· Page Last Updated 24 May 1999
Upwind performance is a bit disappointing, most especially with the short rig. I've had lots of correspondence on this, and no two people seem to agree. Probably because there just isn't enough data!
I read about your difficulties with the reefed rig, and have done some simple calculations that may shed some light on the problem. As I see it, induced drag should be lower (as a percentage of total force), not higher, in the reefed condition. Induced drag is proportional to lift coefficient squared, and if airspeed is doubled lift coefficient must drop by a factor of 4 to maintain the same side force. This should reduce induced drag by a factor of 16. The lower aspect ratio will increase drag, and parasitic drag of rigging and such will rise, but not by nearly as much. Side force is of course limited by righting moment and will be nearly constant in upwind conditions regardless of wind speed.
So what do I think it is? I think the dominant effect is an increase in side force on the foils due to the reduction in effective rig height by reefing and/or allowing the upper leech to open. Since righting moment is constant, any reduction in rig height will proportionally increase the side force that can be resisted by the fixed moment. In upwind conditions, the critical boatspeed is roughly constant (just past planing speed), and every rig height reduction increases the force required from the foil, increasing its induced drag and putting it that much closer to stall. Things to try might include consciously footing a bit to increase speed when reefed or using a larger daggerboard. I suspect the foils also because of your previously described tacking difficulties, again due to asking the foils for more force than they can provide at low boatspeed.
Read about your rig/reefing problems in AYRS Digest. A quick & dirty calculation:
Aspect ratio is defined as span (= luff) squared divided by sail area. For a practically rectangular sail as yours, this is the same as luff/foot [(luff x luff)/(luff x foot)].
Induced drag is directly proportional to aspect ratio, so when you reef down to AR= 2.26, induced drag is increased by 2.54/2.26%= 12% - assuming that lift remains the same. Induced drag is proportional to the square of lift.
But the lift cannot remain the same:
If you are fully powered in 8 kn. of wind with the full main, lift must decrease a lot to allow you to cope with 16 kn., even with the reefed sail. So even with the decreased AR induced drag will be much less:
This something else will need to be a) flattening the sail as much as possible b) sailing/sheeting at a narrower angle to the wind (you need to pinch to keep the boat upright, or sheet even further outboard).
Roughly, the kinetic pressure on your reefed sail will be about 3 times bigger in 16 kn. than in 8 kn. This means lift coefficient must be 1/3 only, for the same heeling moment. Since induced drag is proportional to the square of the lift, induced drag (and associated tip vortex) will be only 1/9th of what it was in 8 knots, even if AR is a little smaller. In light air, induced drag is a big issue, in heavy air it is not...
Huh? Complicated, but I don't think investigating into the tip vortex will help solve your problem. The reason must lie elsewhere. My gut feeling is that your case illustrates well the problem of the cat rig: If you make big enough a sail for the light air, you cannot cope in the breeze, because adjusting power is so much more limited than with the sloop rig (jib). To be able to handle a cat boat efficiently you need a Finn-style free standing rig with heaps of bend, and a more triangular sail.
Your choice of a rectangular sail makes it worse. First of all, when you reef, aspect ratio goes down. With a triangular sail it remains unchanged. Even more important, when you try to feather (twist off) a rectangular sail, you lose all power. With a triangular sail, you dump the top part, but the foot still remains powerful. In other words, the triangle is much more forgiving & gives a wider groove, while your fathead sail keeps you right on the edge of not going anywhere, or tipping over. The fathead sail only makes sense if you are limited in mast height and sail area at the same time - else it's better to make a longer mast and a sail with less roach to get the same area...
There is a loss around the boom... but I don't think the height makes a big difference. A phenomenon called side edge suction makes up for some of the losses. One of the most surprising results we had in the wind tunnel tests we did recently was that lifting the jib up away from the deck had hardly any influence on performance...
I still think (intuitively more than with reason) that your problem is in the difficulty to depower the rectangular sail - it's either on or off, more sensitive to the angle of attack than a triangle... a very interesting topic for a study, if ever I find the time.
UK Moths when we started mothing used to reef in 20 kts +, with a slab off the bottom, or use a low roach full luff sail. (Caws had the storm sail, I used a slab) performance..... with both was similar. Gutless downwind, and much slower than full rig, but less pitchpoling, especially with the storm sail. upwind performance similar, and slightly faster than full rig, generally a bit easier to cope with the big gusts without flapping ( which is very very slow and can lead to blowing over.... all drag and no lift) But now in the age of the T-foil ( and also we are more experienced ) the races get cancelled before we need the small sails. So my experience of the reefed rig was that it was easier & faster upwind, and slower downwind. ( but the smaller rig was only needed to stop the pitchpoles) I suppose your reef is bigger in proportion. The stop/go thing beating may be too much kicker. Moths use fairly slack kicker with lots of twist in big winds, sheeting fairly close to centreline. The sail is backed at the top, but pretty flat. ( the kicker is probably at the same setting as medium winds, but it seems slack because of the twist from the wind loads). The sail twists off and looks funny, but there's still plenty of power
Sailing a 14 or 49er is different.... they have jibs!.... the main doesn't do much, the jib makes the boat go... you can pinch, flap the main and it'll still go. I think perhaps you're pinching too much. When we come back from the nationals, we are sailing upwind much freer... because the others do it, and its faster. Gradually, at home, we sail higher again , till next year, and we remember again. I haven't made my big endplate yet... perhaps you should try it. the 49er sleeve probably extends the luff effect so aspect ratio increases and end plate effect is from deck area etc. generally more efficient. Lots of older designs have supersquat rigs, with pin head sails, so their real aspect ratio must be very small, and tip losses huge. ( and they are slow too!)
After much work-wasting discussion with a friend who works on this sort of thing (albeit on a very much smaller scale) we've come up with some (useful?!) suggestions: As a wind/smoke tunnel doesn't seem feasible, some sort of larger particle tracking velocimetry might be the way forward - if you can get some synchronised digital cameras, then sailing in the dark in a heavy snowstorm under your own illumination should do the trick. We couldn't think of a way of measuring the pressure distributions over the sail (which would give a good indication of what was going on). Alternatively, a slightly more radical option would be to fit inclined winglets (a la Airbus) to limit the losses. Would certainly be a talking point at opens.
(These are extracts from a phone conversation).
Aspect Ratio - seems unlikely - not too many gains to be made over about 3, and have got to consider effect of hull/water
Twist/aspect ratio effects can be experimented lots by playing with twist...
Leeway/board effects might be significant try board up with big rig try checking out leeway
I am not an aeronautical engineer nor an international calibre sailboat designer , but I'll give you my two cents on induced drag.
the airplane people use a formula that goes like this : Cdi = CL2 / pi * ar * e CL2 is coefficient of lift squared pi is 3.14 out as many decimal places as you can tolerate ar is aspect ratio e is efficiency factor of the planform in use. .8 for rectangle 1.0 for elliptical run the numbers if you can calculate cl and then multiply by your sail area and you get induced drag in pounds (if I remember correctly.) The bottom line is that induced drag is inversely proportional to aspect ratio so your 2.26 rig has 12% more induced drag than the 2.54 rig. If that number is tiny than 12% more is still tiny. I wonder if the fact that the rig is not as tall and "sees" less wind due to wind gradient is as much a factor? Both of them together may then become noticeable upwind.
I was going to suggest that you try the sail in a less dense medium in heavy weather & ! I sailed the 300 yesterday with it's reef in for the first time and it was a lot more manageable even though it a very small reef (probably around .70m2 It certainly made the gybes survivable due to the lower c of e. My guess is that your problem is the wopping roach that you've got, ok if its opening and closing with the gusts but when its windy I suspect its fully open most of the time which has got to help induce the air flow upwards into a vortex, way beyond the directional differences from the boundary layer.......must admit I've never been convinced about that anyway and have always tried to keep the leach under control - almost parallel and I'm convinced that's good for upwind speed due to minimal drag. Even more so I've you're going to power upwind by dumping main rather than feathering. That's the best thing about the 300 the kicker is really powerful.
When you're powering the 300 upwind the apparent moves forward and you keep having to dump.......to the point when the main is almost where the shrouds would be ! then *suddenly* the sail completely collapses and it takes an enormous amount of sheeting to stop yourself from going in to windward! I don't have the upper body strength to sail like that for long buts its a lot faster than the feathering technique.
Long woollys from the leach & on the sail in the head reason are going to be your best bet to see what's going on.
There seems to be a large difference between your small and big rigs. It'll probably take a while to sort out when you should change down. I think the aspect ratio isn't to low as the light weight crews in the 12 footers get down to round 2.3 for their number 4 rigs and they don't seem to have problems with tip losses, but this in 20+ knots.
One technique I've found for tacking a large roached main in a blow is to let the vang off a bit just before a tack, holding the leach tension with the main sheet, then going through the tack as normal and once you're on the new tack and making progress pull the vang back on. Letting the vang off means that the sail can twist off easily and hence reduce the drag. The only problem with this method is that the boat has to be setup with a really good vang system.
Sailing yesterday with the big rig in moderately overpowered conditions... Still difficult to get the boat really in the groove, Occasions when I was seemed to involve pointing very low, too low to be competitive. Its noticeable that once planing fast its possible to bring the boat up closer, but its very critical.
Had a little time to do some development sailing after the race, not enough for full parameters. Sailing with a very tight leech, much less twist than I normally have boat seemed to plane a little more readily higher, but was - naturally - intolerant of gusts shifts etc. and needed lots more work on the mainsheet. Sailing with the board up a bit was odd, it didn't seem any more doggy than with the board right down, but it did seem extremely difficult to point. Trying to work out whether the boat is making leeway is difficult from out on the wing because you're so much further out from references and shrouds. The rudder flow does appear to be making a bit more of an angle of attack when the boat isn't "in the groove" , but its so difficult to tell, and the rudder is loaded - i.e. does take sideforce.
Someone is working out some numbers on centreboard loads for me, and it does seem possible that the board is overloaded. This boat has a good amount of righting moment, and load on the foil should be proportional to righting moment...
I'm personally beginning to incline towards foil overload as a possible issue. Pity its so difficult to test! Fortunately (well not fortunately, but you know what I mean - the boat was engineered and constructed so that ripping out the daggerboard case and putting in a larger one is not a major structural exercise. I shall think and test some more though, because its still time off the water and money - my Cherub daggerboard is, I think, way too big.
I was probably the instigator of the induced drag theory, which has been discussed quite intensively. One should be cautious about applying "classic" aircraft theory here. The classic expressions are valid for wings operating in uniform flow (which sails don't) and "aspect ratio" is an artificial figure which can lead one to a flawed understanding of induced drag.
I'm fairly confident that the lack of pace is not due to hull drag. Even a relatively broad reach in lightish conditions, with little drive available, popped the boat onto an easy plane, so I don't think that there is any question of an excessive drag hump.
The square top main adds a great deal of area where there is most wind (and remember the dynamic pressure, which supplies the aerodynamic force, is proportional to the square of the wind speed). To my mind, this can have three possibly important consequences, all related to very careful control of twist.
If the added area is being worked very hard (ie. too little twist, and remember the effective angle of attack is different, due to the wind gradient) it loads up the tip tremendously. This can give one a very distorted load distribution, which could push up the induced drag significantly.
It might be possible to stall the upper parts of the sail much earlier than the rest, due to too little twist. Some tufts could clarify this quite easily.
The extra sail area at the top may simply be adding too much viscous drag, due to the higher wind speed at that height, if there is too much twist to take advantage of the available drive.
Personally, I don't think too much of theory number 3, but experimenting with a lot of twist and tufts could illuminate the question.
The centreboard remains a bit of a question. But if the boat is going slowly due to a rig problem, it might overload the centreboard, which would make matters worse.
Building the Hull
Building the Hull - Earlier Entries
Hull Design
PlusPlus Mk 2
Rig Design
FAQ
Picture Gallery
Main Page
· Page Last Updated 24 May 1999