[Ayrstest] somestuff 6

[Dave Culp]

Delivered-To: daveculp@atlas.dnai.com
X-Sender: tonybull@pop-server.san.rr.com (Unverified)
Date: Wed, 30 Aug 2000 21:54:58 -0700
To: Dave Culp <dave@dcss.org>
From: Anthony Bull <tonybull@san.rr.com>
Subject: Re: AYRS DIGEST 11-14 August, 2000
Status:  U

>  On another list, there was a discussion about apparent wind upwind 
>which caused me to post a little story designed to make one examine 
>these concepts carefully.
>[Post it here again if you like, Tony. It was great fun!  --DAC]
>

OK.... Here it is:

   One fine day a figure stood on the south bank of an estuary, looking
north.  It was a fine day,indeed, and behind him on the airstrip the
windsock showed the wind to be in the west. It was in fact blowing at
exactly 10 knots, and blowing due east. The estuary was remarkable; the
water was always smooth, no matter what, and the wind velocity was constant
all the way across to the other side. The wind velocity was also constant
from the surface up to about 100 feet above the water,and there was no
change in this constant velocity profile no matter which way it blew.

   The waters here experience interesting tidal flows.  In fact, at the
moment, the water was flowing at exactly 10 knots, and it was flowing due
east.  It would do this for several hours before abruptly stopping at slack
water, later in the day.

   Our figure on the shore sees a craft out on the water. It appears to be
moving east at 10 knots!! "Why," he thinks, "there is no wake.  Must be one
of those hi-tech multi-hull things.  Very efficient hull and rig, I
believe."  He raises his binoculars to his eyes. The bows are pointing due
north! "Hmm...", he thinks, "moving east at 10 kts, heading north, .... 90
degrees of leeway angle!!!!  Now I'm not so sure about this hi-tech stuff.
They may be fast, but I don't think I could live with that kind of lee way".

   "No way..." says the multihull mariner out on the boat, "no way my luck
can get worse.  Such a fine day, but here I am dead in the water...not a
breath of wind. No steerage way either. Oh well, I guess I'll pop this GPS
card out of my hi-tech, low battery-drain, but orphaned, Newton 2100; pop
in the cellular modem and download multihulls digest."

   The multihull mariner, a man of infinite resourse and sagacity, may have
built his vessel from bamboo and tarp with his own hands, but he is
internet-savvy. He may be becalmed now, but he can read his e-mail while
waiting for the breeze.  But he has not, O Best Beloved, seen the windsock
standing out in the 10 knot wind. (And you *can* forget the suspenders).

   Suddenly the windsock droops.  The wind has gone!!  The multihull
mariner, however, feels a breeze spring up out of the east.  A good 10
knots!!  Way to go!!  He sets his sails and gives our observer on the shore
something more to marvel at. "Wow," the figure thinks, "look at that
multihull thingy go!  He's having a great time, and there isn't even any
wind blowing!"  He sneaks a look over his shoulder and sure enough, the
windsock is limp.   "WOW!!  Way cool!!"

   Way.  Party on Garth. For what is this we see??  A line of nasty jagged
rocks runs north-south across the estuary, their black tips just breaking
the surface to the east of our mariner.  He sees them.  A pod of orcas
perhaps.  Must be a SeaWorld around here somewhere for them to have learned
to keep in line like that while they swim towards him . Perhaps it *is* a
hazard; but at least it is to windward.  He knows all about the dangers of
a lee shore. Anyway, his multihull digest is fully downloaded, so he'll
just head up into the wind, drop the anchor and read a while.

   Our mariner is about to get a surprise as the anchor gives him a means of
reference to the ground which he lost when he removed his GPS card.  I
think he will be interested to see how the anchor chain sets.

   The figure on the shore, still below the drooping windsock, now sees
something else of interest; two more craft are out there on the windless
estuary. One is a strange beast, with a prodigious stack of kite-foils
flying from the rail.  Somehow this macho speed-sailor has managed to
launch his kites apparently in only 10 knots of breeze...but he can sail
far faster than the wind. The other craft is also strange...a two winged
craft which has just managed to pull itself up onto three foils... now that
it is up, it is fairly whizzing around.  These two intrepid exploiters of
interface energy have also noticed the rocks. They have no place on their
craft for tedious ground tackle. Can they truly sail away from this
apparent hazard?  If they can't sail away, can they sail north-south, or
close to it until slack water?  Which of the two is more pleased with his
craft's beta right now? :-)  Is it truly possible to truly sail apparently
downwind apparently faster than the true wind. But wait, there is truly no
true wind... look at the windsock...apparently there is apparent wind..
Heck, should we say sayonara to these two?

   Our confused observer decides to head for the bar.  As he leaves, the
windsock fills again, this time the wind is 15 knots out of the east.  All
is well, but will our questions be answered?  (This is your cue, Dave)

   Crossing the huge runway to the bar, our figure sees a strange wheeled
cart with a futuristic rig moving swiftly on the concrete.  He overhears
one of the onlookers say,  "I had one but the wheels fell off.  Good thing
actually. Those new low rolling-resistance wheels and zero friction hubs I
installed have taken at least 3 degrees off my beta."

   The barman gives our onlooker his usual pure gin and tonic.  "Fine day
today!" he says.


    Tony