VMG / VMC

VMG

Most of us are familiar with the term VMG, Velocity Made Good, which is the speed you travel up against the wind (sometimes indicatated as a positive speed) or down, “away”from the wind (sometimes indicated as a negative speed). This is a trade off between the boatspeed and the distance traveled to find the sweetspot for the highest possible VMG. Theoretical there is one sweetspot for a given windspeed, in real life a lot of factors will affect this and you sometimes need or can trade off speed and distance to deal with current situations. In a polar it is easy to find the VMG on the graph perpendicular to the speedaxel as shown in the picture, where the yellow fields start . Just find the highest point on the graph.

Upwind VMG

Here the max VMG is on a TWA 43 deg and a boatspeed of 5,5 kts. This is generally calculated as (cos 43) x 5,5 kts and gives us a VMG of 4 kts. The angle can also be described as a difference between the heading and the true wind direction (TWD). So if we were to sail a heading of 80 deg and the winddirection (TWD) is 37 deg we get a true wind angle (TWA) of 43 deg on port tack, sometimes described as -43 deg ( port-red-negative). This formula can then be used all over the polar to calculate the VMC.

Downwind VMG

VMC

Velocity Madegood to/on Course. Why do we want to know VMC? The average speed in any race is important. In upwind/downwind racing we mostly sail on max VMG to maximize the speed upwind and downwind just to try to get to the next mark as quickly as possible. In longer races which will include a lot of reaching the average speed is still very important so we want to sail the boat in highspeed angles, upwind/downwind is less important and we need something else to measure our actual speed. Now our reference is no longer the winddirection but a fixed point somewhere we would like to go, the next mark, an Island, a point/cape or just a wpt. Most boats increase in speed with windangle to a certain point. This can be used sometimes if the winds are shifting. Some polars has dents in it which can be used even if the winds are not shifting.

If we were to sail this boat with this polar we have a dent in the polar which makes things interesting. In this case the brg to the next mark and the TWD will put us on a lowspeed windangle. On both sides the boat will go faster but will the speed be enough to kompensate the longer route..?

Reaching on TWA 85 deg with a targetspeed of 6,7 kts. The maximum speed in this direction. If we apply the same principle on this as we did on VMG we can graphically see we have a lot more speed to squeeze out of the boat.

Sailing at TWA 114 deg gives us a boatspeed of 8,4 kts. The VMC is then cos(difference in angles) x boatspeed, cos(114-85) x 8,4=7,6 kts, a lot faster than 6,7 kts. Sailing in this direction however will not take us to our destination so we can only use this if the winds are shifting and doing the turn for us. Depending on the shift we can use some or all of it. If the polar has a dent like this we can allways change course and use the bump on the other side. This will cost some when changing directions but in the long run it will be worth it.

From upwind/downwindsailing without electronics we come in contact with a kind of speedchart mostly used on downwind legs. It’s basically a table based on how much speedchange is needed to compensate a higher angle. If we use the chart on this example we can clearly see that we are well over minimum speedchange for the 29 deg change in angle, so yes, we will easily cover the longer route with this speedchange.

In the 7 kt col on the 30 deg line we find 8 kts of speed, in the polar we get 8,4 kts, well over the limit. If we calc this we get even better numbers.
The factor is 1/(cos diffangle). So if we were on  TWA85 and wanted to sail on TWA114 we get a diffangle of 29 deg. To brake even we need a minimum speed of 6,7 kts x (1/cos 29=1,14) = 7,6 kts.

Sailing on the bumps will increase the average boatspeed. Turning the polar 90 deg and comparing it to VMG, it’s a lot like tacking upwind or gybing downwind, same principle, to reach the mark.

It can sometimes get a bit confusing and some of the confusion comes from the use of VMG on GPS. A GPS will use the term VMG to display the actual speed towards a waypoint, this is not really the correct term but as the GPS is not dedicated for sailors alone this is what we have to live with. Only sailors will have the use of separating VMG and VMC just due to the fact that sailors use the wind for propulsion and need to know the angles. So when sailing the VMG should in most cases be read as VMC on the GPS and when motoring it’s VMG.