[C320-list] True Wind
Scott Thompson
surprise at thompson87.com
Fri May 29 14:37:07 PDT 2020
Hi Troy:
At the risk of raising somewhat esoteric issues, you are wrong about the
true wind calculation being erroneous because it does not account for
current. True wind calculated by our wind instruments does account for
current, since current creates apparent wind measured by our masthead
instruments. I suspect you are confusing true wind (wind relative to the
water) and ground wind (wind relative to the ground). True wind is the
sum of the apparent wind vector and the vector of boat speed through the
water, as approximated by compass heading and knot meter speed. Ground
wind is the sum of the apparent wind vector and the vector of boat
movement over the ground (as measured by GPS). The difference between
true wind and ground wind is current. See post #4 in this thread for
further explanation:
https://www.cruisersforum.com/forums/f121/differences-between-ground-apparent-and-true-wind-direction-73563.html.
My Raymarine chart plotter has options to report both. It does not
display ground wind by default, but it does display current. The
difference between ground wind and true wind can be substantial if there
is a strong current, and this is especially noticeable if the current is
running perpendicular to a light ground wind.
Anyone who has raced under those conditions knows what I'm talking
about. The anchored race committee sets a starting line that appears to
be perpendicular to the wind as they measure it. But that makes it
perpendicular to the ground wind because the committee boat is anchored
to the ground. The racers believe the line is biased because they
experience a true wind that is different because they are drifting with
the current even when head to wind. I've experienced this situation
often near Annapolis. The committee boat flag will be perpendicular to
the starting line and yet people have trouble crossing it on starboard
because of the current.
Scott Thompson
On 5/27/2020 8:27 PM, Troy Dunn wrote:
> I guess I’m in the 'not knowing true wind drives me nuts club'
>
> I don't really have a rational reason for this. We sail in apparent wind
> and even the 'true wind' our wind instrument calculates is erroneous when
> you factor current into the mix, but I think a lot of people would argue
> that the error induced in calculating true wind from our apparent wind
> using this method is no big deal. I think that statement may be true for
> casual cruisers but probably not for racers. I'm not a racer, nor do I
> prefer to sail WoW DDW without a preventer, hence the no rational reason
> comment. But...I guess because it still annoys me...using SOG when your
> paddle wheel is broken would be a nice feature to have on the MFD, I’m not
> sure...maybe you can do this on some plotters? I haven’t tried to figure
> that out yet in lighthouse 3.0. Now that the sailing dashboards are
> finally back in this version I'll have to poke around.
>
> If you want to dive deeper down the true wind rabbit hole... here's a
> great thread.
> https://www.cruisersforum.com/forums/showthread.php?p=2624124#post2624124
>
>
> Sorry to further sidetrack the OPs topic.
>
> Regards
>
> Troy Dunn
> Hull#514
--
Scott Thompson
Surprise, #653
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