sail aerodynamics

[Schoonertack]

I am not there, but, the sail has two sides, entry and exit as you are thinking about it. So before you buy a new main to match the cut and CAMBER of your new jib, see if you can adjust your jib sheet angle to flatten the roach or leech of the jib and likewise when you are pointing that high flatten the back half of the main with outhaul and boom vang. let me know if that helps. As someone else said on this thread the big boats all sail with the main slightly backwinded by the jib/jenny/ etc.. This does not indicate air flow through the slot.

[Javelin]

I see what you are driving at however what I am trying to define is how do you know when the slot is at optimum.
Using the tell tails helps set individual sails.
Using the leach tails on the Genoa and main helps get the twist and exhaust right.
Getting the slot right has largely been done by eye, dipping under the boom and adjusting the sheet lead to match the back of the main with the Genoa Leach.
The width of the slot is harder the set though.
What is the critical factor.
Do you narrow the distance between the Genoa leach and the main until the main just luffs or luffs a lot or until the lower Genoa leach tail starts going mad or etc etc.
Like I said tell tails are an easy indicator, what is the "easy" indicator for slot efficiency?
If we are getting drive because of the pressure differential then slowing the air flow down through the slot should increase the differential, however it is obvious that at some point you will go to far.
What is that point and how do you measure it?

[Schoonertack]

Javelin, wish it was that easy. just like a sails camber depends on wind speed, the slot does too, and the only reliable way I have ever heard of dertermining all of this is match boat sailing and changing only one thing at a time. Which is the whole point of club sailing. The reason America's cup has become so expensive is that you really need two identical boats to see what combination is fastest on any given wind speed and heading. Years ago when I first started sailing I had a hard time understanding mainsail camber. the controlls are easy enough but why in light airs, little camber, increase for a little more breeze and then as wind speed picks up further continually flatten the leach? I have not heard of any reliable indicator of slot efficency, leech telltales flapping indicates you have seperated the so called boundary layer, and a soft spot in the main is supposed to indicate the same, but what is not said is that air flow even on laminar flow wigs is not completely laminar without boundary control devices, air bleeding, pumps etc. The flow does seperate from the airfoil does become turbulent, but that does not matter. You are looking for lift to drag ratios and in a boat lift drag/heeling forces. That still means time in a boat to find the right combinations. Have fun

[Alixe]

But how come a mainsail is triangular, a jib has another different shape and a genoa too?

Alixe

[redcoopers]

It's not necessarily...

Gaff rigs - a quadrilateral - were used (almost) exclusively until the development of the marconi - triangular - mainsail. While gaffs have more sail area than a marconi rig, it can be more difficult to obtain the required leech tension while sailing upwind. Furthermore, you have an extra "halyard" to pull up the gaff.

I'm unsure, but I think that currently, most racing rules discourage the use of the gaff. For a pure racing boat, it would be used only to gain extra downwind sail area.

Anyway, it's a lot more complex and can't go to wind as well as a marconi. However, it sure is an absolutely beautiful sail... My view, is that it's almost sinful to equip a classic boat such as a bristol channel cutter without a gaff

-Jon

[Javelin]

I sail at a reasonably high level in my class and the opportunity to sail against others in the top 3 or 4 in Europe is limited to competition only!

Experience with two boat tuning was certainly beneficial when sailing mid fleet where the gains are much more apparent but gains now are far harder to come by.

If the Genoa is over or under sheeted by just 1 inch upwind we lose out. So sheets are marked and fairlead positions are all callibrated so we can find a fast setting. However fear of losing tends to stop us experimenting too much. If however we fall over some data that shows us that in theory at least X is worth trying I'll go for it.

Given a set camber, a set wind speed of say 5 knots, a set angle of incidence etc, you should be able to measure the lift and the corresponding airflow around the sail and therefore the differential in windspeed between the windward and leeward sides.

If this is true then surely it is possible to do the same with varing slot widths to identify at least the optimum width at x speed, x angle, x camber, x overlap etc.

If this were done over a range of conditions then it follows that data would be produced which could be used as a good starting point and an indicator of how far you are from the theoretical optimum.

[Ian]

Actually, gaff sails don't have to set with excessive twist. In recently designed gaff rigged boats the mast is usualy too short, so the peak halyard has a very unfavourable angle with respect to the gaff. Gaffers that carry topsails usualy have enough mast length, even discounting the top-mast, and the sail sets well. Acording to wind tunnell tests (marchaj) the gaff sail planform is superior off the wind. Marconi rigs are moving in the direction of getting rid of the useless pointy bit at the top of the sail. An eliptical or rectangular planform is superior. Of course, when a marconi-rigged boat is off the wind, it has a wide choice of large downwind and reaching sails to hang off that tall mast which much more than makes up for the inferiority of the main.

re:

Originally Posted by gonzo

...It makes the air flow separation on the leeward side of the main shift aft.

reply: Am I right in thinking that this shift in air flow separation would effectively stop the mainsail from stalling, allowing it to produce a propulsive force?

regards, ML

[brian eiland]

From the archives of the multihull forum <http://www.steamradio.com/pipermail/multihulls/>
I retrived this posting by Tom Speer:
_________________________________
Tom wrote:
There's been discussion in the past here about the flow around sails and the wakes they shed. I just ran across this picture,

<http://www.ship.saic.com/pictures/acup-flow.jpg>

of the flow around an IACC yacht. Although it doesn't say exactly, I believw what you're looking at is the velocity on the sail surfaces and in a vertical plane behind the boat. The three lines at the bottom show traces of air particles approaching from near deck level.

What I found striking was the dark blue regions in the wake indicating the vortices shed by the rig. The upper one is more in line with the hounds than it is with the head of the main. There's also a concentrated, powerful vortex shed off of the foot of the rig, too, with a substantial wake extending some distance up from the boom.

We're looking at the windward side of the sails, and it's interesting to see yellow and green colors on the jib in the slot, indicating that the flow there is actually slower than freestream, not faster as is often believed. All in all, it's a cool pic
________________________
Brian writes:
This referenced site now leads to a short movie clip. I don't seem to remember this from the past, but rather just a couple of still photos, and in particular a close up of the 'hounds' area. Am I wrong Tom, or was it another photo I am thinking of that concentrated on the wake from the hound area??

At any rate, while I was reviewing a considerable number of papers, including A. O. Smith's "High-Lift Aerodynamics", and Marchaj's "Aero-Hydrodynamics of Sailing" to come up with a few other aerodynamics of sails forum submissions, I ran across this site again and though it might be of interest to some on these two forums.

PS. Its nice to have the archive section of the 'steamboat' multihull forum back up and running again. And the new format of the BoatDesign.net forum is very nice as well. The search feature in the BoatDesign forum is particularly nice!

[brian eiland]

Opps, that movie site is at <http://www.ship.saic.com/overview_fans.html>

[brian eiland]

Here is another interesting visual site:
<http://www.wb-sails.fi/>
and go to the "streamlines and swirls" under the article section.

Also click on the animation in the first, second, third simulation.
Did the 3rd dimension (vertical) of flow just force its way in on the 2 dimensional analysis under which we ordinarily consider such flow?? In other words we have in the past generally restricted our thinking to the 'plane' of flow parallel to the sea surface, choosing to ignore these 'minor'(?) vertical flows.
I am concerned with some of this vertical movement in relation to the narrowing sail 'slot' with increasing mast height while the apparent wind is ever increasing with this mast height. This fact has always concerned me with the traditional Bermuda rig, and is one I try to address with the parallel headstays on my mast aft rig.


Here's a 'full scale' wind tunnel:
<http://www.wb-sails.fi/news/470StreamAnim/V60.htm>
I know this one has been mentioned previously, but I also have another question about it. Does someone know the actual sail combinations being utilized by the V60's at this moment?? Was it there masthead Code 0's as I suspect??

[dougfrolich]

very interesting thread! I reall do not have anything to add.

[brian eiland]

Originally Posted by Javelin

I sail at a reasonably high level in my class and the opportunity to sail against others in the top 3 or 4 in Europe is limited to competition only!

Experience with two boat tuning was certainly beneficial when sailing mid fleet where the gains are much more apparent but gains now are far harder to come by.

If the Genoa is over or under sheeted by just 1 inch upwind we lose out. So sheets are marked and fairlead positions are all callibrated so we can find a fast setting. However fear of losing tends to stop us experimenting too much. If however we fall over some data that shows us that in theory at least X is worth trying I'll go for it.

Given a set camber, a set wind speed of say 5 knots, a set angle of incidence etc, you should be able to measure the lift and the corresponding airflow around the sail and therefore the differential in windspeed between the windward and leeward sides.

If this is true then surely it is possible to do the same with varing slot widths to identify at least the optimum width at x speed, x angle, x camber, x overlap etc.

If this were done over a range of conditions then it follows that data would be produced which could be used as a good starting point and an indicator of how far you are from the theoretical optimum.

_______________________
Brian replied:


I have noted your postings, and particularly your sailing at “a reasonable high level in my (your) class.” With that sort of a comparison available, I think that we all should make an attempt to solve your problem, answer your questions, and consider your observations about that other competitor’s ability to outsail you to windward. We might all lern something in the process, both practical and theoretical.

Would you happen to have a copy of, or access to, a copy of Tom Whidden’s book “The Art & Science of Sails” ?? On page 96 he presents an interesting dwg by Arvel Gentry that depicts several different examples of what happens to the wind flow in the sail slot, as well as shifts in the stagnation streamlines when the relative settings of the main & jib sails are changed. Whidden’s summary at this point, “Addressing the big picture, a correctly trimmed headsail slows the wind in the slot JUST the proper amount, so the air on the lee side of the main does not separate when it is trimmed at a tight angle. A correctly trimmed main places the jib in a lift, meaning the boat can be pointed closer to the wind without the jib luffing."
(I could attempt to add this dwg to this posting if I could figure out how to do it)

Am I correct to understand that you bought both a new jib and a new main?? Do you still have the old main?

Can you describe the shape of these sails?? Both old and new? I would be looking for the geometry at a minimum of three heights; 25,50, & 75%:
a) the camber (expressed as a % of the local sail chord)
b) the position of max camber (% of local sail chord)
c) the twist (expressed in degrees relative to sail foot chord)
d) the entry angle
e) the exit angle

I would also like to know:
a) how the jib sail is attached to the forestay, and any variations existing in the class?
b) the thickness and shape of the luff reinforcement (rope, etc)?
c) and any class variations allowed/existing there?

Did you notice anything different in the construction of the leading edge of your competitor’s jib sail?

I assume you have visited the ‘Quest for the Perfect Shape’ at <http://www.wb-sails.fi/> ? I don’t agree with all of their observations, just most of them.

Hope we all can participate in resolving your competitiveness

[brian eiland]

Subject: Sail Aerodynamics, Sail Wake
_________________________________________________________________
Tom Speer wrote in the steamboat multihulls forum:

> Taking a second look at it
> <http://www.ship.saic.com/pictures/acup-flow.jpg>
I'm not sure what's shown on the back plane. Whatever is shown on that plane is probably different from what's shown on the sail - and I may have been mistaken in what I said earlier. You can see the boundary layer in the wind at the water surface.
It can't be static pressure, because static pressure would be essentially
constant from the water up, away from the boat . I don't think the colors
show the velocity magnitude because there are high velocities near the
trailing vortices, so the velocity should increase, then drop off at the very
center. Dynamic pressure drops off to zero at the water surface and there's a
total pressure deficit in the core of the trailing vortices, so dynamic
pressure is a possibility. So total pressure would be a good guess.
> The component of the velocity in the true wind direction is another
> possibility.
>
> The caption on the FANS page mentions pressure, so the sail colors could be showing static pressure. If that's the case, then there may be no
correspondence between the colors on the sail and the colors on the back
plane. And my remarks about the flow in the slot being slower than
> freestream could not be justified by the evidence in the picture.
______________
Brian responded:
Just in case there is some confusion over the still photo verses the movie I
would refer the viewer to either:
<http://www.ship.saic.com/overview_fans.html> and click on movie
<http://www.ship.saic.com/pictures/aone_small.mov>

What I am finding in my review of some of this computational methods of
investigating sail forces and flow analysis (CFA, CFD, vortex lattice models,
etc) is that generally there are so many assumptions made upfront in order to
simplify the equations so the computer can solve them, that the results get
skewed quite a bit from reality, ie, a quote from one of the annalist;

"CFD = Computer Fluid Dynamics. CFD is a great tool for visualizing and
explaining flow phenomena. While the latest flow software is very powerful and capable of calculating amazing things at astonishing accuracy, the old saying "garbage in, garbage out" is more true than ever. Besides of presenting the problem in a meaningful way, one needs lots of knowledge and experience to interpret the results correctly. Simulation through CFD is especially useful at giving qualitative information - when it comes to quantitative results or hard numbers, you have to be even more cautious when drawing conclusions about the merits of one design over another. Wind tunnel tests are needed to calibrate and validate the CFD code before reliable results are obtained."

"With the power of modern CFD at the desktop, it is too easy to produce
beautiful pictures with little connection to reality. Often these pictures are
produced by flow experts with little sail-specific knowledge, and then
interpreted by sail designers without sufficient understanding of the CFD
tool, and as a result you get just that - pretty pictures."

Brian continues:
And take a look at the LACK of a trailing vortex off of the upper tip of the
mainsail....I don't think this is reality.

I also find interesting the totally screwed-up flow lines on the portion of
the mainsail above the hounds....certainly makes one question the
effectiveness of this sail area, AND the negative effect the vortices off of
the fractional jib have on the mainsail's upper portion. I believe both Tom &
I have questioned the fractional rig verses mast head rig subject in previous
discussions.
________________________________________________________________
Tom Speer continues:

> However, I think the important point is the huge vortex coming off the boom. It shows that the sail rig does not act like a a wing with double the
geometric aspect ratio of the sail rig due to surface effects, and the
> optimum planform is not a semi-ellipse. The vortex is also strongly
> affecting the region of the mainsail where the chord is the greatest.
> People usually concentrate on the vortex at the top, but the vortex at the
foot may be more significant. It's worth considering how to shape the
> mainsail so as to reduce the strength of the vortex and to move some of the sail area away from its influence. One answer is to use a wishbone boom and round the clew so the planform of the whole sail rig looks more like a
sail-board rig.
______________________
Brian replied:
This is just example where the analogy between an aircraft wing and a sail rig
differs quite a bit, "it shows that the sail rig does not act like a wing with
double the geometric aspect ratio...." And this is another example of where
theory and reality differ, particularly when trying to analyze sail rigs with
aircraft wing technologies. I intend to point out some other examples soon,
and raise them for comment.

Per Tom's observation about the boom end of the rig, and substituting a
'sailboard' type bottom, I not so sure that the aerodynamic gain could be
traded for the neccessity to 'shape' the conventional mainsail (vangs,
outhaul, etc) to operate properly with the non-parallel, sometimes fractional
headsail.

Pardon my plug for my unconventional mast-aft design
<http://www.runningtideyachts.com/sail/>
one might note that the flows off of the bottom of my 'mainstaysail' (and my
headsail as well) should not create as much negative vortex action in this
region. And note, I did utilize a wishbone boom on my mizzen sail to
accomplish what Tom has suggested as an optional boom arrangement for the
conventional mainsail.

[nibbit15]

Originally Posted by sailwave

I've seen the "slot effect" referred to as a "myth" recently but folk seem to sail fast by using it as a mental model... Is it a myth?

The slot effect is a myth. Don't get confused by all the aerodynamic models people try explain to you - most of them are degree level or beyond. If you want to sail fast then get a fast sailor to show you how to trim sails and if you really want to learn about aerodynamics or CFD then talk to students/graduates/lecturers from a university that does aeronautical engineering. Lastly if you want great advice on sail set up, talk to a competitive sail loft like North or Hyde.

Huw