Why do we use symmetrical boards in our cats? it seems to me that each board really only needs to give lift in one direction, and wouldn't we get MORE lift out of an assymmetric section?
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<br>I can't imagine that I'm the first to ask this question, but at the same time it seems like a no-brainer. maybe some of you design hotshots can set me straight.<br><br>

ease of production. with symetric blades, you only need one set of molds.<br><br>

When going in a stright line, do you want lift from the rudders? I'm no nautical engineer, and I'm not really sure, it seems the angle of attack when turning produces the lift, not the shape of the foil when going straight.
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<br>Perhaps some of our more eloquent posters can shed some more light on the subject.
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<br>Todd Bouton<br><br>

assymetrical blade will generate lift but at the expense of drag. A symetrical blade does strongly resist the sideways motion and creates much less drag. As an example: look at the H16 assymetrical hulls. There's no need for boards on these boats because the hulls themselves generate lift to resist the force of the wind trying to push the boat sideways. Typical centerboard (symetrical hulls and boards) boats will outpoint an H16 because they do not loose speed to the drag associated with the assymetrical hulls.
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<br>Second example: look at the wings of a small Cesna airplane next to a jet. The jet's wings are much more symetrical to achieve more efficient high speed flight. The Cesna has more asymetrical wings to carry loads at slower speeds.<br><br>[color:blue][b]Jake
<br>[color:blue][b]Nacra 5.2 (2112)
<br>[color:blue][b]Hobie 18 (???)

OK Guys all good points, however the main reason is the boards must produce lift in both directions. Starboard and port tack require the boards to lift in opposite directions. Here is a simple comparison: Consider an aerobatic aircraft. They typically run symmetric airfoils because they must fly just as well upside down as they do rightside up. A symmetric foil requires some angle of attack to produce lift, whereas a cambered or unsymmetric foil produces lift at zero angle of attack. It might be possible to have two oppositely cambered foils in each hull and raise the windward one, allowing the leeward board to do all the lifting, however a cambered board would never work on a monohull...<br><br>

Read with interest recently that 505's (mono dinghy) are building boats with a centreboard case that allows the board to rotate side to side... so when sailing on either tack, it will produce greater lift to windward. Haven't heard or read any comment on how effective it is, or how they stop it pivoting when they're reaching, etc. I wonder of our greater speed on cats would just produce too much drag for this idea to be of any use?
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<br>Gary Eastment
<br>Taipan 4.9 AUS171<br><br>

I think they are not used due to cost and convention. A good assymmetric section should be able to reduce or eliminate leeway with no significant drag penelaty. The crew would have to lft and drop the boards on each tack so the windward board did not fight the leeward one. The open 60 monos (the fastest offshore monos in the world) have used assymmetric boards in conjucntion with canting keels for years. Each tack the crew has to drop the new leeward and raise the old leeward board. These race at sea so dont tack that often. Also all the open 60 tris use assymmetric canted boards in their floats to provide both vertical lift and lateral resistance. It works really well.
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<br>Cheers
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<br>Simon<br><br>

Hmm...
<br>I got to thinking about this long ago, when I first saw a boarded cat. I assumed guys 'waaay smarter 'n me had tried it, or analyzed it and found it wanting. So I had me another think about why it might not work. What I came up with was along this line:
<br>Let's presume an impossible ideal; you can vary your hull plan and dagger profile and angle-of-attack on the fly to produce optimum lift to windward with the least possible drag - - what would that look like... nevermind how your bows point, the path throu space taken by the sail could only be lower than a certain few degrees off straight upwind. The less drag acting on your boat, the tighter you can sheet. Tight sheeting alligns the force generated by the sail progressively closser to 90 degrees abeam. Therefor it decreases thrust while increasing heeling moment. What's usefull about tight sheeting, is that you can generate force from airflow that is closer to being truely fore-and-aft. When you're really sheeted in hard, pointing up as high as your conventional boarded cat can efficiently sail ( max VMG), the sail is producing less force overall, because it is flat, and less thrust, because the force vector is practically perpendicular to your boards. So drag becomes very significant in limiting your velocity. Two kinds of Drag operate on a high point of sail - hull drag and sail drag.
<br>Since the question seems most directly concerned with hull drag, which is the sum of board drag and hull friction, and board drag must increase with lift, (drag is the coefficient of lift - there being no such thing as a free lunch) clearly the lowest drag, as stated elsewere in this thread, is a symetrical board...
<br>So, I hypothesized way back then... those hi-tech boards must work like... (as also mentioned in this thread) aerodynamic plane wings, (like the ones my fellow RC flyers flew next to my house), getting their lift from angle of attack... It'd be hard to measure, but these long, fine-hulled beauties must point a tiny degree above their real line of travel... albeit a much smaller degree than less 'pointy' cats. That's all leeway is, really, your bow pointing above your net line of true travel.
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<br>Back to pivoting boards, which I never thought of till your fascinating post. (..I got here the long way, I Know! lol) So there's this cat, symetrical boards... pointing slightly above her line of travel... inducing non-linear travel through the water for her hulls, while her boards fall into that low-drag, max lift angle of attack they're shaped for....
<br>I do believe if you allow the boards to cant slighlty, to put them in that angle, while allowing the hulls to go straight through the water, you might reduce hull drag somewhat. The payoff is, when you reduce hull drag, you can flatten the sail more, which reduces sail drag... YES!
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<br>Hmmm....
<br> however, your degree of cant to the boards would have to be proportional to the speed of the cat... I'm guessing you'd want to reduce it as the cat picked up speed, no? So you'd have to build a simple reliable, sand-proof spring linkage to use flow-pressure to induce canting, and you'd naturally want a lock-out for, .... um, for what, anyway? You could engineer it so that both boards would remain parallel to each other, you wouldn't have them fighting each other that way, and if you set your spring-linkage right, the boards would always be canted to produce lift in a manner most likely to allow the bows to point in the boat's true line of travel, so under what conditions would the boards be used where you *wouldn't* want the 'zero-leeway' effect on your cat's hulls? hm....
<br>think
<br>think,
<br>think,
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<br>OKay, linkage designed, looks like this... Say you're sailing on port tack. Both conventional boards experience sideways force, in that they're acting to prevent leeway to starboard. From the 'point of view' of the mounting closets in your hulls, both boards are pressing up to port. So if you used spring loaded felt guides that run vertically up and down just behind the leading edge, one such guide on each side of each rudder, by tuning the strength and elasticity-curve of thes springs, you could allow the leading edges of both boards to move slightly to starboard when on a port tack, and vice versa. You could accomplish the same goal by just allowing your daggerboard trunk to be slightly 'sloppy' in it's forward-end's fit, while being nice 'n precise on the aft edge. it just wouldn't be tunable...
<br>Okay, now for the hull-speed cant-governer. Take the spot where the board's leading edge passes through the deck... when the board is pushed aft by water pressure, this is the point where the structure of the hull resists the dagger's growing inclination to tip over. So, here, you put a tapered wedge that, as higher speeds push the top leeding edge of the board forward into this wedge, it in turn, acts to center the board. You might have to put a spring in here, with a roller guide for the leading edge. Then you just tune this spring to adjust your cant and speed-related transition.
<br>A nice snug fit at all times at the line of contact between the trailing egde and the entire hight of the board closet will keep the board from leaning inboard and outboard during these transitions.
<br>Hey!
<br>In fact, if you do it fine enough, you could just dispense with the felt covered spring loaded guides on the sides of the leading edge... without them, the board is supported and stabilized by: a) contact along the entire in-hull trailling edge, which could be rollers contacting the board just forward of the trailing edge, on both sides, top and bottom, b) the wedge guide at the deck surface of the forward leading edge of the closet. Again, put rollers on this, and put a second wedge just under the first. The top wedge is fixed in place to the hull; the one under it is spring loaded, pushing the board-top aft, when hydropressure allows. As the board top is pushed aft, the clearance to the stationary wedge opens up, allowing the leading edge to cant to windward, in response to the lateral forces it is generating as a reaction to it's primary job of resisting leeway. At the highest speeds, the board-top-leading edge is pushed tightly into the wedge, progressively centering it.
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<br>whadya think?
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<br>Me, I'm worried about the lack of support at the leading edge where the board exits the hull bottom.
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<br>hm...
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<br>think, think
<br>GOTIT
<br>okay.. rip out those leading edge wedges... keep the trailing edge rollers. now put in two sets of top-and-bottom rollers, one set on each side of the leading edge. Each set is mounted on one piece of stainless, which in turn pivots on a long pin running up and down, just in front of th leading edge of the board. One pin for each side, allowing the top and bottom rollers on that side to move in and out together, but independantly of the rollers on the other side of the board. Stick a tang up from each stainless casting, put a v-roller on each tang, and when the board-top presses hard on these tangs, the stainless castings will turn in, pressing the rollers in, centering the board.
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<br>yes.. much better. the board is now guided at all for 'corners' of its inhull segment, so it'll be, if anything, more stable than conventional boards, it can cant in proportional response to water flow pressure. For weight you've added nine nylon rollers per board, w axles etc, plus those roller pivots,,, hm.. could you make 'em outta carbon? Wonder if the weight cost 'd make up for the drag savings.
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<br>Anybody out there builds one, kindly invite me for a ride! :-)
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<br>Sail Fast,
<br>Ed Norris<br><br>

A well known principle but never really caught on in cats. I think the Mystere Twister F18 was experimenting with gibing boards.
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<br>wouter<br><br>

Make sure you close off inevidable cap of at bottom for water willl be push trhough the daggerboard well create alot of darg once again. Tornado used skirts for that. This is often a problem with boards that haven't a close fit in side their wells.
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<br>Wouter<br><br>

Julien Bethwaite did a study on this in his book "High Performance Sailing" I don't have the book with me, but he tried it on a Moth or something like that and I believe had positive results. Otherwise in my opinion, I think it would be advantageous in the daggerboards, but probably not a good idea in the rudders. In the past on this forum there have been a few discussions about how assymetric rudders are causing all kinds of problems for other sailors.
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<br>TB<br><br>

I say assymmetric dagger on a cat on bouys DON"T, gibing boards on a cat, MIGHT BE WORTHY A LOOK.
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<br>Remember that there is a difference between the two !
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<br>Assym boards won't help you much for you always have to lift one board. so you optimize the characteristic of one board but lose alot of total board area at the same time. Netto result could well be the same or even negative.
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<br>Wouter<br><br>

Ed,
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<br>What do I think? Firstly that you have too much time on your hands! :-)
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<br>Seriously, I think that it would all be a fair bit of hassle, maintainence and weight for potentially little gain... but some clever thinking from you nonetheless! Better get a patent on that post!
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<br>As you say, there is no such thing as a free lunch, so wouldn't the board in creating the lift required to prevent leeway in turn create drag roughly equal to the drag 'removed' from the hull?
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<br>Is there another benefit that I am not seeing? I'm assuming that the 505's wouldn't bother if it were not worthwhile? Perhaps the lift to windward improves VMG to the extent that the drag penalty is overcome...
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<br>Gary Eastment
<br>Taipan AUS171<br><br>

Hi Gary,
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<br>As I visualize it, the board will retain the same Angle of attack as a conventional boad; by canting it in the hull, I suggest we're just allowing the hull to point in the boat's true line of travel, (the net sum of headway plus leeway) reducing hull drag.
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<br>Regards,
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<br>Ed
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<br>PS Wouter, good point about the skirt - - or you could use a shield that slides back'nforth flush with the hull, and tight on the board if you wanted perfection!
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<br>Ed<br><br>

yep. I knew there had to be an easy answer.<br><br>

""I say assymmetric dagger on a cat on bouys DON"T, gibing boards on a cat, MIGHT BE WORTHY A LOOK.
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<br>Remember that there is a difference between the two !
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<br>Assym boards won't help you much for you always have to lift one board. so you optimize the characteristic of one board but lose alot of total board area at the same time. Netto result could well be the same or even negative.""
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<br>EA Bethwite's book high performance Sailing had a section on asymetrical boards. He concluded from testing that for a board of the same size there was no lift/drag advantage to a asymetric board. He also stated that later when he realy thought about it, the advantage of an asymetrical board might be that it could be much smaller than the equivelent symetrical board.
<br>Makes you wonder wether a cat with 2 boards anyway might be the perfect test platform. You might not need both down at once and they might be smaller if they were asymetric.
<br>Eric
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The last Little America's Cup victor,, Cogito, had assymetric boards. I spoke with team leader Steve Clark about them last year,,, as we were considering an experimental effort along those lines. He said the were effective over a narrow range,, and of course you have to tack the boards (raise and lower alternately) to windward, and downwind as well (unless you pull them all the way up). This would be easier in practice on a centerboard boat, than with daggers, I think. I have to wonder if the innovators in the A-boat class have not tried it at some point. Jim Boyer ought to know.
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<br>Keep at least one hull in the air !!
<br>Alan
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Wow!
<br>If so, Eric, that'd be a swell advantage in weedy conditions!
<br>Regards,
<br>Ed<br><br>

Back to way at the top of the threads someone mentioned the H16 and it's asymetric hulls. The H16, P16, H14, P18.... boardless boats were designed with asymetric hulls to produce lift w/o boards (notice that flat side is out and curved side is in...air/water foil. Ok... the biggest problem with the asymetric "lifting" hulls of these boats is the aspect ratio. Since the hulls are very long vs. their draft, they are low aspect, producing less lift for a given speed at the expense of more drag. Now, lets look at the I-20/18. Their boards are high aspect. With a hight aspect foil, the lift to drag ratio if increased significantly (more lift--less drag). What Jake was trying to say earlier, but used different words( I think) is that if you look at something like a jumbo jet/cargo plane their wings have more draft/curve in them to produce more lift at a lower speed w/more drag and limited top speed. While if you look at a higher performance aircraft... I like to use the voyager as an example (ya know the little white plane that flew around the world) it had very very high aspect wings (foils to us) wich created a high lift to drag ratio... ok... they did that for efficiency. While a jet fighter which flys at higher speeds also has a less drafty/flatter wing because it has more air flowing accross it and can produce the lift it needs with less draft at higher speeds with less drag... (yeah...drag like reading this post.......Zzzzzzzzz)
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<br>What does all this boaring crap mean to us? If we had high aspect asymetric daggers we could use a smaller board and get the same lift as a larger symetric board with less drag(increased board efficency with lower wetted surface). HOWEVER!!! since in some conditions (ligh air) we need more lift we would need more board or a board with more draft, which gives more drag. Sooooo do you optimise the board for a wind/hull speed range? Guess so....what do you do then when your boat is sailing outside the design range? There in lies the problem. Designing a board that is asysmetric and produces the optimum lift at all times is IMPOSSIBLE. As hull speed varies, lift varries...drag varries... what is best at one point is not at all others. The one thought though is... if you build a board with enought lift at slow speed then you can just pull it up at higher speeds to reduce drag/heeling forces. Well even this isn't great, at higher speed you need a less drafty board (board... like me writing this post....and you if your reading it). So nothing will ever be 100% optimal, however their is an A class boat out there with Asymetric boards... don't remember which one though.
<br>So... I think yeah.. if you want to remember to tack your boards all the time, but go faster... asymetric, but if you wanna K.I.S.S. (keep it simple stupid) then go high aspect sysmetric. I'm not gonna try to calculate the difference in drag vs. lift vs. too much weight on my right cheek vs. my left. I'll leave that up to someone else... bet you can't guess who!
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<br>Well I've rambled for WAY too long....
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<br>Will<br><br>

Thanks, Ed. Got it now. Like I said, better put a patent on the post!<br><br>