Code Zero Sails, aerodynamic questions

I was looking to restudy some of the aerodynamics of the 'code zero' sails that we see an increasing usage of since their original development on the Whitbread boats. We now see more use of these type of sails on very fast multihulls (particularly foiling ones) that bring their apparent winds so far around on the nose.

To my mind's eye, I often tend to look at them as a big genoas, as I found this article that expresses the same. I've made a copy of that article here, as often these things can disappear after some period of time in this modern world of SO MUCH information. I hope it does not, as there are some good illustrations that accompany the article.
http://northsails.com/tabid/27323/Default.aspx


"Code Zero" (A0) Asymmetric Spinnakers
Part genoa, part asymmetric reacher... is the A0 (aka "Code Zero") right for you?

 

another reference....
http://www.uksailmakers.com/Sails/Type-of-Sail/Code-Zero-Sails.html

 

Sure seems to speak to some superiorities of a large overlapping genoa sail that I have been so criticized for expounding on my aftmast rig

 

It's not exactly surprising that a sail two or three times as large as a normal #1 is a bit faster than a normal #1 in certain conditions and wind angles. The fact that bigger sails go faster at times is not news.

Once you calculate the cost of the extra sail, the very high loadings involved, the roller furling gear that is normally involved and the hassle that is normally involved, the Code Zero can look like a pretty bad deal from some angles. I remember being on a Volvo 60 when they were experimenting with Code Zero development in 2000 or early 2001. The luff loads were frightening, and not just to me but also to the Volvo racers on board. In some ways it underlined the problems involved with large foretriangles.

I'd like to hack my old 150% No 1 into a cheap Code Zero, because now I've modified the boat to a short overlap rig the old sail is just sitting around and the boat doesn't go as fast as it used to close reaching. A cheap hack could be worthwhile. However, the details of moving to a Code Zero just underline how much of a pain big headsails can be, and how brilliant the fractional rig with a short overlap headsail is.

 

More sail area is faster than less sail area, subject to the law of diminishing returns.

With race boats a lot depends on how the rules work and sail area is measured/permitted.

When total area is restricted/measured most boats end up with a fractional rig with a short overlap headsail because that's the most performance per square foot of sail (and incidentally the most controllable).

When area is unmeasured but spar size is restricted/measured many boats end up with a masthead rig with a big overlapping headsail because that's how you can hang the biggest area off a given spar. If you have an extra 30% rag that is only half as efficient as the rest you've still added 15% power.

The Code zeros came about, IIRC (and I might be wrong), because upwind sail area and downwind sail area were restricted separately on the W60s(which is sensible), but the rule was jiggered so an upwind sail (the code zero) counted in the rule as a downwind one (which I submit was less sensible as it rather defeated the object).

 

You both seem to agree that more sail area in general is faster.

And gggGuest, this comment by you somewhat reinforces one of my pet peeves, ...is it mother ocean that is designing our rigs, or some handicap racing rules :?:

But that is not exactly the discussion I was hoping for with this subject thread. What I was looking for was a better understanding of why, or how, the big overlap area of these code zeros (or genoa-like) sails contribute to better up-wind performance,...for that matter how did (or did) the older style big overlapping genoas contribute to up-wind performance?

Most of the forward drive of these sails must be coming from the forward portions of these sails. But is there a contribution from that aft overlapping area in the up-wind sailing range? What are the more specific aerodynamics of this situation? Are there positive of negative influences with other sails, or the mast or whatever?

I think I need to look back thru some of Tom Speer's contributions, as I believe I remember reading something about this (but my memory is a little shorter these days )

 

Excerpt from the first article I posted:

And from the second article:

 

As soon as you compare any two different rigs you end up having something equivalent to a rating rule, be it only in your head. Best is a relative value, not an absolute one.

Performance square foot for square foot, getting you round a race track, non overlapping fractional is best. But why should that be the only comparison? What about performance per dollar/pound in materials, or performance per foot length of spar, or performance per pound weight aloft? Even performance per man hour of maintenance...

All these are valid ways of comparing different rigs, and quite probably different rigs will give different answers.

Performance per square foot around the racetrack is the most conventional, and perhaps the easiest, and we know the answer for that. But if you were a lobsterman on a working boat who might plan his fishing grounds according to the wind direction maybe performance per pound/dollar invested capital would be have been more important, at least until the day he got embayed by a gale off a rocky coast...

 

Ah ha, here is one of Tom's postings on another forum that I was looking for:
http://steamradio.com/pipermail/multihulls/2001-September/007046.html

How Sails Work, the slot effect (Yeah, another long one)

Just found this with the help of Google. I need to re-read several times myself at this point

 

Three angles to consider:

1.) It is a very large sail. When set, it is probably the largest one flying. One large sail is generally more efficient than several smaller ones, with the same total area.

2.) It has a very clean leading edge. When well attended to, it produces more lift at a tighter angle to the wind. Its full, fore and aft, shape may limit how tight it can be pointed into the wind, but once within that limit, it produces more drive than a flatter Genoa. So it may actually be faster despite not being able to point as high.

3.) Much of its power is relatively low up the mast, so it delivers more drive per heeling moment.

These three potential explanations may be the simplest around, but may also be the truest.

 

As gggGuest already noted, what is the alternative in racing? If you don't restrict sail area we know what happens - design gets centred around loading on more and more sail in a design spiral that leads to diminishing returns. Boats become extremely expensive and difficult to handle, and designs become specialised around handling the huge rigs rather than being efficient or economical.

As already pointed out ad nauseum here, even without rating rules getting involved, most high performance designs move to small jibs and big mains. Big overlapping sails are in many ways creations of the rules you disdain.

Yes, if you add a massive overlapping headsail a boat will go faster in some conditions, but in many conditions it will be a complete PITA to handle as well as being expensive and requiring problematic storage.

 

If fractional jibs are all so superior I wonder why these guys are flying an overlapping sail


http://www.sailmagazine.com/multihulls/the-worlds-fastest-sailing-multihulls/?utm_source=sail-enewsletter&utm_medium=email&utm_content=textlink&utm_campaign=enewsletter


...or the new foiling Gunboat G4
http://www.boatdesign.net/forums/multihulls/gunboat-g4-uptip-foils-50397.html




https://vimeo.com/125378004

Interesting that its the mainsail that can't be released fast enough to prevent the capsize of that new Gunboat,...but the code/genoa gets released in plenty of time.
http://www.boatdesign.net/forums/multihulls/gunboat-g4-uptip-foils-50397-7.html#post733502

I thought that is just not suppose to be....those big, bad, genoas are just going to capture the wind and cause the capsize.

I would enter another alternative theory here,...its more difficult to get a sail to spill the wind that is restrained by a multiple-part tackle than a sail that is being controlled by a single sheet line,...a genoa sheet vs the multi-purchase gear utilized on most mainsails.

 

There are practical consideration, as mentioned, like handling, weight etc. I also suspect that maintaining an optimal camber or shape is more difficult with a larger sail, though modern construction methods of these very costly sails will allow larger sails that maintain their shape further back to the trailing edge.

Most of the lift comes from the first 25 to 30 percent of the cord length on an optimal surface, making a very large surface with a less well defined shape may indeed produce more lift (or drive), but you do not get as much (i.e. efficiency goes away, L/D is much lower), and in certain circumstances I am sure you can actually loose headway made good when not in optimal conditions. Also, taking longer to tack and reset the sails would loose small amounts of time as well, which would add up if the type of racing you are doing requires a lot of course changes.

I know from my former work in aerospace, that double, triple or more slotted flaps and LE devices will add more lift per SF of area, but the drag goes way up. You will never achieve best L/D when in landing mode, the object is to just get as much total lift as possible, the higher drag actually helps the pilot control the glide slope angle using the throttle. The demands on sails is very different of course, but there are times when best L/D would be beneficial I would think, and times when beating into the wind where high lift would be beneficial, particularly if you were head to head with another competitor when approaching a mark.

 

Lift, Drag, and Titanium Sails

If these negatives are so bad, why do these sailors continue to use these overlapping sails. Why don't they shift that extra sail area they are seeking to a more efficient form,...perhaps onto another sail??

We need to be a little more careful in our use, and definitions, of the terms 'lift' and 'drag' when comparing sails on boats verses wings on aircraft, ie; a very hi lift force on a aircraft wing can mean a very big leeway force on a sailboat.
http://simple.wikipedia.org/wiki/Lift_%28force%29

I have to wonder if a lot of these Code sails are in fact the UK Titanium sails
http://www.sail-world.com/UK-Sails-Titanium----little-black-number-/121938

I think I would like to have a pair of these Titanium sails on my twin-headsailed, aft mast rig design.
http://www.boatdesign.net/forums/sailboats/aftmast-rigs-623-7.html#post198605

 

'Dumping Effect'

I tried to take into account this dumping effect that Tom describes with this posting and illustrations here:
Sails in Combination