[C320-list] True Wind

[Jack Brennan]

My solution to this is to put on the autopilot and pop another beer.

Jack Brennan
Sonas, 1998 Catalina 320
Tierra Verde, Fl.
Dolphin Cruising Club of Tampa Bay


Sent from Mail for Windows 10

From: Scott Thompson
Sent: Friday, May 29, 2020 5:39 PM
To: C320-List at Catalina320.com
Subject: [C320-list] True Wind

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