Does anyone know if A-Cats make use of asymmetric dagger boards to eliminate leeway? I don't see anything in the rules which would stop them.... just curious

There is no particular advantage for assymmetry, and the concomitant drag would, if anything, be costly.

really? I don't think there would be significantly more drag as the same sideforce would be needed but you could generate this without the angle of attack produced by the leeway, you may even have less drag since the hulls would track straight throught the water

I know C-Class cats make use of this

Think of airplane wings - aysemetric wings are used on low speed aircraft. High performance wings (less drag) are fully symmetrical. On the scale of things between an airplane wing and our foils in the water at the speeds we sail, the relationship is very similar (Reynolds numbers). Generating the same lift, a symmetrical foil will provide less drag through the water. I don't know what the C-class guys were doing...but more can be done with the angle of the board to present lift to windward and/or lifting force (and, you won't have to gybe a symmetrical foil like you would an asymmetrical foil).

Sorry Jake I think your wrong.
whilst the reynolds numbers might be similar for dagger boards and high speed aircraft, I haven't checked, you've got to remember that the rules change when you have to start considering compressibility and shock waves.

theres no reason why a symmetrical foil should produce less drag at the same lift coefficient as an asymmetric one, I suspect it would be the opposite.

I don't see you having to gybe asymmetric boards, you simply have an opposite board in each hull and keep the windward one lifted and switch them when you tack/gybe

like you say angeling the board in relation to the hull would give the same affect to a symmetrical board, does anyone do this?

sorry thats beggining to sound like a bit of a rant

HangOver, whoever you are..

I have looked a bit into this, and the differences are not that large on a beachcat. Depending on who you ask, you might get different answers..

In theory there is a small advantage to an asymmetric daggerboard setup but on a singlehander with spi you are overtasked as it is. Steve Clark reported trying asymmetric boards on an A-cat, without any significant advantage.
The C class cats are able to pull it off, as was the Formula28s, so it might be you need a certain size of the boat before it pays off.

Martin Schoon did a small study on the matter which is quite interesting. http://hem.bredband.net/b262106/Boat/dagger.html

For an A cat, ref. what "Uncle boat", Steve Clark, said..

I'm in a generous mood, so I'll be patient:

Here's the thing: Experienced boat designers aren't going with assymmetrics. I figure these guys probably have good reason. Assymmetric foils aren't a new innovation, so I'm sure these experienced designers are familiar with assymmetry.

The Man, Steve Clark, has been mentioned here. if I recall, the COGITO, The Boat, has Bruce foils (quote from Wikipedia, for the curious)

"A Bruce foil reduces torque by moving the foil far out from the centerline of the hull, and angling it so that a perpendicular line through the center of the underwater portion of the foil will intersect the center of pressure of the sails. Like a traditional centerboard, the Bruce foil is generally a [color:"red"] symmetric[/color] design, such as the NACA 00xx series, which relies on the leeward slip of the hull to provide the angle of attack needed to generate lift. When mounted on the lee side, the Bruce foil generates an upwards lift in addition to lateral force, and when mounted on the windward side, it generates a downwards force.
Bruce foils can be mounted in pairs, one on each side, as used in some trimaran designs. In this case, the leeward foil works in conjunction with the leeward ama to provide lift, and the windward foil can be retracted to reduce drag.
In theory, the force from the foil and the sail will always be balanced, so capsize should be impossible, but in practice that is not the case. Since the foil's force is relative to the hull's movement through the water, and the sail's force is relative to the wind moving past the sail, sudden changes in wind speed or direction can unbalance the foil and cause heeling. In addition, a foil on the windward side presents an unstable situation, since any lifting of the foil out of the water, by mechanisms such as heeling or wave action, will result in reduced downwards force, potentially leading to capsize. A lee foil provides more stability, as loss of lift will force the foil back into the water.

Advantages and disadvantages
The main advantages over a typical centerboard or leeboard is that the Bruce foil allows the sails to remain nearly upright and keep the full force of the wind available to drive movement without the crew needing to move their weight. The main disadvantage is that the sails remain upright and keep the full force of the wind available, rather than depowering with increasing angle of heel as happens with traditional foils. The Bruce foil also produces increased drag as a result of the vertical component of force, as well as a yawing force due to the off-center placement of the foil, which may require alteration of the steering system to compensate."

Cogito have standard asymmetric daggers and symmetric rudders. No bruce foils on that boat as far as I know. Very few beachcats have bruce foils, I have only heard about one production model I think.

The answer to your question is that there is nothing in the Acat rules to stop you from asymmetric dagger boards or gybing dagger boards. Foils that lift the boat clear of the water are not allowed.

If you have asymmetric dagger boards, you'll have to gybe/adjust them on each tack, unless you design something that does this automatically, which would be neat trick, seeing that you want them to gybe if opposite directions upwind vs down. Lets just pretend that on either upwind or down or both you'll have to do this manually.

With one person on an Acat its going to add time to your tack to gybe your boards, and I think that you'll find that its slower in the tack to gybe your boards, vs any advantage you gain using them (symmetric or not)

The really fast guys - Lars Guck for example, complete their tack and are off on a new tack so fast it looks like a big splash in the water. He walked a group of us through his tack at a pre-regatta session and he's thought through the tack, and he does it the same each time, each step is optimized and he doesnt waste any motion in getting to a new tack.

I would work on getting things more simplified as opposed to more complicated, so your tacks are that much faster.

A cat sailing is easily 60% skipper skill, 40% boat/sails, etc. Thats the reason Ben Hall didnt walk away with the worlds title, he ended up right about the same as he usually does at the worlds, even with a solid wing that is theoretically much faster than a soft sail.


Bill

Hi all,



The only serious use of symmetric foils on aircraft is for aerobatic aircraft, because they want to have similar flying characteristics in normal and inverted flight. I know of not any modern aircraft, even if high performance, supersonic or whatever which uses symmetric foils. And that for a good reason. Especially at transsonic speeds the differences are significant.


The Reynoldsnumber is nothing constant at all, double speed - double Reynoldsnumber. Furthermore there are other differences (e.g. Mach number). Generating the same lift an asymmetric foil has less drag than a symmetric one.

However, if you compare aircraft with sailing boats, you have to consider that the wetted area of the daggerboards and rudders is small compared to the wetted area of the hull(s), while on aircraft wing and empennage have a significant contribution to wetted area. Hence on boats, the non-induced drag is dominated by the hull and not the foils, while on aircraft the foil contributes strongly to drag.

A bit off-tobic, but there is also a common misunderstanding for induced drag: The wrong saying is, that iunduced drag depends on apsect ratio. The correct one is induced drag depends on span, and induced drag coefficient depends on apsect ratio.

To get an understanding how different water and air actually is, just have a look at an aircraft propeller and a ship propeller. Other number of blades, blade aspect ratio and sweep angle and -if you have a closer look- totally diffrenet foils.

However you can use low speed airfoils, Reynoldsnumber 500000-6000000, in water. But you have to be careful, that the lowest cp value isn't below the cavitation limit. Actually I have seen a lot of rudders on boats which start to cavitate before reaching their lift limit.

Finally if you want to use the full benefit of better foils, you have not only to 'exchage' the profile, but also to adapt the area of rudder and daggerboard. Example: You have a NACA 0012 with let say cLmax of 1.4 and low speed (Reynoldsnumber) and now you get a purpose designed section (which need not to be asymmetric) with a cLmax of 1.7, than you could reduce the area with ratio 1.4/1.7 and you would still have the same manoeuvrability.

Cheers,

Klaus

Angled symetric daggerboards are almost as efficient as asymetric daggerboards and tack by themselves when properly designed. It is quite simple, actually. Take a look at Gybing Heads





This is a proven go-fast resource that has been in use for the last forty years in the Penguin class in Brazil (gybing centreboard). I read somewhere that the 505 class also use it.

There's no reason why an A Class wouldn't benefit from the same.

All the best,

thanks for the response everyone, I don't actualy sail an A-Class I was simply curious..

I hadn't considered any of the 3d effects which does seem to negate any drag benefits but this isn't the only potential benefit, don't forget that by negating leeway you are able to sail a shorter course to windward

In which term efficient? In terms of maximum lift they definetly not. In terms of drag, they are as well worse, but the difference is small. You can have look at my posting on the page before.
I can you assure that a symmetric foil with incidence is not much different than a symmetric one without incidence, and that an asymmetric one (but not when two are used at the same time) is always better than a symmetric one, apart from the fact, that it is not symmetric...

Cheers,

Klaus

It works different. A boat with angled board or asymetric board will actually point lower while travelling the same path over water - only faster.

The benefit is that angling the daggerboard (or using an asymetric one) changes the yaw attitude of the hull so that it will use more "foil generated lift" and less "hull generated lift" to keep forces in balance.

Since the foil's lift/drag ratio is far better then the hull's lift/drag ratio, the total drag is reduced and the boat can sail a bit faster.

A secondary benefit is that, due to heading lower, the sails can be trimmed at a higher angle of attack to the wind, also improving speed - or enabling to point the same as a normal boat, but travelling over a better (higher) path upwind.

Take a look at the self-explanatory picture. The asymetric section daggerboard could be an angled symetric daggerboard as well. The difference is negligible.

(note: the drawing does not show the "hull generated" contribution to lift in the conventional boat diagram, but it exists)



To Klaus: the difference between the lift/drag ratio of an asymetric and an angled symetric foil is tiny compared to the diference between any of them and the lift/drag of the hull generating lift at an angle of attack. For this reason, the choice of an asymetric or angled symetric foil would be almost irrelevant.

Still, the angled symetric foil is better because it self-tacks upwind and become a fixed symetric daggerboard when raised (even slightly) upwind, while an assymetric daggerboard will require active management in a cat.

On the other hand, the asymetric board will alwasy fit the cassette perfectly, while the gybing board will leave a gap between its leeward side and the cassete wall when fully down.

All the best,