sail aerodynamics

First off thanks for that interesting reply/analysis.

Please realize Daiquiri that I am NOT trying to claim my aftmast rig design as superior to existing sloop rigs, particularly in the beating situation. Rather I am saying that it is not so bad as a lot of naysayers want to make it out to be. I'd be real happy with that 5-10% difference, particularly if it was on the plus side, and without a traditional mainsail. And this test report lends support to my claims about the dble headsail with overlap as not being so bad afterall.

I see that you have never visited that aftmast discussion. Here is one posting of what I was after:
Aft-mast Origination & Justification

....and this....

Now that is very interesting. I had not noticed that. I had just been looking a dwg they gave in fig 5.

It appears this needs more investigation to find out the reasons for this discrepancy

I think that is more the 'nature of the beast',...typical for traditional boomed mainsails.

I could have utilized a staysail type boom on my inner headsail and ended up with a more efficient 'deck sweeper'

Its an aerodynamic study, not a hydrodynamic one

I don't speculate on how they intended to rig the full size vessel of this design, but I might think they would consider making all the sails roller furling/reefing. That is what I intended.

and this fellow Kojii executed:

 

The truth is - the "Aftmast rigs???" thread has grown too big. Once a thread exceeds 10-15 pages, it becomes very difficult to katch up with the discussion without a strong practical motivation to undertake the long effort. And right now, quite frankly, I am short of time and proper motivation. I will gladly read all the relevant technical papers you can suggest, but in this moment I do not have at disposal the time required for reading a 67-page forum discussion.

And, regarding my previous post - I was not trying to dismiss the merits of the aft-mast at all. I have just critically reviewed the test report which you have posted.

 

I agree, that subject thread has grown too big. But I don't know how it might be restarted without letting a lot of that history of discussion, both positive and negative, just sort of fade away and be forgotten.

And I did not think you were dismissing any merits, I just chose the occasion to reinforce my positive views on the rig.

 

.....an email I just received today from the author of that paper,...and with some very interesting links....WOW


Dear Brian,

Thanks for your email, I have been fascinated to discover that there is such an active and enthusiastic community discussing interesting research topics.

I am indeed an author of this paper (Fossati, et al. 2008, HPYD3). I was in my first year of my PhD and Fabio Fossati and Alberto Zasso were my supervisors. As you might know, sadly Fabio passed away last year.

It is important to remark that we have not performed these tests to demonstrated the superiority of unconventional sailplans, but to compare few different realistic sailplans for a specific yacht; the tests were commissioned by Vallicelli Yacht Design and Co. I don’t think that from these experiments we can draw general conclusions such as that two jibs are better than a jib and a mainsail. Unfortunately the design space is very wide and, as such, it cannot be fully explored. Also, there are very large non-linearities in sail aerodynamics (e.g. laminar-to-turbulent transition, separation and reattachment, etc.) that lead to local minima. Therefore, while a configuration can be better than an other one in some conditions, it is likely that for a small difference in one geometric parameter (e.g. the sail camber, position of the tack, etc.) the ranking between the two configurations is reversed. As sailors we know how difficult it is to rank two different (well designed) sails; to rank two different sailplan concepts is even more difficult.

The tests that we performed are far from perfect, and I have specific concerns on the CFD simulations because of the very low grid resolution that I used at the time. However, regarding the concerns that were raised by Daiquiri, the naked mast and the foot of the mainsail are accounted for in the tests. In fact, the aerodynamic force coefficients are computed measuring the forces with the sails, subtracting the forces without the sails (including the naked mast windage), and dividing by 1/2 rho U^2 S (where rho is the air density, U the nominal wind speed at 10m height at full scale, and S the sail area, which includes the additional sail area mentioned by Daiquiri). The windage subtraction is source of large uncertainties in the results, but unfortunately there is nothing else that can be done for wind tunnel tests. Conversely, the normalisation with the sail area allows comparing the efficiency of two sailplans irrespectively of their size. This is an important aspect to take into account in the design because if there are geometric constrains (e.g. the maximum distance between the tack and the sheet block, as in this paper), the two sailplans might not be scaled to have the same sail area. Therefore there are two different conclusions to draw: (1) which is the most efficient sailplan. (where efficiency can be defined in different ways such as, for instance, which one gives the highest Cx - see figs 14-16 of the paper), and (2) which one is the sailplan providing the maximum boat speed (e.g., in first approximation, the highest CxArea - see fig. 17). While the results show that the unconventional sailplan is more efficient than the standard sloop, the latter has the potential to generate much higher boat speed in light wind conditions, where the penalty of a high CMxArea does not play a major role.

I hope this clarification helps and I invite you to post it on the blog on my behalf. I also take the opportunity to point you to two other websites where you can download many relevant papers:

http://goo.gl/PyioaM

http://www.sailingfluids.org/downloads

For those who have not read it, I suggest "Recent advances in sailing yacht aerodynamics” (http://goo.gl/ZFWRJB)

Best regards,
Ignazio Maria Viola

Dr Ignazio Maria Viola, Institute for Energy Systems, School of Engineering, University of Edinburgh www.homepages.ed.ac.uk/iviola

 

Excellent job Brian, thanks for contacting Mr. Viola.
However, putting aside the technical aspects of his reply, right now I feel like commenting just one small passage which has struck me as soon as I read it - this one:

I didn't know this, I remember that last year I was still reading about Mr. Fossati's involvement in various research projects regarding sailboats.

This is a huge loss for the sailing community.
Fabio Fossati was the vanguard of the modern research of sail aerodynamics. He has given a remarkable contribution to our knowledge about the physics of sailboats, and his work was one of pillars on which the evolution of modern VPP methods and software tools stood.

Very sad news indeed, that man will always have my greatest respect.

P.S.:
Found more about it here:
http://www.nautechnews.it/2015/12/0...archer-and-itc-member-of-the-orc-passed-away/
http://www.sailingscuttlebutt.com/2015/12/03/eight-bells-fabio-fossati/

 

Recent Advances in the Study of Sailing Aerodynamics

I just read (tried) thru this document rather quickly, and found it very interesting (albeit above my 'payscale'...ha..ha). I'll just post the 'Conclusions' portion.
Very interesting what he has to report on the various hi-tech wind tunnels around the world.

 

Fully Understanding Aerodynamics,..Witticism

1) "I am an old man now, and when I die and go to heaven there are two matters on which I hope for enlightenment. One is quantum electrodynamics, and the other is the turbulent motion of fluids. And about the former I am rather optimistic."
- physicist H. Lamb, 1932 address to the British AAS


2) Henri Coanda's old experimental work on boundary-layer attachment was marginalized, even ridiculed, rather than merged with the rest of aerodynamics or included in college textbooks. Air is nonlinear, with no simple math solutions which simply explain either flow-attachment or turbulence. As a result, a big piece of aerodynamics concepts is missing.
After he dies, Nobelist W. Lamb supposedly was hoping to "ask God" how turbulence works. He could have instead asked God an equivalent unsolved question: how do wings really work? Wings work by creating vorticity from nothing, which is also the signature of turbulence. The lifting force is inescapably a product of turbulence, of nonlinear vortex-shedding, so many experts turn away.

 

Why? Jibs on America’s Cup Catamarans

Why? Jibs on America’s Cup Catamarans

If those mighty wing sails are so efficient, why are they putting a jib out front?

…particularly when sailing upwind, close hauled (which they do very often when creating those conditions with their big foiling speeds) ??

Why would they put that ‘soft sail’ out front? (not the code sail, but the jib)

(ggod little video clip of wingsail rigs....https://www.youtube.com/watch?v=hbPTnF6liF8)

 

so they will tack faster and...get a sailmaker to sponsor them

 

?? I don't see these jibs being utilized any any tacking maneuvers ??

 

When off the foils they will need them
Boats would be faster and point higher without jibs

 

So may I ask a simple question; do you believe that the top sailors and designers choose the most efficient rig option, or not?

To be honest, it seems that it's not all that useful to make a big deal about the classes that use sails that support your concepts, but then ignore the classes that don't support your concepts. There is an enormous variety of rig types among the fastest boats and an enormous variety among the most popular boats. Anyone who points to a rig as evidence of a claim they're making can just be countered by someone pointing out an equally efficient or popular class with a totally different rig, can't they? Isn't the important lesson therefore the fact that different rigs suit different craft, and therefore that pointing to any one rig on any one type is of little value?

PS - actually every time I look at the enormous variation in the rigs of the most popular and fastest classes it seems to be proof that people aren't actually conservative when it comes to rig design as so often claimed. I think the list of the half-dozen most popular classes includes only one conventional sloop rig. When the list of the most popular sailing craft includes sprit rigs, wishbone rigs, wing masts, freestanding rigs, conventional sloops, lateen rigs and vertically-furling boomless fathead rigs, it seems that any claim "people won't buy my rig because it's too different" is rather far-fetched.

 

" do designers give you the best rig"
For what?
A moth will blow anything away to windward
A volvo boat goes to windward like a hobie 16..as not designed to do that, neither rig or hull
M32 match races all wish they had jibs but the boat wasnt designed to do what its doing now.
I dont know of any yacht racing that doesnt have design rules

 

"For what"? Sorry, I accidentally edited out the brackets where I pointed out that there were many different definitions of efficiency.

Yes, different rigs and craft perform very differently in different conditions, which is why we can't just look at any one type on any one course and draw very wide conclusions.

Speed sailing and one design centreboarders don't have rules that restrict the designer in the choice of basic rig type, and many of the yacht rules are a lot more open than some people like to admit. Classes like Open 60s (remember the ketch fad years ago?), shorthanded multis, Transpac rule (won by a ketch in 2013), ORR and IRC (where ketches can win) and others are pretty open.

For some reason, there's a strong tendency for people to claim that the rating rules are a lot more restrictive than they really are. People claim that (for example) IRC and the old RORC rule banned rotating masts (not true) and that sloops have always been favoured, when rules regularly give enormous allowances to other rigs.

 

All true CT but as its all art and not science its constantly evolving and both the rig and hull influences eack other and they are both evolving.