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5/13/2016 4:36 am  #1


Polars: which, where, when, why & how

Having recently started with Exp, I'm on a steep learning curve. My style of learning software has generally been to scan the menus and dive in, referencing user guide,manual or in extreme cases tutorials as needed, while teaching myself the tool.

It became clear the other day while taking the North U buoy course that I didn't  understand the many polars that are available, where and when they are used and why.

To that end I am stating my limited understanding and hoping that it will be improved by those with further progress along the path pointing out my errors. (Fergal, you suggested it...)

Data:

"Nav": I loaded the US Sailing VPP, masthead corrected polars for my boat here.

"Perf": I also loaded them here, but didn't scale or edit as yet

"Start": I originally missed loading here, and had some odd behavior in simulated starts without them. Thanks to those who pointed out the omission.

In the North U buoy course Peter suggested deprecating the downwind numbers to reflect no-spinnaker pre start, which makes a lot of sense. As my Asym and Genoa polars are near identical I'll hold off for some calibration before editing here.

"Heel": from another post I took the hint to review the supplied Farr 40 heel, and realized I could extract the "phi" term from US sailing and enter for TWS x TWA grid to tell EXP my predicted heel.  Those curves were not monotonic in mid speeds as TWA changed, which makes sense.

There was also a "Leeway" in the Farr 40"set, which with a CCA centerboard yawl, I suspect I need, but don't as yet have data for.

The polar5-9 I take to be for future use, or what if display.

Usage:

Nav seems to be the basic set that predict sailing speed.

I'm not clear what the difference in Perf vs Nav usage and thus what the specific data should be.
Is it with Optimal routing?

Start seems eponymous, the data should reflect performance during reaching and maneuver, rather than asymptotic optimized speed, combined with the acceleration, Rate of Turn and braking parameters. I'd guess that these numbers should reflect less than a minute of acceleration, and so at higher TWS might approach Perf, but at low would be significantly less, especially if your boat is higher displacement/sail area and your starting box is constrained.

Heel and leeway again seem obvious in intent, yet not clear if they modulate other calculations; eg leeway affects lay lines, heel angle ?

Last edited by sheldon.haynie (5/13/2016 4:43 am)


Semiconductor Development Engineer, Retired Viticulturalist, Occasional sailor
"Lioness" 1962 Hinckley Bermuda 40 Custom Yawl
 

5/13/2016 5:23 am  #2


Re: Polars: which, where, when, why & how

The Perf polar is the "best you can get".  In other words:  if you would always sail the boat 100% these are the polar figures you should aim for.  As explained in the manual, used for performance figures in Exp.
Nav is the 'average'  performance you would actually achieve when sailing (as nobody will ever sail 100%). Used for routing calculations etc.
Start is used during start only for the reasons you mentioned.

 

5/13/2016 7:21 am  #3


Re: Polars: which, where, when, why & how

I bought Expedition in 2012 and I am still on a steep learning curve. Practice, the help file, the webinars, Will Oxley's book, and this forum have all contributed significantly to whatever limited proficiency I have acquired. I will try to address some of your questions and I hope that my explanations can be supplemented by others and corrected if my understandings appear flawed.

​Nav Polar: This should be the MH polar when buoy racing as it is used to calculate laylines. If no MH polar is available as the Nav Polar for buoy racing then use the 10M polar as the Nav Polar but scale the polar up to the MH velocities that you will encounter, using the method / formula set forth in the Help file. This should be the 10M polar when routing as you will be using 10M winds from the grib file. If no 10M polar is available as the Nav Polar for routing then use the MH polar as the Nav Polar but scale the grib winds up to MH velocities, again using the method / formula set forth in the Help file.

Perf Polar: This should be the MH polar to be used to monitor boat performance in real time. If no MH polar is available as a Perf Polar then use the 10M polar but scale it up to MH wind velocities that you will encounter, again using the Help file to obtain the scaling method / formula.    

​Note: It follows from the above that it is often necessary to switch scaling percentages in the Nav Polar during a point-to-point event when using the Nav Polar for both routing and predicting time to laylines. This can be obviated by using two PCs with one dedicated to the performance monitoring and a second dedicated to routing and Race Schedules. "Commodity level laptop" is a term I first encountered here on this forum (Thanks JB) and it really is germane to utilizing Expedition in the ocean racing circumstance.

​Heel Polar: Used to generate / display polar / target heel as a value that should not be exceeded in heavier breezes or possibly aspired to in very light stuff. This will not be used in the calculation of leeway; only the real time heel measured by the heel sensor is used as a value in the calculation of leeway.         

​Leeway: You will need to establish a "Leeway Factor" or "Leeway Coefficient" (often referred to as "K")  that will be applied in the calculation of degrees of Leeway made along with BSP and measured Heel. Obtain a factor by massaging the Leeway, BSP and Heel (from the USS VPP data) for your boat in the leeway equation that you can get from the Help file. You will not obtain a uniform value for Leeway Factor throughout the range of upwind sailing but just try to get an average or "most relevant" number; I prefer to emphasize narrower TWAs here. Don't sweat the values with the board up as you will be going too fast by then for leeway to be big enough to significantly influence your calculated TWAs. You will enter the Leeway Factor into the calibration of your instrument system which will use it to calculate degrees of leeway made which will be added to your TWA by either your instrument system or Expedition; depending on what Expedition receives from the processor. Alternatively, you should enter your Leeway Factor into Expedition if your instrument system is not capable of generating TWAs using leeway. 

 I hope this helps and does not tend to confuse.         

 

5/13/2016 7:58 am  #4


Re: Polars: which, where, when, why & how

Appreciate the assistance, chagrined to re-RTFM and see the answer that I missed the first N times of reading..

I have the MH scaled polars from US SAILING, and have created a heel table by extracting their "phi" term which was based on a measurement of a sister ship (cert #80). 

I have used that off line to synthesize a prospective leeway based on the equation of
Leeway = K* heel/BSP^2  and loaded to polar 5 as "leeway deg" to allow me to display in a number box, and see if it's anywhere close used initial value of "K"=12 as with a 4' near full keel I am not particularly weatherly.

I'm not clear on whether the H5000 "hercules" will automagically add a leeway calculation from heel/bsp or if it will take a COG/HEADING and back out the SET/DRIFT vector, Running H link I am expecting Exp to work on "calibration corrected" but not calculated terms? 

Somewhere in all the myriad channels, I should be able to extract predicted Set/drift from GRIB/tidal models, and measured HDG/COG/SOG/BSP after basic calibration of HDG/BSP and get a better value of "K: 

Given that we rate a 174 PHRF this is a bit like shaving the wheel spokes on Grandpa's old Caddy to cut the unsprung weight at the Gymkhana, but how the heck else am I going to compete with J-24's who can tack 3x to one of ours? 
http://cdn.boardhost.com/emoticons/happy.png


 


Semiconductor Development Engineer, Retired Viticulturalist, Occasional sailor
"Lioness" 1962 Hinckley Bermuda 40 Custom Yawl
     Thread Starter
 

5/13/2016 8:47 am  #5


Re: Polars: which, where, when, why & how

To establish a K factor for leeway of my boat I sailed it on a fresh water lake without current. Then I recorded what the instruments gave back for set and drift.  By trial and error I then settled down on a K factor that minimized drift/set (as set/drift is calculated after correcting for leeway).
Did anyone else use this method and how did it work out for you?
 

 

5/13/2016 8:57 am  #6


Re: Polars: which, where, when, why & how

Concerning your H5000 using leeway in determining TWA, it will if you set it to do so in the H5000 Cal settings (the button pushing is set forth by Nick in the Help file; Help/Search/Leeway/B&G H3000 and H5000).
The resulting TWA is received by Expedition as "TWA/track" (as opposed to "TWA/hdg"). A coherent explanation of this is in the Help file and valuable remarks on this subject can be found in Will Oxley's book. 
Concerning "re-RTFM" there are not enough "re"s in the universe to enumerate how many times I have to do it. 

​I evaluate / refine my "K" from stripchart analysis after sailing, not in real time. In real time I look at the total boat and set an event to record when the boat feels "weatherly" or if it might be "sliding". That event will help me set my tests on the stripchart and get a better grasp on what was going on. Exporting the tests to an xcel file and applying the K formula to a cell can also help get a handle on leeway with the objective of refining K. 
I would be most hesitant about using set/ drift as determinants in any calculation of leeway, no matter what the H5000 or Expedition produces; and current data from a grib file represents the summit of an amalgam of approximations and estimates.
The best estimate of leeway made is obtained by a visual examination of your wake.  
1) Look at your wake and estimate degrees of leeway made (the angle of the wake off the centerline of the boat
2) Promptly set an event in Exp with that estimate in the notes for that event
3) After sailing compare your estimate of leeway with the leeway recorded in the stripchart test at the time of the event 
4) Reconcile the disparate leeways by massaging K
5) Repeat ad nauseum             

Good luck

 

5/13/2016 9:06 am  #7


Re: Polars: which, where, when, why & how

ZeeZee's method is a great way to go. 
​When I search for the best K value on the Great Lakes I massage K until Course (HDG + Leeway) = COG, after looking over the side at something which can establish that there is no wind-driven set & drift. 

 

5/13/2016 9:18 am  #8


Re: Polars: which, where, when, why & how

For those of us doomed to sail in currents (e.g. SF Bay)  I'm planning to try to sail parallel to set where feasible, to get spot measurements of COG - HDG, and do the math with Excel's Solver  to back out "known" set/drift from GRIB's in a region that's not too complex flow (e.g. get a few miles South of Yerba Buena Is or reach N/S on the East Side of Treasure Island. 


Semiconductor Development Engineer, Retired Viticulturalist, Occasional sailor
"Lioness" 1962 Hinckley Bermuda 40 Custom Yawl
     Thread Starter
 

1/26/2020 10:41 am  #9


Re: Polars: which, where, when, why & how

A further question: on the light wind side... 
We just spent 6 hours racing in the "Three Bridge Fiasco" where the winds were max of 10 kts and more commonly under 4kts. 
The standard US SAILING VPP had 6 kts as the minimum TWS.
There's interpolation between TWS values, and 0 kts at 0 TWS is an obvious point.
Is there any value to having more points between 0 & 6  ? 
Specifically, how many more TWS steps make a difference for predicting Target angles/Speeds and routing when the wind is light? 

is "3" kts sufficient or do I need 2 & 4 ?  
I can use tests and take the data to a curve fitting program to extract those values. 

 


Semiconductor Development Engineer, Retired Viticulturalist, Occasional sailor
"Lioness" 1962 Hinckley Bermuda 40 Custom Yawl
     Thread Starter
 

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