Analysing Upwind Performance

[farjoe]

Originally Posted by BOATMIK

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

What you say makes a lot of sense. Therefore would the solution be more sail, to be always powered up as in windsurfing and 18ft skiffs?

And also AC boats.

[BOATMIK]

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

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]

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]

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]

Originally Posted by tonyr

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]

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]

Originally Posted by 249

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.

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]

Originally Posted by 249

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]

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

[farjoe]

Originally Posted by asathor

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]

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]

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]

Originally Posted by 249

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 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