[seafarer]

In high winds and heavy weather I have found that the navigators estimates can vary wildly, even when asked for over short times. Remember, it's just an estimate, based on the specifics of the moment. You say your speed is 6kn, but in those seas, it will vary quite a bit from moment to moment.

You have to treat the nav's estimates as just that, guidelines to better aid your judgement, but not hard and accurate, precise measures. Always allow yourself a good safety margin on based on your own periodic looks at the fuel gauge.

The only time the nav's estimates will be anywhere near consistent from time to time, is in flat calm (no wind at all) seas.