Ellen MacArthur wrote:
"Jeff" wrote |
Hello?! If the boat is moving 4 mph over ground, but the current is
| only 2 mph, then the boat must be moving 2 mph through the water!
| Thus the rudder works.

Well, it doesn't work very good. :-O~

| Consider also: you've been plopped in the ocean with no position
| revealing instruments, but you do have speed and wind gauges. You
| sail for some time and then get rescued. Your rescuers ask if you
| encountered any current. What can you tell them?

Nothing but I can tell them if my rudder worked or not. If there's wind
but no current then it will work in irons because the boat goes backwards.
Look at it this way. The rudder feels a current going by it. (if it could feel).

NO NO NO! This is your mistake. The rudder does not feel the current
because the boat and the rudder are always being pushed by the
current. If the boat were anchored, then it could feel the current.
Drifting free, there is no way to know there is a current. There is
no observable affect.

Another analogy: if you're flying on a plane, at a steady speed, do
you feel the chair pushing you at 500 mph? In one of Galileo's works
on "relativity" he asked if a fly in a cabin on a boat would be
affected by the boat's forward motion - would it fly any differently?

This is all the same thing. When the medium in/on which you're
traveling is in constant motion, its very hard to detect that motion.


How fast the current goes past land doesn't matter. Only what matters
is current passing the rudder. If the wind is pushing you back at the same
speed the current's going back the rudder feels no current.

Again, NO. The current is already pushing you back at the speed of
the current. This is unobservable to you, except that it alters the
perceived wind. If the wind also pushes you back that will be
"through the water" and you will sense that as sternway.

Oh, and it's
the same for trying to back the sail by hand. Even if the wind's blowing
10 mph if you're pushed backwards at 10 mph the sail won't feel any wind.
It'll think it's calm out.

As I said, if the current is the same strength as the true wind (and
going in the same direction) it will feel like you're becalmed. In
fact, it is indistinguishable from being becalmed. But this only hold
when the wind and current are the same. In general, you subtract
(in a vector way) the current from the true wind and you have the
observable wind.