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Board » General Discussion » Performance diagrams

Posted by
lamo


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2008-01-30 16:25:11
2008-02-03 16:32:24 - Performance diagrams
I'm curious about the VPP diagram for the 40 ft monohull. At high wind speeds, the lines sometimes cross each other, meaning that when the wind is increasing, the boat speed will decrease. I have not seen this in other VPP diagrams and it seems very unexpected to me. How do you explain this?
Lars Möller
Hometown: Karlskrona, Sweden
Posted by
jakob


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2007-09-04 10:05:30
2008-02-03 16:49:09 - Re: Performance diagrams
Hej, Well I am somewhat curious too... Sometimes things happen fast here... One can imagine situations where a higher wind speed results in lower boat speed. When beating this is often the case due to increased drag. When reaching this can also happen when e.g. a certain efficient sail cannot withstand the higher wind loads and thus cannot be used. So, such things can happen BUT: In this case I can agree that we have exaggerated the characteristics too much. Sorry.
...one of the guys behind the game...
Posted by
lamo


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2008-01-30 16:25:11
2008-02-03 17:13:02 - Re: Performance diagrams
Tks for your reply. However, an increased drag can only happen if the speed is increased. So, I'm afraid I'm still not convinced.
Compare for instance the VPP on this site:
http://www.shipman.dk/articles/?m0=6&m1=97
Lars Möller
Hometown: Karlskrona, Sweden
Posted by
pipedream


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2008-02-06 09:19:15
2008-02-10 12:26:02 - Re: Performance diagrams
generally I agree that the polar lines shouldn't cross. But.. increased windage and heeling (causing increased rudder and increased displacement)can slow the boat down.
Posted by
jakob


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2008-02-10 15:15:14 - Re: Performance diagrams
Hi. Fun discussion!. Lamo: one theoretically obtained polar does not make much as evidence unless you know much about the underlaying models. I can show you several polars with crossing curves. Secondly, I don´t think you read my last post thoroughly. Decreased speed does not have to be caused by increased drag. It might be caused by decreased thrust, as I was arguing the last time. I also agree with Pipedream that heal, waves etc complicates life. So, polar-lines might cross, nothing strange there. Ok?

...one of the guys behind the game...
Posted by
lamo


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2008-01-30 16:25:11
2008-02-11 22:46:12 - Re: Performance diagrams
Hi jakob

I guess you have studied polar diagrams a lot more than I. Still I like to understand what I see. For instance, the diagram for the VO70 makes sence as I see it. When reaching at TWA 130, the boat speed will always increase as the wind speed increases. Still the lines cross when close hauled, which you might expect because of increase wind and wave resistance. So far so good.

What I could not (and still cannot) understand is the feature in diagram for the 40 ft monohull at TWA 150 degr, where you have
10 m/s > 10 knots
12 m/s > abt 12,5 knots
14 m/s > abt 12,8 knots
18 m/s > abt 12,2 knots
22 m/s > abt 13,5 knots
In this case it's hard to imagine the behaviour of the 18 m/s line. Wind or wave resistance could hardly have this effect when reaching at 150 TWA.
Lars Möller
Hometown: Karlskrona, Sweden
Posted by
mytest


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2007-12-28 22:45:33
2008-02-12 06:39:43 - Re: Performance diagrams
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SNIP
Posted by
jakob


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2008-02-12 08:08:17 - Re: Performance diagrams
Hi, First of all, as I admitted a couple of posts ago I agree that this polar is not perfect, It is hand-manipulated by us. The behavior you are referring to could still be a result of that no optimal sails are available for the 18m/s wind. So, principally, the characteristics are possible BUT I agree that in this particular instance the "bucket" is fairly unlikely. We will in the future try to come up with even more realistic polars. Thank you for communicating with us!
...one of the guys behind the game...

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