What Is VMG ?
We’ve haven’t written anything here for a while so this is us getting back into the habit…
VMG (Wind) - Distance travelled towards the wind divided by time.
VMG appears on a lot of marine instrument systems but is in our opinion not well understood.
The standard definition is;
VMG = Velocity Made Good
But Velocity as any GSCE Physics student will tell you is calculated based on time and distance travelled.
To understand VMG you need to understand the distance travelled part.
So where are we actually travelling to ?
There are actually two distinct forms of VMG.
Velocity Made Good relative to the wind
Velocity Made Good relative to the course towards a waypoint.
The latter in NMEA terms is actually VMC Velocity Made good on Course.
VMG can come from two calculations;
VMG (Wind) - Distance travelled away from the wind divided by time.
The distance towards the Waypoint divided by the time component - then standard calculation that the GCSE Physics student would understand that most GPS units are able to calculate. Its about how fast you are travelling towards your destination when you remove all the other factors like tacking/gybing and tide. This is also more correctly referred to as Velocity Made Good on Course or VMC.
The alternative is to look at the Wind direction and calculate the progress of the boat either upwind or downwind. Its like putting an imaginary waypoint directly upwind or downwind of the boat and working out how quickly you are progressing towards that point.
The key difference between method 1 and method 2 is that method 1 works if you are sailing to a mark on a beam reach. So if you are sailing offshore then calculation 1 is more useful. Calculation 2 works if you are sailing on a Windward Leeward course where (if the race officer laid a good course) you are nearly always trying to optimise your route to the windward or leeward.
So what’s my instrument actually showing ?
Skimming the manuals for the most popular instrument manufacturers;
VMG (Waypoint) or aka VMC - Distance travelled towards a defined point divided by time,
Garmins GN120 will display the VMG (Waypoint) as it calls it so thats actually VMC. They also do VMG to the startline -> which in effect is another VMG to a fixed point. The smaller instruments GMI10/20 will display VMG to either Waypoint or Wind.
B&G and Raymarine are using VMG based on the progress towards the wind.
NKE talk about “…the component of velocity that is in the direction of the mark, should it be upwind or downwind. In the case of sailing upwind, it is actually the speed towards the wind. It is a useful indication” Which I think makes it the velocity towards the Waypoint on the assumption that the Waypoint is to Windward.
The Garmin, B&G and NKE systems are clearly doing a lot more maths in the background to determine the best course to windward or leeward.
This is also particularly important if you are using the data to drive an autopilot. The more sophisticated autopilots can use the VMG to pick the best course to windward or leeward based on the polar information for the boat.
The autopilot computer is combining the polar data with the current wind, tide and sea conditions to pick the optimal heading for the best VMG.
So what else do we need to consider ?
Well in our humble opinion VMG Wind is not a great number to steer to.
Steering to VMG leads to weaving and reduced progress towards your goal.
Think about it this way; whats the best way to raise your VMG Wind.
Obviously sail closer to the wind right ?
But if you do that the boat will eventually slow down - It won’t be an instant slow down due to the boats momentum. Initially the VMG Wind goes up but as the speed drops off you will find yourself sailing a higher course at a slower speed with ultimately a lower VMG.
VMG is actually a number that is more useful to the Navigator than the helm; and what’s useful is an average VMG over a period of time. So in the last minute our VMG was X as opposed to the sub-second update rates you might see on a cockpit display.
If I’m seeing a prolonged heading wind shift my VMG Waypoint might go down but my VMG Wind will stay the same if the helm is steering to the VMG Wind or apparent wind angle.
If its going the other way I should see my VMG Waypoint go up … but the VMG Wind will potentially go down if my helm doesn’t spot it and affectively turns the boat away from where you actually want to go.
We have polars for our boats and know the optimum angle to sail for a given wind speed, upwind or downwind; We know the target boat speed for a close/beam reach so we can tell the helm the wind angle to aim for and the boat speed they are looking for. As the Navigator I can then validate the progress I’m making using the VMG Wind or more likely VMG Waypoint. I can see the impact the tide and leeway are having on my progress towards my destination; and I can chose the tack/angle of attack accordingly.
So what should I actually tell the helm to follow ?
We use two numbers;
COG - Course over ground
BTW - Bearing to Waypoint
But generally we get the helm to sail to water speed and apparent wind angle.
Use a combination of data points to understand your boats progress
As the helm your best outcome is to match the COG/BTW which basically means that the boats Course (not to be confused with Heading) is directly at the target Waypoint.
Clearly on a beat thats not going to happen until you reach the layline but on any kind of reach those numbers will tell you whether you are pointing the boat in the right direction.
The biggest thrill coming into the JOG finish line at Cowes from the mainland shore is matching those two numbers up and ferry gliding a boat across the tide at 10knots with a spinnaker up. The VMG Wind in that situation doesn’t matter; what matters is that you are sailing the shortest course to the last Waypoint on the line.
You don’t actually need the data on the instruments in a lot of cases to do that; you can line the target mark up with a building on the Cowes show line and follow the transit they create in.
Ultimately though the Helm is steering the boat to the best speed/wind angle that keeps the BTW and COG within a couple of degrees of each other.
Too high and you will have to go low and slow in the run in to the waypoint or mark. Too low and you may be forced to sail higher as you reach the mark and have to drop the spinnaker early.
So the helm will weave with the gusts, the lulls and the waves to get the target speed through the water but keep an eye on the BTW/COG numbers to make sure they are staying on the part of the race course that the Navigator thinks will get the boat to the next waypoint quickest.
One minute they might be above the BTW the next below but on average the long term COG matches the BTW.
Other things to consider
The difference between the SOG and the Water Speed,
This gives you an idea of the amount of tide you are in and is a good sanity check that the Navigator has put the boat on the right part of the race course. Sailing in areas like the Solent being in the slower or faster moving water can be worth an extra one to two knots of boat speed.
The water depth.
Deeper water generally flows faster. Its another sanity check for the place you are sailing on the race course. Should you be sailing against the tide in the deep water ?
Data Visibility for the whole crew.
Put distance to waypoint and speed though the water where everyone can see them.
This has two advantages.
One its a sanity check for what the Navigator is doing;
and it stops everyone asking “are we nearly there yet”
Set the route up on the chart plotter before the start.
Keying the route into the chart plotter before the race start takes the pressure off the navigator. Most plotters will automatically increment to sailing to the next waypoint when you reach the current target. So the Navigator can setup the course for the helm to follow before the start and spend the rest as ballast on the rail. Ideally they will have visibility of the course via a tablet repeater for the chart plotter so they can answer questions without diving down stairs…. but don’t try and put the course in on the tablet the process is painful especially if its raining.
Put extra waypoints in to guide you to the fastest part of the course.
It also makes sense to put waypoints in to guide the helm around the parts of the race course that you think are favoured. So if you want to do Cowes to the Nab Tower put a couple of waypoint in to help the helm keep the boat in the fastest (or slowest) moving part of the tide. In areas with significant tides or wind development patterns the straight line route to the destination is rarely the quickest.
So what can we conclude from this ?
There are a couple of things to take away.
Make sure you know which VMG you are looking at. You will probably have to read the manual for your instrument system to work that out.
Make sure you have all the right data points and they are correctly calibrated. To get an accurate VMG Wind you need good wind angle data.
Make sure everyone else on the boat understands which VMG they are actually looking at.
Use a transit to sanity check what your instruments are telling you.
A helm steering to instruments will tend to oversteer if the VMG data is used as a progress target and that is slow.
That’s given everyone something to think about… !
Bill/January 2021
For the record the thumbnail photo that goes with this post is from Rick Tomlinson and is still one of favourites from the last couple of years.