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yeah i wasnt sure how much it made sense to make one for the small waves. i thought it would be fun for the big ones though. or semi big ones we ger here during hurricane season
have any of you goes ever seen one in person.
my friend the welder has some left over high grade steel that i didnt no that he had so i might give this foild board thing a go.
i guess what i really need to know is what kind of foil i need to have to make the thing ride above the water like that.
For basic info on the hydrofoil assembly, go to:
http://www.uspto.gov/patft/index.html
Click on: “Patent Number Search” (in the green area). For the patent #, enter: 6,786,785 . This will not be Laird’s hydrofoil, but rather the latest air-chair/sky-ski (can never remember which brand is associated with Woolley and which with Murphy) patent. Nevertheless the hydrofoil assemblies are virtually identical to those used on the Airboards (except perhaps how they mount to their respective boards/hulls). Check out the description and drawings for dimensions, reasons for various design factors, etc. Also check out some of the earlier patents for some of this info (in particular: 5,100,354 - Woolley and Murphy - the original patent).
Keep in mind that these airboards are considerably more demanding to ride than the Air Chair and Sky Ski since one does not have the stabilizing effect of the tow line once you let go. Think of it as riding a unicycle that you have to balance in both pitch and roll (it is inherently unstable in both directions)–but less forgiving than a unicycle (in part because you can’t start and stop it’s forward motion).
You must also respond quickly to changes. For example, assume (for the sake of argument) that you are going 30 ft /sec (~20 mph) and moving parallel to the sea surface along your pathline. Now suppose that you angle a little higher on the wave face, or move a little farther away from the breaking crest, and as a result the slope of the wave face along your pathline changes by 6 degrees (0.1 radians). This means that now the board is either rising farther above the sea surface, or descending toward it, at a rate of about 3 ft/sec. The spacing between the foil(s) and hull is 29 inches, or 2.4’. Assuming that you start out with the sea surface midway between the foil(s) and hull, this means that you have about 0.4 sec to recognize that you are rising or descending and make the appropriate corrections before either the board impacts the sea surface, or the foil(s) broach the surface. Certainly presents an additional workload for the rider compared with riding a board with a planing hull.
It is also not true that a hydrofoil will not maneuver with a turning circle much smaller than those appearing in the videos (at least the turns that I’ve seen). The primary problem with the Airboards would seem to be a loss of control–the smaller the turning circle->the greater the centripetal acceleration required->the greater the bank angle required-> the more unstable and less forgiving of small errors in balance the board becomes. Moreover, as you turn, the pitch angle of the board must also (and simultaneously) be changed by the rider to provide the required lift to keep the board at a constant elevation above the sea surface. And to make matters even worse, since the wave face has compound curvature, one must also be constantly changing the pitch angle (even if the loading were constant) to maintain the desired elevation. Lots of tasks that the rider must quickly and accurately accomplish when aggressively maneuvering.
As with planing hull boards, everything is a compromise. For example, the primary design objective of the HYPO (HYdrofoil paiPO) board was maneuverability–and that objective has essentially been achieved. However, in return, the speed potential is not as great as for a Laird style board, and it is much more susceptible to the presence of surface chop. (On the other hand, it can be paddled into waves rather than requiring a tow in).
As regards fabricating the hydrofoil unit out of steel…
The hydrofoil units built by Air Chair and Sky Ski have an aluminum strut and fuselage, and aluminum foils (Type 305 T6 aluminum, as I recall). I understand that there has been some interest in reducing the weight of the unit by the guys riding the Airboards. Hence I do not think you would find a unit fabricated out of steel to be satisfactory since it would weigh considerably more than the present aluminum units. Plus there would be the problem of corrosion/maintenance (unless the “high-grade steel” you refer to is a stainless steel suitable for use in sea water, e.g. Type 316…or 316L if you’re going to be welding). On the other hand, if you’ve got megabucks, a suitable machine shop, and your friend can weld titanium, that might be a good lightweight alternative.
mtb