|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
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
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
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.