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10/29/2016 1:54 pm  #1


Simplified directions to create polars

It would be useful if someone could give a simple, clear description of how to create and modify their polar files. I know it is possible to buy yet more software that can help, and I know it is possible to buy a crude set from US Sailing, but a simple, concise discussion of creation and modification would probably benefit a lot of people.
- John Swanson

 

10/29/2016 5:26 pm  #2


Re: Simplified directions to create polars

I am not skilled enough to make the explanation concise and simple and I have found that the best instructional aid for me was Peter Isler's NorthU webinar tutorial. Also, the US Sailing package is not really "crude" but has a lot of good stuff in it with which to create a heel polar and and refine your Leeway factor calibration in addition to Expedition-usable MH and 10M polars which need no scaling.

My polar creation method moves along these steps, which can probably be shortened or rendered simpler by a proficient user of PCs or xcel.   
1. Open as a text file a set of polars for a boat similar to yours that you will find in Nick's polars that come with Expedition; in the Expedition Data Folder / Polars. Using "Save As", save this polar with a name that works for you or your boat. You may also elect to retain this new polar in a location other than the Expedition Data Folder 
2. Select all of the values in this text file (all row and column headings and entries in all cells) and copy them.
3. Paste these values into a blank xcel worksheet that you have opened and named as you wish (but the name should be readily associated with the name in step 1). Now you have a polar worksheet.
4. Work in this worksheet, replacing the values that are there (True Wind Speeds, True Wind Angles, Boatspeeds) with values that are germane for your boat. These values are obtained from analyzing logfiles in Stripchart for those periods that you have recorded as representative of the boat being sailed at its optimum.  
5. When you have constructed this polar save the worksheet in case you have to make minor corrections or significant re-configurations later.   
6. Select all of the values from the worksheet and copy them.
7. Paste all of the values copied in step 6 into your polar text file from step 1.
8. Select and delete all of the original values that were in this text file when you saved it from Nick's stuff in step 1.
9. You should now have a usable MH polar which can be your Perf polar but which must be scaled to serve properly as your Nav polar when you utilize 10M grib file winds with the router.
10. Have a look at the polar and investigate the values that might be making any gross "bumps" which you can "smooth out" by
(a) dragging the curves in the "Edit" screens or
(b) altering the values in the text file or worksheet.

I hope others might make this explanation more coherent or substantively usable.                   

Last edited by Joe Goulet (10/29/2016 5:30 pm)

 

10/30/2016 7:53 am  #3


Re: Simplified directions to create polars

Joe,
This is good, thanks. And what I meant by "crude" is to really say that although they provide some very useful info, there are a lot of details that still may need to be tweaked based on details of how one's boat is actually sailed, the condition of the sails, etc.
- JSwanson

     Thread Starter
 

10/30/2016 12:42 pm  #4


Re: Simplified directions to create polars

I think you pretty much nailed it Joe, nice job!

 

10/30/2016 4:47 pm  #5


Re: Simplified directions to create polars

Thanks for the feedback, guys. However I forgot to advise about one potential hangup which, if overlooked, can end up driving you crazy.

For every TWS you must insure that your TWAs increase as you move from left-to-right in that TWS row. Do not attempt to superimpose your non-target TWA columns if your target TWAs end up with a substantial spread, as is often the case in the downwind TWAs. Expedition does not require that the target TWAs line up in one column and Expedition will not accept a polar file in which the TWAs do not increase from left-to-right along each TWS row. This can drive you crazy when you try to build a polar and Expedition will not accept it but will instead warn you that "TWAs not increasing".

Good luck.    

 

11/01/2016 6:52 am  #6


Re: Simplified directions to create polars

A couple of extra tips:

1 - you can open the text file directly from within Excel, and when saving keep the same *.txt formatting

2 - if you are starting from scratch, keep your TWA columns to a minimum, say 0, uptarget, 70, 110, downtarget, 180. This reduces the workload in clocking/dragging points to smooth within the Expedition edit polar facility. As you learn more about the boat, you can add TWA columns as necessary.

3 - lots of polars from VPP here: http://jieter.github.io/orc-data/site/
 


:::Campbell Field::::::www.fieldyachting.com:::
 

11/01/2016 7:18 am  #7


Re: Simplified directions to create polars

Perhaps all of this information can be used to add into the Expedition book or at least into the help files. A similar level of detail and discussion could be made for other areas, such as sail charts.
- John Swanson

     Thread Starter
 

11/02/2016 2:34 am  #8


Re: Simplified directions to create polars

Another approach is to write a program to rearrange the ORCi polars .slk files by choosing a particular set eg Best Performance. 
Expedition tolerates "ragged" tables (not all rows of equal length) but creates an error message anyway which doesn't seem to stop the table from being used. This can come about from having 2 equal or duplicate values like the optimum target downwind Twa equal 180 with the ORCi table showing a separate set of values for Twa=180 also. In this case I interpolate between the previous value say 165 and 180 values to make an artificial value to make the polar table  regular and not ragged.
 


If you rest you rust
 

11/02/2016 1:26 pm  #9


Re: Simplified directions to create polars

Thanks Campbell,
It is definitely faster to open a file doing it as you suggested "within Excel".

Struggling here to try and obtain Tgt Bsps from the data in the files on the otherwise excellent reference sight that you provided the link to. Presently "winging" the tgt bsp from the polar plot from the site with an eye to refining that parameter in the Expedition Polar Edit upwind-downwind non-polar plot plot pages using the result from dragging to the correct tws-twa combination.    

 

11/03/2016 12:28 pm  #10


Re: Simplified directions to create polars

If you do it in Excel,

target bsp = target vmg / cos(target twa)

 

 

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