Understanding Polars
I have a set of polars for my C&C 34+ (below), but I am unsure of how to interpret the downwind data. There is no information I have found on how the angles were determined. I am interested in best angles for downwind work without a spinnaker going wing on wing with a whisker pole. I am guessing the data is with a spinnaker and I am not sure if that matters. The issue is whether to go wing on wing direct to a mark and go a shorter distance vs. wing on wing or not on a lesser angle according to the polar chart. I have tried to measure this on my boat and have not found a big enough difference to convince myself I should tack downwind. Dave

S/V Aries
1990 C&C 34+
New London, CT

David,
I'm not sure there is one right answer. I th.ink a lot depends on sea
state. If you are wing on wing and the boat is rolling like crazy on a
long leg, the discomfort may dictate gybing downwind. If seas are calm in
10 knots of wind, as long as you can do at least 4.9 knots wing on wing and
sail directly to the mark then that is faster.
BTW, in 20 knots I am not sailing at 173 degrees off the wind no matter who
is driving!
Hope that helps!
Joel
On Fri, Apr 5, 2024 at 1:49 PM David Knecht via CnC-List <
cnc-list@cnc-list.com> wrote:
I have a set of polars for my C&C 34+ (below), but I am unsure of how to
interpret the downwind data. There is no information I have found on how
the angles were determined. I am interested in best angles for downwind
work without a spinnaker going wing on wing with a whisker pole. I am
guessing the data is with a spinnaker and I am not sure if that matters.
The issue is whether to go wing on wing direct to a mark and go a shorter
distance vs. wing on wing or not on a lesser angle according to the polar
chart. I have tried to measure this on my boat and have not found a big
enough difference to convince myself I should tack downwind. Dave
[image: C-C_34_Plus_Polars_Angles.jpg]
S/V Aries
1990 C&C 34+
New London, CT
[image: pastedGraphic.tiff]
Please show your appreciation for this list and the Photo Album site and
help me pay the associated bills. Make a contribution at:
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Thanks for your help.
Stu
--
Joel
David, I don't pretend to understand the polars, but I do understand your question, having asked it many time on the race course, please let us knwo what you eventually decide, whether with the polars or not, thanks!
Richard1986 C&C 37 CB, Ohio River, Mile 596;
Richard N. Bush Law Offices2950 Breckenridge Lane, Suite NineLouisville, Kentucky 40220(502) 584-7255
On Friday, April 5, 2024 at 01:49:35 PM EDT, David Knecht via CnC-List <cnc-list@cnc-list.com> wrote:
I have a set of polars for my C&C 34+ (below), but I am unsure of how to interpret the downwind data. There is no information I have found on how the angles were determined. I am interested in best angles for downwind work without a spinnaker going wing on wing with a whisker pole. I am guessing the data is with a spinnaker and I am not sure if that matters. The issue is whether to go wing on wing direct to a mark and go a shorter distance vs. wing on wing or not on a lesser angle according to the polar chart. I have tried to measure this on my boat and have not found a big enough difference to convince myself I should tack downwind. Dave
S/V Aries1990 C&C 34+New London, CT
Please show your appreciation for this list and the Photo Album site and help me pay the associated bills. Make a contribution at:
https://www.paypal.me/stumurray
Thanks for your help.
Stu
Personally, I think sailing to polars can be challenging. I prefer to
use VMG. VMG rules for windward/leeward race courses! Bear with me on
this.
First, a couple of definitions used by Touche's Nexus instrument system.
Velocity - speed and course
VMG - velocity made good wind based. The velocity directly upwind or
downwind.
WCV - waypoint closure velocity. The velocity made closing on the set
waypoint
Okay, if your instrument calculates VMG (wind based), simply steer the boat
to maximize VMG. Done!
If your instruments do not calculate VMG (wind based), here's how to get
your GPS to closely approximate it. First, many GPS systems show VMG which
is actually WCV when a waypoint is set. If you set the upwind (or
downwind) mark as the waypoint, WCV will incur more error as you approach
the layline. Visualize it. As you approach the layline, your WCV will
decrease to zero as you near 90 degrees to your course to the mark. (If
you overstand, WCV will turn negative as you've sailed past your mark.)
So, set a waypoint 10 miles directly upwind or downwind of the race mark.
Now your GPS WCV will closely approximate VMG (wind based). Steer to
maximize WCV and your course(s) to the mark will be optimized.
--
Dennis C.
Touche' 35-1 #83
Mandeville, LA
Thanks, Dennis. How did you arrive at 10 miles.
Sent from my iPhone
On Apr 5, 2024, at 5:31 PM, Dennis C. via CnC-List cnc-list@cnc-list.com wrote:
Personally, I think sailing to polars can be challenging. I prefer to use VMG. VMG rules for windward/leeward race courses! Bear with me on this.
First, a couple of definitions used by Touche's Nexus instrument system.
Velocity - speed and course
VMG - velocity made good wind based. The velocity directly upwind or downwind.
WCV - waypoint closure velocity. The velocity made closing on the set waypoint
Okay, if your instrument calculates VMG (wind based), simply steer the boat to maximize VMG. Done!
If your instruments do not calculate VMG (wind based), here's how to get your GPS to closely approximate it. First, many GPS systems show VMG which is actually WCV when a waypoint is set. If you set the upwind (or downwind) mark as the waypoint, WCV will incur more error as you approach the layline. Visualize it. As you approach the layline, your WCV will decrease to zero as you near 90 degrees to your course to the mark. (If you overstand, WCV will turn negative as you've sailed past your mark.)
So, set a waypoint 10 miles directly upwind or downwind of the race mark. Now your GPS WCV will closely approximate VMG (wind based). Steer to maximize WCV and your course(s) to the mark will be optimized.
--
Dennis C.
Touche' 35-1 #83
Mandeville, LA
Please show your appreciation for this list and the Photo Album site and help me pay the associated bills. Make a contribution at:
https://www.paypal.me/stumurray
Thanks for your help.
Stu
Purely arbitrary. 3-5 miles would probably work.
--
Dennis C.
Touche' 35-1 #83
Mandeville, LA
On Fri, Apr 5, 2024 at 4:33 PM Matthew Wolford via CnC-List <
cnc-list@cnc-list.com> wrote:
Thanks, Dennis. How did you arrive at 10 miles.
Sent from my iPhone
On Apr 5, 2024, at 5:31 PM, Dennis C. via CnC-List <
cnc-list@cnc-list.com> wrote:
Personally, I think sailing to polars can be challenging. I prefer to
use VMG. VMG rules for windward/leeward race courses! Bear with me on
this.
First, a couple of definitions used by Touche's Nexus instrument system.
Velocity - speed and course
VMG - velocity made good wind based. The velocity directly upwind or
downwind.
WCV - waypoint closure velocity. The velocity made closing on the set
waypoint
Okay, if your instrument calculates VMG (wind based), simply steer the
boat to maximize VMG. Done!
If your instruments do not calculate VMG (wind based), here's how to get
your GPS to closely approximate it. First, many GPS systems show VMG which
is actually WCV when a waypoint is set. If you set the upwind (or
downwind) mark as the waypoint, WCV will incur more error as you approach
the layline. Visualize it. As you approach the layline, your WCV will
decrease to zero as you near 90 degrees to your course to the mark. (If
you overstand, WCV will turn negative as you've sailed past your mark.)
So, set a waypoint 10 miles directly upwind or downwind of the race
mark. Now your GPS WCV will closely approximate VMG (wind based). Steer
to maximize WCV and your course(s) to the mark will be optimized.
Actually, 3 miles might be a bit short. Excuse the tech analysis but if
one assumes you sail 0.5 miles from the rhumb line between the upwind and
downwind marks and the arbitrary waypoint is only 3 miles away, then 0.5/3
is 0.1667 the sine of which equates to an angle of about 9.5 degrees. That
would mean about a 10 degree error in your course.
Conversely, 10 miles would be 0.5/10 or .05. That equates to about 3
degrees error.
Dang, my brain hurts.
--
Dennis C.
Touche' 35-1 #83
Mandeville, LA
On Fri, Apr 5, 2024 at 5:11 PM Dennis C. captbuy@gmail.com wrote:
Purely arbitrary. 3-5 miles would probably work.
--
Dennis C.
Touche' 35-1 #83
Mandeville, LA
On Fri, Apr 5, 2024 at 4:33 PM Matthew Wolford via CnC-List <
cnc-list@cnc-list.com> wrote:
Thanks, Dennis. How did you arrive at 10 miles.
Dennis said exactly what I was thinking. I can't think of any time when VMG shouldn't be the metric. Most boats have polars that are flat or concave between 170 and 190. As I understand it this is likely because the boat has more wetted surface area when it is flat in the water. Regardless of spin or wing you should get crew weight to the leward side and cheat to the wind as much as the boat will allow to maximize VMG. A preventer helps for safety and a whisker pole helps keep the headsail open. With both you can probably come up as much as 10 or even 20 degrees - again, as needed to maximize VMG. All things being equal, jibe when your angle to the mark is slightly more than double the amount which you're off 180°. This will also be about the same time that your VMG as a percentage of wind speed will start to drop. My apparent wind meter has demarcations between 160 and 200. I try to stay out of that region...it sucks.
Josh Muckley
S/V Sea Hawk
1989 C&C 37+
Solomons, MD
Apr 5, 2024 17:31:27 Dennis C. via CnC-List cnc-list@cnc-list.com:
Personally, I think sailing to polars can be challenging. I prefer to use VMG. VMG rules for windward/leeward race courses! Bear with me on this.
First, a couple of definitions used by Touche's Nexus instrument system.
Velocity - speed and course
VMG - velocity made good wind based. The velocity directly upwind or downwind.
WCV - waypoint closure velocity. The velocity made closing on the set waypoint
Okay, if your instrument calculates VMG (wind based), simply steer the boat to maximize VMG. Done!
If your instruments do not calculate VMG (wind based), here's how to get your GPS to closely approximate it. First, many GPS systems show VMG which is actually WCV when a waypoint is set. If you set the upwind (or downwind) mark as the waypoint, WCV will incur more error as you approach the layline. Visualize it. As you approach the layline, your WCV will decrease to zero as you near 90 degrees to your course to the mark. (If you overstand, WCV will turn negative as you've sailed past your mark.)
So, set a waypoint 10 miles directly upwind or downwind of the race mark. Now your GPS WCV will closely approximate VMG (wind based). Steer to maximize WCV and your course(s) to the mark will be optimized.
--
Dennis C.
Touche' 35-1 #83
Mandeville, LA