Ok, Still seems like a lot of math in the heat of the battle.
What I would generally do, since you're gybe angle is 80 which is close
to 90 I'd site down the traveller and if the mark
is close to that line you're pretty much on the layline. You could
likely find another transient maybe a winch that was 10 deg
up if you really wanted something at 80.
That said, if this is working for you go for it. Let us know (or me at
least) how it works out. I'm interested in seeing some results.
Which GPS are you using? On the two boats I race on (mine below) we
have garmin chartplotter, on the J-105 we have
all raymarine with a plethora of data sources and displays to give us
all kinds of useful and useless information.
Cheers,
Jeff Nelson
Muir Caileag
C&C 30 - 549
Armdale Yacht Club
On 2021-10-12 4:28 p.m., Bob Mann wrote:
Jeff,
You are correct in that it should be the leeward mark is at a bearing
of 80*, and yes, leeward port layline in this example.
TWD and TWA are not the same. TWA is a reference to the boat while
TWD is a reference to compass headings.
"Since you likely know the course to the mark, and you've already
determined your gybing range (80 deg). Isn't it easier to add or
subtract 80 to
your course to see if it matches the bearing to the mark?" The point
of the math was to get to knowing that +/- 80 as bearing from current
heading. Now that we have the math we know for the future that
2*gybing angle is bearing to mark. Not everyone has a chart plotter,
but my GPS will tell me bearing to the mark. We don't know the course
to the mark; we've calculated the bearing to the mark and
adding/subtracting that from our current heading tells us the heading
we'll be on after the gybe.
Course and TWD aren't constant and thus this isn't a perfect
calculation. The bearing to the mark is the theoretical moment at
which we need to gybe before we've overstood the mark and sailed too
far. Rounding to port, we want to gybe before this bearing. Rounding
to starboard, we want to gybe after this bearing.
From the example, TWD is 270 and current course is 230, so off the
wind [stern true wind angle or STWA] by 40. The gybing angle is
2*STWA, or 80, so course after gybe is 230+80, or 310.
A different example: TWD is 75 and current course is 135. STWA is 60
so gybing angle is 120, bearing to mark needs to be 120, and course
after gybe is 15.
Just like tacking, experience will help to determine if a fudge factor
needs to be added.
Bob
On 10/12/2021 8:33 AM Jeff Nelson via CnC-List
<cnc-list@cnc-list.com> wrote:
I'm really not following your math here.
Perhaps some clarification:
TWA = True Wind Angle (I think you refer to that as TWD True Wind
Direction below) ie 270?
Course = 230 deg
Tacking Angle = 80 deg
New Course = 310 deg
Course to Leeward Mark = ?
This statement is confusing to me:
"So, when the windward mark is at a bearing of 80* I'm on the leeward
layline"
In this example we are going to the Leeward Mark (downwind mark). I
believe you are heading to the port layline,
The windward mark would be the one you previously rounded.
With all of the above...It seems like an aweful lot of math to do in
the heat of the battle. Since you likely know the
course to the mark, and you've already determined your gybing range
(80 deg). Isn't it easier to add or subtract 80 to
your course to see if it matches the bearing to the mark? This can
be achieved from your chart plotter BTW (Bearing to
waypoint) or via handheld compass if you can see the mark. Maybe
that's what the math below does, if I plugged in
the right numbers, but the closest I could get was 180 - course (230)
= 310 but if I multiply that by 2 I get a course of
260 which wouldn't be your layline, as you need a course of 310.
Also, if I use any of those course numbers, they will
remain constant, whereas the bearing to the mark will constantly
change as you get further from the rhumbline (dead down wind).
Cheers,
Jeff Nelson
Muir Caileag
C&C30 - 549
Armdale Yacht Club
On 2021-10-11 10:07 p.m., Bob Mann via CnC-List wrote:
My geometry for calculating leeward layline:
If my ideal heading for the current conditions is 230* and TWD is
270*, that means my stern has to swing 40* to become dead downwind,
and another 40* to get to the reciprocal course after the jibe. In
a perfect scenario, this means an 80* swing. So, when the windward
mark is at a bearing of 80* I'm on the leeward layline. Bearing to
the mark can thus be calculated as 2 x (180-TWA).
Bob
On 10/08/2021 11:15 AM Bob Mann <sailrm...@comcast.net> wrote:
I still need to review the geometry, but I believe that when the
bearing to the mark is equal to 180-TWA, one has reached the
downwind layline. I also found that when the masthead is pointing
at the mark, that also indicates the layline has been reached.
Bob
On 10/06/2021 11:17 AM Jeff Nelson via CnC-List
<cnc-list@cnc-list.com> wrote:
Hmmm...Interesting question.
So, Ed's answer was pretty good, I'll add a few thoughts here:
AW isn't an ideal guide, True wind is what is needed so that you
can keep the angle to true wind to be constant on
either Gybe, assuming you know what the best angle is.
If you can find polar's for your boat, they will give you an idea
of ideal.
As Ed mentioned, VMG is your friend in determining what the best
angle is (especially if no polars are available).
You can experiment in different wind speeds to find a reasonably
good angle for windspeed and VMG.
Here's a good article to get you thinking:
http://www.oceansail.co.uk/Articles/VMGArticle.php
Hope this helps.
Jeff Nelson
Muir Caileag
C&C 30 549
Armdale Yacht Club
On 2021-10-06 12:02 p.m., Bob Mann via CnC-List wrote:
I'm fairly new to flying a spinnaker. I did it last weekend in a
race and I believe I overstood the mark as we were sailing 110 AW
before the gybe and 85 AW heading directly to the mark.
How does one determine when to gybe?
Bob
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