dht@druri.UUCP (Davis Tucker) (08/09/85)
>Back to basic airfoil theory -- why does a sailboat sail upwind? >Because a low pressure area develops on the leeward side of the >sail (or, conversely, a high develops on the windward side). >This pressure difference is also the cause of heeling; not only >does it drive the boat *forward* it also sucks it *down to leeward*! 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. There is a large amount of bleed of pressure through the fabric of the sail, even if it is mylar or some other exotic. 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. 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. 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. 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". 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. >Now, suppose that you could take that low pressure to leeward >and make it suck your boat up and forward instead of down and forward. >You would then be developing lift and forward drive instead of heel >and forward drive. THIS IS EXACTLY WHAT A FULLY ARTICULATED RIG BUYS YOU. What happens on a sailboard is that the lift vector of the sail is not fixed two-dimensionally as it is on a fixed rig. The lift can be used to essentially lift the board out of the water somewhat, reducing wetted surface and reducing heeling moment to almost nil. Might I point out that sailboards do make leeway, and do heel to some extent (or would, given a theoretical situation of the sailor not actually standing on the board, thereby allowing it to assume its natural position). Heeling is caused by a number of factors, not the least of which is the center of lateral resistance. Note that in a regular monohull, say a Laser, that you will hardly heel at all if you remove your daggerboard. You won't go anywhere but sideways, but... (in fact, this is a very good racing trick in medium-air, crowded starts - not many expect it). The reason why it seems that sailboards do not heel is that the sailor constitutes so much of the weight of the entire rig, and therefore counteracts the heeling moment that is actually present. To prove my point, try this: stand on the leeward side of your board, or at the centerline, in a fair breeze. You will notice that the windward rail tends to ride up. Another reason why it seems that boards do not heel is that the board itself is so flat and heel-resistant that it is very difficult to make it heel (as with a catamaran until it reaches a certain point). 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. >When the wind starts to blow on a conventional rig, it heels, thus >reducing the sail area presented to the wind. When the wind blows >on an articulated rig, the sailor reduces the sail area presented to >the wind by inclining the rig *to windward*, and the result is lift. >On a sailboard, the experience is absolutely thrilling: you are leaning >back hanging all your weight on the sail, i.e. taking all your weight >off the board, and the board, which suddenly displaces say 50 pounds >(instead of the ~200 pounds it had with your full weight on it)... This is essentially correct, but it misses an important point - that wind is three-dimensional. In a condition of streets of cumulus clouds with a 20 kt. breeze, the wind is actually moving in a spiral down the middle of the street (where the clouds aren't). In some other conditions, especially heavy air, wind is moving just as rapidly downward as it is in any other direction - just ask a hanglider about downdrafts. Another factor that is only beginning to be explored is that in very heavy, flat-out conditions, the sail of a fully-articulated rig is using the "ground-effect" to generate greater pressure between the sail and the water, similar to the flying of a seaplane at very low altitude to conserve fuel. There is a cushion of denser air being carried by the sail which can counteract the effect of downdrafts. I have not read any detailed analysis of this, other than passing references. A very good book on micro-meteorology is Alan Watts' "Wind And Sailing Boats". >So THAT'S why I say that the fully articulated rig makes heeling obsolete >(just like this technology here makes paper obsolete). >I have seen one of Dick Newick's big trimarans with a huge hairy universal >joint at the bottom of the mast (and big hydraulic cylinders on each of >the stays and shrouds for radical adjustments of their length). >And I did hear that the first monohull to finish the last OSTAR also >had a fully articulated rig of some kind. 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. 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. >I believe that the fully articulated rig is the most important sailing >breakthrough since the development of the fore 'n' aft rig. I would rank the catamaran, the hydrofoil, the fully-battened rig with rotating mast, the wing mast, the solid-wing sail, and most definitely the introduction of fiberglass as greater sailing breakthroughs. >Will we see f.a.r.'s in Perth? Maybe after they let multihulls race :-). Actually, you're right. A monohull would not have the beam to crank a mast over to windward more than a few feet. >If you need to be convinced, try board sailing. If you thought sailing >a conventional rig was fun . . . . . 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). Davis Tucker AT&T Information Systems Denver, CO