The setting 'minimum speed to sail' is the minimum Speed Over Ground (SOG) your boat needs to have in order to sail. If the boat is moving slower than this savvy navvy will suggest you turn the engine on. 

For example: if there's moderate but favourable wind, you might get a decent boat speed of six knots, but because of a strong current your actual speed over ground is more like three knots. 

If you have set your minimum sail speed at four knots, savvy navvy will recommend that you turn on your engine for that part of the route. 

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