David, This may start a "my brain hurts" discussion. I just read Sailtimer's discussion here:
http://www.sailtimerapp.com/VMG.html In that article are links to additional articles which may be enlightening. Since I don't have the app, I can only surmise what the display looks like in action. From seeing a screen shot on their site, it looks like it shows you a series of optimized tack legs that you simply follow. In their article they criticize GPS based VMG and rightly so. However, their slightly negative opinion of wind based VMG is subject to debate. Let me take a somewhat circuitous route to reply. You probably already know much of what I will say. As I have said often on this list, GPS VMG is just another datapoint for the racer. For those that haven't read my opinion on it, here's a brief synopsis. First, some definitions. - Velocity is both speed and direction. - VMG - velocity made good. - GPS VMG - when a waypoint is set, this is the vector component of how the boat is closing on the waypoint. In the Nexus instrument systems, it is called waypoint closure velocity, WCV - Wind based VMG - the vector component of speed and course directly upwind or directly downwind. Must have input from wind instrument, knotlog and compass. - SOG - speed over ground - COG - course over ground If you use GPS VMG in windward/leeward racing, you need to be careful. Many racers will set a waypoint at the windward mark. Early in the windward leg, GPS VMG closely approximates WCV. However, as you near a layline, GPS VMG will tend to zero. You are sailing perpendicular to your course to the waypoint. Wind based VMG will still be accurate but will be showing how quickly you are moving upwind, not towards the waypoint, or windward mark. So, my guess is that the Sailtimer app first learns the tacking angle for your boat. Then, based on that and wind based VMG, back calculates your optimum tack(s) from the waypoint (if set) to your boat's position. As the wind changes, it moves the points at which you should tack and subsequently moves each tack leg correspondingly. In it's simplest form, it may calculate the intersection of your boat's current SOG and COG and it's calculated layline from the mark. My opinion, as a racer, is that, IF the lifts and headers aren't dramatic and/or long lasting, SailTimer's suggested tracks may work well. This would be even more true for our slow to accelerate heavy displacement boats where frequent tacking would actually penalize performance. I question whether it would be optimum for the newer, lighter more agile sport boats. I also wonder how it would react to a continuous shift. What we racer call sailing a "great circle route". Dennis C. Touche' 35-1 #83 Mandeville, LA On Mon, Oct 14, 2019 at 8:07 AM David Knecht via CnC-List < [email protected]> wrote: > I had an email debate last week with the folks who make the Sailtimer app > and wireless wind instruments. They claim their app will determine optimal > tacking angles and adjust them in real time. I was trying to understand > what the software did and how it was doing calculations and getting very > confusing (to me) answers. As an example, I asked what the software would > do if there was a header. My presumption was it would detect the shift and > give you some feedback or recommend tacking. It should not be hard to > figure out that you are going slower toward the mark (VMG). We agreed taht > VMG was problematic because it changes as you approach a mark, but their > approach was equally problematic. Here is the response I got: > > *Headers and lifts are actually another antiquated racing method, that are > very clumsy in the age of GPS and computers. They were great in the 1920s > when it was impossible to do trigonometry every second in a boat heeled > over and crashing through waves. But they make you choose some arbitrary > length of time to get an average wind direction. And they make an > assumption that the wind is going to go back to average later. If a lift > happens for 2 minutes, why call that a lift and not say that it is the real > wind? Too many assumptions. * > > *They are not necessary; why not just always sail on the optimal course > to get you to the waypoint fastest? * > > *If the wind changes while on the proposed course, the green line moves, > and you just keep on following it. There is no such thing as lifts and > headers from some arbitrary time interval in which the wind direction is > averaged. Your goal should be simply to always follow the optimal tacks. * > > That answer makes absolutely no sense to me. Their optimal course is > based on polars as near as I can tell. More importantly, they are arguing > that there is no advantage to tacking on a header. Yes, there is a > tactical argument as to whether you would tack on every shift in a large > keel boat where tacks are slow relative to continuing straight, but in any > significant shift, my years of racing experience plus the math of the > sailing angles argues to me that Sailtimer's explanation is bogus. Am I > missing something? Dave > > S/V Aries > 1990 C&C 34+ > New London, CT > > > _______________________________________________ > > Thanks everyone for supporting this list with your contributions. Each > and every one is greatly appreciated. If you want to support the list - > use PayPal to send contribution -- https://www.paypal.me/stumurray > >
_______________________________________________ Thanks everyone for supporting this list with your contributions. Each and every one is greatly appreciated. If you want to support the list - use PayPal to send contribution -- https://www.paypal.me/stumurray
