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> mailto: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> mailto: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 > > > > > > > > > > > > > > > Thanks to all of the subscribers that contributed > > > > > to the list to help with the costs involved. If you want to show > > > > > your support to the list - use PayPal to send contribution -- > > > > > https://www.paypal.me/stumurray Thanks - Stu > > > > > > > > > > > > > > > > > > > > > > https://www.avast.com/sig-email?utm_medium=email&utm_source=link&utm_campaign=sig-email&utm_content=emailclient > > > > Virus-free. www.avast.com > > > > https://www.avast.com/sig-email?utm_medium=email&utm_source=link&utm_campaign=sig-email&utm_content=emailclient > > > > Thanks to all of the subscribers that contributed to > > > > the list to help with the costs involved. If you want to show your > > > > support to the list - use PayPal to send contribution -- > > > > https://www.paypal.me/stumurray Thanks - Stu > > > > > > > > > > > > > > > > > > Thanks to all of the subscribers that contributed to the list to > > help with the costs involved. If you want to show your support to the list > > - use PayPal to send contribution -- https://www.paypal.me/stumurray > > Thanks - Stu > > > > > Thanks to all of the subscribers that contributed to the list to > > help with the costs involved. If you want to show your support to the list > > - use PayPal to send contribution -- https://www.paypal.me/stumurray Thanks > > - Stu >
Thanks to all of the subscribers that contributed to the list to help with the costs involved. If you want to show your support to the list - use PayPal to send contribution -- https://www.paypal.me/stumurray Thanks - Stu
