Analysing Upwind Performance [Archive]

SeaDrive

11-23-2004, 01:48 PM

Someone else will have to tell you whether an improvement on 50 degress is possible in a small catamaran. A lot of small boats don't achieve the nominal 45 degrees from the wind in practice. However...

Your symptoms are either
1) you can't point higher because your sails no longer draw and start to luff, or
2) the sails stay full when you point higher, but the boat slows way down, or stops.

Probably the first. In this case, the most likely culprit is the sails, or the way the sails are set and trimmed. For example if the jib lead is wrong, some part of the jib luff will lose the wind too soon. If the lead is too far forward, it wil be near the foot. If the lead is too far aft, it will near the head. If the lead is too far outboard, it will be the whole luff. If the lead is too far inboard, it will backwind the main too much. It's easy to find instructions for trimming sails. The big sail lofts have pamphlets for free, and there must be web sites for Hobies and other popular catamarans. Sorry to say, if the sails are old there may be no cure short of new ones.

Case two where the sails draw but the boat stops would be due to the centerboard(s) or daggerboard(s). Foils have to have enough forward motion to work, and the higher you point, the more force they have to generate, and the faster you have to be going. Once the boat slows down, it starts going sideways. Make sure your foils are not damaged and the proper size. There may be some technique involved. To get to better performance grooves, it may be necessary to get moving fast on a close reach and then point up. Look for material on 'changing gears' on sites about sailing catamarans.

farjoe

11-23-2004, 02:36 PM

Thanks for your reply.

I myself tend to think that the main culprit is probably the jib. This is because I point mainly using the tufts. At 50^ the top one is lifting up frequently. Any closer and the sail starts to backwind. The sail is fairly old but seems comparable with others which do better upwind. I tend to blame it on 2 things:

1. the jib is set on furling gear and and it's luff is tightened at the top and the clew some 50mm behind the forestay. The luff tape, which is of the type with double luff rope and is about 50mm wide seems to add extra camber right up front.

2. The mast has no backstay and therefore the forestay can only be tightened by pulling on the mainsheet. However this cannot be pulled too tight since you will end up with no twist in the main and the leech telltales curling back. This also tends to add extra camber up front.

I say the above, well aware of the old saying that "a bad workman quarrels with his tools"

Regards

tonyr

11-24-2004, 08:38 PM

"Nominal 45 degrees". Hmmmm. Some time ago I was firmly told in this forum that modern 12 metres can point well upwind of that - 35 degrees off the true wind was mentioned. I am still waiting for someone to give me a full technical reference which DOCUMENTS scientifically that it really is possible to sail closer to the true wind than 45 degrees (anecdotes don't count).

Any takers?

Tony.

249

11-24-2004, 09:43 PM

Tony, what do you call "anecdotal" evidence and what is "scientific"? Is a VPP, (which is just a computer programme's attempt to take information from a wind tunnel and from a tank test and then ally them according to formulae constructed by someone) more "scientific" than something written by someone who sails 12 metres?

I've only done one 12 metre regatta, but man do they point.....

Fajoe, in light conditions even high-performance cats don't seem to point at 45 degrees. We seem to need 8 knots or so to get pointing. I don't really know why; maybe in light airs, boatspeed is such a high degree of the apparent, that the apparent shifts waaaay forward?

farjoe

11-25-2004, 12:45 AM

The 50^ I give is the average angle I do over a leg and I measure it at home by downloading the data from my GPS onto my home computer and taking the average on each leg. I think that is a good measure because it weeds out theoretical angle improvements due to wind shift. When i spar against my friend on a J80 I know for sure he is doing better than I because I see him pointing better by about 15^ consistently.

Finally to the assertion that multihulls point worse especially at low speeds...I don't understand why it should be so and hence the reason for my original mail.

regards

249

11-25-2004, 03:56 AM

farjoe

11-25-2004, 09:15 AM

<>>>We know they are footing well, so that's why I think it may be that they have the apparent a long way forward - but I'm probably wrong.<<<

My boat is a 24ft Micro called Strider. When I am comparing to the J80 which is more or less the same size, the 2 boats would be going at the same speed in winds of 8knots or under but he would be going tighter. Therefore the reason cannot be apparent wind.

>>>But that's cats in general. How do you find out how your boat is going? Well, have you looked up the class tuning guide? Have you asked the top skippers how high they are pointing, using telltales as a guide? What's your twist like? Many fast cats don't need much because their sails are so flat. What sort of boat is yours?<<<

I am afraid I cannot perform the comparison you mentioned above simply because there are no similar boats around here.

One statement I find hard to accept is that cats always point worse than monos. There must be logical reasons.

regards

tonyr

11-25-2004, 11:26 AM

Guest 249 - I mean measured in controlled conditions on the water or in a test tank, and not just "go look at any good boat and you will see", or "I did last week". I have been sailing myself in all sorts of boats for the last 50 plus years, and I have yet to see any hard evidence that any conventional sailboat can point (i.e. course made good) much higher than 45% to the true wind, if at all. If a good 12 metre can do this, then show me!

We all know about special craft with propellers on deck driving ditto in the water, and similar contraptions. That is a separate subject.

Tony.

249

11-25-2004, 03:52 PM

Farjoe;

Sorry, I thought when you said "small", you were talking 16' or so.

The apparent wind COULD be a factor iin the boats I mentioned as they are 16-18 footers that go much faster upwind in the light than a dinghy like a Flying Dutchman (that's the 3 time world champ FD, not a random turkey).

Skippy

11-25-2004, 06:14 PM

farjoe Analysing Upwind Performance
I guess it is because I can only manage about 50` to the wind in my small catamaran, but I am obsessed with trying to understand why other boats point better than me.

249
Fajoe, in light conditions even high-performance cats don't seem to point at 45 degrees. We seem to need 8 knots or so to get pointing. I don't really know why; maybe in light airs, boatspeed is such a high degree of the apparent, that the apparent shifts waaaay forward?

Hello forum! I've been lurking here for a while and you just picked the subject I'm most interested in, so I thought I would contribute. I'm not a professional designer, and I'm only an amateur sailor, but I've spent the last year or so studying sailboat design for fun, and weatherliness has always been my top priority. I like the idea of being able to cruise at a good clip on any point of sail.

First of all, my understanding is that while cats don't do their best as close to the wind as a monohull or a tri of similar length and displacement, the cat still gets better VMG than the mono by falling off and going MUCH faster. So it seems to me the answer to the question lies in what happens to the cat above 50 degs, or why it doesn't beat the tri. And I think what happens is the opposite of what happens on a reach: In a good stiff breeze, the mono doesn't have enough beam to hold the sail up, and the tri has its leeward float plowing through the water. With the cat on the other hand, the windward hull is just barely skimming along, maybe even threatening to fly. So you have the ideal situation, one long narrow hull taking almost all the boat's weight.

When you're close hauled, you have the opposite problem. You don't have enough power to hold the cat's windward hull up, so it plows through the water along with the leeward hull. The mono doesn't really do better than the cat, at least not much better, but it doesn't benefit from falling off because of the narrower beam. And the tri has the ideal situation: the boat is balancing mostly on the main hull, especially in lighter air or if you can get some ballast to windward.

Actually, there should be one situation where the cat does do the best when pointing: in a really strong gale where you can carry the windward hull even at 45 degs. But it might still do even better if you fall off just a little. The only other issue I can think of is that some cats might not have deep enough daggerboards or not have any at all, or if the boat is just too heavy.

farjoe

11-25-2004, 11:37 PM

249,

You don't have to apologise. I also have a 16fter so your points are still relevant anyway.

Skippy

[I]

First of all, my understanding is that while cats don't do their best as close to the wind as a monohull or a tri of similar length and displacement, the cat still gets better VMG than the mono by falling off and going MUCH faster.

You are right with this statement for winds above 10 knots. I can beat most monos around here except perhaps the all carbon ones at twice the length and 3DL sails.

The situation below 10 knots is another matter, at least upwind. At these wind speeds the reduced wavemaking advantages of narrow cat hulls seem minimal and I think friction resistance is probably more for 2 immersed hulls than one.

My all up weight is probably in the 1000kgs range including a crew of 2 and I always retract my daggers when not in use so their condition though not perfect are probably better than the monohull ones which are always immersed.

regards

249

11-26-2004, 02:29 AM

Skippy;

I think you're right about the tri in light winds.

The boat I sail and the ones I sail against are performance cat; ie Tornado (current rig, 2 traps, big main and spinnaker, national champ); A Class (18' long, about 30' high wing mast, 75 kg fully rigged, all carbon, last year's world champ as skipper); Nacra F 18 (kite, 180 kg, former world champ); and the Taipans (16' long, 28' high wing mast, 104 kg fully rigged in two-person (sloop) mode and 96 kg in singlehanded mode, the fleet includes the national champ sometimes. All have deep foils by cat standards, the Taipan and A are very light.

So it's not weight or foils. Our jibs are old (2 new ones on order) but were are certainly not miles off the pace compared to the average cat sailor.

I'm still really a dinghy/windsurfer/yacht sailor at heart, but I know the cats point well in moderate and heavy air. I'm just shocked at the way we lose that pointing in the light and (as is obvious) I really don't know the reason, either!

By the way Skippy, with modern cats (which often have small hulls like an As) the windward hull is sometimes dropped in the water upwind in a big breeze. The extra wetted surface is more than cancelled out by the lee hull not being so submerged that it's driving through waves too deeply, and keeping the rig more vertical reduces downforce. We picked this up from last year's A Class world champ.

mistral

11-26-2004, 06:51 AM

I'm still really a dinghy/windsurfer/yacht sailor at heart, but I know the cats point well in moderate and heavy air. I'm just shocked at the way we lose that pointing in the light and (as is obvious) I really don't know the reason, either!

may be the foils at low speed don't generate enough lift to let you point so high as you do in heavy air (and top speed)

Mistral

BOATMIK

11-27-2004, 12:37 AM

farjoe

11-27-2004, 08:17 AM

BOATMIK

12-02-2004, 09:46 PM

Hi Farjoe,

Not quite as simple as adding more sail area.

If you can add more sail without increasing drag proportionately then it is a good way to go - also providing the boat has enough stability to carry the extra sail.

Coupla of the many things that create drag

Flapping or luffing Sails - too much sail area can result in drag from this source. An option is to go for a solid wing like a C-class cat (no flapping or luffing even when pointed breathtakingly close to the wind) or a highly refined rig like an 18 foot skiff. The skiff is an interesting point - they generally have three different rigs. So if it is blowing hard you put on a smaller rig so the sails won't have to luff or flap as much - thus less drag - thus higher pointing. "Always powered up" is ideal - ie maximum power the hull can cope with without drag producing heeling and no flapping or luffing of sails.

Waves - boats point higher in smooth water because the hull has less drag. If you sail a fast dinghy or cat it almost always is worth sailing lower in rough water to get more speed and retain that speed. If you try to point every time you hit a wave the boat will slow and because it is pointing high it will take (relative) ages to get the speed back up.

Boatmik
www.ozemail.com.au/~storerm

249

12-02-2004, 11:04 PM

Is it aerodynamic drag that is causing our cat to point lower? I don't think so.

The rig on our boat (Taipan 4.9) was developed from A Class cats. It has a large wingmast, a high-aspect planform, square top, flat shape, It produces so much power and so little drag that in the conditions we're talking about (light upwind) the T4.9 (in one-up cat rigged form) is competitive with the A Class cats (according to the current national Taipan champ who is also the current world champ in F18s and A Class cats), faster than the F18 cats, and pretty close to the Tornado.

The similar-length mono we sail also has a wingmast (a small one) a reasonably effective roachy fully-battened mainsail but it would create MORE aero drag, I think. Yet the mono points higher, by a significant amount.

I'm assuming the cat has a lower-drag hull due to its narrower entry angles, similar weight, and the fact that the mono doesn't have a very low wsa shape. Plus, we know the cat is much faster in those conditions.

Downwind in very light airs, I think the mono may actually be much closer to the cat than upwind, in VMG terms. I don;t know, but I think that indicates it's not hull drag, doesn't it????

water addict

12-03-2004, 07:38 AM

From Tonyr:
"I have been sailing myself in all sorts of boats for the last 50 plus years, and I have yet to see any hard evidence that any conventional sailboat can point (i.e. course made good) much higher than 45% to the true wind, if at all. If a good 12 metre can do this, then show me!"

How do you want it shown? I have a J105 polar plot show that when the true wind is >10 kts that it points roughly 40 degrees to the wind. Why do you not trust "anecdotal" evidence? Do you think people would lie to you or something?
Watch the next Americas Cup on TV, you'll see them crossing tacks at an angle much tighter than 90 degrees. I'm not sure if that's "anecdotal" for you or not.

tonyr

12-03-2004, 03:20 PM

Water addict. Thanks for the comments. The polar plot is exactly the sort of reasonably hard evidence I was looking for. Assuming that the instruments producing it were accurate (and why not, for our purposes?!) then we are now able to say that we have evidence from at least one modern boat on one occasion that it can point 40% to the true wind. I assume that it was in relatively smooth water, of course. Is that the established limit? Is there one? How high can America's cup boats point?

Re: anecdotal evidence - no, no suggestion of lying. Just want the evidence to be adequately indisputable for my purposes, remembering that I was taught in a course in applied physics (over 45 years ago) that the 45% course was a practical limit, in seaman's terms. Apparently it isn't any more, and I find that fascinating.

Perhaps we are getting somewhere.

A more educated Tony. Getting wiser by the minute.

Skippy

12-03-2004, 05:27 PM

we are now able to say that we have evidence from at least one modern boat on one occasion that it can point 40% to the true wind. ... Is that the established limit? Is there one?

From a theoretical point of view, there is no limit. As long as the sail has a small amount of area across the wind, the wind will push on it at least a little, and as long as the boat is at a small angle relative to the sail, a small amount of the sail's force will propel the boat. And if you can build a very long, ultralight boat, maybe a trimaran or proa or even a hydrofoil, theoretically the speed is limited only by the minimum drag on the hull, windage, inefficiencies of the sail (or airfoil), and other practical considerations.

BOATMIK

12-03-2004, 09:43 PM

Regarding "249"'s Taipan in light wind

>I'm assuming the cat has a lower-drag hull due to its narrower entry angles, >similar weight, and the fact that the mono doesn't have a very low wsa >shape. Plus, we know the cat is much faster in those conditions.

In light winds the main determinant of hull drag is wetted surface. When a multi has two hulls in the water it will have considerably more wetted surface than a monohull. Particularly as the mono can be heeled easily and the crew weight moved forward to lift the flat aft sections of the boat out of the water.

You can move crew-weight forward with a cat but because of the fineness of the bows they immerse quite deeply with the result that the increase of wetted surface at the bow balances the loss at the stern.

If it was possible to put the displacement of hte boat into a SINGLE hull with semicircular section then that would be the optimum cross section. But cats with two hulls in the water have a considerable wetted surface penalty.

From memory Bethwaite's book "high performance sailing" quantifies the above.

farjoe

12-04-2004, 03:25 PM

Is it aerodynamic drag that is causing our cat to point lower? I don't think so.

The rig on our boat (Taipan 4.9) was developed from A Class cats. It has a large wingmast, a high-aspect planform, square top, flat shape, It produces so much power and so little drag that in the conditions we're talking about (light upwind) the T4.9 (in one-up cat rigged form) is competitive with the A Class cats (according to the current national Taipan champ who is also the current world champ in F18s and A Class cats), faster than the F18 cats, and pretty close to the Tornado.

The similar-length mono we sail also has a wingmast (a small one) a reasonably effective roachy fully-battened mainsail but it would create MORE aero drag, I think. Yet the mono points higher, by a significant amount.

I'm assuming the cat has a lower-drag hull due to its narrower entry angles, similar weight, and the fact that the mono doesn't have a very low wsa shape. Plus, we know the cat is much faster in those conditions.

In my opinion cats have in general more drag than monos, above and below the waterline at low speeds. Both small and larger multihulls seem to have a higher and more vertical sheerline than monos. It is also the same underwater. For the same length and displacement 2 hulls will have more friction drag than one. At least, this is what I concluded when I tried to work out some numbers some time ago.

I also do not agree that narrower entry angles mean less drag...unless you are referring to wave drag.

All of the above is unfortunately theoretal opinions. I wonder if there is anybody out there who can give us some solid numbers.

249

12-05-2004, 06:16 AM

I'll work through some rough numbers re wsa over the next few days.

Yes, I was referring to wave drag. I thought basic theory re PC etc also indicates that fine-bowed hulls are better at low speeds??

Boatmik;

I'll locate my wsa formulas and run some rough figures. I'm still not convinced the overall drag of the cat is higher than that of the mono. I'm fairly sure the extremely flat, high-aspect rig of the cat has lower drag than the dinghy rig, and very, very rough figures make me wonder about the amount of extra wsa of the cat.

At the upper end of the conditions I'm thinking of, many dinghies are fastest dead upright, so it's not all about heeling reducing the dinghy's wsa, surely.

Bethwaite is wrong on several points re cats (and sailboards). For example, good cats don't really have "disappointing" light air performance like Frank says. Our 16' cat has rolled straight over the top of the 3 time world Flying Dutchman champ in sloppy waves and glassy sea. Considering we are giving away 3' that's not too "disappointing".

That's not to say you're wrong, but any means.....but I'm not sure it's just drag.

farjoe

12-07-2004, 03:19 AM

I'll work through some rough numbers re wsa over the next few days.

Yes, I was referring to wave drag. I thought basic theory re PC etc also indicates that fine-bowed hulls are better at low speeds??

249,

I cannot wait to see your result. When I had done it I had fixed the length and the displacement for both boat types. I had also assumed a perfectly circular bottom section. I do not remember if I had maintained the same draft.

asathor

12-09-2004, 09:55 AM

farjoe

12-09-2004, 11:19 PM

How about entering Leeway into the discussion.

My Freedom 21 with its fat carbonfiber mast and shoal keel does not go to windward either but I am working on that this winter with keel modifications and I will attack the mast/sail next.

Currently it performs like a "typical" Cat rig - when I fall off the wind a bit the speed goes up and VMG stay the same or goes up. I can get up to almost hull speed tacking at 120d (with enough wind). On the other hand closehauling seems to generate far too much leeway and little progress (my sails are original).

I am thinking about a "windward assist" daggerboard for additional control when direction and not speed is the answer. Does anyone have any experience with adding such a device?

Asathor :?:

How are you measuring your leeway?

I myself estimate this by looking back from the boat and align 2 points on land. I look back some time later to see how much I've drifted sideways. Not very scientific and I am never sure whether the angle I see is due to a wind shift or really leeway.

Could you explain more about your "windard assist" daggerboard? Are you referring to the gybing daggerboard being discussed currently in another area of this web site?

farjoe

asathor

12-10-2004, 09:54 AM

I do not have an accurate wind direction "meter" (masthead) so I use the compass in combination with a GPS to determine the discrepancy between displayed tacking angle vs actual track/tacking angle.

My current thought is to try a slim blade "rudder" mounted in front, maybe with a little bit of stearing, like a shaped 1x4 just to see what difference it makes. It would be easy to rig a line to raise and lower it with. Basically the idea is to add directional stability as far away from the center of rotation as possible so general leeway and smaller puffs don't have as much effects on direction but leave an adequate amount of rudder power. Raising it or adding stearing would be necessesary to maintain good tight maneuvering (think dual axel trailer).

Leebords is a god idea but I am not planning on cutting holes in the hull until I know if it will make enough of a difference.

Asathor

sigurd

12-14-2004, 07:20 AM

Maybe the multis you are comparing with are optimised for more speed than the monos. Maybe they would point better in light airs if they had larger boards or more profile to the sails?

brian eiland

12-20-2004, 01:28 PM

Well, at the cat club I sail, we have a couple of former world champs who don't point much higher in light airs than we do, so it can't be the skippers.

They sail low-windage boats like A Classers, so it can't be windage - and in light airs windage isn't important anyway, is it?

They have very good daggerboards (like those on last year's world A Class champ), so it can't be bad foils....these are not Hobies we're talking about.

They have superb sails, so it can't be the sails.

The As are about the fastest thing there is upwind in light airs, so it's not that they are bad designs.

So it can't be any defects in these particular boats....so (unless I'm very much mistaken about the angles, which may be the case) it must be that cats just don't point too well in light airs. We know they are footing well, so that's why I think it may be that they have the apparent a long way forward - but I'm probably wrong.

But that's cats in general. How do you find out how your boat is going? Well, have you looked up the class tuning guide? Have you asked the top skippers how high they are pointing, using telltales as a guide? What's your twist like? Many fast cats don't need much because their sails are so flat. What sort of boat is yours?

One thing that is noticeable in our class, which comes in cat-rigged singlehanded and sloop-rigged doublehanded versions, is that the cat rig points much higher in the light airs.

Am I correct in assuming your A class cats are uni-rigged like those in the US? I didn't understand your "cat rig points higher" comment....with or without jib?

Per another thread I had included this interesting observation by Eric Hall (http://boatdesign.net/forums/showpost.php?p=5685&postcount=3) when discussing his uni-rig experiments, "More recently I ran across a news article in the Sept issue of Seahorse magazine which discusses the very interesting full scale prototyping work being carried out on a J-90 class boat by Eric Hall of Hall Spars. Eric is now on his third-generation, free standing ,carbon wing rotating mast, with a una-rig mainsail. His “ thought process (and maybe not entirely logical) was: If biplanes became monoplanes and monoplane wings shed wires, why not an unstayed una-rig upwind” Boy, you would surely think this was the ideal upwind rig. In responding to an inquiry on upwind performance, Eric responds, “ first, of course, the boat would be improved upwind with a No.1 jib. Generally, we could not point as high as the others here (Block Island) and therefore had difficulty holding lanes.” He goes on to say, “this is a very interesting project that we especially want to succeed. I have been accused of a missionary zeal, which frankly keeps moving it along. It’s a real problem sometimes keeping focused on what we are trying to do in view of all that is ingrained in our minds about what makes sailboats work. Anyway we are having fun…

another reference
http://seahorsemagazine.com/2000-June/blackwing.html

CT 249

12-21-2004, 08:05 PM

Re "Am I correct in assuming your A class cats are uni-rigged like those in the US? I didn't understand your "cat rig points higher" comment....with or without jib?"

Yes, our A Class cats are uni-rigged, and dominant at world level so they are obviously sailing at the best possible pace.

The class I'm sailing is Taipan 4.9s, developed by world-class A Class builders in the late '80s IIRFC and still the fastest 16' around anywhere it seems. They are sailed two-up with jib, or singlehanded with no jib. The singlehanded version definitely points higher in light to moderate airs in my experience, and in that of others too. I'm not 100% sure it's true, but I'm about 95%+ sure.

The same thing occurs in Mosquitoes, which are like a baby Tornado (16') fromt he '70s with a small low-aspect rig. The cat rigged one-up version points better in light airs than the two -up sloop. In the Mossie, the 2 up and 1 up are the same speed overall. In the Taipan, the cat rig is faster in the light, slower in a breeze, about 1.5% slower overall.

Over the last 3 or 4 years, the guys who race Taipans in our nationals have won 3 or 4 A Class worlds and the F 18 worlds, with many other top placings (ie guys who hopped into F18s from Taipans were on something like 4 of the top 10 F 18s at the worlds, despite their lack of experience in F18s), so it's not that the T 4.9 sailors are idiots who aren't getting the best from their boats......well after a few beers lots of us are idiots, but some of the other guys are still getting the best from their boats.

Tim B

12-22-2004, 03:19 PM

You're bang on, boatmik, it's the hull resistance that becomes the problem. Remember also, that the hulls are lifting to windward (think about it) and therefore have an induced drag factor (I admit that the mono has a Cdi also). Wavemaking is an interesting question though, often measured as CDt-Cdf (CDt - total Cd , CDf - Skin friction) and it is typically called CDr (residual drag ((I'm talking in Coefficients here))) CDi can also be allowed for (if you know side force), and so CDr will become CDw (non-dimensional wave drag) now then, at very low Froude numbers, wave drag is very small (for cat or mono) however, CDi is rather high because V is small. As the Froude number increases, the wave drag becomes a more major issue, the cat, however, beats the mono hands down because of it's finer sections.

Now then, the rig... Use the shrouds to balance the tension in the jib luff, this will change the shape of the mainsail less than using the mainsheet. One must remember that cats tend to have quite large roaches, and this tends to move the centre of effort back, thus more lee helm, and less pointing. If this is the case, check the position of your rudders, are you holding much helm on? this will also slow you down, you should be able to balance the boat with sail force and weight. If you are having problems with this, play with the rig in different places, you'll know when you get it right ;-) .

You're measurement system seems good, but what is the wind doing. When designers quote minimum pointing angles, we mean relative to an apparent wind which has constant strength and direction, in practice this is very rare. The maximum speed upwind, however, may be given a speed and an angle, dependant on the speed of travel, relative to the true wind. The maximum speed upwind will be at a greater angle than the minimum pointing angle.

So yes, the pointing angle (to apparent wind) may be 30 to 40 degrees, but the maximum speed upwind (to true wind) will be at 40 degrees to 50 degrees (in most cases).

Tim B

Doug Lord

12-22-2004, 03:38 PM

Tim, you said in the previous post: " One must remember that cats tend to have quite large roaches and this tends to move the centre of effort back, thus more lee helm and less pointing."
You didn't really mean that did you?

Cee Tee 249

12-23-2004, 07:33 AM

Well, there still seems to be a consensus that cats have higher wetted surface area. I was going to measure our Taipan 4.9 (which is pretty close to a 10 year old A Class) but I didn't get a chance before it was chartered for the nationals. However, because all of our boats have pads moulded to fit the hull, one can get a pretty good eyeball estimate of the wsa of the hull section. I still reckon the cat wsa is surprisingly low.

Martin Fischer's site mentions that the A Class cats in 8 knots have a wsa of 24.3 ft. Martin has a PhD in fluid dynamics and is the designer of teh Capricorn F18, probably the world's fastest F18 cat (ie about 5th and 6th in worlds with amateur sailors with little F18 experience, up against the pro teams who have been sailing F18s for years). So I assume his figures are correct.

A Flying Dutchman has a wsa of 70 feet (Baader), a Finn about 39.6 IIRC (from articles by Saarby) and a Laser about 35 IIRC (from a study by Shevy Gunter). So even if the A Class figure of 24.3 only includes one hull and we have to double that to 48.6, we still have a lower wsa than an FD and something vaguely similar to the much shorter Laser and Finn.

By the way, I have studies on the Finn and Laser that indicate that when heeled and sailed bow-down, wsa drops only by 5 ft2 or so IIRC. So the cat is still way ahead on wsa compared to its overall length and speed.

Yet when racing things like an FD, we are much faster but lower upwind in light airs.

So in summary, the argument that a cat has higher wsa doesn't seem to hold water. I know Frank Bethwaite uses it, but the way that a good modern cat can slide past a Bethwaite B 14 or something shows that Frank's line that cats are light-air dogs is not correct.

grob

12-23-2004, 02:34 PM

That number of 24. sq ft seems a little low about 30 sq ft would be more like it for a 75kg boat with 75kg crew, perhaps this was assuming flying a hull i.e. one hull carrying all the weight then 24 sq ft seems more like it.

I'm suprised at those numbers for the monohulls as I thought that in general a catamaran does have higher WSA than a monohull, if you are comparing like with like. i.e. boats of the same weight, those other numbers are probably for heavier boats with heavier crews.

That is one of the reasons that cat sailors try to fly a hull to cut down on the WSA, two hulls supporting 150kg gives around 30sq ft, one hull supporting 150kg gives around 25 sq ft.

Also those numbers represent the lightest of all the cats, most cats weigh in at 150kg with 150kg crew. Then you are probably talking about a WSA of 50 sq ft

CT 249

12-26-2004, 05:44 AM

Yep, the wsa for an A Class does represent that of the lightest sort of cat. However, of course, even at 50 ft2 the wsa of the cat is much lower than that of a mono of equivalent length.

I have a suspicion that the old ideals about cat hulls have a high wsa came from the early Vee shaped cats. Today, of course, cat hulls are very much U shaped which is a more effective shape in terms of wsa for volume.

Skippy

12-26-2004, 02:03 PM

Lorsail mentioned in the leeboard thread
http://boatdesign.net/forums/showthread.php?t=5944&page=2#post35356
cat hulls that are flat on the outboard side and rounded inboard.
Isn't that design intended to fly the windward hull, and wouldn't it point poorly if both hulls are significantly in the water?

Doug Lord

12-26-2004, 02:42 PM

Chris, the difference in wsa between equal dispacement mono and a cat is that the cat when sitting level has about 41% more wsa. There is another thread that goes into this(search under wetted surface) or you can simply construct two half cylinders one 2' by 10' the other 2.8292' by 10' and figure the "wetted area" not incl the ends. The bigger cylinder will have the same volume as two of the smaller ones but substantially less wetted surface.This compares two semi circular shapes.I think it serves to illustrates the basic princible.
Skippy, when I raced Hobies we would always try to keep the weight on the lee hull even in light air for two reasons: 1)-to allow the "foil shape"(asymmetry) of the lee hull to work,and 2)- to keep the wetted surface down.
Now, I never calculated it out back then-it was racing scuttlebutt that seemed to work..

marcofailla

12-27-2004, 04:32 AM

Hi to all
I would also greet with much affection JoeF
an old "virtual friend" that sails the same mediterranean sea I sail

I would enter in boatdesign forum with the issue Joe arise
why cat point less than other "usually lower" perfomance boats
I think reasons are multiple and much of them have been dissected in this thread
The only reason why that happens I haven't seen around here (exuse me if I miss something) concerns the Reynold's number of cat sails
This number tell us how to "standardize" different sized "wings" in different condition of "speed" and "air pressure"
well
A class cats, for example, have sails with very tall mast and with short chord sails
this make the best of possible choices for upwind performances(low drag and high lift) but render them very susceptible to the wind speed in the sense that the groove where these types of rig can operate at their best is little if compared to monohull sloop configuration
when cat rig operates in higher Rn all is OK and its best, but when wind speed lowers Rn goes down very quickly in short chord sail, much quickier than long chord sail like those of monos.
In these conditions cat sails stalls much easier,
cat usually are lighter and more prone to pitch even in little waves in low wind conditions, every little pitch movement will make the sail to stall, this effect is amplified by the rig height
in low wind a traditional sloop rig configuration and paradoxically a boat with more inertia (weight) will pitch less, stall less and thus point higher
this obiovsly does not mean that a heavy and very short masted sail boat will reach the upwind mark before than a cat
who sails cats knows that often even a 50° upwind beat let us turn the mark well ahead of sloop monos beating at 45° or less
The feeling of being not able to follow with an A class cat friends sailing sailing a 470 or a 420 is very common
it happens often to me with my a class
simply I cannot make their root becouse I sail a different sail machine that has different requirements...
to finish I (try) to attach a quasi-polar chart of vairous boats including 12 meter, tornado and 18 skiff, all in 10 knots conditions and the comments aqttached to this table
cheers
Marco

Al Bowers comments:

"...None of these boats goes downwind fastest at 180 to the true, they all tack downwind...Be aware that these are from different sources, and salt vs fresh water will affect these numbers, just as wave conditions will (most of the time none should be able to get numbers this good). Also, the numbers are all for 10 knots. I had to interpolate the 12 M numbers as I only had 7 and 12 knots wind. The skiff and the Tasar are planing everywhere they are over 7 knots; both show a characteristic "double hump" in their polars which is symptomatic of planing boats. None of the other four boats have a double-hump and all are displacement boats (this does not mean "hull speed" applies though!). Hull speeds (FWIW) are: 5.5, 6.5, 5.0, 6.2, 7.0, and 10.5 knots respectively. Also, this only applies in about 10 knots of wind. In higher and lower wind speeds, these do NOT scale up or down. Especially in lower wind speeds. Anyone who has tried to sail a small cat against a large leadmine in light air will tell you that the cat will get slaughtered everytime. In light air (below 5 knots) the boundary layer across the earth is laminar and there is very little wind down low. At 6 knots or higher wind speed the boundary layer is turbulent and there is more wind down low and that is when cats "come alive" and we slaughter everyone else. Well, maybe not the Aussie 18s, but then for the money those folks spend, we can let them win. Just follow them until they capsize, then be sure to wave and smile cheerfully as you sail by..."

ggGuest

12-27-2004, 07:47 AM

The bigger cylinder will have the same volume as two of the smaller ones but substantially less wetted surface.This compares two semi circular shapes.I think it serves to illustrates the basic princible.

While that's true Doug, you also have to remember that, with the exception of Moths, most monohull dinghies are vastly over flat from the cylinder - typically waterline beam will be three or four times the depth - whereas modern high performance cqtamaran hulls are quite closs to a semicirular underwater section. Its really quite easy to draw a 14ft catamaran hull with less wetted area than a typical 14ft dinghy hull.

Doug Lord

12-27-2004, 01:36 PM

gg, you're right: my simplistic example is only valid when comparing two types designed for minimum wetted surface.
This morning ,for the hell of it, I compared the wsa of a 1990's vintage I14 with a 14' cat with 14/1 beam to length ratio hulls. The results: I-14=36.52 sq.ft , Cat- 38.7 sq. ft.
So the cat still had more wetted surface sitting flat but just barely....

farjoe

12-27-2004, 10:02 PM

It is very good to hear from you again.

You bring up another interesting angle to the problem. The issue of pitching is an area which seems to affects us multihullers a lot more than our single hulled friends. This seems to be especially true for us Mediterranean sailors who seem to get more than our fair share of short, sharp chop. In this regard my 24ft boat with circular underwater cross-section definitely suffers a lot more than the monohulls and even my 16ft cat which has a more wedge shaped cross-section.

Al Bowers comments:

".... Anyone who has tried to sail a small cat against a large leadmine in light air will tell you that the cat will get slaughtered everytime. In light air (below 5 knots) the boundary layer across the earth is laminar and there is very little wind down low. ..."

This is precisely the kind of condition which I am trying to understand although in my case I am comparing with a J80, in flat water, sailing upwind in 8 knots. Downwind, in the same conditions, he is no match even with him setting a spinnaker, so this probably means that my WSA is not inferior. Incidentally can anybody quote a figure for WSA for the J80?

From all the above excellent comments , I tend to point even more to the issue of windage. I certainly have a higher freeboard ( on 2 hulls) than the J80 and my rectangular beams do not help either.

regards

marcofailla

12-28-2004, 06:30 AM

Al Bowers comments:

".... In light air (below 5 knots) the boundary layer across the earth is laminar and there is very little wind down low. ..."

I am not in perfect agreement with the fact that a laminar flow implies a minor wind speed at the ground (through the entire sail height...) for various reasons

1: turbulent vs laminar flow on sea surface... it is not at all an easy task to manage... except for large wave conditions, one could assume that on a practically flat surface like sea is, laminar to turbulent transition happens only due to two factors: waves and wind speed...huuumm seems to me that this would give us much surprises, but really i think that one could assume flow as laminar unless very large waves occur

2: turbulent flow is much worse than laminar flow in assuring a lift on a whatsoever sail (think at airplane turbulence that can make the subsequent take off a disaster if it starts with a little gap between the two flights...)

3: cat rig are higher than comparable (in length) monohull... so this should favour multis

to conclude I would like to add that to me you cannot compare as a mesure of total drag the straight WSA of a cat and a J80 (provided we are talking of a ballasted J boat 80)
simply becouse cat have that WSA induced drag as a major part of hull drag while J80 have wave induced drag as the major part of drag and this is not related to WSA

windage is an issue, but it is hard to crunch in numbers

however catamarans usually are designed for overall speed, hard to design a cat that can conbine best of mono and best of multi, unless you don't have different set of sails and mast as windsurfers (and mono) have

don't forget that sloop rig counts much more on forehead sail than on main
while cat rig are thinked on the opposite concept
this is like to compare a wing and a flap (sloop config) to a slat and a wing (Cat config with little jib)... this could be also an issue

moreover the fact that you have a leading edge 3 cm thick (roller furling) or 15 cm thich (mast) makes a lot of difference in terms of upwind ability
indeed, I think that a sloop sailing with only the main cannot point at all

cheers MArco

gggGuest

12-29-2004, 12:35 AM

[I]Al Bowers comments:

moreover the fact that you have a leading edge 3 cm thick (roller furling) or 15 cm thich (mast) makes a lot of difference in terms of upwind ability
indeed, I think that a sloop sailing with only the main cannot point at all

cheers MArco

My empirical experience suggests that's not the case Marco. Single sail dinghies manage some of the highest pointing angles. Also I've sailed the same monohull dinghy with and without a wingmast, and there's not an enormous difference.

Answers to the original question aren't going to be decided unless someone has some hard data, but for my money its mostly to do with apparent wind. When you sail high performance boats that achieve a reasonable fraction of wind speed you soon learn that in order to go upwind you have to slow down. In my boat in a decent breeze the difference between beam reach setting on the jib and upwind setting is maybe two or three inches on the jib sheet...If you go from the reach to the beat its a question of stuffing the boat upwind and you get the pointing angle as the boat slows down.

Empicical evidence suggest that boats that are slow for their size/rig efficiency etc tend to go upwind like snakes, boats that are faster have to foot off because with the extra speed the apparent wind goes back. This goes from the little British Moth dinghy in the UK, which is notorious for pointing way higher than most classes. certainly far higher than the much quicker International, right up to things like the 12metres and IACC boats, also slow for their length, which point like crazy.

Now you can draw the graphs, and given an equal rig there's a direct relationship between speed and pointing angle. If you have a boat that is sailing at 25 degrees to the apparent wind then if it is travelling at 4 knots it can manage 45 degrees to the apparent wind without losing sail power, if you slow down to 3.1 knots then you can point it up to 40 degrees from true.

To really predict performance from that you have to factor in the drag curves as well, and I lack both firm data and the capability to produce a mathematical model. However what I have read suggests that, whicle craft are displacement sailing, drag increases relative to speed faster for the monohull than the multihull. I'm not going to attempt to model it, but suggest that this means that the multi will achieve its optimum vmg upwind travelling faster and pointing lower than the monohull.

This does seem to match the empirical experience reported here, so my conclusion would be that the reason multihulls tend to point lower upwind in the light is because they are quicker and achieve a better VMG doing so. Consequently when designing the sheeting angles and the like for a jib rigged multi a competent designer would ensure that the sheeting angle was sycj that the jib would tend to luff if stuffed up into wind too far.

marcofailla

12-29-2004, 02:47 AM

I have not experience in dinghy sailing nor in wingmasted dinghy sailing
but I fairly agree with you
from your explanation, however, it is not easily said why, regardless from VMG, a lighter and faster boat that slows down going upwind is not able to point as high as a monohull that is heavier and as slow as the slowed multi
in other words to me it is not a so simple matter and as you said unless one have data to reason on, we can make only hypotesis that are true to some extent in most of cases but it is hard to have an idea of their relative strength

cheers
Marco

PS
Joe I am going quite fine
I am finishing my residency period here in catania and for sure I will find sooner or later the calm to continue working on my project
and you?
I hope you are fine too
best wishes for the new year
yours
Marco

farjoe

12-29-2004, 07:17 AM

Marco and GggGuest,

I have yet to be convinced about your assertions on Drag and Apparent Winds at wind speeds of less than 5 knots and for Multihulls which are not beach Multihulls. The sail area/ displacement for the J80 is listed as 35m^/1300kg...That of my Strider is 31m^/900kg(estimate).

It appears to me that the wave drag component of the total drag at these speeds is pretty negligible. At least the J80 leaves hardly a ripple in these conditions.

At these wind speeds the 2 boats do practically the same speed and therefore, in my view, the resultant apparent wind angle should not be all that different.

At higher speeds I agree totally with what you say.

regards

P.S.

Marco, I am still at the old email address if you want to drop me a line.
All the best

SeaDrive

12-29-2004, 08:48 AM

...things like the 12metres and IACC boats, also slow for their length, which point like crazy.

I know this is a sidebar issue, but can you elaborate on the statement that IACC boats are slow for their length?

Doug Lord

12-29-2004, 09:19 AM

If you compare ACC designs to modern designs like maxZ86's they ARE slow for their overall length...
I've read that C-Class cats have the abiliy to point really high while still makng good speed. If you want to investigate this contact Steve Clark.

mistral

12-29-2004, 09:56 AM

ACC are slow because their box rule keep them slow; the'y re quite heavy (IMOCA60 weight 10t, they weight 24t), very narrow, and with a short keel compared both to maxi and open; their ratio best upwind speed/best downwind speed is really poor (less than 1.4 maybe); there's one thing they do better than other boats: thay have an impressive ability to point up their nose going upwind, ACC regattas on TV showed clearly tacking angles upwind smaller than 90° in each conditions form 5 to 20 knots, maybe near to 80° more than to 90° in average wind; ACC boats cannot be compared to other classes as they're designed and imroved for a single particular kind of competition and they sail in a very narrow corner of wind/wave conditions, out of this conditions they risk to became a wreckage in a couple of minutes. Quite absurd for such expensive boats, don't you think ?????

Mistral

brian eiland

12-29-2004, 11:18 AM

Dear gggGuest,
I think you got it right on a number of points, and without a lot of 'technicality'.....it's mostly to do with apparent wind and good efficiency from the sailing rig.

Obiviously if you manage to create more wind (greater apparent wind) as many lighter-weight, slender multihulls do, then your chances for greater performance are enhanced. But you must make efficient use of this increased wind. This is the job of the sailing technic (helmsman), and a function of the rig design itself.

Most competitive multihulls can create a substantial amount of apparent wind. And they generally utilize it most effeciently by sailing no higher than 45 true, tacking thru 90-100. This generally maximizes their VMG. However, I have been in numerous situations where, if we slowed down to a speed just barely above that of a real competitive monohull, we could work our way to windward of him, and boy were they surprised! (I must admit it took a moment of inattentiveness on their part to pull this off...playing the slightess wind shifts). We concentrated on getting the greatest driving efficiency from our rig at these higher pointing angles...proper slot, proper sail shape for this higher incidence angle.

When I speak to the efficiency of the rig there are so many factors to be considered. I've addressed a few of these in the past suggesting the superiorty of the headsails over the Bermudian mainsail. I'll leave this for another time.

However, I will make a note about your statement, "single-sail dinghies manage some of the highest pointing angles". Your inference here being that the uni-rig configuration might somehow be superior, or more efficient. Very likely it is in the case of dinghies! Please note that in small dinghies it is almost always a case of 'disjointed interaction' between the mainsail and a jib; ie, they are generally never 'masthead' rigged, and the booms are located substantially above the foot of the jib....three distinct, interactive areas....lots of rig drag potential in sloop rigged small boats. For some interesting discussions and illustrations visit http://www.wb-sails.fi/ and look thru their site for extensive studies on the one design 470, etc.

Richard Petersen

01-03-2005, 12:20 AM

Is it as simple as both boats when traveling 90 degrees to the wind present the same side area and wind resistance. As the boats rotate into the wind the single hull wind area is decreasing. As the twin hull boat rotates into the wind it starts to " unmask " a second hull area which is now causing more air turbulance on the down wind bow and on the upwind stern to a lesser degree. On a single hull the wind force slides over and around the hull once. The twin is creating a series of turbulant obstacles and drags before it releases the same volume of air. The air flow in the tunnel should become most powerful at 45 degrees due to the air traveling the full length of the tunnel. More wind resistance longer, more rudder, slower speed,etc.-------- I also design competitive water wings.

CT 249

01-03-2005, 01:38 AM

Sorry, Richard, are you saying that the catamaran is actually SLOWER upwind in the light when you say "more wind resistance longer, more rudder, slower speed,etc"?????

A good cat is certainly not slower upwind in light airs than a good dinghy; our 16' cat will and has, whipped the 3 time world Flyung Dutchman champ upwind in the light because our speed is so much better, but our pointing is lower. Or am i reading your post wrong?

Brian, masthead-rig dinghies have been tried, with a notable lack of success. Booms are located substantially above the foot of the jib in the fastest dinghies (49ers, 18' skiffs). I looked at dropping the boom on my Int Canoe (we tack around the back of the sail, not under it) but in all the decades of canoe sailing, no-one has found droppin the boom to be a major boost according to the asking around I've done.

Even one of the oldest major dinghy classes, the Snipe, carries its boom in the top position. The boat was originally designed to drop it low in a breeze but it has been found better to have it high.

Having said that, in windsurfers we find "closing the gap" to be very important....I'm just pointing out that the most innovative dinghy classes (those that created the modern assymetric spinnaker, the practical foiler, hiking devices etc) have found the big jib arrangement to be slow and that boom height is not a major issue.

Of course, maybe the Bethwaites, the creator of the C Class cats like Cogito, Uffa Fox, all the 18' skiff and Moth sailors and everyone else are all wrong.....but somehow I doubt it.

Richard Petersen

01-04-2005, 12:35 AM

The original question was. Why can not my cat point as far into the wind as other boats can. My assumptions for reasoning out his statement. All single, dual and triple hulls are sized and built of same speed ,turning, tonnage, sail equipment and abilities of the crew. What is left, is who can still move foward into the wind, at what angle, and why?

SeaDrive

01-04-2005, 09:09 AM

Below is a section from an essay by Richard Woods, posted on his website. It makes some of the same points as have appeared earlier in the thread.

"Self Tacking Jibs
These sound like a good idea for short handed crews. But they are only any good when actually tacking or sailing close hauled. They are no good for example when you want to sail off an anchor and need to back the jib, while when reaching you will need to barber haul the sail to control twist. If you have a roller reefing headsail then you can forget it as far as sail shape is concerned (a rolled sail becomes more baggy the more vertical the leech as the cloth rolls on top of each preceding turn).
Monohulls like the Star and Etchells have self tacking jibs. But self tackers work better on these boats because to increase VMG (speed made good to windward) a monohull has to point higher as it cannot go faster through the water as it reaches maximum hull speed in only a light wind. A multihull, on the other hand, can increase VMG by sailing faster even if it doesn't point as high, so it can usefully use a different style of rig."

SeaDrive

01-04-2005, 09:16 AM

IF you had inboard jib leads, a taut jibstay, and flat-cut sails (or sails cut with exquisite precision to the sag of the jibstay), you would be able to point as high as the J-80. But your boat was designed to sail a bit more off the wind, and get the best VMG by sailing faster.

There was an article in Sailing World this year about an Etchells championship (IIRC) where the winner said he tuned his boat for max speed without worrying about pointing higher than everyone else. If you are sailing in shifty winds, sailing a little lower with the same VMG gives you more leverage in the shifts.

brian eiland

01-04-2005, 10:09 AM

Those were a couple of good observations SeaDrive.

Below is a section from an essay by Richard Woods, posted on his website. It makes some of the same points as have appeared earlier in the thread.

"Self Tacking Jibs
These sound like a good idea for short handed crews. But they are only any good when actually tacking or sailing close hauled. They are no good for example when you want to sail off an anchor and need to back the jib, while when reaching you will need to barber haul the sail to control twist. If you have a roller reefing headsail then you can forget it as far as sail shape is concerned (a rolled sail becomes more baggy the more vertical the leech as the cloth rolls on top of each preceding turn)."

And I might add to this observation by Richard, that the 3-point staying of the most multihulls does not condone taunt forestays, and certainly not a masthead foresail.

So small jibs are the order of the day, so why not make them self-tacking. This makes sense from a handling point of view, but NOT necessarily from an efficiency point of view. Put another way, the 3-point rigging method sort of dictates big tall mainsails and small fractional jibs.

brian eiland

01-04-2005, 12:24 PM

For a visual representation of headsail shape change with forestay tension one might visit North Sail's presentation at http://www.northsails.co.uk/media/fast/genoa/gt-step5.htm and click on the "backstay oval" at the bottom of the page.

sjors

01-07-2005, 02:38 PM

The more combined drag from hull(s) and rig the lower the pointing angle.

Multihulls often have considerably more wetted surface than a similar sized dinghy particularly when that dinghy is heeled and the crew move forward to get the broad aft sections out of the water.

Certainly a single semi circular section has a minimum wetted surface for volume but when you have two of them it just don't work that way.

As the wind increases and the boat accelerates the multi has lower wavemaking drag so goes much faster.

MIK
You are right about the smaller wetted surface of the monohull.

When two boats are compared with identical performing rigs and daggerboards, the speedgoverning factor in light wind is the sail-area to wetted-surface ratio. I think if you look at the lift to drag ratio of the rigs of a J80 and Aclass cat, the two can be compared, as well as the daggerboard and the keel.

I sail mostly on yachts and the best light wind boat is in most cases the heaviest! When a boat gets heavier, the ability to carry sail grows faster than the wetted surface.

I am very curious if this little catamaran beats the J80 on a reach or broad reach in very light winds. The comparison shouls be made with the same sails when sailing upwind. I put my money on the J.

When the wind increases, the catamaran goes above hullspeed without dramatically increase in hulldrag. Allthoug the J80 is a sportsboat, the boat experience an increase hulldrag resulting in less speed.

If you are interested in these matters I highly recommend the book from C.A. Marchai "aero-hydrodynamics of sailing" ISBN0-7136-5073-7

CT 249

01-07-2005, 05:40 PM

Sjors;

You say monos have lower wsa, but what about the figures from Martin Fischer's A Class (18' LOA) with its wsa of 24.3 or 48.6, compared to the 19' Flying Dutchman's wsa of 70ft2 or the 15' Finn's wsa of fractionally under 40ft2?

Could you point me towards some figures re the wsa of modern cats v modern multis? I wonder if changes in design have changed the relative figures. Older multis tended to have Vee hulls (large wsa, of course) whereas older monos tended to have rounder sections, so a lot has changed since Marchaj's initial work. His current editions still use older-style boats if I remember rightly.

Are you SURE the J/80 rig would be as effective as an A Class rig? In that case, why did the A Class drop the fixed stick/sloop rig they originally tried? Why didn't C Class cats stick to a "normal" rig like the J/80s? Have you sailed with an A Class rig? It really offers serious performance (at some significant hassle) whereas the J rig is largely designed for convenient moderate-pace sailing.

farjoe

01-08-2005, 02:17 PM

I am very curious if this little catamaran beats the J80 on a reach or broad reach in very light winds. The comparison shouls be made with the same sails when sailing upwind. I put my money on the J.

Well, with it being a Saturday and the fact that my boat is currently on the dry I went down and took some measurements. I also collected some data from the internet for the J80 for comparison.

Strider J80
sail area 31 35
LWL 6.8 6.7
WSA (sq. m) 4.1*2 ??
WSA (sq. ft) 44.7*2 ??
displacement 900 1300
area above WL 6 5.1

Most numbers are in metric units. The "area above WL" has been put in to get an idea of the hull windage and is simply the broadside area. The value for the J80 is an eye ball comparison to mine(85%). I am sure that at 45^ to the wind this ratio is a lot lower. I would be great if somebody can provide the WSA for the J80.

To answer Sjors question the J80 beats we downwind with just the main and jib in force 1. In force 3, I am faster even if he carries a spinaker.

CT 249, I think you should be careful when comparing the WSA of small cats to larger ones. This is because most small cats do not need antifouling and therefore the effect of surface roughness comes into play. Also, due to the fact that small cats can be easily righted, their sail area to displacement ratio tends to be higher than that of bigger Multihulls simply because the latter cannot afford to capsize.

regards

farjoe

01-08-2005, 04:31 PM

Just realised that the table I had set out with spaces came out unreadable, so here's another try.

_______________ Strider____ J80

sail area _________ 31_______ 35
LWL ____________ 6.8______ 6.7
WSA (sq. m)_____4.1*2______ ??
WSA (sq. ft)___ 44.7*2______ ??
displacement____ 900______ 1300
area above WL____ 6_______ 5.1

amolitor

01-21-2005, 09:12 PM

Even a little bit of leeway affects the apparent wind adversely. Catamarans go fast
which rotates apparent wind forward, and have less hydrodynamic force to conteract
leeway. When trying to point, both leeway and headway cause the apparent wind to
rotate foward.

The more forward the apparent wind, the higher the lateral component of aerodynamic
force, and hence the more tendency to leeway.

The point at which, with optimal sail trim, all this junk balances out under a specific set
of conditions is the pointing angle of the boat.

AC boats go slower, and have awesome high-lift keels, creating much more lateral
hydrodynamic force. This makes the apparent wind farther aft than in a catameran
zipping forward twice as fast, and making a lot more leeway.

Draw it out, if I am right, even a little leeway hurts your apparent wind, without
headway's compensating increase in apparent wind speed.

tspeer

01-23-2005, 08:38 PM

...Draw it out, if I am right, even a little leeway hurts your apparent wind, without
headway's compensating increase in apparent wind speed.

Actually, I think the case could be made for saying, "The leeway angle should be as great as possible, short of stalling the keel/board."

The apparent wind angle, beta, is best measured from the boat's velocity through the water instead of relative to the boat's centerline - see attached figure. Beta is the sum of the aerodynamic and hydrodynamic "drag angles" [arctan(drag/lift)]. The boatspeed/true wind speed ratio is sin(gamma-beta)/sin(beta), where gamma is the point of sail (head to wind => gamma = 0). Note that what counts is to minimize the drag angle (maximize L/D), regardless of where the boat is pointing! This is why it's more relevant to measure apparent wind from the velocity vector instead of the centerline.

The lift on the keel will be that required to offset the side loads from the sail rig. Below stall, the leeway angle will be proportional to the lift produced by the keel. The other attached figure shows the drag breakdown of a typical foil and an arbitrary increment (CD=0.08) to represent all the other drag contributions of the hull. When considering the foil alone, the maximum L/D occurs at a comparatively small lift coefficient/leeway angle. But when the hull drag is included, the maximum L/D is much lower and occurs at a much greater lift coefficient/leeway angle. The reason is a lot more lift is required to "dilute" the hull drag in the L/D.

Now the drag due to lift is not dependent at all on the area of the keel - it depends (inversely) on the square of the keel depth. But the profile drag of the keel does depend on the area. As does the leeway angle (inversely). So you can make the leeway angle arbitrarily small by increasing the keel area. But if you maintain the same depth, all you're doing is increasing the parasite drag. This is why a fin keel yacht goes upwind better than a full keel yacht of the same depth.

So if you start with a full keel and start cutting away at the the chord of the keel to reduce the area, the lift will remain a constant (it's determined by the sail trim) and the drag due to lift will remain a constant. But the drag will go down and the leeway will go up. Until the keel is so small that it stalls under the applied load. Then the drag will increase rapidly. So you want to stop cutting away at the keel short of that point.

The result is the keel operating at a high lift coefficient, and thus a comparatively high leeway angle.

amolitor

01-23-2005, 08:50 PM

If I understand this properly (no guarantees!) what you're saying is that the
higher the leeway up to a keel stall, the higher the hydro lift, which certainly
makes perfect sense to me! (up to a keel stall, check, points on keel design
well taken -- thanks, I did not know a lot of that stuff)

However, notice that as leeway increases as measured against your total
velocity (the big light blue arrow representing true velocity) rotates clockwise.
So, if we make the simplifying assumption that your Beta remains constant,
the apparent wind is rotating forward, so eventually your sailplan falls
apart.

In the limiting case you have a stall-proof keel, and you're going dead
sideways! Your sailplan is generating no lift at all ;)

Anyways, all I was trying to say is that leeway affects the apparent wind
in some sense 'more' than headway. This means that in a high-leeway design,
the highest angle you can sail relative to TRUE wind is lower, all else being equal.
You may be pointing like crazy against the apparent wind, but that doesn't get
you to the mark!

I am fairly sure that this is what's going on with catamarans.

---

Oh, and furthermore! I see a little more now. The more hydro lift you have,
the more efficiently you sail. Your sailplan "works better," but maximum
pointing isn't about sailplan efficiency, it's about the point where everything
falls apart and you stop going forward!

tspeer

01-23-2005, 11:34 PM

What's going on with catamarans isn't any different than what's happening with other sail craft. I learned the importance of loading up the "hull" when I was working on a landyacht VPP. I had arranged things so that the maximum aerodynamic L/D occurred below the stall angle of attack. I expected the VPP to say the best sheeting angle was that for max L/D, but instead the VPP said performance was better when sheeted past max L/D to the point of stall. What I realized was the extra sheeting was loading up the chassis and inproving its L/D more than was being lost by the rig being oversheeted.

It's easy to get near zero leeway angle on any boat - just luff the sails. But that doesn't do anything for windward performance.

Let's say you employed a jibing centerboard. You could get the situation shown in the attached figure, which has zero leeway. Since the wetted area and depth are exactly the same as before, there's very little change to the drag if the sails are sheeted to maintain the same force as before. So the relationship of the force vectors - and the path through the water - are essentially the same as before. What's changed is the hull orientation relative to the course through the water. If you were to jibe the board the opposite way, you'd end up with more leeway on the hull - and much the same set of force vectors. The point is, leeway angle itself doesn't really tell you anything about the boat's performance.

I think what you're trying to get at is the rotation of the apparent wind vector becomes the limiting factor in high performance craft. And you're right about that. For example, when I put in zero rolling resistance in my landyacht VPP, I still got a finite speed. Beta, being the sum of the aerodynamic and "hydrodynamic" drag angles, was equal to the aerodynamic drag angle, since the chassis drag angle was zero. So when the apparent wind was so far forward that the resultant aerodynamic force was perpendicular to the direction of travel, that was the limiting speed.

But the apparent wind angle depends on the lift and drag, not directly on the leeway angle. It's much the same with sail trim. In order to get the same sail force in the figure below, the sails have to be eased relative to the hull compared to the original case, so their angle of attack is the same. The angle of attack of the sails is roughly independent of the orientation of the hull because they can be sheeted (above centerline if necessary) to get the desired angle of attack.

In the end, it's the lift/drag ratios that determine performance, not the leeway angle or the sheeting angle themselves.

amolitor

01-23-2005, 11:51 PM

> What's going on with catamarans isn't any different than what's
> happening with other sail craft

Then, why do they tend to not point at well? I think you may answer this in your remarks, though. I think it's simply because they don't generate as much hydrodynamic lift as keelboats, and I think that's what you're saying. We're just coming at the same information from two different angles. Heh heh. Angles.

It's foolish of me to suggest, and I apologize for doing so, that there's any specific thing that causes something else. It's a complex system where all the parts interact to create a specific performance profile.

By the way, it's perhaps worth pointing out that the lateral hydrodynamic force ALWAYS equals the aerodynamic lateral forces unless the boat is accelerating sideways. Leeway is caused by the boat "slipping sideways" until enough lateral hydrodynamic force to balance the aerodynamic. Lifting keels/boards exist to provide "free" lateral force in the right direction, so you don't have to do it all with bulk resistance (or whatever the right word is).

You know this, I am pretty darn sure, but I am not finding it particularly clear from your postings.

tspeer

01-24-2005, 01:50 AM

I don't think the difference is in the lift. As the apparent wind comes forward, the crew sheets in the sail to maintain the same angle of attack - that's what you're doing when you trim by the telltales. So the lift vector stays the same magnitude but changes direction. This actually increases the required hydrodynamic lift, as you can see in the diagrams by decreasing beta.

Instead, the difference is in the drag. A keelboat has a very distinct increase in drag with speed - much more than increasing by the square of the speed. A catamaran's drag does not increase as rapidly with speed. And a landyacht or iceboat's drag increases very little with speed. The less the drag increases with speed, the more it makes sense to foot rather than point. A modern multihull can point as high as a monohull, but if they do, they don't go to weather any faster than a monohull. So they foot for better speed and Vmg because they can. If a keelboat tried the same trick, it wouldn't pick up enough speed to make up for the extra distance.

If the drag increased with the square of the speed so the lift/drag ratio stayed the same, the polar plot of the yacht's performance would consist of two circular arcs. The point of sail for best Vmg would be 45 degrees plus beta/2, implying that a lower performance yacht would actually need to foot rather than point. But beta isn't constant as the point of sail changes - it increases as the keelboat tries to foot, and that drives them to point instead.

However, the theory shows that there's something fundamental about sailing 45 degrees to the true wind for best windward performance. The optimum is going to be in that neighborhood no matter what kind of sailing craft it is.

The notion that a multihull can't point is flat out false, so I disagree the premise of your original question. In my F-24 trimaran, I routinely sail up through the lee of bigger monohulls, sailing both higher and faster. The misperception that multihulls don't point well is a left-over from the early first generation mulithulls that had excessive windage and wetted area (due to V-shaped sections), and whose designers thought they could get away with shallow keels or no board at all. Or they were designed with no boards for the purpose of sailing off the beach like the Hobie 16. Today you see more attention paid to rounded topsides for less windage, semicircular sections, and deep daggerboards to reduce induced drag. As a result, modern multihulls point much the same as performance monohulls.

amolitor

01-24-2005, 02:07 AM

> The notion that a multihull can't point is flat out false

First of all, did I miss it, or have you not mentioned this helpful datum before? This is something I did not know, and was assuming was the other way about.

Secondly, with regard to drag. Is the point here that for multihulls that speed is cheaper with regard to drag, so by sailing deeper they get enough extra speed to give greater VMG? Or, another way to put it, those neato polar graphs "lobe out" farther further off the wind than a monohull?

I'm not seeing how drag makes the boat point less well. Well, it depends on definitions, perhaps. If 'pointing' simply means how far up it goes without starting to go backwards, anyways. If 'pointing poorly' means, instead, that the boat goes forward at high angles (the sails are pulling), just really really slowly, I guess I'll buy that.

I've been thinking entirely about how far up it can go and still be going forward, which as far as I can tell depends (almost) entirely on apparent wind and sailplan. Or, perhaps I am thinking of how far up it can go and still have the sails applying lift forward of the beam, which is almost but not quite the same thing.

brian eiland

01-24-2005, 01:40 PM

.....excerpt....A modern multihull can point as high as a monohull, but if they do, they don't go to weather any faster than a monohull. So they foot for better speed and Vmg because they can. If a keelboat tried the same trick, it wouldn't pick up enough speed to make up for the extra distance.

If the drag increased with the square of the speed so the lift/drag ratio stayed the same, the polar plot of the yacht's performance would consist of two circular arcs. The point of sail for best Vmg would be 45 degrees plus beta/2, implying that a lower performance yacht would actually need to foot rather than point. But beta isn't constant as the point of sail changes - it increases as the keelboat tries to foot, and that drives them to point instead.

However, the theory shows that there's something fundamental about sailing 45 degrees to the true wind for best windward performance. The optimum is going to be in that neighborhood no matter what kind of sailing craft it is.

The notion that a multihull can't point is flat out false, so I disagree the premise of your original question. In my F-24 trimaran, I routinely sail up through the lee of bigger monohulls, sailing both higher and faster. The misperception that multihulls don't point well is a left-over from the early first generation mulithulls that had excessive windage and wetted area (due to V-shaped sections), and whose designers thought they could get away with shallow keels or no board at all. Or they were designed with no boards for the purpose of sailing off the beach like the Hobie 16. Today you see more attention paid to rounded topsides for less windage, semicircular sections, and deep daggerboards to reduce induced drag. As a result, modern multihulls point much the same as performance monohulls.

I observed the same thing Tom, as I mentioned above in #50:

Obviously if you manage to create more wind (greater apparent wind) as many lighter-weight, slender multihulls do, then your chances for greater performance are enhanced. But you must make efficient use of this increased wind. This is the job of the sailing technic (helmsman), and a function of the rig design itself.

Most competitive multihulls can create a substantial amount of apparent wind. And they generally utilize it most effeciently by sailing no higher than 45 true, tacking thru 90-100. This generally maximizes their VMG. However, I have been in numerous situations where, if we slowed down to a speed just barely above that of a real competitive monohull, we could work our way to windward of him, and boy were they surprised! (I must admit it took a moment of inattentiveness on their part to pull this off...playing the slightess wind shifts). We concentrated on getting the greatest driving efficiency from our rig at these higher pointing angles...proper slot, proper sail shape for this higher incidence angle.

tspeer

01-24-2005, 04:34 PM

...
Secondly, with regard to drag. Is the point here that for multihulls that speed is cheaper with regard to drag, so by sailing deeper they get enough extra speed to give greater VMG? Or, another way to put it, those neato polar graphs "lobe out" farther further off the wind than a monohull?

Yes, that's it exactly.

...I've been thinking entirely about how far up it can go and still be going forward, which as far as I can tell depends (almost) entirely on apparent wind and sailplan. Or, perhaps I am thinking of how far up it can go and still have the sails applying lift forward of the beam, which is almost but not quite the same thing.

You're getting closer. As the boat slows down, it takes a larger leeway angle to produce the necessary lift to counter the sails. Eventually, the keel stalls, and the boat really starts going sideways - using drag to counter the sails as well as (greatly reduced) lift.

If it weren't for the keel stalling, then it would be due to the net aerodynamic force coming abeam. That's what limited the speed when I put in zero rolling resistance in my landyacht VPP.

amolitor

01-24-2005, 05:20 PM

So, which happens first? I see a couple of scenarios as a boat heads up (slowly) until it starts to pinch. I don't think these are not readily distinguishable from the deck, I think you'd have to do Science:

1) the keel stalls first, the boat starts to skid sideways, causing the sails to luff
2) the sailplan falls apart first, the sails start to luff before the keel stalls (and then as the boat slows, the keel stalls)

do you have a sense of which one happens first on real boats? Or, does something else happen?

tspeer

01-25-2005, 12:25 AM

Something else. The crew trims in the sails to keep them from luffing. Keel stalls, sails stay full. Both leeway angle and apparent wind angle increase.

You seem to think the sails will be luffing at the limit of a yacht's abilty to point. I suppose there are boats that won't allow the sails to be sheeted in adequately, and for them luffing might be the limiting factor. But you certainly can't generalize that.

Go try it out for yourself. Pinch up and watch what happens.

amolitor

01-25-2005, 02:51 AM

Well, in all the boat's *I* have driven, which ain't that many to be sure, the jib starts to get soft and fall in right around the time the boat stops wanting to go forward. These are various ratty sails on charter boats, some genoas sheeted to the spreaders. Smaller jibs that certainly COULD be sheeted in more if only the crummy old boat had a track there etc.

So, sure, in theory on a boat with the right setup you could simply keep sheeting in until the net lift was aft of the beam, and then the keel stalls, and you start slithering around the water.

I guess I am asking about, say, a Santa Cruz 27 with a beat up dacron 135% genoa sheeted to the spreader, and it is unreasonable to expect you to know if the keel or the sail stalls first ;) It sounds like the answer might well be 'it depends on your boat.'

Skippy

01-25-2005, 08:45 AM

Tom, what about multihulls? Doesn't a catamaran hull stall out before the daggerboard? And doesn't it generate fairly good lift below its stall angle? With a monohull, it seems like the hull never generates enough lift to make a difference.

Jocko

08-08-2005, 11:54 AM

By way of comparison, if you view a sailboat's upwind performance as being analogous to a glider's (engineless plane) descent profile, the ability of your boat to climb to windward is identical to the pilot's ability to fend off excessive altitude loss. A gliding pilot with sailing experience sees up as windward, down as leeward - managing that glide angle requires controlling two factors simultaneously: lift and drag. The sailor must also control the lift and drag forces of underwater components to effectively/optimally sail upwind. Here is where the connection lies. A cat in a light wind has too much drag from being cursed with wetted area - hence the angle suffers. (Not a cat sailor - has anyone tried hiking to leeward in light winds? - wouldn't it make the cat perform more like the tri?) The cat improves its' angle in stronger winds by creating underwater lift with less drag when its' wetted area decreases.

My suggestion - focus on making the underwater parts most efficient - foils! and hull drag, with windage a close second (third? - I've lost count)

Next - the suggestion that 50 degrees is good since the idea of 45 + half the drag angle is optimal is only a player for the 'rocket' ships like ice yachts and land yachts which can achieve speed increases when they fall off the wind. Most sailors don't see much speed increase when the angle grows - unless you're on an awesome cat - hence most people benefit more from angle fixation, as per the AC crews. (OK, why do I feel like I'm about to be blasted by those who really know what they are saying)

Skippy

08-08-2005, 02:51 PM

Be careful about taking that analogy too far. The high-pressure (windward) side of the wings is the undersides, not the upper surfaces. In sailing, you lose power when you pinch because the true wind direction doesn't change, and by sheeting in, you're turning the sail's power to a less favorable angle. Whereas on gliding, you stall when you nose up too high because your power source is gravity, which is then similarly in a less favorable direction. But that's a gravitational issue, not an aerodynamic one.

"has anyone tried hiking to leeward in light winds?"
Not me personally, but yes, people have mentioned doing that with good results.

"OK, why do I feel like I'm about to be blasted by those who really know what they are saying"
A good sense of intuition? :p

CT 249

08-08-2005, 06:30 PM

But as Tom mentioned (and I gave figures to indicate the F18's good wsa v a Flying Dutchman) a good modern multi may not actually have significantly higher wsa. In fact some of them have such small foils (to reduce) wsa that the small foils may be the problem, perhaps.

Our Formula 16HP has foils that extend about 18" below the hull, and they are comparatively big for a performance cat. In contrast monos from Laser to International Canoes and skiffs have big foils. Only the modern Moth lacks a deep CB, partly because it's a small low-drag boat with a low-drag rig and it's also not at its best in light airs.

I assume that the multi's small foils "rotate the lobe" to favour footing - does that sound right Tom?

jam007

09-24-2005, 08:32 AM

Reading this thread I was inspired in making the graph below.
It shows Velocity made good and Boat speed for different boats and headings.
As shown has the SeaCart 30 (A racing trimaran) a higher TWA for its maximum VMG since the boats speed increases rapidly with higher TWA angles.
All boats has a minimum AWA when they stop going forward. That is when the part of the sailforce directed forward at the smallest possible sheeting angle just is sufficient to counteract the aerodynamic drag form rigg, sail and hull. In the graph the VO60 seems to come close to this minimum angle as the speed slows rapidly as the AWA gets close to 20 degrees while the SeaCart 30 seems to have a few degrees left to pint before that.
The data on SeaCart 30 and VO 60 is taken from the homepage of SeaCart 30 and is for 10 knots of true windspeed. The data on the Dinghy from Marchaj's "Sail Performance".
I have included the leeway angle in AWA. Iow. AWA is calculated from the direction of motion.

I have searched for ACC yatch data and would really like to plot them. Can anyone direct me to where I can find a polar for ACC yatch? (Does not have to be exact, no secrets revieled;) )

tazmann

09-25-2005, 01:11 PM

Hello
As far as upwind performance, there are to many varables to say any one thing is the cause but you should be able to point higher than 50% granted your boat speed will suffer. Cats are usualy designed for off the wind performance, You cannot compare to the J80 for upwind light air sailing, Thats what there built for. I raced santana 20 one design for a few years and its amazing the deference between the same boats , it all boils down to rig tuning and sail trim ( if the boats and sails are equal) for the wind and sea condition. The first couple races all the other boats were pointing 10-15% higher and going faster than I was (real frustating) so my learning curve went way up. I guess what Im trying to say is you have to learn you particular boat, what works on one doesnt allways work on the other. With cats you should be able to fall off the wind and go for boat speed make a few tacks and come out ahead of the J80
Tom

tspeer

09-25-2005, 06:26 PM

...
I assume that the multi's small foils "rotate the lobe" to favour footing - does that sound right Tom?

You do want to foot in a multihull. You can pinch with the monohulls and go at comparable speeds. However, it's better to foot off for better speed. It's not so much that the lobe is rotated, it's more like the lobe is extended.

A fun sport in a modern mulithull is to pick out a larger monohull going to weather, and climb up under their lee until you pass in front, sailing both higher and faster.

Jocko

09-26-2005, 12:36 PM

Actually, I think the case could be made for saying, "The leeway angle should be as great as possible, short of stalling the keel/board."

Disagree: Edmond Bruce's tank testing showed, and he firmly believed,published that leeway angles > 5 degrees were undesirable. His statement to this is basically that greater leeway than this meant too little board area/too large sail area and vice versa.

... Note that what counts is to minimize the drag angle (maximize L/D), regardless of where the boat is pointing!

Totally agree! Amen - I'm with you on this. :cool:

Now the drag due to lift is not dependent at all on the area of the keel - it depends (inversely) on the square of the keel depth.

Again from Bruce: his tank testing showed that airfoil theory didn't necessarily apply to keel design - especially w.r.t. surface piercing foils. Indeed, his testing showed that surface piercing foils operated most efficiently with aspect ratios near unity. I believe that monohulls with deep keels (ACC et al) would probably be far enough away from the surface piercing realm as to believe that span loading was key. I believe that Cheekee Monkey now has foils outboard, similar to many modern tris - they perhaps don't need(benefit from) the aspect ratio beneficial to monohulls.

... So if you start with a full keel and start cutting away at the chord of the keel to reduce the area, the lift will remain a constant (it's determined by the sail trim) and the drag due to lift will remain a constant. But the drag will go down and the leeway will go up.

Stop! The drag (wetted surface) will go down, but the leeway (increasing) will necessarily increase the hull's drag. Total drag - determining the all important hull's L/D ratio - will not necessarily improve. I believe this is where Bruce and Speer diverge, no? Is it not reasonable to believe that increasing the leeway beyond 5 degrees creates a greater drag penalty from the hull than improvement from the foil? Hence the optimum shown in Bruce's tank testing?

...Until the keel is so small that it stalls under the applied load. Then the drag will increase rapidly. So you want to stop cutting away at the keel short of that point.

At this point, the hull would likely be slewing sideways at something like 15 degrees when equipped with high aspect ratio foils, or 25 degrees if equipped with low aspect ratio/full keel equipment. Surely that angle is going to overwhelm the drag numbers.

... The result is the keel operating at a high lift coefficient, and thus a comparatively high leeway angle.

:eek: Say it ain't so! :eek:

tspeer

10-09-2005, 01:48 AM

My main point is, for the designer, reducing leeway does not necessarily improve performance. You can make the leeway arbitrarily small by adding more lateral area. But this extra wetted surface will slow you down.

The article I've seen by Bruce on optimum centerboard size ("Design for Fast Sailing", AYRS, p. 65 - 68) had a geometric aspect ratio of 3. He assumed the board had an effective aspect ratio of 6 because of the end-plate effect of the hull, but his data had an average lift curve slope of 0.064/deg, which implies an effective aspect ratio of 3.24 (an Oswald efficiency factor of 1.08).

When he looked at sizing the centerboards, he maintained the aspect ratio, thus increasing the depth for the larger board. This is not the same as maintaining the same depth and cutting away at the chord. Under his assumption of constant aspect ratio, the board alone reached its optimum L/D at 6 degrees, and this stayed the optimum angle because he maintained the aspect ratio. Had he maintained the same depth, he would have gotten a different result.

Here's Bruce's table:

AR = 3
Angle(deg)____0_______2_______4_______6_______8______10______12
CL___________0_______0.14____0.26____0.39____0.52____0.64_____0.73
CD___________0.035___0.037___0.042___0.059___0.085___0.123____0.161
L/D__________0_______3.8_____6.2_____6.6_____6.1_____5.2______4.5

Here's the same table multiplied by the areas Bruce examined (3, 6, 9) to get the lift and drag divided by the dynamic pressure. The necessary lift area will be proportional to the sail area at a given boat speed/wind speed ratio.

Boards sized as Bruce sized them (constant aspect ratio):
AR = 3__________Area = 3_____e = 1.08_____Depth = 3__________________________Design Pt
Angle(deg)______0_______2_______4_______6_______8_______10______12__________10.15
Lift Area________0_______0.42____0.78____1.17____1.56____1.92____2.19___________1.94
Drag Area_______0.105___0.111___0.127___0.180___0.261___0.378___0.495__________0.387
Profile__________0.105___0.106___0.108___0.136___0.181___0.257___0.338__________0.263
Induced_________0.000___0.006___0.020___0.045___0.080___0.121___0.157_________0.123
L/D_____________0.0_____3.8_____6.1_____6.5_____6.0_____5.1_____4.4___________5.0

AR = 3__________Area = 6_____e = 1.08_____Depth = 4.24_______________________Design Pt
Angle(deg)______0_______2_______4_______6_______8_______10_______12__________4.97
Lift Area________0_______0.84____1.56____2.34____3.12____3.84_____4.38__________1.94
Drag Area_______0.210___0.223___0.255___0.361___0.522___0.756____0.989_________0.306
Profile__________0.210___0.211___0.215___0.271___0.363___0.515____0.676_________0.242
Induced_________0.000___0.012___0.040___0.090___0.159___0.241____0.314________0.064
L/D_____________0.0_____3.8_____6.1_____6.5_____6.0_____5.1______4.4__________6.3

AR = 3__________Area = 9_____e = 1.08_____Depth = 5.20_______________________Design Pt
Angle(deg)______0_______2_______4_______6_______8_______10_______12__________3.26
Lift Area________0_______1.26____2.34____3.51____4.68____5.76_____6.57__________1.94
Drag Area_______0.315___0.334___0.382___0.541___0.783___1.134____1.484_________0.365
Profile__________0.315___0.317___0.323___0.407___0.544___0.772____1.013_________0.321
Induced_________0.000___0.017___0.060___0.134___0.239___0.362____0.471_________0.044
L/D_____________0.0_____3.8_____6.1_____6.5_____6.0_____5.1______4.4___________5.2

For his notional design, he needed a lift area (CL*Area) of 1.94, and for these designs the best choice was the area 6 board, operating at an angle of 5 degrees.

But suppose he had compared the boards on the basis of constant depth and varied the chord to change the area. Here's what his table would have looked like:

Boards sized keeping the depth the same:
AR = 6__________Area = 3_____e = 1.08_____Depth = 4.24_______________________Design Pt
Angle(deg)______0_______2_______4_______6_______8_______10_______12__________10.15
Lift Area________0_______0.42____0.78____1.17____1.56____1.92_____2.19___________1.94
Drag Area_______0.105___0.109___0.118___0.158___0.221___0.318____0.416__________0.325
Profile__________0.105___0.106___0.108___0.136___0.181___0.257____0.338__________0.263
Induced_________0.000___0.003___0.010___0.022___0.040___0.060____0.078__________0.062
L/D_____________0.0_____3.9_____6.6_____7.4_____7.1_____6.0______5.3____________6.0

AR = 4.83_____Area = 3.73_____e = 1.08_____Depth = 4.24_______________________Design Pt
Angle(deg)______0_______2_______4_______6_______8_______10_______12__________8.01
Lift Area________0_______0.522___0.969___1.453___1.938___2.385____2.721_________1.94
Drag Area_______0.130___0.136___0.149___0.203___0.287___0.413____0.541_________0.287
Profile__________0.130___0.131___0.134___0.168___0.225___0.320____0.420_________0.225
Induced_________0.000___0.004___0.015___0.035___0.061___0.093____0.121_________0.062
L/D_____________0.0_____3.8_____6.5_____7.2_____6.8_____5.8______5.0___________6.8

AR = 3__________Area = 6_____e = 1.08_____Depth = 4.24_______________________Design Pt
Angle(deg)______0_______2_______4_______6_______8_______10_______12__________4.97
Lift Area________0_______0.84____1.56____2.34____3.12____3.84_____4.38__________1.94
Drag Area_______0.210___0.223___0.255___0.361___0.522___0.756____0.989_________0.306
Profile__________0.210___0.211___0.215___0.271___0.363___0.515____0.676_________0.242
Induced_________0.000___0.012___0.040___0.090___0.159___0.241____0.314_________0.064
L/D_____________0.0_____3.8_____6.1_____6.5_____6.0_____5.1______4.4___________6.3

AR = 2__________Area = 9_____e = 1.08_____Depth = 4.24_______________________Design Pt
Angle(deg)______0_______2_______4_______6_______8_______10_______12__________3.26
Lift Area________0_______1.26____2.34____3.51____4.68____5.76_____6.57__________1.94
Drag Area_______0.315___0.343___0.412___0.608___0.902___1.315____1.720_________0.387
Profile__________0.315___0.317___0.323___0.407___0.544___0.772____1.013_________0.321
Induced_________0.000___0.026___0.090___0.202___0.358___0.543____0.706_________0.066
L/D_____________0.0_____3.7_____5.7_____5.8_____5.2_____4.4______3.8___________5.0

If you only looked at the area 3, 6 and 9 boards, it might look like the previous result. But this is only because the profile drag is increasing rapidly at the higher angle of attack. The true optimum is at 8 degrees angle of attack. Had the profile drag not been increasing as it did, the chord could have been cut some more, making the angle of attack even higher.

When the drag of the hull is factored in, that also drives the optimum angle of attack higher because the extra induced drag due to the extra lift is offset in the lift-drag ratio by the higher lift.

Note also that the induced drag area is the same for the different aspect ratios when the lift area and depth are kept constant. Induced drag for the same lift depends on depth and not planform area or aspect ratio. This is a fundamental problem when trying to design a shallow-draft keel.

I submit that keeping the depth constant and varying the chord is the way a typical designer actually works. The depth is set by considerations like the desired stability, water depth, etc. So what the designer is free to vary is the chord.

farjoe

10-09-2005, 03:25 AM

Hello
You cannot compare to the J80 for upwind light air sailing, Thats what there built for.
Tom

The interesting question is WHY?

It is not the weight. It may be the lower immersed are(some do not agree) and lower windage. The sail area is similar. Daggerboard area is lower and probably the surface finish is inferior due to the fact that the J80 cannot retract it.

What are the parameters which make boats like the J80 "built for upwind light air sailing"?

farjoe

tspeer

10-09-2005, 11:03 AM

Oops - I forgot to correct the angles of attack for the different aspect ratios.

The change in angle of attack comes from the change in the angle of the wake left by the board. The board sails in a crosswise-current of its own making, and the shorter the board, the larger this adverse current is for the same lift. This self-induced current shows up as a change in leeway.

Boards sized keeping the depth the same:
AR = 6__________Area = 3_____e = 1.08_____Depth = 4.24_______________________Design Pt
Angle(deg)______0.00____1.57____3.21____4.81____6.42____8.05_____9.78___________8.18
Lift Area________0_______0.42____0.78____1.17____1.56____1.92_____2.19___________1.94
Drag Area_______0.105___0.109___0.118___0.158___0.221___0.318____0.416__________0.325
Profile__________0.105___0.106___0.108___0.136___0.181___0.257____0.338__________0.263
Induced_________0.000___0.003___0.010___0.022___0.040___0.060____0.078__________0.062
L/D_____________0.0_____3.9_____6.6_____7.4_____7.1_____6.0______5.3____________6.0

AR = 4.83_____Area = 3.73_____e = 1.08_____Depth = 4.24_______________________Design Pt
Angle(deg)______0.00____1.68____3.40____5.10____6.80____8.53____10.32__________6.81
Lift Area________0_______0.522___0.969___1.453___1.938___2.385____2.721_________1.94
Drag Area_______0.130___0.136___0.149___0.203___0.287___0.413____0.541_________0.287
Profile__________0.130___0.131___0.134___0.168___0.225___0.320____0.420_________0.225
Induced_________0.000___0.004___0.015___0.035___0.061___0.093____0.121_________0.062
L/D_____________0.0_____3.8_____6.5_____7.2_____6.8_____5.8______5.0___________6.8

AR = 3__________Area = 6_____e = 1.08_____Depth = 4.24_______________________Design Pt
Angle(deg)______0.00____2.00____4.00____6.00____8.00____10.00____12.00__________4.97
Lift Area________0_______0.84____1.56____2.34____3.12____3.84_____4.38__________1.94
Drag Area_______0.210___0.223___0.255___0.361___0.522___0.756____0.989_________0.306
Profile__________0.210___0.211___0.215___0.271___0.363___0.515____0.676_________0.242
Induced_________0.000___0.012___0.040___0.090___0.159___0.241____0.314_________0.064
L/D_____________0.0_____3.8_____6.1_____6.5_____6.0_____5.1______4.4___________6.3

AR = 2__________Area = 9_____e = 1.08_____Depth = 4.24_______________________Design Pt
Angle(deg)______0.00____2.43____4.79____7.19____9.58____11.95____14.22__________3.91
Lift Area________0_______1.26____2.34____3.51____4.68____5.76_____6.57__________1.94
Drag Area_______0.315___0.343___0.412___0.608___0.902___1.315____1.720_________0.387
Profile__________0.315___0.317___0.323___0.407___0.544___0.772____1.013_________0.321
Induced_________0.000___0.026___0.090___0.202___0.358___0.543____0.706_________0.066
L/D_____________0.0_____3.7_____5.7_____5.8_____5.2_____4.4______3.8___________5.0

The difference isn't so pronounced - it's now 7 degrees, not 8 - but it's still 37% higher than the 5 degrees that was Bruce's optimum angle.

RHough

11-15-2005, 03:54 AM

Sailing to windward is L/D pure and simple. You need low L/D to go to weather, the higher the L/D the higher the boat points.

If you can increase lift without increasing drag you point higher. If you can decrease drag at the same lift you point higher.

If your boat doesn't point as high as another, your L/D ratio is lower than the other boat's. Not a tough concept.

RHough

11-15-2005, 10:56 AM

When looking at the sail plan going to weather the rig used to be considered a semi-span as far as AR is concerned. The water plane became the centre and any gaps between hull and sail foot treated like a gap in the wing.

Thus a sail with measured AR of 2 would be assumed to have an AR of 4 for induced drag calculation. Is that model still used?

When two sails are some distance apart fore and aft (Freedom Cat Ketch) are they considered separately? In the opposite case of an IOR sloop where the sails overlap for most of their length how is AR determined?

tspeer

11-16-2005, 01:04 AM

...
Thus a sail with measured AR of 2 would be assumed to have an AR of 4 for induced drag calculation. Is that model still used?

Unless the lift can be carried right down to the surface, the effective AR is nowhere near doubled. Even if you don't have a gap, if the lift drops off due to the wind boundary layer near the surface, you lose the effective span extension.

The figure below shows what a lifting line analysis predicts for the minimum induced drag for sail rigs designed for different amounts of gap. This is not the increase you get when you design for no gap and the move the rig up, instead each point is a different design that is the best you can do for a gven luff length. Given that the foot of a typical sail is around 5% of the span or more above the surface, the drag is almost the same as for a surface designed without the reflection plane.

http://www.tspeer.com/Planforms/Fig07.gif

For more sail-like lift distributions,
http://www.tspeer.com/Planforms/Fig08.gif
the induced drag with gap tradeoff looks like this:
http://www.tspeer.com/Planforms/Fig14.gif

When two sails are some distance apart fore and aft (Freedom Cat Ketch) are they considered separately? In the opposite case of an IOR sloop where the sails overlap for most of their length how is AR determined?

Munk's stagger theorem says that when multiple surfaces are lined up in the streamwise direction, the induced drag is the same as for a single surface with the same total loading. It doesn't matter how the surfaces are spaced or how the loading is distributed between the surfaces.

So the appropriate aspect ratio to use is the square of the maximum span divided by the total area.

RHough

11-16-2005, 01:36 AM

Unless the lift can be carried right down to the surface, the effective AR is nowhere near doubled. Even if you don't have a gap, if the lift drops off due to the wind boundary layer near the surface, you lose the effective span extension.

Thanks! If I had thought it through a bit more I'd have seen that. "Carry the lift down to the surface" is the best description of why the AR is not double that I've ever read.

Munk's stagger theorem says that when multiple surfaces are lined up in the streamwise direction, the induced drag is the same as for a single surface with the same total loading. It doesn't matter how the surfaces are spaced or how the loading is distributed between the surfaces.

So the appropriate aspect ratio to use is the square of the maximum span divided by the total area.

I'm getting this ... almost. Cat Ketch two sails of 150ft^2 each and 30ft span would be 30^2/300= AR 3:1 correct?

Sloop with 150ft^2 main and 150ft^2 genoa overlapping 50ft^2 (150% of J) would be either AR 3:1 also or would it be 30^2/250 = AR 3.6? Is the total sail area used or the projected area?

I used to design RC sailplanes so I'm trying to turn my head sideways to understand sails in terms I'm more familiar with.

Thanks again for a very informative response.

gggGuest

11-16-2005, 04:06 AM

You're getting very deep in the theory of foil performance boys. This tends to be based on steady flow and constant speeds. All well and good, but there are traps waiting. A few years ago on a fast dinghy I designed I had to switch from a smallish high speed section daggerboard to a larger low speed section.

The problem was, although the board was just fine moving along, it had a horrible propensity for stalling coming out of tacks and the like, leaving the boat dead in the water for seconds or worse until normal service could be resumes. So in the end the major factor on design wasn't L/D or anything else,but purely the problem of getting through tacks...

And of course in practice our boat may not - even will not - be travelling at constant speed and direction, but instead have both heading and speed constantly changing as it runs through waves and responds to wind changes.

That's why, although I would never want to work without the theoretical background, it has got to be very strongly balanced against and founded in empirical data. The problem is just too complicated. This is, incidentally why the Bethwaites are so strong. They have the theoretical background, but they also have an enormous body of empirical data which no-one else has done, and tend to be guided more by the real world outputs of the empirical data.

RHough

11-16-2005, 10:49 AM

And of course in practice our boat may not - even will not - be travelling at constant speed and direction, but instead have both heading and speed constantly changing as it runs through waves and responds to wind changes.

Of course. However, one does not have to build 10 sailplanes to discover that AR 20:1 has a flatter glide slope than AR 6:1 if one knows some theory.

I don't expect to create a polar diagram of a boat's performance on paper and have the real boat perform exactly as predicted. In my RC sailplanes I was able to select an optimum AR for a given span that performed as predicted. It is certainly possible to predict that one design will have a higher L/D than another.

In non human carrying designs it is possible to do things like calculate max loadings on a spar that the planform can generate then build a series of wings with the safety fudge factor progressively reduced until the spar fails to check the validity of the calculations. I don't have the budget to build multiple rigs and test to failure on a full scale boat.

In the case of a foil stalling out of tacks, I would look for foils that operate far from the stall in steady state so they can tolerate a higher AOA change before they stall. I'd also look at how many degrees AOA has to be reduced before flow is re-established and select on the basis of broadest range with smallest hysteresis loop area. I'd look at what works in aerobatic aircraft rather than sailplanes.

Jammen

10-10-2006, 09:42 AM

In a mono and a tri you have one keel/daggerboard with only on end free against the water. On a cat you hav two end surface wich makes the daggerboard "look" shortern then thay are.

This is the same reason we are not flying biplanes today and one reason twin rigged boats nerver will performe either.

//Calle

truemorc

10-11-2006, 10:42 PM

So we can feel confident that the lift vs drag is the primary issue in the original question on pointing? And mostly the Hydro end of it in this case?
I just want to make sure I have it right. Emphasis on primary...For those of us who are a little less advanced. Thanks. :)

HeloDriver

10-23-2006, 10:39 AM

Hate to be the proverbial wooden nickel, yet I would like to resolve the above issue regarding maximizing L/D for optimum windward performance and seek clarification. Is indeed max L/D our target?

Sailing to windward is L/D pure and simple. You need low L/D to go to weather, the higher the L/D the higher the boat points.

If you can increase lift without increasing drag you point higher. If you can decrease drag at the same lift you point higher.

If your boat doesn't point as high as another, your L/D ratio is lower than the other boat's.

I may have misunderstood from the 2004 Seahorse: Hollom/Oglesby articles 'Go with the flow' that for a given planform, low CDp in conjunction with an proper stall angle was emphasised over L/D.
Though Max L/D appears to be a simpler resolution!

So we can feel confident that the lift vs drag is the primary issue in the original question on pointing? And mostly the Hydro end of it in this case?
I just want to make sure I have it right. Emphasis on primary...For those of us who are a little less advanced. Thanks. :)

There was a similar yet maybe not totally resolved discussion a while back in the thread entitled "Keel design issues" http://www.boatdesign.net/forums/showthread.php?t=4874&page=3&highlight=keel+shape. And like truemorc above..layman's explanation appreciated.

tspeer

10-23-2006, 11:36 AM

Hate to be the proverbial wooden nickel, yet I would like to resolve the above issue regarding maximizing L/D for optimum windward performance and seek clarification. Is indeed max L/D our target? ...

No. Maximizing the L/D of the rig alone, or even maximum aerodynamic L/D of the rig+hull is not sufficient. You must minimize the apparent wind angle, beta, which requires the combination of aerodynamic and hydrodynamic drag angles be minimized.

One factor that enters into this is the hydrodynamic lift from the hull depends on the lift from the rig. The L/D of the hull is limited by the amount of lift demanded by the rig. In offwind sailing, the L/D of the hull is very low because it has drag but is not required to produce much lift. This isn't a fault of the hull design - it follows from the sail trim and point of sail.

In upwind sailing, if the maximum aerodynamic L/D is below stall, it may pay to trim the rig for maximum lift instead of maximum L/D, even though the aerodynamic drag angle will increase. The added lift can improve the hull's L/D and reduce the hydrodynamic drag angle more than the loss in the aerodynamic drag angle, and improve performance.

In general, for the kinds of boats we're talking about, the windage is so high that the rig will stall before it reaches maximum L/D. So there's benefit in improving the maximum lift as well as the L/D of the rig alone. But the really big gain may come from reducing windage.