Expedition Expedition Navigation Software

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5/13/2016 9:41 am  #1


Why scale to mast height?

Of course you need to ensure that wind instrument reading, grib files and polars are based on the same height.
You often read that therefore polars and grib files should be scaled to mast height (as they are normaly based on 10m).
But why?  Isn't it much easier to scale the reading from your wind instrument down to 10m?    This only needs applying a simple correction factor (which most instruments can do) and you can then just use the 10m  grib and polars as they are.   Or am I overseeing something?

 

5/13/2016 10:16 am  #2


Re: Why scale to mast height?

It may very well be the case that instrument wind can be scaled to 10M but it is outside my experience. I have never altered Measured Wind Speed (MWS) or scaled its derivative, True Wind Speed (TWS). I know that Ockam and B&G recommend most strenuously in their user manuals that MWS never be altered from the calibration set at the factory.     

 

5/13/2016 12:41 pm  #3


Re: Why scale to mast height?

Joe, I agree with not messing with MWS.
Also, IMHO, the variation in WS over the height of a mast would be much smaller than the variation in a computer generated polar table for a given boat.  In addition, the variation due to height, would change due to conditions and air stability.  At some point you have to compromise. 
It's interesting that these things can be even considered now, it shows how far tools like Exp have taken us
Cheers

 

5/13/2016 2:28 pm  #4


Re: Why scale to mast height?

The vertical shear implicit in a forecast (GRIB) at 10 km resolution is a wholly theoretical construct to yield an average 10M number a few days in advance. Surface roughness over water & land  factor, as well as macro topography of land vs humidity/temperature affects on density that affect boundary flow and mixing, and fluctuations are of both cyclic and random periods depending on causes. 

Measured speed at your masthead a wholly empirical quantity, albeit affected by upwash, heel, pitch etc and therefore much more direct a predictor of boat performance, where a 10% error would be notable. 

f you have a high resolution forecast where the lateral calculation grid and topography are modeled at the 10-100m level, then perhaps treating the average windspeed forecast with more respect is appropriate. 


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

5/13/2016 10:10 pm  #5


Re: Why scale to mast height?

I know that in menu 'Settings - Weather' I can set a 10m wind scaling % that is applied to Grib files.  But how to scale my polar. The polar I use (and on which I will make refinements later) is based on 10m wind so I need to scale it (down) basically with the same % as Grib files are scaled up (assuming they are 10m as well).

In the Polar menu I can set a % but this impacts the polar speed itself, not the wind.
Do I need to enter calculated numbers for the wind strength in the polar?   So instead of entering polar lines for 6, 8, 10, 12, 14, 16 and 20 knots (which I currently have) enter the polar values for winds of 5.95,  7.91,  9,88  etc??    Or can I enter the scaling % somewhere in the same way it is done for the Grib %?
The Exp manual is not clear in this.



 

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5/13/2016 11:23 pm  #6


Re: Why scale to mast height?

Mostly you would leave the polar scaling at 100% as your really want your polars to reflect what you measure at mast height.

At least, that is my take on it. It isn't an exact thing.

 

5/14/2016 1:22 am  #7


Re: Why scale to mast height?

Understood.  But if the polar that I entered into Exp is based on 10m winds, I need to apply a scaling factor to make it comparable to what the instruments measure (e.g. if the instruments are at 20m).   Is there a place where I can enter that factor?

     Thread Starter
 

5/14/2016 6:09 am  #8


Re: Why scale to mast height?

In the Polar menu I can set a % but this impacts the polar speed itself, not the wind.
Do I need to enter calculated numbers for the wind strength in the polar?   So instead of entering polar lines for 6, 8, 10, 12, 14, 16 and 20 knots (which I currently have) enter the polar values for winds of 5.95,  7.91,  9,88  etc??    Or can I enter the scaling % somewhere in the same way it is done for the Grib %?
The Exp manual is not clear in this.


​My experience with polars "scaled" for diffrent heights is with a 10M and MH polar from USS Sailing in their "Performance Package". From an examination of the polar tables it appeared that the term "Scaling Polars" is something of a misnomer in that the BSP and TWA values remained unaltered but the TWS values were indeed altered.
I suspect it is the case that when polars are "scaled" in Expedition
1) the TWS values in the "scaled" polar table that is used by the processor are altered by the relevant percentage (although they are not displayed)
2) new BSPs for the original TWS values (in the displayed polar table) are generated "internally" and utilized.
To answer your original question, based on my suspicion of how "scaling polars " is accomplished in Expedition, it is not necessary to enter calculated values for BSP into a new polar table.  

Last edited by Joe Goulet (5/14/2016 6:20 am)

 

5/14/2016 10:39 am  #9


Re: Why scale to mast height?

Thanks Joe.
But where do I need to enter the polar scaling percentage that Exp will then use internally (again, for the Grib's it's clear where to enter them, but I don't see a where I need to enter the polar scaling percentage.)

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