VMC

VMC iPhone App

App approved, get it on App Store!

I've created an iPhone app to calculate Velocity Made on Course (VMC). It's really easy to use, and I'm planning a future update to include Apple Watch support.

VMC is a super helpful measurement that a lot of people I've talked to, either don't know about or mix up with VMG.

The concept of VMC is most easily understood when considering sailing directly or nearly directly downwind. As most sailors know, sailing straight downwind, even if going wing-on-wing ("πεταλούδα"), is not really efficient sailing  as it primarily relies on wind pressure rather than lift.

Plus, heading straight downwind can be a bit risky since accidental jibes can happen more easily. That's why you'll often hear that it's best not to go wing-on-wing if the wind is anything more than a gentle breeze. 

This inefficiency when sailing directly downwind is visually represented in a yacht's polar diagram, which typically looks like this:

Usually, the faster and lighter a boat is, the less efficient it is to sail directly downwind.

And it's pretty common knowledge that a broad reach is much faster than a run.

But, if your destination – your next waypoint – is directly (or almost directly) downwind, sailing on a beam reach will make you go faster through the water, but you'll have to sail a longer distance.

So, the big question is: what's the best course to steer to get there fastest?

This depends on tons of things, like wind strength, how choppy the sea is, how heavy your boat is loaded, and what sails you're using.

And to make things even trickier, as you get closer to your destination, your speed towards it (the part of your speed pointing that way) gets slower as the angle between your destination and where you're going gets bigger!

So, even if you know your boat really well, it's tough, if not impossible, to know exactly how deep you should sail for the best progress.

The same idea applies when going upwind, although maybe not as much. But with fast boats, especially multihulls, sailing at 50 degrees upwind instead of 45 can make a big difference in speed, and you MIGHT actually get there faster by going a bit wider.

For a more detailed explanation of VMC, you can refer to this resource: https://www.navigation-mac.fr/vmg-vmc-pour-les-nuls/?lang=en

Ultimately, I created this VMC app for my own use, but it's become something I find genuinely helpful, so I wanted to share it with fellow sailors.

I'm really interested in your experiences and any ideas you have for making this app even more valuable for everyone. Please share your comments and suggestions below!

Screenshots and explanations

Upon launching, the app displays a regional map with the user's current location clearly marked.

Initially, no waypoint is set, and therefore Velocity Made Good (VMG) is not calculated.

To set a waypoint, the user simply taps on the screen.

Once a waypoint is established, the app displays the following information:

where:

• 1. Speed Over Ground (SOG): The boat's current speed in knots.
• 2. True Course Heading: The direction of travel in degrees, visually represented by a green line with an arrow (8).
• 3. Bearing to Waypoint: The direction to the waypoint, indicated by a red line connecting the user's position to the waypoint (7).
• 4. Angle: The angle in degrees between the boat's course and the bearing to the waypoint.
• 5. Distance to Waypoint: The remaining distance to the designated waypoint in Nautical Miles
• 6. Velocity Made Good (VMG): The speed at which the boat is progressing directly towards the waypoint.

Tapping the blue gear icon located at the bottom right navigates the user to the settings screen.

Here, they can customize options such as map orientation, map type, and GPS signal smoothing. The smoothing setting is implemented to mitigate inaccurate readings caused by GPS errors, with the medium (default) setting providing optimal performance in most scenarios.

The app also runs as a full-screen native iPad application, as shown in the following screenshots:

While, as shown in the ipad screenshots, it can technically be used on land, its primary purpose is for marine navigation, hence the units are fixed to Nautical Miles and Knots.

It's also worth noting that in the second iPad screenshot, where the angle between the actual course and the direction to the waypoint exceeds 90 degrees (it is 104 degrees), the VMG is displayed as negative. This indicates that the vessel is moving away from the intended destination.

Despite the obvious overshoot in the second screenshot, out on the water, without a bird's-eye view and in demanding conditions, even seasoned sailors can get too close to, and occasionally exceed, a 90-degree angle relative to the waypoint.

This results in a near-zero or even negative Velocity Made on Course (VMC), meaning you're barely progressing or even moving away from your goal. The VMC app directly addresses this challenge by providing skippers with continuous, precise VMC calculations, enhancing their situational awareness and giving them the control needed to maintain an optimal course.

So, that more or less, covers it!

Fair winds,

George Theotokis

PS:
PRIVACY POLICY as needed by App Store:

"NO DATA COLLECTED!"