BWD wrote: ↑Thu Dec 26, 2019 3:29 pm
Jumping in with the basics.
Planing craft are hydrofoils with the top side dry.
All hydrofoils/lifting bodies/wings/planing craft make lift due to pressure differential and forces on the high pressure side. Even flat plates work - primitive kitefoil, alaia, flat bottom skiff, skimboard, fishermans planer rig etc.
Thickness in a foil organizes the flow over the low pressure side reducing turbulence/drag. Low speed application with high AOA or high load needs a thick foil to not drag or stall, high speed needs thinner foil.
Planing on the surface, the top side is dry, so no worry about flow over the top - as with most boats. Surface piercing boats like pointy bowed multihulls on the other hand have streamlined upper bows made to pierce through and shed waves.
This is the most wrong you can be with your statement:
Jumping in with the basics.
Planing craft are hydrofoils with the top side dry.
There is a clear and definite distinction between lift from displacement, lift from planing, and lift from a foil. Simple thought experiments will give you a basic understanding here, and will illustrate why you are absolute wrong in your opening statement and mostly throughout. Here you go:
1. At rest, a displacement hull will float because the water it displaces is equal to it's weight. Also when at rest, a planing hull will displace an amount of water equal to it's weight or it will sink. Once planing the enclosed volume is no longer required to continue planning. To prove this, you could make 2 boats with a detachable displacement top and a planning (no volume) drive section. As both boats are powered from where displacement forces are overcome by planing forces, you could discard the displacement section of one of them as they are side by side. The drive section without the displacement section would no longer have the weight, the air drag, nor the drag from spray being accelerated by the upper section.
2. Think of the skipping rock scenario. If the rock is set on the water, it just sinks unless it is a rock of a weight less than the amount of water it displaces (possible with quickly cooled lava rock). If a dense rock that would normally sink is thrown fast enough at the right angle to plane, it skips (temporarily planning) and does not sink until the ideal angle is lost (why you spin it) or it slows down to a point where the planing forces cannot make it skip again. Thus planing and displacement are DISTINCTLY different phenomenon. You cannot substitute one for the other, or make any serious statement of likeness in multiple aspects between the two.
3. At rest, a foil section does nothing. When moving at a minimum speed within the correct allowable AOA range, a foil will generate enough lift to lift the weight above, regardless of it's displacement or equivalent underside planing surface. And what happens if you stall the foil? This requires some understanding of how the flow above a foil becomes unattached when stall happens. And what happens to the lift in a stall? The bottom surface is the only thing lifting (oversimplification), and almost all of the lift is gone. This is the first proof that a foil section cannot be seen as generating anything more than insignificant lift (not functioning as intended/useful), when only one surface has attached flow. Thus unlike planing, both surfaces are REQUIRED to have the same media flow, and that media must have attached flow over the top surface, for a foil section to work as intended.
On top of that, what would happen if a hydrofoil breaches the surface of the water "accidentally" or "intentionally", to now function as a planing surface? - It fails! There is no situation where a hydrofoil transitions from foiling to planing and still functions. Should you even attempt to create a proof of concept experiment, the number one consideration would be to solve the issue of the upper surface reattaching flow when it encounters a wave, then violently lifting up, and losing contact with the waters surface on the underside. But there are many others.
4. With displacement, planing, and foiling, think of an example where you try to combine one with another. Lets first say you want a hydroplaning vessel capable of functioning in a displacement mode. This is an example that has been done on a large scale and even down to the kite scale. A large scale example here would be large displacement ship that has a hydrofoil capability. What are the drawbacks of this? Immediately you see that you have an extremely efficient hydro plane system that has to carry around a large amount of "baggage" in the form of all of that unnecessary volume and weight in the displacement vessel. Conversely, you have a stable displacement vessel that has delicate outriggers or an extended draft with a hydrofoil underneath, reducing the maneuvering/shallow water/docking capabilities. Thus when combining one with the other, you reduce performance of both.
In kiting, think about combining an SUP board with a hydrofoil. In this endeavor, you would also reduce the performance of both. The SUP board functioning in displacement mode would not work on waves as the inactive foil would create unwanted and unmanageable forces on the wave face or even in swell. And in the hydrofoilling mode, the extra length, swing weight, and overall weight of the SUP, would rob performance from the hydrofoilling capabilities.
Combining displacement with planing is the same with respect to sacrificing performance of one vs the other. A planing hull only sees detriment from planing from the displacement parts that are out of the water (air drag, weight), while a displacement hull only sees detriment from increasing width to make a quicker and more efficient planing hull (more drag from a wider cross section when at lower speeds).
Thus if you do find similarities between displacement, planing, and foil, you also find that the differences override those similarities and become detrimental, not complimentary. So given the above 4 thought experiments, you should be able to clearly understand the distinction between the 3 types of lift on a vessel. And that clearly illustrates that the differences override any attempt at claiming any basic similarity.
BWD wrote: ↑Thu Dec 26, 2019 3:29 pm
Surfboard rocker/thickness and rail profiles make the foil of a surfboard. There are plenty of times water wraps the rail of a surfboard, most obviously with longboards. The pile of water on the tail of a longboard majorly contributes to a rider being able to noseride, the most blatant example. Effects come into play in other ways too -
remember the top side of a foil shape mainly about reducing drag and keeping control like when trimming for speed etc.
That’s the planing/foiling basics. It mostly comes down to the bottom, though what’s up top majorly affects how well it works in a given scenario.
Right away, throw the longboard references in the garbage. And I have some great reasons for suggesting this. First, there is an easily demonstrable reduction in performance while kiting on any longboard, and even down to "funboards". Using either of these types of large boards is an intentional handicap which forces the rider to work within severely restricting performance envelopes. If you keep thinking in terms of longboard with a kite, you make things worse, not better. Still, longboards and funboards can be a fun challenge, but never a higher level of performance. So while this reference proves my overall point, it is outside of what is necessary to provide that proof.
For the part in red, the top side of a foil shape is NOT for reducing drag. It absolutely provides the lift, as evidenced by a stall. Yes, it gets much more complicated than this, and it is actually the two surfaces (top and bottom) working together that truly provide the lift in a foil. Could you tell us where you acquired your understanding of displacement vs planing vs foil sections?
But in the end, the question boils down to "why are you wrong in your assumptions when the physics are so clear"? The answer to that is deceptively simple - You feel a difference when riding a low volume kitesurfboard vs a standard higher volume prone surfboard. And you equate that difference to something being wrong, as opposed to recognizing that the difference is actually a higher performance envelope when planing with lower volume. I too was of your mindset for a long time. But by kiting my shortboards back to back with my kitesurfboards, doing an in depth examination of the underlying physics, and doing some more testing and thought experiments, I was cured of what I would call an oddly emotional and stubbornly entrenched set of assumptions. All carryover from surfing.
Corrected.