VMG Tool
VMG Calculator – what it does.
VMG, or Velocity Made Good, is the speed you’re actually making toward where you want to go – not the speed showing on your log. When you can’t sail in a straight line to your destination (because it’s directly into or it directly downwind of you and you choose to keep your speed up on a broad reach), your real progress is always slower than your boat speed.
This calculator tells you exactly how much slower.
VMG = SOG × |cos(½ × angle)|Below 180° = upwind tacking. Above 180° = downwind gybing on broad reaches.
Catamarans often gybe at 280–320° to keep apparent wind forward instead of running dead downwind.
The maths is straightforward: VMG = boat speed × cos(½ × tacking angle). The wider you tack, the less of your speed turns into useful progress.
How to use it
Three sliders, three things to set:
- Tacking angle — the total angle between your two tacks. Drag from 60° (very tight upwind, pinching) up through 180° (beam reach, going nowhere useful) and on to 350° (deep broad reach, almost downwind). Anything below 180° is upwind tacking; anything above is downwind gybing.
- Boat speed (SOG) — what your GPS reads on the tack. A performance catamaran might do 10 kn close-hauled; a cruising cat 6-7 kn.
- Leeway — how much your boat slides sideways under sail pressure. Defaults to zero. Add a couple of degrees if you know your boat slips when sailing close to the wind (say up to 3 degrees running tight on a crusing cat . The calculator only applies leeway on upwind tacking angles, fading it to zero as you bear away.
The diagram updates live: red is the port tack, green the starboard tack, and the green arrow shows your VMG vector, and the arc shows the angle between your tacks. The numbers below tell you the result in knots, what percentage of your boat speed you’re keeping, and how long each nautical mile of progress will actually take.
Where it comes in handy
The classic case: you want to sail north, the wind is from the north, and the question is how to do it. Tack through 60°? You’ll do 86% of boat speed toward your goal but you’ll be pinching and sliding and slow through the water. Tack thgrough 100° (50° TWA)? Less leeway on a crusing cat, but you’ve dropped to 64% – the wider tack is probably still faster overall.
Another interesting use case is downwind. Most monohull sailors instinctively run dead downwind because it is direct. But push the slider past 180° and you’ll see that gybing on two broad reaches at, say, a 300° tacking angle still gives you 87% VMG downwind – and your apparent wind stays forward, so the boat sails much faster. Two long legs at 13 knots will beat one slow run at 7 knots, every time. This is why catamarans, especially performance ones, almost never run directly downwind.
Have a play with it!