What is tailwalking and what is wheelies?

[dsgrntlxmply]

[DanWeiss]

Just to clear up a few misapplied terms present in this thread.

First, cavitation rarely -if ever- happens on a windsurfing fin. Cavitation is where a sudden drop in water pressure causes air bubbles to form. The water is, for a moment, literally boiling against the surface of whatever caused the air to precipitate out of the water. That something is usually a rapidly spinning propeller or an abrupt bump along a very high speed powerboat's hull. Simply, a windsurfer does not generate sufficient pressure differential to cause cavitation. What we experience is either stall, where the fin can no longer resist the pressure causing its angle of incidence and it slides sideways, or, more commonly, a windsurfing fin can loose most of its lift component through ventilation. Ventilation is when the air above the water is drawn down the fin for any number of reasons and we suddenly spin out.

Ventilation is somewhat close to cavitation insofar as each involve air on the fin surface but are truly different in cause.

Second, tail walking is a term of art that describes a situation where the board's nose rises wildly due to a lack of mast base pressure and excessive fin power together insufficient to keep the board's nose down The entire board is ripped out of the water and appears to stand nearly on its tail. Another necessary component is a very high flow around the fin, enough to cause the fin to roll upward when the nose rises. You sometimes will see tail walking after bearing off to pass underneath a competitor in a slalom race when sailing overpowered. In most cases, tail walking can be overcome only be very strong and talented sailors who constantly sail on the edge of destruction. Tail walking is a hugely dramatic situation.

Getting the nose to lift, even a decent amount, when overpowered is not nearly as catastrophic. To me, it comes down to degree, tailwalking being of the higher order.

[feuser]

[bred2shred]

[techno900]

feuser,

[raffar]

I liked bred2shred explanation, particularly the last paragraph in his post.

Different angles to the discussion:
One:
We have been taught when "Pumping" a large-finned formula board in marginal winds to include a rotational or skid motion. Among other factors this exacerbates the bend on the fin with every thrust causing the fin's windward side to bend toward the surface. The resulting the wing lift vector force has an upward element and the board lifts in jumps with every thrust minimizing the board's "wet area" and its resistance. This has an overall positive effect that helps getting on the plane ever earlier, which is obviously desirable as opposed to the tail walks mentioned earlier in this thread .
Two:
Counter point - Why fins aren't angled forward? I always thought the trailing edge of the fin angle is the one that generates the lift.
Three:
Hydrofoil fins.
Four: Do weed fins generate more/less lift?
$0.02

[bred2shred]

[DanWeiss]

IF anyone cares, the only reason I hedged on Bernoulli as taught in high school is the explanations assume without any basis one thing: That lift is caused by a foil splitting a fluid will cause molecules to flow faster on one side than the other so they meet each other at the trailing edge to create a pressure differential. Bernoulli isn't wrong, but explanations of what he wrote are dumbed down for mass consumption.

Let's assume that a fin's leading edge meets two water molecules at the same time. One molecule ("A") diverts to the left, the other ("B") to the right. Each travel on opposite sides of the fin. Those teaching Bernoulli claim but rarely explain but did not prove that molecules A and B will reach the trailing edge of the fin more or less at the same time. This flawed assumption undermines Bernoulli's conclusions because the teachers essentially ignore the existence of mass and inertia of the fluid.

Newton's explains differently. Foils for water are designed knowing that water's viscosity never changes. In other words, water's viscosity is the same, more or less, whether in fresh or sea water. Unlike air, water does not compress. And air's ability to compress results in greater mass and inertia of the fluid as a foil passes through it. That's why the trailing edge of most airplane wings -especially flying slower speeds- tip downward. The air displaced is pushed down and continues down, thus allowing the wing to function. The air must be deflected downward in order for a plane to fly. That's confirmed by the law of Conservation of Energy. Changing viscosity explains why airplane wings are designed to change shape depending mostly on speed and sometimes upon pressure changes, each of which changes the viscosity of the air. Newton's ideas on lift can be reduced to two words: air deflection. More mathematically, Force = accelerated mass or F=mA. Bernoulli doesn't contradict Newton but describes it using air velocity.

Much more complexity faces us when trying to describe why fins and sails, for that matter, create lift. Consider the Coanda effect states that moving fluid must be attracted to a nearby surface. The water from a faucet bending to flow around the convex surface of a nearby spoon is a demonstration of the Coanda Effect. It also means that the fluid will flow around the side of the foil with significantly less flow. Rotational flow and vortices seem to be what's actually making wings work but present far too much complexity of thought and computation to make for a 10th grade physics class.

For those interested in reading a more accurate explanation, check out http://amasci.com/wing/airfoil.html#expl

Tons of links on the website. This page gets down to the nuts and bolts, and much of my writing takes source from that direct link.