Beat or Motor Calculator
The harbour is twenty-four miles away, dead to windward, and you have four hours of daylight and forty litres of diesel. Every VMG calculator will tell you your velocity made good. This one tells you whether to beat or start the engine.
The distance is not yours to choose
Sailing two tacks at the same angle to the wind, the distance made good to windward is the sum of both boards multiplied by the cosine of that angle. Rearranged, the total distance you sail is the rhumb line multiplied by cos β ÷ cos α — and the split between the boards has cancelled out. That is the part crews argue about and the part that does not matter. Beat in two long boards or twenty short ones and you sail the same distance; what short tacks buy you is proximity to the rhumb line, which is where a windshift can be turned into a gain rather than a loss.
What does matter is the angle your boat tacks through, and here the difference between boats is larger than most crews assume. Five degrees of pointing, the kind of thing a new headsail or a properly tensioned rig buys you, is worth about four percent of the distance on every windward leg you sail for the rest of the boat’s life. On a twenty-four mile beat that is very nearly a mile.
Work out the true wind from what you feel on deck with the True Wind Calculator, get the rhumb-line distance from the Sailing Distance Calculator, and check your arrival against the gate with the Tide Height Calculator.
Frequently Asked Questions
How much further is it when you have to beat?
Exactly cos β ÷ cos α, where α is half your tacking angle and β is how far the destination lies off the true wind. Dead upwind in a boat tacking through 90° that is 1 ÷ cos 45° = 1.414, which is where the familiar advice to add about 40% comes from. It is worth knowing the figure does not depend on how you split the tacks: one long board and one short costs precisely what a dozen short ones do. Only the number of tacks is yours to choose; the distance is fixed by the geometry.
Is the 'add 40%' rule good enough?
Only for the one case it was built from. A boat that tacks through 80° pays 31%, not 40%, and a boat that tacks through 100° pays 56%. The bigger error is at the other end: if the destination lies outside your no-go zone you lay it on a single tack and pay nothing at all, yet the rule still adds 40% to a leg with no beat in it. On a 24-mile passage that is ten invented miles, which is the difference between arriving in daylight and arriving in the dark.
When is motoring actually faster?
Whenever your speed under power beats your close-hauled speed divided by the windward penalty. The distance cancels out of the comparison entirely, so the crossover is a property of the boat and the wind angle, not of how far you are going: a boat making 5 knots close-hauled on a dead beat through 90° needs only 3.5 knots under power to get there sooner. That is why motoring so often wins on a beat and so rarely wins on a reach.
Why enter the speed I expect rather than my engine's cruising speed?
Because a beat is upwind by definition, and upwind is where the sea is. Punching into a short head sea costs a displacement hull a knot or more against its flat-water speed, and the figure on the engine plate assumes neither wind nor waves. Enter the speed you expect to make good in the conditions forecast. The same caution applies under sail: a boat pinched to its tacking angle in a seaway sails slower than the same boat cracked off a few degrees, so the honest comparison uses honest speeds on both sides.
What does this deliberately leave out?
Leeway, tidal set and windshifts. All three matter and none of them is a constant you can put in a box: leeway depends on your hull and how hard you are pressed, tidal set reverses through the day, and a shift can turn a beat into a fetch or cost you a whole extra board. Every one of them makes the real passage longer than the geometry here, so treat this as the floor on the distance rather than the expectation, and consult the tidal atlas before you commit to a side of the course.
A planning estimate from the geometry of the windward leg. It does not model leeway, tidal set or windshifts, each of which makes the real passage longer. Not a substitute for a passage plan, the tidal atlas, or the forecast.