Analysing Upwind Performance

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.

 

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

 

lee helm

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?

 

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.

 

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

 

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.

 

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?

 

Wsa

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..

 

FarJ an old friend

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..."

 

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.

 

mono/multi wsa

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....

 

Ciao Marco- come stai?

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.

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

 

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

 

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.

 

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