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

All optimization algorithms that can use constraints are victims of
inadequate human input expressed as constraints. In this simple case, it
could be made more useful and less dangerous by applying a time and/or
location constraint. Maybe the program could give you optimal speed of
advance settings once outside the bar. In other words, it could tell you to
go to WOT to catch a current enroute or to slow down because you will be
struggling against a tide somewhere else along the route. At least for PNW
mariners, I hope so. Here, on the less challenging East Coast, we can
probably get a good result. On some inlets, however, you still want to exit
or enter at slack or sometimes exit at ebb depending upon the wind.

You didn't tell us, how do it know about the weather? Even the mouth of the
Potomac can be uncomfortable with opposing wind and tide. I'd sure like to
see it compute Annapolis to Cape May - I always have to figure that one out
carefully.

Ron Rogers
Annapolis, MD
_/)

----- Original Message -----
From: "Joe Engel" joe@jre.com
|
| 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.