Rocket Science: "Money for nothing, and chics for free" Bottom Contours: Venturi Revisited *PIC*

Your spoon.>>> Which ever way it moved in your experiment, there was a force which you > did not mention. You not only had to hold the spoon up (gravity) but you > had to change the flow of the water (in order to get the effect.) To > change to flow of water requires force, and that force had to come from > somewhere.>>> So in order to generate the forces that you believe are there you got to > have some other force holding the spoon in place so to create the > circumstances that allow these forces (which resulted in some lateral > movement) to be generated.>>> “Money for nothing, chics for free.”>>> Throw a surfboard in a river and it will just move with the current, > regardless of its bottom contours, fins or whatever (excluding somebody > paddling it.) Throw a surfboard on the ocean and it will do the same thing. Why? Forward velocity at least equal to wave velocity is necessary to catch waves. A floating object will not move forward in the direction of wave propogation because the net vectors (gravity plus centrifugal force due to the circular motion of water with wave energy passing through it) equal and offset the buoyancy force of the floating object (which is perpendicular to the water surface). On the other hand a moving object at wave speed can generate net positive surfing thrust. Note that this does not have to be a planing hull shape, but can be a displacement hull (outrigger canoe, sailboat), a displacement/planing hull shape (surfboard, racing dinghy) or even a non-planing underwater object (dolphins surf better than we do). Point being that “surfing” occurs with objects other than surfboards, both surface and sub-surface, some of which are non-planing (thus it’s not all about planing theory…)>>> I am not claiming that the circumstances, analogous to your spoon > experiment do not arise, its just that how might they and when might they > arise is an issue, which in my opinion is often overlooked. But there’s > even something more important,>>> … and that’s everything costs something.>>> You can achieve greater lift, but it will cost you, possibly in speed. You > can achieve greater speed, but the cost might be stiffness in turning or > in maneuverability. Which brings me back to my cartoon about the Venturi > effect. The effect assumes a constricted flow, if you constrict the flow, > you pay a price. And its likely that price is a force in a direction > contrary to the flow moving through the constriction. In the extreme > example of a funnel, the price would be the force required to hold it in > place.>>> (Claims of a Venturi effect? See http://www.eatonsurf.com/Bonzer.htm I > have little doubt that the claims made by surfers who use a bonzer are > real. I have problems with the description of what’s going on underneath > the board. Eaton’s explaination is interesting, but in my opinion misses > an important point, the price paid for constricting the flow,… chicken > before the egg?) The “price paid” for restricting a flow, according to physics, is simply a transfer between velocity, pressure, and temperature. Faster moving fluids move at lower pressures. Gasses may also transfer energy in the form of density or temperature shifts. The price paid is not necessarily drag, as you seem to be assuming. It’s not possible to verify surfboard performance empirically due to lack of controls - no two waves are the same, for one thing. But there is lots of subjective data in the form of feedback from lots of surfers riding lots of boards. In my personal experience I’ve only had a few “magic” boards and at least two of them had “Bonzer” type bottoms. One was a 5 fin longboard w/ a venturi shaped channel, wide in the front and narrow at exit, and one was a shortboard with similar bottom design. Both of them rode amazingly well and were fast both down the line and in turning. To me this is at least some evidence that constricting the water flow between the fin set produces useful results (call them what you will, “redirection” if “venturi effect” offends your sensibilities for some reason) I’ve talked to others with similar experience. And the shaper of the 5 fin longboard told me that he’s made other boards without the venturi channel but they don’t work as well (according to feedback from the riders).>>> I do not disagree with the ability of some bottom contours aiding in the > redirection of flow. Redirection of flow comes at cost however, and you > may see it as being worth the price. (I do.)>>> Under many, if not most circumstances, redirection of flow results in some > lift (say in the direction of Upward force in the prior diagram) as well > as in the Forward direction (see prior diagram). And under most > circumstances, the surfer is able to counterbalance that lift with some > change in technique or reposition themselves on the wave (angling the > board in some manner) or even changing the center of mass of surfer and > surfboard, or use it. Same for the lift in the Forward direction. Low pressure areas would not produce lift, as they are under the board. High pressure areas under the board produce lift. Lift in the forward direction is called thrust.>>> No pictures this time. I was going to draw your spoon but decided it was > unecessary, hopefully my point about the unmentioned forces is clear. I > guess I’ll find out soon enough.