Faster thanks to the VMG concept!
|13 January 2012||Posted by Alex from Lagoon under Technical world|
This is not a new insult or a new acronym from the internet culture like “LOL”. These 3 letters represent a fundamental notion in catamaran sailing, sometimes forgotten by skippers: “Velocity Made Good”. This idiom means:
– A calculation of the “efficient speed” when nearing the wind direction.
– You can also say that it is a projection of the boat’s actual speed on a fully headwind route.
Well known by racers, this concept can sometimes seem a bit complex for cruisers and it is why Edouard Paulet, owner of a Lagoon 410 S2 asked me for some advice in this regard when sailing a catamaran.
For math fans, here is the VMG calculation formula for your boat : surface speed times cosinus of the angle to wind: VMG=V.cos(α)
Let’s consider 2 cases: I want to reach a windward buoy which is perfectly headwind, and my surface speed is 7 knots.
– If the boat is on an abeam reach course, the angle to the wind is 90°.The cosinus is thus 0 and my VMG is: 7 x 0 = 0 knots. I will never reach this damned buoy!
– If the boat is motoring on a headwind course, the angle to wind is 0° .The cosinus is thus 1 and my VMG is: 7 x 1 = 7 knots.
The “efficient speed when nearing the wind direction” concept becomes a bit more comprehensible now. Anyway, let’s take a real case with small figures to be more comprehensible:
– If my boat is on a close hauled course with an angle of 40° to the wind, with a 4 knot water surface speed, my VMG is: 4 x cos (40) = 3,06 knots.
– If the angle to the wind is 50°and the surface speed 5,2 knots, my VMG becomes: 5,2 x cos (50) = 3,34 knots.
– If I bear away by 10° and my surface speed reaches 6,5 knots, my VMG becomes: 6,5 x cos (60) = 3,25 knots.
I will reach the buoy quicker with a 50° angle to the wind than with a 40° or 60° angle. It is why the VMG is also called the compromise between course and speed. Be careful though: this term takes neither the drift nor the current into account, as the reference speed is the surface speed (the speedometer speed). If you want to include these two elements you need to use the Speed Over Ground (SOG on your GPS) and the Course Over Ground (COG), which are available from your GPS,for your calculation. These two elements are used by your GPS to offer the VMC : Velocity made on Course (but generally –and wrongly- called VMG on your GPS). It is an “efficient speed over ground” notion, not related anymore to the wind direction like the VMG but to a waypoint.
This VMG occurs also when you sail off the wind. Indeed you can be faster on a broad reach by doing gybes than by sailing downwind.
If you look at the course of the boats on the ARC you can see in red the great circle sailing and in white the course of the 50 foot trimaran Rayon Vert. If we consider that the trade winds close to the Carribbean* are more or less stable in direction we can observe a zigzagging course which allowed the boat to reach her best VMG.
We find something similar with the Lagoon 560 Blue Ocean, winner of the ARC**, but with smoother angles because it is a heavier boat that reaches her max VMG close to a running course under open wind angles.
In the end, all this demonstration only confirms what all seamen know: « the fastest course is not always the shortest »!
* The wind in the Carribean is more or less from the East with some variations. We have to admit that the course of the boats in zigzag cannot be explained by the VMG only,but also because the crews wanted to take advantage of these variations.
** The Lagoon 560 Blue Ocean finished 3rd in elapsed time and 1st in corrected time in the multihull class, in front of numerous reputedly “performance” cats.