Another question about fuel

[Phobus]

Greetings,

I posted a earlier about running out of fuel after I had dived.

Now can some one answer this question. From my present patrol.

The situation, I am running at 6kts. Wind 15 m/s 189 deg
1538 hrs request Nav for duration. Answer 14,521 kms
1948 hrs request nav for duration. Answer 13,521 kms. difference 896 kms
2359 hrs request Nav for duration. Answer 13,166 kms. difference 459 kms

I am traveling from 'A' to 'B' in a straight line at speed of 6 kts. distance covered in first 7 hrs 21 mins circa 42 nm.

So why has my duration dropped by 1,355 kms?

I could expect some descrepancy due to the wind factor, but this amount?

At this rate I could well be out of fuel again before getting home.

[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.

[TomcatMVD]

What's your heading? Wind seems quite strong.
The NO calculation not only depends on the engine RPM but also on the exact speed at the time when asking for range. NO assumes that speed will be constant, and if there's a slight difference in speed between the times you've requested the range, multiplying that for the lenght of the trip, gives you quite a big difference between readings.
That's my best bet.
P.s.:Also, I thing the NO is not error-proof

[Storabrun]

If you want a better estimate you need to bumb TC up a bit (64 or so) and then spam the button for a while. Then you will get values like 11700-12500 or something like that. I'm not suggesting that you calculate the average, just look at it for a while and you will get a feel for it.

What you absolutely can not do is what you just did. The variations are big enough that you could end up with a better range after 7 hours. Obviously you have not refueled even if the numbers seem to indicate that;)