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Board » Technical Support » VO70-v4 and VO70 Polars

Posted by
Zembu


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2008-12-15 20:58:27
2009-01-12 14:11:40 - VO70-v4 and VO70 Polars
Question for Kalle as he part answered my chat question (12/1/09). The old VO70 polar had the 9 and 15 knot wind speed lines showing boat speeds greater than the boat speed shown by the 30 knot wind line for TWA from 0 to 30 and 40 degrees respectively. So we could expect that a current wind speed line from above 6 to 15 knots would likely show intermediate boat speed similarly. The new VO70-v4 polar shows boat speeds for all permanent wind speed lines increasing as the wind speed increases for all angles of TWA. So today I was surprised to see the 21.8 wind speed line plotting outwith the 30 knot line for TWA from 0 to 40. I'm sending a screen shot of the polar. Is the new VO70-v4 polar operating correctly?
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Posted by
jakob


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2009-01-12 14:50:16 - Re: VO70-v4 and VO70 Polars
Hi, well my name is not Kalle but I am a also one of the guys behind the game and maybe I can help. Your observations are correct and I understand your question. The colored curves we have chosen to show does not at all have to show maximum boat speeds, it is in fact extremely unlikely. Thus, your observation is correct and the polar might be "fairly ok too" :-) ok?
...one of the guys behind the game...
Posted by
Zembu


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2009-01-12 15:38:09 - Re: VO70-v4 and VO70 Polars
Jakob, Thanks for your reply. But looking at your manual, para 1.7 "The boat performance" you imply there that the polar diagram is accurate - e.g. you say "For a certain wind speed and TWA you can read the corresponding boat speed..." Your current response implies that we cannot rely on the boat speed curves you have shown. As I said - the old VO70 polar showed that for lower wind speeds up to 40 TWA we could expect greater boat speed than for the 30 knot wind curve. The new VO70 does not give any indication that the same could occur. Old VO70 attached.
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jakob


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2009-01-12 16:10:06 - Re: VO70-v4 and VO70 Polars
Well, two things. 1) The old polar is just that - old. We changed the polar since it behaved a little strangely at low twa:s. The new polar is better in that respect. 2) The actual performance routine in the code is continuous in the sense that the boatspeed can be calulated for all combinations of twa and tws. The graphical representation of that we call the polar is accurate (!) but not showing the boat speed at all combinations of twa and tws. ok?
...one of the guys behind the game...
Posted by
Zembu


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2009-01-12 17:43:45 - Re: VO70-v4 and VO70 Polars
Thanks Jakob, I guess I have to accept it. Reason I'm so fussy is that as an Excel exercise, I was trying to build an Excel file to help me get best course, and a fundamental of that has been to make a print of the polar and scale off the data, have the Excel do graphs - get equations from these, enter the formulae into a look up table etc etc. I see from your answer to Frank Putman on 15Mar2008 that you don't give out the tabular data, so there's no point in asking you for it I guess.
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jakob


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2009-01-12 19:42:27 - Re: VO70-v4 and VO70 Polars
Well yes, so far we have not given away the tabulated data. if you are doing the interpolation yourself try to use cubic splines in both twa- and tws-directions to get results resembling mine ;-)
...one of the guys behind the game...
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Zembu


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2009-01-13 16:03:56 - Re: VO70-v4 and VO70 Polars
Hi Jakob,

Thanks again for your advice. I will see what I can do and maybe will email you with results if I think I'm successful.

Best wishes,

Andy

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