Alan,
Please stop … next I will have to make a semi-solid sail 
(actually, I like that idea).
I can make them, but I’m not sure that I can compare them. I think that I either need three boats and evenly matched sailors, access to CFD software or a wind tunnel.
Jim.
Sorry I will stop now … my workshop is full of ideas, but that what makes this hobby so interesting …never stop learning … have fun 
As discussed earlier in this thread, I’m thinking about methods to compare sail performance. I’ve come up with a possible method, and I’d appreciate some feedback.
Procedure:
The picture below shows the results of a test of 3 sail angles.
With the 3D nature of this arrangement I appreciate that it is not clear exactly what I am measuring. But can this method be used to provide a basic comparison of sail performance?
@Mij:
Interesting… your method effectively bypasses the mathematics involved and just displays the result, effectively returning X and Y values for the resultant force. Clever…
There would be some issues in the measurement, particularly when used for comparison. The main one that I think will give you grief is that you do not have an accurate continuous wind stream but rather a conical shape starting at the fan and spreading outward (both horizontally and vertically) the further the wing is from the fan. To overcome this, on a smaller scale blowing air through a large bunch of drinking straws would give you (more or less) parallel wind. For this scale, it needs to be much bigger of course. Maybe if your wind source was behind a couple of metres of (cardboard?) tube with dividers to break it up into (?) 6 inch squares. Maybe the packing box for a refrigerator - that sort of size…
Somthing like this perhaps…
http://www.rcgroups.com/forums/attachment.php?attachmentid=785766
Moving the fan further away and increasing the fan speed may also be of value
mrpenguin,
I agree. When the sail moves to a new position the nature of the air flow it is exposed to also changes, hence I’ve been getting some instability. I’ve been trying to avoid making a laminar flow system, but it looks like I can’t avoid it.
Jim.
Thanks, thats the words I was lookin for!!!
Another possible method:
This method tries to measure the force that can be translated into forward motion.
The diagram may be unclear so below is some footage of it in operation:
This method is more stable that the previous one and returns a single value. However, whether the single value it returns is useful, I’m not entirely sure.
Yip lost me ! I don’t see what you are doing has got to do with “sail performance” if it is “wind force” then the nature of wind is that it is never constant anyway, attached rough guide
The best “visual” way to tune sails for their optimum performance (and every sail cut is different) is to use telltales
This is where your set-up may be handy as it is hard to see teltales on RC boat when you standing on the shore line, using a anemometer you can check wind speed coming off the fan.
Once you have achieved optimum sail shape settings (camber, twist, luff tension etc.) at different wind speeds then the fastest & easiest way to compare “different sail performance” is to put them on the water and measure boat speed.
If there is another KISS way of doing this, I’m watching with great interest :rolleyes:
Cheers Alan
Ok Alan, I admit that I am floundering a bit. It just seems to me that short of “putting them on the water and measuring boat speed” there must be a way of comparing two sails. Even putting them on the water isn’t an ideal comparison as you have to either have the same sailor at different times (with different conditions), or different sailors with, potentially, different levels of skill.
As you say, telltales provide a good way of tuning a sail to its optimum, but can’t really be used to assess how it will perform against another sail. Even CFD isn’t perfect as there are apparently issues simulating detachment.
What I am trying to do is to measure the lift generated by a sail under different (controlled) conditions. Clearly, without access to a wind tunnel this is challenging, but I wonder if part of the reason that methods haven’t been developed to compare sails experimentally (other than racing or on-water testing) is that it is very difficult for big boats. Perhaps at a model scale there are methods that are simply not viable at full-scale?
I think that the reason that I am interested in this issue is that I would like to compare quite different sails, solid wings, soft wings, wing masts and normal sails, as we discussed previously. The differences are probably more subtle and less important for normal sails which have already been optimised over a long period of time.
I’ll keep trying and report back if I have any success.
Thanks for the data, very useful.
Jim.
I get stuck sometimes too … when I can’t see the forest because of the tree’s !
Funnily enough as “soft sails” are the engine of any sail boat, I’m surprised how there is very little discussion and data about them, with hard wings you can find lot of data through aircraft channels.
You probably know Lester Gilberts site, take look at the section on “How fins and sails work” and you can read on his wind tunnel projects, might find something useful there.
Cheers Alan
P.S Btw: I don’t trust any software that has built in algorithms, never seen one that works probably. Read an article on NASA engineers comparing S/W design against natural physics and they proved S/W to wrong too often to be trusted.
Alan,
On reflection, I think that for method 2 I was only measuring the sideways force, not the lift force. However, method 1 may produce a useful result. The diagram I gave doesn’t really describe the situation very well. Let me try again. In this case only consider the result marked 20deg (yellow dot). In this case I think that the situation was as described in the following image:
The movement of the sail resulted from the difference between the lift force on the sail and the component of the gravitational force in the x,y plane resulting from (predominantly) the weight attached to the base of the mast. As long as the wind and weight are kept constant, the final position of the sail should give an indication of the lift force generated by the sail, thus allowing the lift generated by different sails to be compared. Is there something that I am missing?
Jim.
Additional note: just remembered, the force that may complicate things a bit is the force on the sail exerted by the cord that holds the sail at the correct angle
Maybe not, but I have 
… I stood on my chair trying catch that one … but I missed it ! can you humour my peanut with simpler explanation.
When it comes to forces we are using three elements:
For the best performance, ideally each element needs to be designed and tuned so that it contributes to the optimum performance of the total package or in other words not under or over designed.
Example: A 747 can have powerful engines that provide enough thrust to drive 400 tonnes too lift off speed but if the surface area of the wings does not provide enough lift for the weight of the plane …well let’s just say it gets real ugly at the end of the runway.
Same for to sail as example a “hard wing” maybe needs less surface area than a “soft sail” to have the same amount of lift, I don’t know if these numbers publicly exist. The only way to conclusively really find it out is NIKE …just do it !+
I agree that method 1 is more value than method 2 regarding the results. As I mentioned earlier, you get an X and X result as the sail is pushed away from the wind (X) and horizontally perpendicular to the wind (Y). With the test subject suspended on a comparatively long line, the Z effect is minimal.
To compare another sail, you would need to make certain the total suspended weight was the same - that way you are comparing apples with apples…
Keep at it - there is something important hiding in here I think…
Alan and mrpenguin,
Sorry, I’ve made a few errors which doesn’t help in understanding. Let me start from scratch.
The sail is suspended from above with a weight attached to the base of the mast. A fan is positioned in front of the sail and a cord is attached to the boom:
Because the whole sail is suspended it can move in response to the wind from the fan, however, it is working against a restoring force due to the weight. Here are a couple of images of the arrangement in action.The camera is positioned behind the sail looking towards the fan. In the first image the fan is off, and in the second image the fan is on:
Am I correct in thinking that the extent of movement to the left is indicative of the lift produced by the sail?
Jim.
Alan, I may come back to you at some stage regarding C.E. and C.L.R. I’m not sure how to measure C.E. for a wing sail, and I’m not on top of the hydrodynamics of hulls and keels and their contribution to forward movement.
I believe you are correct in essence. The change in position with the fan on consists of two vectors though, one is directly away from the fan the other is directly sideways.
Lets see if I can elaborate… (see diagram)
Firstly, I will add some nominal measurements to your diagram - when you turn the fan on, let us assume the weight moves 50 units (from the “fan off” position to the “fan on” position on my diagram)
If we assumed for a moment that this was a boat and the wind was dead astern then you have two force vectors:
a) 40 units of force pushing the boat forward (call this “Y”)
b) 30 units of force pushing the boat sideways (call this “X”)
the resultant - the distance between the two points (this is what your dots are measuring) is the hypotenuse of a right-angled triangle i(Pythagoras’s theorem)
502 = 302 + 402
(Of course the sideways force (X) in reality results in heeling and that compounds the whole scenario - lets not go there yet…)
Now lets say you now wanted to compare a second wing angle (nothing else changes) - you change the angle of the wing and measure a new point we will call “point2”
Lets say that this time the distance between “no fan” and “point2” is 70 units directly away from the fan (the “Y” direction) and is 20 units to the side (the “X” direction)
Now your marker at “point2” will be a calculated amount (lets call it “D”) from the starting point.
D2 = 202 + 702
D2 = 400 + 4900
D = SQRT (5300)
D = 72.8
Hope this is of some help…
For the mathematically minded - I AM aware that there is a mixture of units in this explanation - the “units of force” are relative only and are not actual values
Yes and you can have infinite number of results with the different combination of settings of: wind speeds, sail area, mast shapes, angles of attack, luff tension, max sail draft/chord position, sail twist, lateral & longitudinal mast bend … just to name a few.
Tip: try and remove as many variables as possible.
Then you will find that one sail and it’s settings is good on one point of the wind and worse on another compared to another sail … a gain in one parameter area generally means a compromise in another somewhere else, this why the perfect sail boat does not exist ! (excepting ETNZ AC 72-1 of course) 
I applaud your quest in searching for answers through testing, as it is the tutor of experience & we never stop learning … It’s a challenge in itself, good luck !
Cheers Alan
ClaudioD pendulum method is fast and very precise for the purpose for finding the “Static C.E & C.L.R” that works for any 2D shape. I print out on A3 sheetsize & I know Claudio uses A4, results are the same.
A soft sail on a mono-hull the “Active C.E” is changing with hull movement, sail shape and angle to the wind and generally moves slightly forward as boats heels more going upwind, I have no experience with multi-hulls C.L.R and solid wings C.E. or know if they are even different ?
Thanks Alan, point taken. If nothing else this discussion and process has significantly increased my understanding of the behaviour of sails, admittedly I was coming off a low base.
I’ll do some tests with different sails and attempt to determine how effective the method is. If it works it might be best employed (to beign with) to optimise simple things like the size of the gap between the elements of a wing sail, or the best flap angle for a particular angle of incidence. I would like to take the guess work out of some of these decisions.
Something that needs more thought is the position of the cord that sets the sail angle as this will influence the result. To assess the lift generated I think that it might be better if this cord is attached in the direction of the fan. Also, as mrpenguin suggests, heeling is not something that this method deals with, so perhaps it is best used for multihulls.
Jim.
Hi Mji,
you may try to build up your own dynamo-meter and replace the spring with commerce instrument. http://mcpsh.en.alibaba.com/product/287398142-212407497/TRANSPRENT_DYNAMOMETER.html
Then in a windy day you may measure the wind force with and handy meter and cross check the mast sail displacement on the trolley.
It will be not very precise due to frictions but repetitive and have relative measurements when changing sails.
You can also mount the trolley on a 360° table and check for sail apparent wind orientation (~15° from the sail chord)
Cheers
ClaudioD