A Sail Question

Rcsailing.net General Discussion
21

Thanks Dick
I already have SailCut and have used it. The problem is going from program to paper. To print to your printer it breaks the bottom two panels in half. Joining them back together introduces error as does the printer itself. The only laser cutters I have found in town are for sign making and they will only cut something with a backing. So I was stuck. I thought I would hand make a few sets to help me learn what is actually happening when I point and click. I haven’t given up on SailCut, it seems to be a good program, And when I find someone with a cutter that will work I will try it again.

Thanks
Don
Vancouver Island

22

<blockquote id=“quote”><font size=“1” face=“Verdana, Arial, Helvetica” id=“quote”>quote:<hr height=“1” noshade id=“quote”>And when I find someone with a cutter that will work I will try it again.<hr height=“1” noshade id=“quote”></blockquote id=“quote”></font id=“quote”>

Hmmm… seems to me there is a laser cutter in our shop. I think there is a few hundred yards of material in there as well.

Peter R.
www.climatemodels.com

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23

<blockquote id=“quote”><font size=“1” face=“Verdana, Arial, Helvetica” id=“quote”>quote:<hr height=“1” noshade id=“quote”>Greg
Bear with me a sec. I can be thick. If I were to make two sails identical except for luff curve and hang them both on straight masts would not the one with more luff curve hang with more twist?<hr height=“1” noshade id=“quote”></blockquote id=“quote”></font id=“quote”>

Greg
I think I was wrong. I just laid out my sail templates on the kitchen table and started moving them this way and that and I think that more luff curve causes less twist. I thought I would post this just in case somebody took my ramblings for fact. If you put positive luff curve on a sail and then straighten it out you force material into the center area (causing increased draft) but you can’t push more length into the leach so it is going to be shorter in respect to the luff causing less twist. Now think about a jib that is cut with negative luff curve. When you are setting up on the stand the jibstay is tight and you are pulling the luff straight which puts twist into the sail. Then you static adjust your jib twist and put the boat in the water. The wind hits the sail, the jibstay sags which adds draft and reduces twist. Maybe a person should add a little more twist when the boat is on the stand. Sorry for the rambling, but it helps to <s>right</s> write it down. And maybe some one will correct me if I’m wrong.

Thanks
Don
Vancouver Island

Edited to spell write right.

24

Alright - I have another question regarding sails. I have been using the program that Dick mentioned (Sailcut) to design a set of sails. The question I have is regarding sail draft. I have found elsewere on the Web that a good beginning draft would be at 40%. That being said Sailcut allows you to modify the sail depth (chord?) from top to bottom. Is there reasoning behind this? Do you want a sail that reaches its fullest depth in the top, bottom or middle? Or would you want a consistant depth? I am hoping that someone could help me understand this a little better.
Thanks in advance for your help,

Tom
Seawind #80

25

<blockquote id=“quote”><font size=“1” face=“Verdana, Arial, Helvetica” id=“quote”>quote:<hr height=“1” noshade id=“quote”>Originally posted by TWilliams

Alright - I have another question regarding sails. I have been using the program that Dick mentioned (Sailcut) to design a set of sails. The question I have is regarding sail draft. I have found elsewere on the Web that a good beginning draft would be at 40%. That being said Sailcut allows you to modify the sail depth (chord?) from top to bottom. Is there reasoning behind this? Do you want a sail that reaches its fullest depth in the top, bottom or middle? Or would you want a consistant depth? I am hoping that someone could help me understand this a little better.
Thanks in advance for your help,

Tom
Seawind #80
<hr height=“1” noshade id=“quote”></blockquote id=“quote”></font id=“quote”>

I think this is one of those things that depends on the sailmaker. I think I remember Tom Whidden in “The Art and Science Of Sails” (you should read this book-it’s very good)saying that the draft should get deeper towards the top of the sail. I have read just the opposite elsewhere.[:-banghead] It may be that at AC level they use more draft in the top because they’re good enough to use it. Much like the draft, the 40% is kind of standard but the AC boats may move it back to 50%. The boat points higher but if your steering is off by more than a couple of degrees the boat goes slow. Someone else will probably have a better answer.

Thanks
Don
Vancouver Island

26

Tom,

you need to be careful with your terminology. The chord of the sail is the straight line distance from the luff to the leach at a section you are interested in examining. The draft of the sail is the amount of curvature in that section measured as a ratio of the maximum distance of the curve from the straight chord line. The max draft position is the distance back from the luff where the max draft occurs (again measured as a percentage of chord length).

A typical draft for a sail might be 10% to 12%. A typical position of the max draft might be 40% to 50%.

As far as the amount and position of draft at the foot of the sail versus the top of the sail, I too have heard different theories on this. I think al ot of it depends on twist. when you add more draft or move the draft further forward, you increase the entry angle. this means you need to sail at higher angles of attack (of the sail relative to the aparent wind) in order to align the wind at a positive angle relative to the entry of the sail. If you then add twist as well, then you need to head down even more…

Take a look at this: http://www.onemetre.net/Download/Entry/entry.htm

That helps explain how the entry angle effects pointing and how the draft, draft position and twist affect the entry angle.

As you know the optimal VMG is not always achieved at the highest pointing angles. In lighter winds, the speed you gain by footing a bit will more than make up for the lost windward progress. but in higher winds, you can pinch up and make better VMG than if you foot. So in heavier winds, you need to reduce your entry angle so that you can pinch up. This means more outhaul to create a flatter sail and may also mean adding some mast bend to flatten the sail in the middle.

I know I did not answer your question (in terms of giving you a number you could put into your sail cutting program). The best advice IU can give in that regard is to measure some sails that you think are fast and copy them. If you have a digital camera this is easy to do with this program: http://uksailmakers.com/accumeasuredl.asp

Using this program you can measure the foot, middle and top of the sail and calculate the draft, draft position and twist. I use this program quite often when I am playling with my rig tensions to get the best shape I can. But these numbers can also give you a good starting point for a new sail design. From there, you are on your own and need to experiment with more draft or less, more twist or less, draft forward at the top of the sail versus draft back and so on.

Good luck!

Will Gorgen

27

Thanks Wil,
As usual your replies are very helpful. Those are the answers I was looking for.
Tom

Tom
Seawind #80

28

I went to the library and checked out Tom Whidden’s book again (third time) to check my post. The biggest reason he gives for low draft at the bottom and high at the top is to reduce induced drag at the bottom of the sail. Less lift=less induced drag. Also to tie in with what Will said if there is more draft at the top than at the bottom it twists the entry angle which helps make up for the change in apparent wind without twisting the whole sail. He also talks about the vertical change in draft somehow emulating the shape of a Spitfire wing (elliptical)that I’m going to have to read a few more times to understand.

Thanks
Don
Vancouver Island

29

Don,

Tom Whitten is talking about elliptical lift distribution when he refers to the spitfire wing. It has been demonstrated that the lowest induced drag results when the lift on a wing as a function of span follows an elliptical distribution. I can point you to a dozen aerodynamic textbooks that will allow you to calculate that for yourself, but the background material needed to understand those equations would require several weeks of careful study (lifting line theory, vortex shedding and so on). Take a look at this page for a primer on the subject from Lester:

http://www.onemetre.net/Download/Downwash/Downwash.htm

The bottom line of all that is that you want the lift on the sail to be distributed in an elliptical form. Most sails are of course triangular shaped rather than elliptical shaped. Lift at any section of the sails comes from camber (draft) andgle of attack (twist) and area (chord). Since the chord distribution is triangular, if you keep the camber and angle of attack constant over the sail, then you will get a triangular lift distribution. In order to turn that triangular lift distribution into an elliptical distribution, you would want to add camber or angle of attack to the smaller sections (the top of the sail) and reduce camber or angle of attack in the bigger sections (the foot) just as Tom Whitten suggests.

The problem with this theory is that sails also create heeling moments. By switching to an elliptical lift distribution, you move more lift higher on the sail which results in more heeling moment. Since you have a limited amount of righting moment at your disposal, you would need to reduce the overall sail lift to keep the heeling moment under control. This would result in a lower overall driving force and of course you would be slower.

So, the optimal sail shape for overpowered conditions often reduces the lift at the top of the sail while maintaing lift at the bottom of the sail in order to reduce the heeling moment. This results in more induced drag (due to the non-ellipticality of the lift distribution) but since the overall lift is greater you can live with higher drag because you still come out ahead in driving force. This is why you see guys adding twist as the breeze comes up.

In lighter winds when the righting moment is not an issue, you would want to try to maintain the elliptical lift distribution. In heavy winds you can deviate from the elliptical lift distribution for the sake of more sail drive with less heeling moment.

Of course many RC sails are closer to an ellipse than a triangle (US1M, 36/600, M, 10R, etc). These sails are shaped that way to try and get closer to the elliptical lift and thereby reduce the induced drag. IACC boats with their high roach sails also have more elliptical shapes than a traditional triangular shaped sail. So Tom Whidden’s conclusion is not universal…

Hope I didn’t make things muddier in my attempt to clarify…

  • Will

Will Gorgen

30

I think whats confusing me is that even though the lift distribution is elliptical the sail itself is still triangular. How does the air know that it’s going around an ellipse and not a triangle? Is induced drag not caused by the sail itself or is it air running into air that causes the drag? I’m not being very clear- I hope you can get my drift.

Thanks
Don
Vancouver Island

31

What is eliptical is the magnitude of the lift versus the span of the sail (foot to head). In other words as you move from the foot of the sail to the head of the sail, the lift at any section of the sail will decrease. You do not want the lift to decrease linearly (1/2 way up the sail you have 1/2 the lift) bt rather you want to lift to decrease as if you were following the curve of an ellipse. Gradual at first and then steeper drop off near the head of the sail.

Here is a figure out of Marchaj showing what is meant by a lift distribution, and in fact an elliptical lift distribution:

Download Attachment: ellipticallift.jpg
74.23KB

As you can see in this figure, there is more lift near the center of the wing than there is out a t the tips. The lift decreases as you get closer to the wing tips. But it does not decrease linearly. Rather it decreases lsowly at first, then more quickly near the tips so that the distribution looks like an ellipse.

Ignoring the foot gap, a sail is “half a wing” so the lift will be greatest near the foot and drop off to zero at the head.

Don’t let the fact that the wing in that figure is shaped like an ellipse draw you to any conclusions about what shape sail you need. Remember that you get lift at any point along the span of the wing or sail from 3 factors:

  1. total chord length

  2. camber (draft)

  3. angle of attack

So if your chord length at any particular spanwise location is shorter than an “elliptical” wing, then you need to generate more lift by adding either camber or angle of attack at that location.

So it is not about “the wind knowing that it is going around an ellipse”. It is about adding lift in some parts of the sail and taking away lift in other parts of the sail so that the distribution is elliptical.

This is why Whidden suggests that you need to add more camber at the top of the sail. The top of a normal triangular sail is shorter in chord than you need to produce an elliptical lift distribution. so by adding more camber near the head you can add more lift there to make up for the lack of sail area.

Again, I don’t feel like I am being clear. This is a third or 4th level abstraction from the air movement. First you need to understand how the wind creates lift (2-D aerodynamics). Then you need to understand what happens when you have a finite span wing (3-D aerodynamics). Then you can optimize that wing to get the minimum drag. It is really hard to jump in with both feet at the deep end of this pool without having waded into the shallow end and leaned about 2-D and 3-D aerodynamics first.

Take a look at Lester Gilbert’s treatment of aerodynamics:

2-D Aero: http://www.onemetre.net/Technicl/SailSect/SailSect.htm

More 2-D Aero: http://www.onemetre.net/Download/Downwash/Circul/Circul.htm

3-D Aero: http://www.onemetre.net/Download/Downwash/Momentum/Momentum.htm

And finally, more 3-D Aero: http://www.onemetre.net/Download/Downwash/LiftLine/Liftline.htm

Once you have gottne the jist of lifting line theory, come back to the figure I posted here and it should make more sense…

  • Will

Will Gorgen

32

Thanks Will, it’s sinking in, just slowly!

Thanks
Don
Vancouver Island

33

Will: Do you make your own sails. You do know what you are talking about!

34

O.K. Reading the circulation theory he talks about circulation around the box HK. He starts at the top left and the particles travel along the top, down the back, along the bottom and then he says that up the front they have 0 speed. How is this? Why would they stop? Wouldn’t you get an awfully big pile of particles in the lower left hand corner of the box? Just a little confused. When I took Grade 11 Physics they introduced a new course and the whole class flunked. I never took it again.

Thanks
Don
Vancouver Island

35

Hoj,

I do not build my own sails (I don’t have the patience for it). However, I do design jet engines, so that has to count for something… I majored in aerospace engineering (Bachelors and Masters) with a concentration on aerodynamics and structures at MIT. After spending that much money, i should know what I am talking about…

Don,

Only the vertical velocity is 0. The horizonatal velocity is not zero (flow is still going through the box from left to right). Lester has actually simplified this point a little too much.

When you study circulation theory, you have to realize that the “circulation” around the airfoil is in addition to the bulk flow from left to right. So you need to add together an undisturbued left to right flow with a circulation flow to get the total flow field. So when the flow goes faster than the bulk flow across the top of the airfoil and slower than the bulk flow along the bottom of the airfoil, you can think of this as being the bulkflow plus some velocity on the top and the bulkflow minum some velocity on the bottom. Then if you ignore the bulkflow, you would see the circulation flow. Hope that helps…

  • Will

Will Gorgen

36

<blockquote id=“quote”><font size=“1” face=“Verdana, Arial, Helvetica” id=“quote”>quote:<hr height=“1” noshade id=“quote”>Originally posted by Don

[…] he talks about circulation around the box HK. He starts at the top left and the particles travel along the top, down the back, along the bottom and then he says that up the front they have 0 speed. How is this? Why would they stop?<hr height=“1” noshade id=“quote”></blockquote id=“quote”></font id=“quote”>
Hi Don

As Will says, all of the air is basically flowing over the foil with a speed of V. The “circulation” flow, if any, is superimposed upon this. To figure out the amount of circulation flow separate from the “basic” flow, we inspect the movement of the particles as they pass over the foil, and ask if, in addition to their basic speed V horizontally, they show any movement vertically. They show some downwash velocity W after they have passed over the foil, but (in principle!) show no appreciable disturbance at some appropriate distance ahead of the foil. It is not that the particles have stopped, they are still moving over the foil, but that they are not being deflected (yet!) when we look at them ahead of the foil.

Lester Gilbert

37

Thank you guys! I’ve got it now-I think. Any difference in velocity(top vs. bottom is only made up at the back of the foil because the air at the front hasn’t moved yet.

Thanks
Don
Vancouver Island

38

[quote]Originally posted by ClimateModels

Just like a master mechanic who can tune every last horsepower out of an engine but will not tell anybody else how he does it, some of us make a living by experimenting and perfecting sail designs. Giving it away does not put food on the table for our families.
Also, if I am racing against you, why would I give you my design secrets? [:D]

This is really a poor answer and I find, all to often, that this is the attitude of most good sailors from sail design to tactics to anything sailing. This attitude is what keeps us from getting many new, good sailors. Even if you don’t want to share even SOME of your knowledge, you could at least point to where you learned from (books, videos, mentors, etc.) instead of keeping it all to yourself. If somebody hadn’t told you where to look at some point, you wouldn’t be making sails today so why would you deny others the same courtesy?

Mike~

39

See below.

40

Mike. Please don’t make posts about a person unless you know them.

See the little smiley face at the end of the statement?
That means I am pulling your leg.
Anyone who has ever asked a direct question from me has always gotten a direct answer. Ask anyone on this board who knows me, they will tell you the same thing.
FYI: I have started 3 different clubs (all still very active and with over 40 members each) as well as helped hundreds of people get started in this great hobby of ours. My compnay has given away more kits and products than most clubs will see in their lifetime.
Our own board moderator has a free set of sails on his IOM that I gave him.

The original post by Don asked if it was a CIA plot or if sailmakers were sworn to silence by their guild.
See the sarcasm there?

Lighten up buddy. Go sailing. Relax.[:-spin]

Peter R.

[/quote]

Visit www.climatemodels.com