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From: Joe Engel (no email)
Date: Thu Jul 03 2003 - 18:34:20 EDT
I was messing with the new VNS 7 this afternoon... playing with their
ETA calculator, which is a very cool feature. But I discovered
something that I should have thought about before. This has probably
been an issue since there has been an ETA calculator.
You can ask that the ETA calculations use tides and currents to
calculate the best departure time, fuel burn, etc. Real nice. But if
you ask it for the best departure time, it will calculate the best time
to leave that will give you the maximum advantage of the currents along
your route. It figures in all of the real-time currents predicted along
your route at the time you should arrive there, based upon your
estimated speed. That sounds great.
So I tried it on a route from Ilwaco WA to Neah Bay WA (West coast of
Washington State.) and it gave me a departure time of 7:27 AM. That
sounded nice. If I let at the ETA calculator's suggested time I would
save 30 minutes in total transit and about 10 gallons of fuel.
But wait a minute... I had better see what the best time to cross the
Columbia Bar is (slack current) just to make sure all is well. Oooops,
the worst time of maximum ebb is 7:45 AM. Not a good time to cross the
bar. Then it dawned on me. The ETA is calculating the best time to
take advantage of the outgoing flow. That's maximum ebb.
I tried it on a few other places where maximum current would put you
into a world of hurt, like Malibu Rapids or Tillamook Bar. It's the
same deal. It makes sense. There is no way for the ETA calculator to
determine what is safe, just what is the best advantage in speed. So if
the ETA calculator figures you can get a 14 knot advantage through the
Malibu Rapids at maximum flood, that's when it will tell you to be
there. The fact that you probably can't survive the ride is not part of
the program.
So the ETA system works well in general tidal currents but check very
carefully if you are planning on shooting one of the many dangerous
rapids, narrows and bars we contend with in the PNW. Chances are ETA
will be using the maximum speed of the current to help you on your way
and that may not be a good thing.
Joe Engel
Portland, OR
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