hogg@utcsri.UUCP (John Hogg) (08/14/85)
"Experts have their expert fun Ex cathedra telling one Just how nothing can be done." -Piet Hein It's nitpicking time, in response to a couple of postings on sailboards and wing theory. The drift of the first is that rigs articulated in the lateral plane are superior to conventional rigs in that they lift the hull and thus reduce resistance; the second poster seemed to disagree, but said basically the same thing, with a few technical terms thrown in. My turn: >Okay, Bernoulli principle time. The main thing to remember about a sail >is that (with the exception of Patient Lady IV and other solid-wing craft, >and even they don't fully conform) is that it is not an airplane wing. It works the same way, and is quite similar to many ultra-light foils. >There is a large amount of bleed of pressure through the fabric of the sail, >even if it is mylar or some other exotic. Mylar has essentially NO pressure bleed. I'll ask a sailmaker friend the next time I get a chance, but I STRONGLY doubt that they even test mylar for this at the MILL, let alone the sailmaker's. >Attached airflow over a thin >interface such as a sail is a very difficult subject, inasmuch as such >factors as tip vortex and surface viscosity and relative air density >affect its calculations to a large degree. The same factors have to be considered in thick-foil fluid dynamics. >You also overlook a very >important factor, the slot effect (jib and main). The slot effect, in >such boats as a Tornado and Nacra, as well as a fair number of monohulls, >provides a large amount of lift by both increasing the pressure on >the leeward side of the mainsail and increasing the speed at which >it flows over the main. Actually, the jib DECREASES the lift produced by the main. This is true for all boats, mono- and multi-hull. However, the jib's performance is improved by a greater amount. It's like the better pointing angle experienced by one boat lee-bowing another. For a more quantitative analysis, consult Marchaj's "Aero-hydrodynamics of Sailing". Fun reading, in SMALL pieces. >Remember that an airplane wing has significant >chord depth, whereas a sail has virtually none. It is not correct to say that >a sail drives a boat forward; it is more correct to say that the lift >coefficient of the sail, in conjunction with the lift coefficient of the >foil surfaces in the water, cause forward motion and sideways motion (leeway). >Or in other words, the center of effort in conjunction with the center of >lateral resistance squeeze the boat like a pumpkin seed between your thumb >and finger. The "lift coefficients" are numbers. The centres are not of interest in this context; their relative positions are only important in determining turning couples. If you want to get technical, you can say that the lift produced by the sail is opposed by the drag of the keel and the drag of the sail is opposed by the lift of the keel and when all is in balance, the boat will move forward at a steady speed - but it's simpler to say that the sail lifts the boat forwards, and implicitly assume the existence of the other three forces. >Also, the pressure difference you speak of does not suck the boat down to >leeward. This is a misstatement of how lift above the water interacts with >lift below the water. Manfred Curry in the early 20's explored this subject >fully, as did C. A. Marchaj later, and it has ben conclusively proven that >if you want to sacrifice speed for pointing, you can articulate your center- >board (or presumably, any other lifting surface) so that you do not make >any leeway - which is what I assume you mean by being "sucked down to leeward". You can certainly mount the hull sideways on the rig and keel, and if the latter is large enough, you will sail merrily along. However, the original poster referred to the rig pushing (or, to be scrupulously correct, dragging) downwards in a vertical direction due to heel. >At any rate, the principles of Bernoulli apply to fluids, not gases, and >the only reason they are used to approximate what goes on in aerodynamics >is because at high speeds (which is what most aerodynamic research is concerned >with), air acts more like a fluid than a gas. At low speeds this is not >necessarily true in all cases. A basic approximation of sailing theory is that air is an incompressible fluid (which can be a liquid or a gas). This is in fact a very good approximation up to speeds approaching Mach 1, at which few of us sail. > (Several accurate and perceptive lines about heeling on sailboards > deleted.) >Also note that the geometry of the vectors involved means that you sacrifice >increased leeway (due to less wetted surface and a lift vector that is trying >to pull the center of lateral resistance (i.e., centerboard) up out of the >water) for increased speed, as you would with a catamaran or any other high- >performance sailing craft. Less weight -> higher speed -> greater lift -> tilting the rig to weather will produce LESS leeway on a sailboard, until the daggerboard is lifted a significant amount (!) out of the water. >A fully articulated rig is not practical on a large boat, or in fact any >boat much larger than one person can control. In one paragraph, you correctly >stated that one of the reasons for the increased speed and apparent lack >of heeling is that most of your weight is over the boat, being held up by the >sail. On a large tri, there is no corresponding mass to suspend over the >boat. As upward lift is generated, there is no opposing force to direct it. >Since the only fulcrum is the base of the mast, and since the force is being >exerted perpendicular to the lever arm of the mast which is held by the >windward shroud, all you are really doing is buying time prior to capsizing >or reducing sail. The force of lift will be directed at some vector upward >along the windward shroud, and since there is no mass over the side of the >boat at the windward shroud that would correspond with the sailor's body >on a sailboard to counteract this force, heeling will still result. The point of rig articulation is not to reduce heeling but to reduce downward force (i.e., apparent weight) by applying an upward force. You don't have to suspend any weight overboard to accomplish this. >Of course, the primary reason it isn't practical is because of Murphy's law. >There's already quite enough complexity and room for chance error in a >rotating mast on a large boat, which has been demonstrated over and over >again, and to introduce a complicated system of hydraulics is, to me, >asking for trouble. The only thing I could possibly think of is a device >I have seen on some hydrofoil sailboats, which is a large, rotating arm >with a freely-articulating foil surface on it. The trick is that the lift >generated by this anti-heeling device is downward, so that it attempts to >drive itself further and further into the water, and thus bringing the >weather hull down with it. When you tack, you bring it around with you. >A possibility. A better plan (which I believe is the basis of these boats that you mention, and has been since at least the 1890's) is to shift your UPWARDS lift to leeward. Pushing down, whether with sails, lead keels, or gorillas on the weather rail, is really an inefficent way to do things. The problem is, it's also often the simplest and most reliable. God, that was fun - giving my unsolicited inexpert knowledge to the world at large. But wait - one last parting shot... >I agree, boardsailing is fun. It doesn't compare with sailing a hydrofoil, >though, and it certainly isn't in the same league as an iceboat or a land- >sailer. To be perfectly honest, it barely compares with sailing a Tornado >or a Hobie 16 or a Nacra. There's something inherently fun about flying >a hull at 20 kt. while double trapezing. Not to pick a fight, but let's >be honest - boardsailing racing, round-the-buoys, is not of sufficient >caliber compared to most sailboat racing in classes of much less popularity. >A minor point (if you're not a racer). Ah, but I don' think that I'll EVER get the hang of doing a railride on a Hobie. -- John Hogg Computer Systems Research Institute, UofT {allegra,cornell,decvax,ihnp4,linus,utzoo}!utcsri!hogg