt that was not forseen in the original design), and a strengthening of the glass in the vicinity of the rider's elbow contact.

Yes, it can easily (the begining rider might say too easily) do lots of S-turns. That's not only a consequence of the small moment-of-inertia of the craft and rider around the roll-axis (a substanitial motivation for the prone position) and the inherent roll instability leading to quick transitions from one turn to another, but equally importantly to the ability of a hydrofoil to carry speed through a maneuver.

For a conventional planing hull board, racing across the face of a 4 foot wave, the induced drag contributes about half of the total drag. It is much less for a hydrofoil-based board. The additional drag resulting from executing a turning maneuver is almost exclusively an increased in induced drag. For the same radius coordinated turn (and bank angle), the "lift" force required is the same for a planing hull and a hydrofoil-supported hull. Thus the percentage increase in the induced drag during turning, relative to traveling across the face, is the same for both.

However, as the initial induced drag is so small with the hydrofoil board, compared with the planing hull board, the total drag undergoes a much larger increase for the planing hull board than it does for the hydrofoil board. So the planing hull board loses substantially more speed than the hydrofoil in executing the turn. But that's for equal turns. The hydrodynamic efficiency of the hydrofoil allows even sharper turns for the same loss of speed as a lesser turn with the planing hull board.

You stated that aside from S-turns, it [the hydrofoil] seems more of a novelty than any great surfing advantage. I guess that could be either correct, or incorrect, depending on your definition of "great". Although it's difficult to compare maximum speeds (if, for no other reason than it's difficult to find identical waves even at a single break), the best numerical calculations presently available indicate that the HYPO board will have a speed advantage of 12-22 percent over a state-of-the-art planing hull board (the range indicated depending on the skill of the HYPO rider). I like speed, and I like maneuverability--what's wrong with that? That's certainly what I perceive to be the primary performance discussions on this message board.

Also, keep in mind that the board is still in the state of development, and the rider(s) is(are) still learning how to ride it (one of the captivating aspects is that it rewards the proficient rider with substantial improvements in performance--but requires considerable effort to become skilled). As the rider(s) ability(abilities) increase, it is inevitable that one will become aware of more tricks than can be executed with it(some undoubtedly unique to this design--I already have some in mind, or partially executed).

One other comment...

At the present time, only one person (a stand-up surfer, but ridding the HYPO prone) besides myself has ridden the craft . World-class bodyboarders and a world-class stand-up surfer--as well as lesser talented bodyboarders and stand-up surfers--have expressed an interest in trying it (pending a conjuction of what they considerable to be suitable waves and their free time). But no kneeboarders (and I have asked a reasonable number). Are kneeboarders that closed-minded...and so lacking in curiosity? Are you so wedded to kneeboarding that you're also unwilling to ride in another position (in this case prone)--even when physics and ergonomic considerations indicate the alternate position to be optimal?
Experience gained is in proportion to equipment ruined.

March 2007 Update

The board (HYPO) is still in development. In particular, I am presently building (in my spare time) the "patent/full-featured version" (vs the "prototype version" shown in the pics at Rod's website). The primary differences between the prototype version and the full-featured version are:
  1. A slightly different hull shape and size (the prototype was built using the tail section of a broken longboard).
  2. The foils, struts, and hull will be separate and removable components so the board can be disassembled for convenient storage, transport, etc..
  3. The main wing will use a foil section with less drag than the Clark Y used on the prototype (chosen for the prototype because it is a very "tolerant" foil section).
  4. The canard foil will be available in three different parabolic arcs--flat, 9-deg span-wise exit angle, 16-deg span-wise exit angle (to tailor the handling to different types of waves).
  5. The prototype is constructed using polyurethane foam and polyester resin. Most of the full-featured model will be constructed using EPS foam, epoxy resin, and bi- and uni-directional glass and carbon fiber.
  6. The full-featured version has a pair of moveable "control sticks". The surfer's right hand controls the deflection angle of the "rulerons" (flap-like control surfaces on the trailing edge of the main struts). These act similar to the ailerons on an airplane and control roll. This will make the board easier to balance (especially in extreme maneuvering) as well as further enhance the maneuvering capability. The left hand controls the rigging angle of the canard (forward) foil. This boosts paddling speed (tests with mods to the prototype board indicate a 50% increase in burst paddling speed) thus making it easier to catch waves and less critical take-offs. It will also permit the rider to set the foil for maximum speed or maximum maneuvering--and all points in between--vs the compromise fixed angle of incidence of the original prototype configuration.
I am currently building the tooling/molds for the major components (hull, struts, main and canard foils, etc.) of the full-featured version. In some cases, the component for the full-featured board will be built off the mold; in others, the mold will be built off the hand-crafted component for the full-featured board. Thus once the full-featured version is complete and tested, it will be relatively easy to begin building identical boards for sale on a semi-production basis (if there is a customer demand :-)   If you would like me to do so, I can add your name to a list of persons who would like to be notified when the boards will be available for sale. Also, if you happen to be in the San Diego area, I invite you to try out the prototype board if you wish.

I don't have many pictures of the prototype in operation (as I mostly surf at night). Below are some more pics including three pics of a ride on a small wave at the south side of the S Jetty at Oceanside and (to be complete) a candid pic that a photographer for SurfShot took in the parking lot at Swamis in 2005.

Thanks again for your interest,

Terry Hendricks

Click on pics for a large image

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Last updated on: 07/08/13