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# How to calculate seconds per mile

## The rating equivalency of a speed difference In sailboat racing rating is given in seconds per mile. When deciding how much difference in rating a speed advantage is, it is convenient to be able to convert between a speed difference and a rating difference. This gives a handy yardstick to decide if a particular change is worth its cost.

So exactly how do we convert speed gain into seconds per mile? In the figure, Green is covering a mile 6 seconds faster than Red. This makes Red’s rating 6 seconds/mile less than Red’s.

## Seconds per mile formula: Vs is base boatspeed
Δ is the speed difference ## Straight-line speed advantage for a Vmg difference

When there is a Vmg difference, the boatspeed equivalent is higher. For a given gain in Vmg, the boatspeed is about 140% faster. The exact amount is 1/Cos(Bt) where Bt is the true wind angle.

This speed difference to rating function is used in many situations where you need to make tactical decisions, for instance;

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