Great summary Dennis, thanks for that. Ken H.
On 13 October 2013 22:35, Dennis C. <capt...@yahoo.com> wrote: > Did some more scratching around on wind based VMG. I hope I'm correct on > all this. :) > > First, let's define some terms and abbreviations > > > - VMG - Velocity Made Good. Velocity directly up or down wind > - WCV - Waypoint Closure Velocity. Velocity towards a waypoint. > (Shows on GPS as VMG) > - TWA - True Wind Angle. The angle of the boat to the true wind. > - TWS - True Wind Speed. The speed of the wind across the water > - AWA - Apparent Wind Angle. The angle of the wind across the boat** > - AWS - Apparent Wind Speed. The speed of the wind across the boat** > - BSP - Boat Speed. The speed of the boat through the water** > > Note: The only one of the above that requires a GPS is WCV. The remainder > require either a wind sensor, a knot transducer or both. The ones marked > ** are observed, the rest are calculated. > > Also note that TWA should not be confused with true wind direction, a > compass bearing. None of the above require a compass. > > The calculation of VMG is a bit obtuse. > > In order for your instrument system to calculate VMG it must first > calculate TWA. It does this from BSP, AWA. and AWS (apparent wind speed). > It's vector addition or trigonometry. > > Here is a decent explanation and calculator with diagrams. > > http://www.sailingcourse.com/keelboat/true_wind_calculator.htm > > Next, it calculates VMG. Simply put, > > VMG = BSP x cosine (TWA) > > Here is another site which may help visualize it. > > http://lagoon-inside.com/2012/01/faster-thanks-to-the-vmg-concept/ > > Now, let's point out that VMG does not consider current. It does, > however, contain leeway because of the apparent wind component. > > Now let's look at WCV. Many GPS units display VMG. This is really WCV. > It is the velocity at which the boat is approaching the active waypoint set > in the GPS. A stand alone GPS has no idea what the wind is and therefore > can not calculate wind based VMG. WCV will work at the start of a race or > cruising leg, but will become less useful as the boat sails up the course. > This becomes apparent as the boat approaches the windward or leeward > laylines and the racing mark has been set as the active waypoint. As you > approach the laylines, WCV will tend to zero as you are sailing past the > waypoint at a 90 degree angle and are no longer closing on the waypoint. > In fact, many GPS units will display negative WCV once you have overstood > the layline. A useful tool if you also have instruments that calculate VMG. > > So how do you use your GPS to approximate VMG for a windward/leeward > race? Simple. Before the race begins, turn head to wind and observe the > true wind direction. Next, enter a waypoint in your GPS that is 20+ miles > beyond, and directly upwind of the windward turning mark. > > Now sail your race maximizing WCV. In some ways, this may be better than > maximizing wind based VMG because it accounts for current. > > Darn, my brain is tired. I need to go chat with my friend Johnnie Walker. > > Dennis C. > Touche' 35-1 #83 > Mandeville, LA > > _______________________________________________ > This List is provided by the C&C Photo Album > http://www.cncphotoalbum.com > CnC-List@cnc-list.com > >
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