Sail forces and sail plan ?

Ok, you have asked for it.

First of all, don't play a lawyer with me, please. It doesn't work with me because I deal with laws, lawyers and their verbal tricks and strategies on nearly daily base. The unfortunate collateral consequence of my job (engineering, engineering consulting and building yards management, before you consume your nails asking).
Do not deviate the discussion and do not distort my words just because you are unable to defend your point.

Second point. I have raced many sailboats, dear Rick, ever since I was 8 yrs old. Had a stop of 4-5 yrs at the age of 12 because I fell in love with swimming and waterpolo. But then I got back to sailing and windurfing and I still love it.

Now about you and your bleeding vanity.
Every further word you are writing here is making me doubt increasingly about your true understanding of one or several of the following topics:
- thin airfoil theory, it's numerical modelling, applications and limits;
- vortex panel method, as above,
- thin airfoils in real flows;
- different boundary layer models, how and when are they used, the underlying assumptions behind them and what are their range of validity;
- data processing, interpolation and regression analysis;
- extrapolation and risk inherent to various methods;
- sail L-D diagrams, their relationship with sailboat handling and choice of optimum sail angle with respect to the course;

I'm pissed off like hell because you have made me waste nearly 30 minutes of my life for replying to the crappy post you wrote above.

 

At this point, it's your problem.

 

1) You do realize the difference between camber and thickness, do you?
2) You are insisting that JavaFoil is perfect for the task, so it's up to you to demonstrate it. Happy googling.

 

daiquiri

"...Every further word you are writing here is making me doubt increasingly about your true understanding of one or several of the following topics:
- thin airfoil theory..."

Ahh, there is the flaw...'theory'.

Rick just uses computer programs which requires numbers for an input and numbers as an output, he is only providing a wall of numbers and coloured plots, as if that justifies the answer and position??? Anyone can provide an answer by tapping numbers into a program...

His replies demonstrate a lack of comprehension of the theory of the subject matter at hand, just punching numbers into a program requires no understanding of the theory involved nor whether the answer being given can be validated or not. Classic 'GIGO'.

Understanding theory is paramount when using software programs, even more so when using potentially dangerous software programs that are so easy to use it 'looks' like child's play!

Classical weak ploy when being asked to validate a statement, change tact and ignore the question by asking another unrelated question.!

 

Just decided to look at those fancy colour plots and the numbers.

Says Reynold number starts at 100,000.

Well, if this is the case the length of the sail at 10knots = 280mm at 20knots is 140mm and at 30knots 95mm!!

I don't recall a sail of 95mm being very good...hmmmm

 

I took the time to answer the original post with useful information. All you have provided is your penchant for water polo took you away from sailing. That is not very useful to the original request.

If you cannot be bothered to provide an answer or to show where a particular answer is wrong then you should go to the drivel thread. Because that is all you are offering.

In fact you even agreed that JavaFoil is accurate for the narrow range of operation that a windward sail would operate in.

When I see something of value that you can produce and you are prepared to put your name to your claims and opinions you might gain a little more credibility.

Rick W

 

"...If you cannot be bothered to provide an answer or to show where a particular answer is wrong.."

So, please explain why you are using a Reynolds number of 100,000, where does this come from?

 

I see you don't wish to provide where the numbers come from and why?

 

On the picture in post #13, shouldn't the sails be at an different angle in the flow field? I don't think that the soft sails can fill up in this direction. New physics?

 

The color coding shows the pressure. The "cold" colours meaning negative pressure and the "hot" colors meaning positive pressure. THere is a scale of the pressure shown on the right of the plot.

For the foils I have set, their positioning and the chosen angle of attack you will see that windward side has positive pressure. Possibly with the exception of the leading edge of the jib. This would indicate a luffing condition.

The model was not intended to demonstrate any prescribed conditions. It was simply provided as a quick example to show fcfc how JavaFoil could be used to put some numbers behind what he wants to do with sails.

fcfc can choose whether he uses it or not. Many others do and I believe it provides great insight along with some useful numbers. A good example of this is in this discussion:
http://www.boatdesign.net/forums/sailboats/sail-aerodynamics-457-11.html#post90633

At least it is something useful to advancing his understanding - better than what anyone else has offered.

Rick W

 

"...At least it is something useful to advancing his understanding - better than what anyone else has offered.."

Yes, i suppose sails that are 95mm in size, winds that defy the know laws of physics, numbers that have no meaning etc....all very useful to someone who also doesn't understand and wont question them.

Most of us here give fcfc credit for having intelligence and a naval architecture background.

Funny how mr numbers doesn't reply with more numbers when asked for an explanation of their derivation, just slience....or could it be just another 3m wave factor and one must just accept it because it has been spoken, so it must be so!

 

Rick;

I cannot find the terms "luffing limit" or "lift limit" in use in sailing. Can you please provide more information about what you actually mean by them, because I may be misunderstanding you; it appears as if by your terminology the sail when actually luffing is at its "lift limit"; I'd have thought that was the luffing limit????

Terminology apart, the fact is that skilled sailors spend a large amount of their time with the telltales NOT streaming. A few like to spin the lee telltales when "pressing the headsail" (to use US slang) at the low side of the groove, normally when accelerating.

Most helms (actually every skilled driver I've sailed with or against) sail with the windward telltales lifting or stalled in certain conditions. Therefore the performance of a rig at such an angle of attack is actually extremely important in terms of winning races.

The basic point is that practical experience demonstrates that the performance of a sail at other than theoretically ideal angles of attack is actually extremely important in reality.

 

The therms "positive pressure" and "negative pressure" would be alone a sufficient reason to make you fail any serious exam in Aerodynamics.

Regarding your post #36, I've given you a series of explanations on why JavaFoil cannot be used to perform a reliable analysis on a single 2D sail sheet, let alone extending that result to a complete, finite-length, masted sail.

But you don't want to listen, because you refuse to admit to yourself that you don't know everything about anything and that there are still many things to learn.
Or maybe you simply didn't get the point of my warnings about using the software tools for tasks different from what they have been made for, and outside of boundary where the physycal models used by the software are valid.

Remember that thread on inclined propellers? I've learned something new from you over there, Rick, and I had written it and gave you my thanks for that.

I'm plain HAPPY when I can learn something new, that's the main reason why I have joined this forum. In return to that gained knowledge, I'm offering my knowledge in aerodynamics - which is not small at all, though you are reluctant to accept that someone might know more things about some particular field than you do.

Let me end with a following citation - who knows why did it come into my mind...:

"There is no more blind man than the one who doesn't want to see."

 

Absolutey. If you examine Ad Hoc's Lift vs. Drag diagram in the post #17 and apply it to the actual boat course, you will immediately understand why is it so.

 

Exactly; that is what I stated earlier. In given circumstances the sails are used over a very wide range. They are flexible foils with a wide range of adjustment. The ability to sail on the edge of flow separation, either windward or leeward, will depend on the type of boat, the point of sailing and sea conditions. I was happy to have all tufts streaming unless it was very light wind and calm sea.

However to say that sails are very complex and operate over a wide range does not give any meaningful answer to fcfc's question. I have provided him reference to a tool that will give meaningful answers and considerable insight into multi-element sails. You do not need to know the maths behind it to use it effectively - the same as you do not need to know the maths behind a motor car to drive it. There are a lot of factors that come into play but the paper he referenced shows this in considerable detail - it does lack the method of getting numerical answers as it only references software tools that I expect are beyond the availability of most. JavaFoil will enable analysis to a satisfactory level of accuracy for comparison purposes.

Rick W