Tornado-Cat

Boats and foils comparison

Recommended Posts

7 hours ago, MaxHugen said:

Interesting, didn't know that!

They were overstressed at one point. I'm guessing a challenger finals race with Artemis when winds were stronger than anticipated, maybe even the first day of challenger finals.

Share this post


Link to post
Share on other sites
1 hour ago, Horn Rock said:

Hate to be an anal prick, but the word you're looking for is impediment. Impedance is current resistance measured in ohms.

Fair

Share this post


Link to post
Share on other sites

Thought I'd have a closer look at possible maximum foil cant angle, assuming level flight, so I used an excellent pic of Te Rehutai from dead-on behind by AirflowNZ, posted by @rh3000 .

It's pretty accurate, using an overlaid Rules diagram, and shows the MWP (green dashed line) which is defined as 520mm below the foil arm axis (or rather, vice versa). This puts the very bottom of the skeg/bustle at 600mm below it.  Added a water level at 100mm below that, and then canted the foil to 25°.  Doesn't look like they can go a lot further:

image.png.67285d7d61934e5ee2442aa635f9e39c.png

  • Like 3

Share this post


Link to post
Share on other sites

Would/will be interesting to see the impact of one of these boats in disturbing the air of one behind when going downwind. On hand the large camber they are all running with will mess it up a lot one suspects. On the other hand it would be a header for anyone behind them so they could sail deeper.

Wonder how far it extends?

Share this post


Link to post
Share on other sites
9 hours ago, Horn Rock said:

Hate to be an anal prick, but the word you're looking for is impediment. Impedance is current resistance measured in ohms.

Wow.  What an anal prick! ;)

Share this post


Link to post
Share on other sites
6 hours ago, MaxHugen said:

Doesn't look like they can go a lot further:

 

But they do.  We see the tip out of the water much more than you're showing.

Share this post


Link to post
Share on other sites
15 minutes ago, Ex-yachtie said:

But they do.  We see the tip out of the water much more than you're showing.

And how often can they maintain flight at just 100mm (4") above the surface? It's quite easy to picture how much of a foil at 25° is out of the water if you merely double that amount to 200mm. As for canting it to 30° as someone suggested, well, I don't think so.

image.png.364cb9573187e4d4b2fabce3336c2bd0.png

Share this post


Link to post
Share on other sites
46 minutes ago, MaxHugen said:

And how often can they maintain flight at just 100mm (4") above the surface? It's quite easy to picture how much of a foil at 25° is out of the water if you merely double that amount to 200mm. As for canting it to 30° as someone suggested, well, I don't think so.

image.png.364cb9573187e4d4b2fabce3336c2bd0.png

Nice drawing, and pretty close to what I'm getting from my model. The model is alive and back again, this time with vortex data somewhat integrated. I just ran the first few polars to see how it's doing, but there are a few things left to fine-tune.

I created tables of lift, induced drag, and CoE for four different sail plans (main + jib #3; jib#2; jib#1 and code 0, but that code0 needs some more work). In these polars below, I ran jib #1 all the way up till 13kts of wind, jib#2 for 15kt wind and jib#3 for 20+ kts of wind.

speed_pol.png.93ee46136a864d508f6f9f2b65190b06.png

I'll have more details on how the model is working now, just wanted to share this on cant angle and sail CoE quickly:

f_cant.png.a152edf83e900d8a7fd36ce084a04c0c.png  coe.png.412677434718eb2505c3e57a6023fb1a.png

First, as I said before, at least in the way I look at it, these are highly correlated. The cant angle here is measured differently from how you measure it. I use the angle of the line connecting the foil arm rotation point to the end of the foil arm relative to horizontal. With the curves of the foil arm, it works out so that 25 degree on my graphs equals 30 degree on your drawing. 30 degree on mine equals 20 on your scale etc.

So for the most extreme conditions I ran so far, I get maybe 2.5 degree more cant than 30 on your scale, but I'd say that's pretty close. 

The zig-zag on the lines is due to the limited "resolution" of my model. I'll explain that later, but I have to finish some work now.

Just one more thing quickly, I incorporated other foil drag components in the model which are based on some very questionable math :D, just wanted to throw it out there to see maybe someone can tell me how far off these numbers are... These are taken in 15kts of wind at 45deg and 130 deg TWA at about 29 and 39 kts of boat speed, respectively.

fdrag_tws15_twa45.png.ba47edf3872e68b7beb7067e045213c8.png  fdrag_tws15_twa130.png.dbd2599d150c62f1589382266ff1afd1.png

 

 

 

 

  • Like 1

Share this post


Link to post
Share on other sites
10 hours ago, MaxHugen said:

Thought I'd have a closer look at possible maximum foil cant angle, assuming level flight, so I used an excellent pic of Te Rehutai from dead-on behind by AirflowNZ, posted by @rh3000 .

It's pretty accurate, using an overlaid Rules diagram, and shows the MWP (green dashed line) which is defined as 520mm below the foil arm axis (or rather, vice versa). This puts the very bottom of the skeg/bustle at 600mm below it.  Added a water level at 100mm below that, and then canted the foil to 25°.  Doesn't look like they can go a lot further:

image.png.67285d7d61934e5ee2442aa635f9e39c.png

The assumption being that they are not healed to windward 

Share this post


Link to post
Share on other sites
52 minutes ago, Xlot said:

@erdb: you sure about the units for Total Foil Drag?

 

Oh, you're right. Forgot to convert to kN. Here it is:

15kts wind

TWA 45, speed 29.6 kts                                               TWA 130, speed 39.2 kts   

  fdrag_tws15_twa45.png.adc1fe709b4fb4666fa522b09d3a9696.png  fdrag_tws15_twa130.png.53814e6090a9b1d6c2f2d2711cbd7ac6.png

Share this post


Link to post
Share on other sites
8 hours ago, erdb said:

The zig-zag on the lines is due to the limited "resolution" of my model. I'll explain that later, but I have to finish some work now.

Good stuff mate!  I've got twist factors kinda sorted, now have to adjust until I can get transverse moments balanced. I'm using 3 mains, and 6 headsails.  I'm not so happy about my "resolution" either, but I don't have a practical way to get 100,000-400,000 data sets from XFoil, so I've used 500.

Unless you have an example of an input file I could use for XFoil, that would take all the params, and dump into 1 file - or even 9 (one for each sail).

Share this post


Link to post
Share on other sites
14 minutes ago, MaxHugen said:

Good stuff mate!  I've got twist factors kinda sorted, now have to adjust until I can get transverse moments balanced. I'm using 3 mains, and 6 headsails.  I'm not so happy about my "resolution" either, but I don't have a practical way to get 100,000-400,000 data sets from XFoil, so I've used 500.

Unless you have an example of an input file I could use for XFoil, that would take all the params, and dump into 1 file - or even 9 (one for each sail).

I'm curious how you're handling twist and sail forces. How do you have that many data sets (or data points?) from xfoil? I have a somewhat similar method in my model where I have different cl, cd datasets for various foil flap angles. First I used separate data tables for the cl and cd values. In each table, I had a row for each 0.2 deg AOA step, and a column for each 1deg flap angle step. When the algorithm got to the point of calculating lift and drag for the foil, it just went to the respective coordinates of the two tables to get the cl and cd values. It was kind of slow, so what I'm doing now is that I fit polynomials for each flap angle to describe the relationships of AOA - cl and AOA - cd, and just use that function to get the values. I was able to get pretty close to the real curves with the polynomials, and it's much faster to run a function than look up values in a table. Python has some easy to use tools for this kind of stuff, but you can do it excel too/ Fit a polynomial trendline on your graphs, select show equation and just take the coefficients manually, plug them into your algorithm.

Share this post


Link to post
Share on other sites

Here is how I run the model now using vortex data. I turned the whole algorithm around. I used to start with setting the sails at various angles, get the sail forces and see at what speed they balance out the drag. Now I go the other way. I start with setting up a base table or worksheet, which has a good range of potential boat speeds with 0.2 kt steps. For each boat speed, I have a good range of possible rudder vertical forces with 100 N steps and for each of these speed and rudder combinations, I have a range of foil cant angles with 0.2 deg steps. It's a huge table, for a speed range of 10 kts of speed, 10 kN rudder force and 10 deg of foil cant, it's 250,000 rows or combinations. I actually start with an even bigger table that has much wider ranges for each parameter, and then narrow it down closer to speed and force ranges expected for a given TWA and TWS. Creating this table takes quite a few minutes, but now it's saved in a csv file of ~ 120 MB that I can load pretty quickly when I start the program. Even with this huge table, the resolution of the model is limited by these steps in speed, force and angle.

I run the following analysis for every point of sail and every true wind speed:

For each possible speed - rudder vertical force - foil cant angle combination --> calculate foil forces needed + sail forces needed for balance on X,Y,Z axes --> get center of effort (CoE) for the sails required for pitch and roll moments using these forces  <--- check if the CoE height is the same for roll and pitch balance <--- check vortex table if sail forces and CoE is possible by setting sails at reasonable AOA and twist angles. Eliminate all the scenarios where balance is not possible, and choose the setup from the remaining scenarios with the highest speed.

For each row  with a specific speed, rudder and foil cant value, I calculate rudder drag, foil drag and foil lift necessary for balance. From foil lift, I calculate backwards what the foil AOA needs to be at that speed (this is where I use those polynomials with flap angle input), and using that AOA, I rotate the boat (pitch and yaw angles) to match the foil AOA. Using these rotation angles, I update the coordinates of center of gravity, and CoE for the foil.

From the drag calculations and from the foil forces, I get the required sail forces (lift and drag) for longitudinal and transverse force balance. Using these forces, I can also calculate where the sails' CoE needs to be for pitch balance, and where the CoE needs to be for roll balance. Here, I can already narrow down the possible scenarios, because I only go forward with speed, rudder force and cant angle combos, where the same CoE height satisfies both pitch and roll balance. From the original 500,000 possibilities, I'm usually down to 50 to 100 possible speed, rudder force and foil cant angle combinations.

The final step is where I bring in the vortex data. Using the vortex spreadsheet, I created tables that have lift, drag and CoE values for a good range of AOA and twist values. I use these tables backwards. Since I know the required sail forces (drag and lift) and the required CoE, I check the vortex table if it's possible to achieve these values with a given sail setup. This usually limits the possible scenarios for a given TWA and TWS to only a few rows, and I choose the one with the highest boat speed. This gives me one data point for one specific boat setup and one specific pair of TWA and TWS. I managed to optimize this process to get a data point calculated usually within 15 to 30 seconds (had some fun figuring out how to do multithread processes for this).

I can create a list of commands in an excel file for batch processing that the program reads in to run various TWS and TWA combos or various boat parameters (like sail areas, foil areas, angles etc) to create polars. I leave it running for the night, and next day I get a bunch of wiggly lines :D.

The slow part is the vortex table lookup, but I only have to do that on a limited number of rows. Still this is another weak point of the model in terms of resolution or precision. I could make vortex tables with smaller steps of AOA and twist changes, but that increases the look-up time a lot. I wish I understood the vortex calculations so I could code that into the program instead of just looking up values. Finally, for the vortex calculation handles the main+jib combo as one airfoil. This is fine for lift and induced drag, but the part that's missing is how the presence of two airfoils instead of one changes viscous drag. Right now, I'm only calculating with drag as if one airfoil represented the whole sail area. Since induced drag is much higher than viscous, I hope it doesn't shift my numbers too badly.

Here are some of the examples:

This was the boat speed polar I've already posted. The lowest wind speed was 8 kts where I could get the boat foiling. I'm clearly not at the level of ETNZ yet... I'll try to play around with various foil AOA, flap angles, foil areas, sail areas etc to see what's the minimum wind speed where the boat can fly. 

speed_pol.png.0814beaad0d9062318d624f27a4986e3.png

This shows how the foil vertical lift and the rudder downforce balance out each other (and the weight of the boat):

foilz_rudz.png.99c6811b95d05df46fddcb50c0935c92.png

Just like with the previous version of the model, I only get positive vertical lift from the rudder at low wind speed, low boat speed upwind situations. Otherwise, it's always a downforce. The craziest angles and the hairiest situations are between 90 and 120 TWA in 20+ knots of wind. Some pretty big forces there!

Here are the sail forces (X is driving, Y is heeling force):

1871066381_sailforces2.thumb.png.67cb87b42d54c9a10a056b573c90b57e.png

  • Like 1

Share this post


Link to post
Share on other sites

 

2 hours ago, erdb said:

I'm curious how you're handling twist and sail forces. How do you have that many data sets (or data points?) from xfoil?

I section each sail into 4. I work out the area for each, plus a chord width at the centroid-y height. Chord width is used for the RE calc.

For each sail section, I guess an AoA, using some data I've gleaned from pics etc. These also vary by speed, but I have to do that in 10knot blcks (eg 21-30 knots etc), otherwise the manual data-gathering from XFoil is too much, since I currently don't know how to create an input file to run a big range for me. 

I won't admit to how long I spent getting even the 500 data points I have... but some of that time was spent trying to get the headsails to converge. The mainsail was no prob.

Once I have CLs etc for each section, forces are worked out by section, totalled, and then a moments calcs gets me the CE for the sail.  For each sail I have 5 "twist factors" - ie, 5 sets of AoAs for each section, for increasing overall twist. Finally I combine the forces for the 2 sails, and do another moments calc for the combined CE-y value.

PS: not using Vortex, as I don't really know how it's doing the calcs. So for me, that would be relying on a black box, which doesn't work for me.

Share this post


Link to post
Share on other sites
13 minutes ago, MaxHugen said:

 

I section each sail into 4. I work out the area for each, plus a chord width at the centroid-y height. Chord width is used for the RE calc.

For each sail section, I guess an AoA, using some data I've gleaned from pics etc. These also vary by speed, but I have to do that in 10knot blcks (eg 21-30 knots etc), otherwise the manual data-gathering from XFoil is too much, since I currently don't know how to create an input file to run a big range for me. 

I won't admit to how long I spent getting even the 500 data points I have... but some of that time was spent trying to get the headsails to converge. The mainsail was no prob.

Once I have CLs etc for each section, forces are worked out by section, totalled, and then a moments calcs gets me the CE for the sail.  For each sail I have 5 "twist factors" - ie, 5 sets of AoAs for each section, for increasing overall twist. Finally I combine the forces for the 2 sails, and do another moments calc for the combined CE-y value.

PS: not using Vortex, as I don't really know how it's doing the calcs. So for me, that would be relying on a black box, which doesn't work for me.

You know you can use the aseq command in xfoil to run a range of AOAs for a foil, right?

Once you are in OPER, use the pacc command to specify a filename to save your data, then use "aseq 0 15 0.2"  for example to get curves between 0 and 15 degrees with 0.2 steps. Then type pacc again to finish collecting data. If you have a problem with convergence, you can increase the iteration number by typing "iter 100" for example. It also helps to use the "init" command before running a new analysis.

Vortex basically does what you're doing, but splits the sail into more segments and it also calculates the induced drag more accurately. I understand what it does, but I didn't have time to go through all the steps of calculations. Sometime I have to work on stuff that pays the bill <_<.

What is the max twist on the main you've seen on pictures?

I limited twist in vortex to 15 degrees, although since it's looking at the whole sail plan, it's not the same as the twist in just the main alone.

Share this post


Link to post
Share on other sites
On 12/2/2020 at 7:50 PM, D_Dog said:

Thanks. I'm up to speed on the TLA's now...

Just more attempts to upstage the famous UpTits

Share this post


Link to post
Share on other sites
4 hours ago, erdb said:

Here is how I run the model now using vortex data. I turned the whole algorithm around. I used to start with setting the sails at various angles, get the sail forces and see at what speed they balance out the drag. Now I go the other way. I start with setting up a base table or worksheet, which has a good range of potential boat speeds with 0.2 kt steps. For each boat speed, I have a good range of possible rudder vertical forces with 100 N steps and for each of these speed and rudder combinations, I have a range of foil cant angles with 0.2 deg steps. It's a huge table, for a speed range of 10 kts of speed, 10 kN rudder force and 10 deg of foil cant, it's 250,000 rows or combinations. I actually start with an even bigger table that has much wider ranges for each parameter, and then narrow it down closer to speed and force ranges expected for a given TWA and TWS. Creating this table takes quite a few minutes, but now it's saved in a csv file of ~ 120 MB that I can load pretty quickly when I start the program. Even with this huge table, the resolution of the model is limited by these steps in speed, force and angle.

I run the following analysis for every point of sail and every true wind speed:

For each possible speed - rudder vertical force - foil cant angle combination --> calculate foil forces needed + sail forces needed for balance on X,Y,Z axes --> get center of effort (CoE) for the sails required for pitch and roll moments using these forces  <--- check if the CoE height is the same for roll and pitch balance <--- check vortex table if sail forces and CoE is possible by setting sails at reasonable AOA and twist angles. Eliminate all the scenarios where balance is not possible, and choose the setup from the remaining scenarios with the highest speed.

For each row  with a specific speed, rudder and foil cant value, I calculate rudder drag, foil drag and foil lift necessary for balance. From foil lift, I calculate backwards what the foil AOA needs to be at that speed (this is where I use those polynomials with flap angle input), and using that AOA, I rotate the boat (pitch and yaw angles) to match the foil AOA. Using these rotation angles, I update the coordinates of center of gravity, and CoE for the foil.

From the drag calculations and from the foil forces, I get the required sail forces (lift and drag) for longitudinal and transverse force balance. Using these forces, I can also calculate where the sails' CoE needs to be for pitch balance, and where the CoE needs to be for roll balance. Here, I can already narrow down the possible scenarios, because I only go forward with speed, rudder force and cant angle combos, where the same CoE height satisfies both pitch and roll balance. From the original 500,000 possibilities, I'm usually down to 50 to 100 possible speed, rudder force and foil cant angle combinations.

The final step is where I bring in the vortex data. Using the vortex spreadsheet, I created tables that have lift, drag and CoE values for a good range of AOA and twist values. I use these tables backwards. Since I know the required sail forces (drag and lift) and the required CoE, I check the vortex table if it's possible to achieve these values with a given sail setup. This usually limits the possible scenarios for a given TWA and TWS to only a few rows, and I choose the one with the highest boat speed. This gives me one data point for one specific boat setup and one specific pair of TWA and TWS. I managed to optimize this process to get a data point calculated usually within 15 to 30 seconds (had some fun figuring out how to do multithread processes for this).

I can create a list of commands in an excel file for batch processing that the program reads in to run various TWS and TWA combos or various boat parameters (like sail areas, foil areas, angles etc) to create polars. I leave it running for the night, and next day I get a bunch of wiggly lines :D.

The slow part is the vortex table lookup, but I only have to do that on a limited number of rows. Still this is another weak point of the model in terms of resolution or precision. I could make vortex tables with smaller steps of AOA and twist changes, but that increases the look-up time a lot. I wish I understood the vortex calculations so I could code that into the program instead of just looking up values. Finally, for the vortex calculation handles the main+jib combo as one airfoil. This is fine for lift and induced drag, but the part that's missing is how the presence of two airfoils instead of one changes viscous drag. Right now, I'm only calculating with drag as if one airfoil represented the whole sail area. Since induced drag is much higher than viscous, I hope it doesn't shift my numbers too badly.

Here are some of the examples:

This was the boat speed polar I've already posted. The lowest wind speed was 8 kts where I could get the boat foiling. I'm clearly not at the level of ETNZ yet... I'll try to play around with various foil AOA, flap angles, foil areas, sail areas etc to see what's the minimum wind speed where the boat can fly. 

speed_pol.png.0814beaad0d9062318d624f27a4986e3.png

This shows how the foil vertical lift and the rudder downforce balance out each other (and the weight of the boat):

foilz_rudz.png.99c6811b95d05df46fddcb50c0935c92.png

Just like with the previous version of the model, I only get positive vertical lift from the rudder at low wind speed, low boat speed upwind situations. Otherwise, it's always a downforce. The craziest angles and the hairiest situations are between 90 and 120 TWA in 20+ knots of wind. Some pretty big forces there!

Here are the sail forces (X is driving, Y is heeling force):

1871066381_sailforces2.thumb.png.67cb87b42d54c9a10a056b573c90b57e.png

Wake me up when the racing starts.

giphy.gif

Share this post


Link to post
Share on other sites
3 hours ago, erdb said:

You know you can use the aseq command in xfoil to run a range of AOAs for a foil, right?

Once you are in OPER, use the pacc command to specify a filename to save your data, then use "aseq 0 15 0.2"  for example to get curves between 0 and 15 degrees with 0.2 steps. Then type pacc again to finish collecting data. If you have a problem with convergence, you can increase the iteration number by typing "iter 100" for example. It also helps to use the "init" command before running a new analysis.

Vortex basically does what you're doing, but splits the sail into more segments and it also calculates the induced drag more accurately. I understand what it does, but I didn't have time to go through all the steps of calculations. Sometime I have to work on stuff that pays the bill <_<.

What is the max twist on the main you've seen on pictures?

I limited twist in vortex to 15 degrees, although since it's looking at the whole sail plan, it's not the same as the twist in just the main alone.

I don't use XFoil, only the GUI interface XFLR5, so I unfortunately don't know the XFoil commands.

I don't understand what 15° twist means in Vortex. Twist where? Mainsail, headsail, both, which section of a sail...?

I've measured maybe 10-11° twist at the main top quarter. The max. would theoretically be at whatever angle gives zero lift at that section, but I limit it for the main to 4° due to mast thickness. I implement twist first at the top, naturally, then progressively increase it down the sail through the other sections. The 1-5 twist settings incrementally increase the twist for each of the 4 sections. All done by guesswork though.

 

Share this post


Link to post
Share on other sites

@erdb could I impinge on you to write me an input file for, say, the jib_c6.dat foil file, that can run a set of calcs for speed range 1-70 knots (mach 0.0015 - 0.1050), and for each speed, a range of AoAs from -2 to 13°, and dump it all into one text file? It would need to include an RE number for each speed too.

I wouldn't mind installing XFoil and trying this.

If the input file can't accept all those ranges in one instruction set, I could probably use VBA to generate individual input files from a db such as MS Access, and run them sequentially... I think.

Share this post


Link to post
Share on other sites
1 hour ago, MaxHugen said:

I've measured maybe 10-11° twist at the main top quarter. The max. would theoretically be at whatever angle gives zero lift at that section, but I limit it for the main to 4° due to mast thickness. I implement twist first at the top, naturally, then progressively increase it down the sail through the other sections. The 1-5 twist settings incrementally increase the twist for each of the 4 sections. All done by guesswork though.

Erratum :  Sorry, that was camber that's limited. But I didn't end up using camber by section, otherwise too much data to get manually, so used the centroid-y position for the whole sail, and used an average chord camber and width at that point. Still, it's the AoA that's the biggest factor of course for the twist calcs.

Max. AoA for a top section of a mainsail is currently 0°, but may reduce down to -2°.   I already use -2° for some headsails where they more or less have zero lift.

Share this post


Link to post
Share on other sites
5 hours ago, MaxHugen said:

I don't use XFoil, only the GUI interface XFLR5, so I unfortunately don't know the XFoil commands.

I don't understand what 15° twist means in Vortex. Twist where? Mainsail, headsail, both, which section of a sail...?

I've measured maybe 10-11° twist at the main top quarter. The max. would theoretically be at whatever angle gives zero lift at that section, but I limit it for the main to 4° due to mast thickness. I implement twist first at the top, naturally, then progressively increase it down the sail through the other sections. The 1-5 twist settings incrementally increase the twist for each of the 4 sections. All done by guesswork though.

 

I looked at both. For me, xfoil is easier to use. You only really need a few commands, and if you forget, you can always list them by typing "?".

Good question about vortex AOA. Maybe @Basiliscus could chime in to correct me. As I understand, AOA is representative of the whole wing - either the middle of the span or the segment where the CoE is. The twist is the difference in AOA of the foot vs the head. However, because the whole sail plan is modelled as one airfoil,  the foot AOA involves both jib and main whereas the head AOA is only the main. So a 15 deg twist is the angle difference between the line connecting the forestay and the aft end of the boom vs the top edge of the main.

The thing is that since the vortex look-up is at the very end of my algorithm, I don't even use these numbers (the AOA and the twist) I only need vortex to tell me whether a certain combined sail force and a certain CoE can be achieved with the sail plan. The limit on what's an acceptable twist is important though and I may need to tweak that number up or down - not sure about it.

5 hours ago, MaxHugen said:

@erdb could I impinge on you to write me an input file for, say, the jib_c6.dat foil file, that can run a set of calcs for speed range 1-70 knots (mach 0.0015 - 0.1050), and for each speed, a range of AoAs from -2 to 13°, and dump it all into one text file? It would need to include an RE number for each speed too.

I wouldn't mind installing XFoil and trying this.

If the input file can't accept all those ranges in one instruction set, I could probably use VBA to generate individual input files from a db such as MS Access, and run them sequentially... I think.

The way it works is that you have to set the Re to a certain value, and then run curves for a range of AOAs. If you give me a few Re values (let's say up to 3-5 different values), I can run the curves for those and put it in a table. Maybe you can interpolate between them for the various Re numbers you're getting for different AWS and cord length.

4 hours ago, MaxHugen said:

Erratum :  Sorry, that was camber that's limited. But I didn't end up using camber by section, otherwise too much data to get manually, so used the centroid-y position for the whole sail, and used an average chord camber and width at that point. Still, it's the AoA that's the biggest factor of course for the twist calcs.

Max. AoA for a top section of a mainsail is currently 0°, but may reduce down to -2°.   I already use -2° for some headsails where they more or less have zero lift.

Some people claim that with active control of the head of the main, you could achieve "negative lift" to provide righting moment or in other terms further lower the CoE of the sail. There were rumors of this with the AC50s as well, although I'm not sure it's ever been proven.

Share this post


Link to post
Share on other sites
1 hour ago, erdb said:

I looked at both. For me, xfoil is easier to use. You only really need a few commands, and if you forget, you can always list them by typing "?".

Good question about vortex AOA. Maybe @Basiliscus could chime in to correct me. As I understand, AOA is representative of the whole wing - either the middle of the span or the segment where the CoE is. The twist is the difference in AOA of the foot vs the head. However, because the whole sail plan is modelled as one airfoil,  the foot AOA involves both jib and main whereas the head AOA is only the main. So a 15 deg twist is the angle difference between the line connecting the forestay and the aft end of the boom vs the top edge of the main.

The thing is that since the vortex look-up is at the very end of my algorithm, I don't even use these numbers (the AOA and the twist) I only need vortex to tell me whether a certain combined sail force and a certain CoE can be achieved with the sail plan. The limit on what's an acceptable twist is important though and I may need to tweak that number up or down - not sure about it.

The way it works is that you have to set the Re to a certain value, and then run curves for a range of AOAs. If you give me a few Re values (let's say up to 3-5 different values), I can run the curves for those and put it in a table. Maybe you can interpolate between them for the various Re numbers you're getting for different AWS and cord length.

Some people claim that with active control of the head of the main, you could achieve "negative lift" to provide righting moment or in other terms further lower the CoE of the sail. There were rumors of this with the AC50s as well, although I'm not sure it's ever been proven.

Modelling the twist was not that hard in one respect, but trying to get all the variations included - like AWS + Re + Camber + AoA per section per sail - was too many data points to get without automating it.  Plus camber and AoA by section has to be guessed in any case, so accuracy is, well, you know.

Plus which the TWA should be modified as well as twist.

And somewhere down the calc track, the only goal is VMG!

I thought about negative lift at the head of the main, but rejected it. The mast profile is 9-10% of the entire "foil" profile. It can only be rotated to one position, and that would probably be where they want the most drive from the sail, certainly somewhere in the lower half of the sail. That means that at the top section of the sail, the tapered mast plus skins at at a "negative lift" angle, versus the "drive" section, would look like this:

image.png.63f5826fab1d8ed6b2751a1939d49059.png

You can see the problem with this.  No lift, but heaps of extra drag!

Share this post


Link to post
Share on other sites
59 minutes ago, MaxHugen said:

Modelling the twist was not that hard in one respect, but trying to get all the variations included - like AWS + Re + Camber + AoA per section per sail - was too many data points to get without automating it.  Plus camber and AoA by section has to be guessed in any case, so accuracy is, well, you know.

Plus which the TWA should be modified as well as twist.

And somewhere down the calc track, the only goal is VMG!

I thought about negative lift at the head of the main, but rejected it. The mast profile is 9-10% of the entire "foil" profile. It can only be rotated to one position, and that would probably be where they want the most drive from the sail, certainly somewhere in the lower half of the sail. That means that at the top section of the sail, the tapered mast plus skins at at a "negative lift" angle, versus the "drive" section, would look like this:

image.png.63f5826fab1d8ed6b2751a1939d49059.png

You can see the problem with this.  No lift, but heaps of extra drag!

Yes, but what if the camber is completely taken out at the top? I don't know just wondering - it may be possible.

main.thumb.png.998a1021ef20112a5487bfffdd5a9159.png

For your sail segment calculations, you may be able to do some reasonable shortcuts. The slope of the cl vs AOA curve is pretty close for all reasonable foil shapes below ~10 - 12 degrees of AOA. Vortex uses 0.1046  (cl / degree) as a default and I compared that to my various foil shapes I tested in xfoil, and it's pretty close. Close enough that any error will be much smaller than what comes from all the other assumptions/guesses in the model. Since induced drag depends on lift, that won't be affected by the airfoil profile either. The only thing that's different is the viscous drag, but it's a much smaller component relative to the induced drag. Even if you have some errors there due to using only a few different Re numbers versus nailing it perfectly for every cord length and AWS, the overall effect on the model will be very small.

This is why I switched to vortex, because it does a better job at calculating the CoE and induced drag for the complete sail plan including the jib - taking the planform shape into consideration as well. While I do have an error in viscous drag, that error is much smaller than what I'd get from less-than-perfect induced drag calculations. The problem is that you can't really calculate induced drag separately for the jib and the main, because they really work together as one combined airfoil. 

 

 

 

 

Share this post


Link to post
Share on other sites
14 hours ago, erdb said:

For your sail segment calculations, you may be able to do some reasonable shortcuts. The slope of the cl vs AOA curve is pretty close for all reasonable foil shapes below ~10 - 12 degrees of AOA. Vortex uses 0.1046  (cl / degree) as a default and I compared that to my various foil shapes I tested in xfoil, and it's pretty close. Close enough that any error will be much smaller than what comes from all the other assumptions/guesses in the model. Since induced drag depends on lift, that won't be affected by the airfoil profile either. The only thing that's different is the viscous drag, but it's a much smaller component relative to the induced drag. Even if you have some errors there due to using only a few different Re numbers versus nailing it perfectly for every cord length and AWS, the overall effect on the model will be very small.

This is why I switched to vortex, because it does a better job at calculating the CoE and induced drag for the complete sail plan including the jib - taking the planform shape into consideration as well. While I do have an error in viscous drag, that error is much smaller than what I'd get from less-than-perfect induced drag calculations. The problem is that you can't really calculate induced drag separately for the jib and the main, because they really work together as one combined airfoil.

Your suggestion of a shortcut had me very interested. However, my data does not agree that CL/AoA is reasonably close to a constant, namely Vortex's 0.1046 per degree.

In the data below are stats for the mainsail. The headers at top are "Camber-AWS",  and at left the AoA.  Values are CL/AoA.  All data was derived from XFoil, using my custom "AC75 Mainsail" foil design.  Unless I have misunderstood you, or have a major problem in all my data, none of the values even remotely resemble Vortex's "CL factor".

Assuming my data is OK, then using the Vortex factor grossly underestimates the CL and therefore all force calcs.

image.thumb.png.976ceee075a8c411976083c12384a706.png

It does however show that the CL is virtually independent of the AWS (used for RE calc, which is included in XFoil calcs plus the AWS itself) for a given Camber.

I'm quite concerned that I may have made a significant error in my CL calcs, as you mentioned that "compared that [Vortex's val] to my various foil shapes I tested in xfoil, and it's pretty close."  Would you send me your foil dat file, so I can check some XFoil polars to see what I get?

Share this post


Link to post
Share on other sites

 

Came across this. Look familiar? 

I bet there's some jealous chase and spy boat drivers with bad backs would look longingly at these. 

Not to mention the person paying the fuel bills on 1800HP tenders. Yours for $USD300000

  • Downvote 1

Share this post


Link to post
Share on other sites
4 hours ago, MaxHugen said:

Your suggestion of a shortcut had me very interested. However, my data does not agree that CL/AoA is reasonably close to a constant, namely Vortex's 0.1046 per degree.

In the data below are stats for the mainsail. The headers at top are "Camber-AWS",  and at left the AoA.  Values are CL/AoA.  All data was derived from XFoil, using my custom "AC75 Mainsail" foil design.  Unless I have misunderstood you, or have a major problem in all my data, none of the values even remotely resemble Vortex's "CL factor".

Assuming my data is OK, then using the Vortex factor grossly underestimates the CL and therefore all force calcs.

image.thumb.png.976ceee075a8c411976083c12384a706.png

It does however show that the CL is virtually independent of the AWS (used for RE calc, which is included in XFoil calcs plus the AWS itself) for a given Camber.

I'm quite concerned that I may have made a significant error in my CL calcs, as you mentioned that "compared that [Vortex's val] to my various foil shapes I tested in xfoil, and it's pretty close."  Would you send me your foil dat file, so I can check some XFoil polars to see what I get?

I think your numbers are good. CL/AOA refers to the slope of the CL curve. More precisely it's (change in CL) / (change in AOA). These were my curves for the mainsail with different cambers

 original2.thumb.png.0a2213e7a0d38ba9c0f58b936656c3b0.png

If you look at the CL curves on the left, you see they all run close to parallel until 10-12 degrees of AOA, their slopes are pretty much the same. For the 2% camber line for example, CL is ~0.25 at 0 degree AOA, and ~1.25 at 10 deg AOA. Slope = (1.25 - 0.25) / (10-0) = 0.1 just by eyeballing it. The starting CLs at 0 deg AOA are different for the different cambers, but the rate of change with AOA is very close until you get close to maximum CL. This will also be quite independent of Re, too. It makes sense to me that Re only affects your CD, because Re will determine where the laminar flow trips. Basiliscus mentioned that those low drag buckets that corresponds with situations where the flow stays laminar for most of the cord may be too optimistic for real life where the airflow is always a bit turbulent to begin with. He suggested to trip the flow with the xtr command either at 0.25 of the cord or even ahead of the foil to get more realistic numbers. I haven't tried this, but maybe if you did that, then the CD would also become less dependent on Re.

 

 

 

Share this post


Link to post
Share on other sites
15 minutes ago, erdb said:

I think your numbers are good.

Thanks, I can breathe a sigh of relief!

I'm reconsidering my previous decision to use an average camber for a sail when calculating the effect of twist. That xtab table I did above really pointed out just how large the difference in camber makes to CL.

This may account for the fact that even using the smallest mainsail and headsail, with the greatest amount of twist, I can't balance out the transverse moments beyond about 40 knots AWS!

Share this post


Link to post
Share on other sites

Played around with the model a bit. Tried to figure out what's the lowest wind speed I can make the boat fly. I think it comes pretty close to what's been reported. 

speed.JPG.c394e6e3a752f20d4dddcca0f4a177f6.JPG

I could make the boat fly at 7 kts of wind with a 150 m2 main and a 155 m2 code 0. At this wind speed, only the code 0 works, max sized jib (90 m2) is not enough to fly.

However, in just a little more wind (8 kts), the 90 m2 jib is already a tiny bit faster. Then, at 10 kts of wind, I couldn't even balance it out with the code 0.

I'm sure in real life they have more ways to depower than what I can model with sheeting angle and twist, but it still shows how narrow is the range for the code 0. Plus, if there's any chance the wind picks up by only a few knots, they are in trouble. Here are the AWA and AWS curves:

awa-aws.JPG.88f9e8e2d7046b18aa51fdc1afe28a33.JPG

Some really tight apparent angles needed here. Of note, in 7 kts of wind, the min TWA upwind was 65 degrees, and VMG was 8.9 kts. Downwind, the lowest I could go was 107 deg TWA with a VMG of 7.5 kts. So faster than the wind, but only by a little. The wind limit is 6.5 kts, right? It's going to be interesting if they have races around the lower wind limit.

In 8kts of wind, best upwind is 58 deg TWA with a VMG of 11.9 kts. Best downwind angle is 112 deg TWA with a VMG of 10.6 kts. So definitely faster than the wind.

In 10 kts of wind, the AC75s come to life, they can sail to 45 TWA upwind with 15.6 kt VMG, and downwind TWA is 135 deg with 20.6 kts VMG, more than twice the wind speed. Quite impressive! How is that analogy? If you had no wind at all, but a river flowing at 10 kts, you could go against the current with 10 kts relative to the shore with an AC75!:o

Share this post


Link to post
Share on other sites
22 minutes ago, Stingray~ said:

Joop Slooff, author of The Science behind Sailingand Australia II and the America’s Cup; the untold, inside story of The Keel, takes us on a deep dive of the boats to be sailed in the 36th America’s Cup

https://www.sailingscuttlebutt.com/2020/12/06/americas-cup-assessing-ac75-design/

Good article, thanks! 
 

This quote was a highlight: “The British boat looks like a radical departure from common knowledge.”

Share this post


Link to post
Share on other sites
1 hour ago, Ex-yachtie said:

Good article, thanks! 
 

This quote was a highlight: “The British boat looks like a radical departure from common knowledge.”

I can't agree on the comment that ETNZ hull is designed for sailing rather than flying :blink:

Share this post


Link to post
Share on other sites
2 hours ago, Ex-yachtie said:

This quote was a highlight: “The British boat looks like a radical departure from common knowledge.”

This opinion caught my eye too, itals mine

As mentioned above and explained in the Scuttlebutt article of about a year ago, the aerodynamic efficiency of the rig and sails, plus the hull on which they are mounted, requires that the hull should have zero vertical lift and be as close to the water surface as possible, with zero heel.

Share this post


Link to post
Share on other sites

So we can call BS on the aero/WIG/downforce hull nonsense for the 100th time :D

Share this post


Link to post
Share on other sites
3 minutes ago, uflux said:

So we can call BS on the aero/WIG/downforce hull nonsense for the 100th time :D

Tend to agree :D 

But like he says, paraphrasing, ‘this sh*t is complicated!’ 

Share this post


Link to post
Share on other sites
8 hours ago, Ex-yachtie said:

Good article, thanks! 
 

This quote was a highlight: “The British boat looks like a radical departure from common knowledge.”

They are apparently SA readers... used one of my diagrams:

image.png.8586ad36aed1e3006e151109c3e6155a.png

  • Like 4

Share this post


Link to post
Share on other sites
9 minutes ago, MaxHugen said:

They are apparently SA readers... used one of my diagrams:

image.png.8586ad36aed1e3006e151109c3e6155a.png

Learning a lot from you @MaxHugen

  • Like 1

Share this post


Link to post
Share on other sites
3 minutes ago, Kiwing said:

Learning a lot from you @MaxHugen

More like the whole of SA!   

I enjoy SA because it's like a big "think tank".  I'm learning heaps along the way. Even if I did have to revisit my high school maths and physics! :)

Share this post


Link to post
Share on other sites

My biggest regret of moving was throwing out my physics text books.  But the brain is quite a lot slower these days.

I enjoy 

Things like this who would have thought you could solve that but even the background people are interesting.

Share this post


Link to post
Share on other sites
1 hour ago, MaxHugen said:

They are apparently SA readers... used one of my diagrams:

image.png.8586ad36aed1e3006e151109c3e6155a.png

You must have enjoyed having it credited to you?

  • Like 2

Share this post


Link to post
Share on other sites
8 hours ago, uflux said:

I can't agree on the comment that ETNZ hull is designed for sailing rather than flying :blink:

I don’t think that’s what he said. 

Share this post


Link to post
Share on other sites
12 minutes ago, Ex-yachtie said:

You must have enjoyed having it credited to you?

Haha... absolutely revelling in the glory! :D

I've noticed a number of times in these last few months that SA is obviously perused by a larger audience than we might think, considering their unintended "give-aways".  There was even one of the team members (apologies for my sh*t memory) who commented on something specifically said in SA.

Not bad for a bunch of people without any access to CFD, farms of computer processing power, etc etc!

May the force be with the SAAC team!

Share this post


Link to post
Share on other sites
6 minutes ago, MaxHugen said:

Haha... absolutely revelling in the glory! :D

I've noticed a number of times in these last few months that SA is obviously perused by a larger audience than we might think, considering their unintended "give-aways".  There was even one of the team members (apologies for my sh*t memory) who commented on something specifically said in SA.

Not bad for a bunch of people without any access to CFD, farms of computer processing power, etc etc!

May the force be with the SAAC team!

Plus your drawings are really nice. What do you use to make them?

Share this post


Link to post
Share on other sites
1 hour ago, MaxHugen said:

Haha... absolutely revelling in the glory! :D

I've noticed a number of times in these last few months that SA is obviously perused by a larger audience than we might think, considering their unintended "give-aways".  There was even one of the team members (apologies for my sh*t memory) who commented on something specifically said in SA.

Not bad for a bunch of people without any access to CFD, farms of computer processing power, etc etc!

May the force be with the SAAC team!

The NZ Herald lurks here. Just FYI.

Share this post


Link to post
Share on other sites
44 minutes ago, Ex-yachtie said:

The NZ Herald lurks here. Just FYI.

Because they're understandably embarrassed to be seen in public?

Share this post


Link to post
Share on other sites
2 hours ago, erdb said:

Plus your drawings are really nice. What do you use to make them?

Inkscape.   I find it excellent for what I use it for, and the learning curve was quite short.  Highly recommend it!  Best of all, it's SVG, and priceless.  :)

Share this post


Link to post
Share on other sites
23 hours ago, MaxHugen said:

Inkscape.   I find it excellent for what I use it for, and the learning curve was quite short.  Highly recommend it!  Best of all, it's SVG, and priceless.  :)

Looks great seems to work similarly to Solid Edge that I'm using but with more colors and overall nicer look.

Thought you may be interested in this to compare with your calculations. These are the various sail coefficients and other parameters I get from my polars.

First, speed and AWA for various wind speeds for reference:

speed2.JPG.22f316c01a3f4531c5a74563b2ccdd3c.JPG  AWA.JPG.160d6427b9c2a3a4c5a15f01c88a220e.JPG

Here are the cl and cdi values (for the whole sail plan). Code 0 for TWS 7, all others with main and jib.

sCL.JPG.c463afdb6371801a058739aeba8e3571.JPG  sCDI.JPG.e63065aafed05fa171a70e56d94a6bdf.JPG

Sails' AOA and twist:

sAOA.JPG.e583ce1e1c72d9c329510ef2f6476dac.JPGsTwist.JPG.a5c5cb1ee6296947fc31d2f1aa72e96b.JPG

AOA refers to the foot of the sail and twist is the difference between head and foot. In these runs, I let twist go up to 20 deg max.

Going upwind at ~ 40 deg TWA,  the foot of the sail is only 2-4 degrees off the centerline. With 20 degrees twist, the head of the sail is at negative 6 degrees relative to the wind. Maybe a little too wild, but it does make the boat go faster so let's hope they can do this with active control at the top. Earlier, when I limited twist to smaller values, I had to use smaller sails and the speed was a little lower, but not a major difference. Keep in mind, these numbers all refer to some theoretical combination of the two sails.

The zig zag in the lines is because of how values are looked up in the vortex tables. The variation is actually pretty small so I didn't bother with trying to smooth it out.

Finally, effective CoE height of the sail plan measured from deck level:

sCOE.JPG.94a0a50eadf7a44fb64c9d7c15dc0200.JPG

 

Share this post


Link to post
Share on other sites

^ " With 20 degrees twist, the head of the sail is at negative 6 degrees relative to the wind. Maybe a little too wild..."  yes, I think so!  I limit twist at any of my section chords to no more than -3°, which is about where I get zero CL. Even that might be optimistic, hard to say. Maybe we'll get a good img out of overhead videos when they start publishing them.

Share this post


Link to post
Share on other sites

with the upwind starts, has anyone made a guesstimate on how much they need to dial down to gain enough speed for lift off? Are the small foils of TNZ taking off later then the LR/ineos ones?

 

 

Share this post


Link to post
Share on other sites
On 12/4/2020 at 4:26 AM, minimumfuss said:

 

Came across this. Look familiar? 

I bet there's some jealous chase and spy boat drivers with bad backs would look longingly at these. 

Not to mention the person paying the fuel bills on 1800HP tenders. Yours for $USD300000

He talks about the extra drag in the water and that it doesn't really plane.  It is like a plow, smooth ride but take some time to get to 40 knots...

Share this post


Link to post
Share on other sites

How much difference does foil size make?

I looked at boat speed vs foil area. Guess which boat is different from the rest?

Speed vs Foil area in 8 (left) and 15 kts (right) of breeze:

TWS08_all.thumb.png.b4adfd5d9bd989d32dd65a7f57568c83.png      TWS15_all.thumb.png.c873f4d7cfc4ee50313146315b406931.png    

A solid 2-3 kts speed advantage for ETNZ on all points of sail at both wind strengths. Under 8 knots of wind, it gets tricky upwind with the small foil, but increasing foil flap angle takes care of that. With INEOS' big 2.25m2 foil, there is an opposite problem downwind, where it may be necessary to apply negative flap angle to make it work in strong winds. (At least in my model with a standard foil profile that's surely different from what the teams use.)

Anyway, it's noteworthy how much speed can be gained if you figure out how to get up, tack and jibe with a smaller foil. Once you're airborne, the extra surface just slows you down.

I still think the secret is that ETNZ has better sail control, so they need less foil flap adjustments and they don't need the anhedral the other teams need to quickly change the direction of lift on the main foil.

others2.JPG.3cd003c7855e2ca7cc1d3c0aee6c801b.JPG

If you sail with one foil wing loaded more than the other, then you have to size it so that it can carry that extra load. On the other hand, if you do all your adjustments with the sail, you can have a completely straight and smaller foil. 

Of course it could also be a better foil profile or that lifting body / bulb shape - who knows?

  • Like 7

Share this post


Link to post
Share on other sites

The challengers will all have their last sets of foils already made. To they either have the tools to compete with ETNZ in that department or not. Too late for any change to direction now as it takes 3 months to built them.

Share this post


Link to post
Share on other sites
7 minutes ago, uflux said:

The challengers will all have their last sets of foils already made. To they either have the tools to compete with ETNZ in that department or not. Too late for any change to direction now as it takes 3 months to built them.

That three months is for composite foils, if they make the foils from steel the timeline would be a couple of days.

  • Like 1

Share this post


Link to post
Share on other sites
8 hours ago, erdb said:

How much difference does foil size make?

I looked at boat speed vs foil area. Guess which boat is different from the rest?

Speed vs Foil area in 8 (left) and 15 kts (right) of breeze:

TWS08_all.thumb.png.b4adfd5d9bd989d32dd65a7f57568c83.png      TWS15_all.thumb.png.c873f4d7cfc4ee50313146315b406931.png    

A solid 2-3 kts speed advantage for ETNZ on all points of sail at both wind strengths. Under 8 knots of wind, it gets tricky upwind with the small foil, but increasing foil flap angle takes care of that. With INEOS' big 2.25m2 foil, there is an opposite problem downwind, where it may be necessary to apply negative flap angle to make it work in strong winds. (At least in my model with a standard foil profile that's surely different from what the teams use.)

Anyway, it's noteworthy how much speed can be gained if you figure out how to get up, tack and jibe with a smaller foil. Once you're airborne, the extra surface just slows you down.

I still think the secret is that ETNZ has better sail control, so they need less foil flap adjustments and they don't need the anhedral the other teams need to quickly change the direction of lift on the main foil.

others2.JPG.3cd003c7855e2ca7cc1d3c0aee6c801b.JPG

If you sail with one foil wing loaded more than the other, then you have to size it so that it can carry that extra load. On the other hand, if you do all your adjustments with the sail, you can have a completely straight and smaller foil. 

Of course it could also be a better foil profile or that lifting body / bulb shape - who knows?

i think you've banged it on the head. smaller more efficient foils, capable of more lee resistance control for upwind resistance and downwind slide, better sail control to get the fattest low speed shape to get up on said fast foils. this is the special sauce.

  • Like 1

Share this post


Link to post
Share on other sites
11 hours ago, Terry Hollis said:

That three months is for composite foils, if they make the foils from steel the timeline would be a couple of days.

Plus, they only count when they’re installed - leaves time for a last-minute rethink

Share this post


Link to post
Share on other sites
3 hours ago, barfy said:

i think you've banged it on the head. smaller more efficient foils, capable of more lee resistance control for upwind resistance and downwind slide, better sail control to get the fattest low speed shape to get up on said fast foils. this is the special sauce.

With due respect and appreciation for MaxHugen’s and erdb’s work, this marinara sauce is a recipe that shouldn’t have escaped the Challengers’ designers a long time ago. Plus, the supposed remedies (with which I agree) are nothing special given the time available - compared with say rebuilding an AC72 shredded to pieces (hint, hint)

 

  • Like 1

Share this post


Link to post
Share on other sites
1 hour ago, Xlot said:

With due respect and appreciation for MaxHugen’s and erdb’s work, this marinara sauce is a recipe that shouldn’t have escaped the Challengers’ designers a long time ago. Plus, the supposed remedies (with which I agree) are nothing special given the time available - compared with say rebuilding an AC72 shredded to pieces (hint, hint)

Decades ago, I wrote a program for a bank to help manage the 3,500 odd properties they owned/leased etc.  One of their accountants got into my data and created a great Power Point presentation for some senior managers, for their 5 year budget.  The managers were happy, as it fit previous forecasts fairly well.

I cajoled the accountant into showing me his calcs, since I was going to incorporate his projections into my app.  As I was going through them with him, I asked where the allowances for inflation of labour and materials costs were.

He went white.

Just a tiny factor really, the rest of the sauce was well done (IMO), but with compounding inflation it turned sour. Poor bugger had to go and explain that to those managers I suppose.

It doesn't take much, and when it appears to fit...

  • Like 4

Share this post


Link to post
Share on other sites
12 hours ago, erdb said:

Of course it could also be a better foil profile

I think it's this cos the ETNZ foils in the last were both enormous and utterly dominant.

  • Like 1

Share this post


Link to post
Share on other sites
5 hours ago, jaysper said:

I think it's this cos the ETNZ foils in the last were both enormous and utterly dominant.

And with talk of surprises from the challs on coming weeks, don't expect etnz to rest. Their final foils will come out "when there isn't time to copy them", and they won't be slower than the current ones.

  • Like 4

Share this post


Link to post
Share on other sites
9 hours ago, Xlot said:

With due respect and appreciation for MaxHugen’s and erdb’s work, this marinara sauce is a recipe that shouldn’t have escaped the Challengers’ designers a long time ago.

This is true but I do thoroughly appreciate the effort to quantify the relative sizes & performance.

 

And there is still the question of how small is too small?

At some size threshold you're going to have a hard time accelerating & staying airborne through turns.

 

I don't think we really got enough good quality pics of all the B1 launches/ but I'd love to see a comparison between B1 & B2 foils.

As I recall the TNZ B1 foils were thought to be biggest & Ineos had among the smallest.

Share this post


Link to post
Share on other sites
10 hours ago, Xlot said:

With due respect and appreciation for MaxHugen’s and erdb’s work, this marinara sauce is a recipe that shouldn’t have escaped the Challengers’ designers a long time ago. Plus, the supposed remedies (with which I agree) are nothing special given the time available - compared with say rebuilding an AC72 shredded to pieces (hint, hint)

 

I agree absolutely. There are obviously thousands of other factors that go into this and I analyzed one of those. Plus, my model only tries to predict maximum speed when foiling in a stable position. This never happens in real life, as there is always something that kicks the boat out of balance a bit and then they have to compensate. So you could have a very fast, but very sensitive boat that at the end loses more, because the speed drops more when it goes slightly out of balance. But, being confortable with a potentially more sensitive setup may suggest better sail/foil/rudder control systems - or better sailors;).

Finally, these courses are ridiculously small for these boats. It's quite possible that half of every tack or jibe will be spent by accelerating to target speed and TWA. How fast you get to your target may be more important than a few tenth of a knot difference in the target itself. 

Share this post


Link to post
Share on other sites
18 minutes ago, hoom said:

And there is still the question of how small is too small?

At some size threshold you're going to have a hard time accelerating & staying airborne through turns.

I noticed NZ a few days ago foiling with both down for a while, which was puzzling. Perhaps they were practising for the circumstances you speak of?

I could imagine the jostling during the pre-start in lighter wind might mean they run the risk of dropping off one foil... or a tactic where you're to leeward of your opponent and try to force him up and off his foil, whilst staying airborne?

But I'm stretching my speculative powers.

Share this post


Link to post
Share on other sites
25 minutes ago, erdb said:

Finally, these courses are ridiculously small for these boats. It's quite possible that half of every tack or jibe will be spent by accelerating to target speed and TWA. How fast you get to your target may be more important than a few tenth of a knot difference in the target itself. 

Agreed. Bring back 2+hr races on unconstrained courses I say.

  • Like 2

Share this post


Link to post
Share on other sites

With the above limitations in mind, here is another parameter that has an interesting effect on righting moment and boat speed: position of the foil on the longitudinal axis of the boat - how far it is from the transom.

According to the rules, the foil + foil arm need to be between 10 - 12 m measured from the transom. As Max and maybe others already pointed out, it seems every team put their foil arms at the very aft end of this range. However, there are some differences in where the foil wing is on the foil arm.

LR's foils seem to be on the front end:

591902505_Lunaside_cr.jpg.8013b6c4e86de0f776fbeda74a241c5d.jpg

AM and INEOS in the middle:

AM-cr.jpg.b386bfea2a9e98eff05c494b3917a9ae.jpg

(BTW this is a much better pic than what I used to measure AM's foil size, and on this one it seems it's very close to ETNZ's foil size with the only difference being the bulb area/shape)

And ETNZ's foils are at the very back.

P1130433.JPG.350cfc9ffb70edb8be2e1196f6fd6038.thumb.jpg.d62921e1bb0713924db7c4431d0b4938.jpg

This all may not matter much, because there could be small differences in the foil arm position, too that we haven't picked up by the yellow line analysis on pictures.

Anyway, I wanted to see what difference moving the foil fore-and-aft makes and why all teams have theirs at the aft end of the box.

I moved the foil's CG and center of effort through the 10 - 12m box. The model wouldn't even balance with the foil forward of 11m, and speed goes up as the foil is moved further aft. It's not a huge effect though. In 15 kts of wind, the speed difference between foil position at 10m vs at 11m is 0.6 kt upwind, 1.2 kts on a reach and 0.4 kts downwind.

Speed_tws15.thumb.png.ecdf1f484677587e30f3966ffe8642c4.png 

I think what's behind of this is that as the foil is moved forward, there's less weight forward of the foil, which means lower rudder downforce for pitch balance, which in turn means less righting moment generated by foil lift / rudder downforce. This comes out nicely from the model; rudder exerts 6kN downforce when the foil is at 10m, but it's positive 1kN if the foil is at 11m (upwind in 15kts of wind). Righting moment on the other hand goes down from ~480 kNm to ~430 kNm.

RudZ_tws15.thumb.png.747bc4a33d5e473f4fd8c1a1b1f6e349.png   RM_tws15.thumb.png.6055ef457bfc462f9faa7ec45c681595.png

Of course with higher rudder downforce, the drag increases as well, so it's not a straightforward advantage. Plus the balance between lateral center of efforts of the foil and sails is important, too, but that's not included in my calculations.

 

  • Like 1

Share this post


Link to post
Share on other sites
8 minutes ago, erdb said:

 

AM-cr.jpg.b386bfea2a9e98eff05c494b3917a9ae.jpg

(BTW this is a much better pic than what I used to measure AM's foil size, and on this one it seems it's very close to ETNZ's foil size with the only difference being the bulb area/shape)

And these are the foils prior to the Dec 6th adjustment where they made them smaller.  I did a rough calc and got about 1.0 m2 of foil flap surface area on the new foils (excluding the bulbs).

  • Like 1

Share this post


Link to post
Share on other sites
18 minutes ago, The_Alchemist said:

And these are the foils prior to the Dec 6th adjustment where they made them smaller.  I did a rough calc and got about 1.0 m2 of foil flap surface area on the new foils (excluding the bulbs).

Not sure about your calcs, 1.0m2 of flap seems lot?    I did some calcs on the NZ "BFB" foil, and flap area was about 0.46m2 :
image.thumb.png.9189cc4756085de8d83a4e2e47df6e0a.png

  • Like 1

Share this post


Link to post
Share on other sites
8 minutes ago, MaxHugen said:

Not sure about your calcs, 1.0m2 of flap seems lot?    I did some calcs on the NZ "BFB" foil, and flap area was about 0.46m2 :
image.thumb.png.9189cc4756085de8d83a4e2e47df6e0a.png

Sorry, not just flap but the foils without the bulb

  • Like 1

Share this post


Link to post
Share on other sites
3 hours ago, hoom said:

Agreed. Bring back 2+hr races on unconstrained courses I say.

No can do with these ridiculous boats.

They require power all the time to trim the foils, unlike the IACCs that required relatively little power outside of tacks and gybes.

Share this post


Link to post
Share on other sites
8 hours ago, erdb said:

Finally, these courses are ridiculously small for these boats. It's quite possible that half of every tack or jibe will

Im not sure if my comment with regards to the sweet sauce was taken as a slight somehow to the amazing experiment that you and max are conducting with cheap obs and even cheaper software...but I have enormous respeCt for you both and the journey you are on.

I hope next cup we hear that you can't comment much as you both are on in opposing teams.

And regards courses, yes small..but it is one method to insert some crew craft into the race.

Share this post


Link to post
Share on other sites
12 hours ago, jaysper said:

No can do with these ridiculous boats.

They require power all the time to trim the foils, unlike the IACCs that required relatively little power outside of tacks and gybes.

Which eco-warrior will be first to chant, PV...PV...PV?

Share this post


Link to post
Share on other sites
7 minutes ago, Sailbydate said:

Which eco-warrior will be first to chant, PV...PV...PV?

Oh God, if they put PV cells on the boats I will 100% stop watching.

Eco warriors like Greta fucking Thunberg make me want to vomit.

Best use of them would be to throw them into a furnace for fuel.

  • Downvote 2

Share this post


Link to post
Share on other sites
8 minutes ago, nav said:

Whoever it was that 'welcomed you back' needs to look at themselves

Take a look at the reaction of this group against Michael Moore after he published "Planet of the Humans" and get back to me.

It was an excellent documentary that pretty clearly pointed out that PVs and wind farms cannot possibly ever fix the problem and they have viciously attacked him ever since.

They are pond scum

Share this post


Link to post
Share on other sites
40 minutes ago, Stingray~ said:

Are the designs give any ‘leeway’ to how the FCS and therefore the foil arms are mounted? 

The position of the foil arm axis point is set in the rules.

image.png.f03762c8eadae847a7357a0608d06bd6.png

Share this post


Link to post
Share on other sites
51 minutes ago, Stingray~ said:

Are the designs give any ‘leeway’ to how the FCS and therefore the foil arms are mounted? 

Placement of pumps, tanks and batteries seam to be open.
Mounting tolerances allow for small  possible built in AoA on the  arms

Share this post


Link to post
Share on other sites
33 minutes ago, justsomeone said:

Mounting tolerances allow for small  possible built in AoA on the  arms

Interesting... thanks, it could be a very big deal if yaw-up and yaw-down are somehow adjustable, for up vs downwind ‘crabbing’ purposes. 
 

Hard to imagine the designs can fine tune it given the loads, but maybe it’s possible? 

Share this post


Link to post
Share on other sites
3 minutes ago, Stingray~ said:

Hard to imagine the designs can fine tune it given the loads, but maybe it’s possible? 

Most likely would have to be inbuilt at build time

Share this post


Link to post
Share on other sites
7 minutes ago, justsomeone said:

Most likely would have to be inbuilt at build time

Have wondered too, if teams can angle the arms differently from out of the FCS. 

Share this post


Link to post