I've really resisted jumping into this one, but....

I guess that if you consider VERY high speeds (approaching the speed of
light), it does go asymptotic. However, at anything significantly below
that, to suggest that there is some "brick wall" velocity that cannot be
surpassed no matter how much thrust is applied is just plain incorrect.

Now, the thrust required may not be achievable by any normal engineering
and forces on the hull may cause any it to disintegrate if built out of
any of the standard materials and methods. And even if you could build
it strong enough, stability and control problems will rear their ugly
heads when you start going fast enough. Afterall, if you could keep it
from pitch poling first, and pushed my Tayana fast enough, you'd get
seperation occuring with the water flow where my hull starts curving
back to the stern. Once the rudder is high and dry in this seperation
area, it'd be a might difficult to control.

So in the real world, I guess that there is a limit to how fast my
Tayana can go no matter how many JATO units you strap onto it. But it
is a pratical matter, not some theoretical "hull speed" value beyond
which the resistance somehow becomes infinite.

A quick thought experiment to show the point: Picture yourself speeding
along on glassy smooth water in a speed boat. You have a small model of
a displacement hull sailboat in your hand (only a few inches long).
Now, since the hull speed of this model is only a fraction of a knot, if
there were a hard and fast limit beyound which it is impossible to go,
then you could not reach over the side and drag this toy through the
water. It would wrench your arm off. If you had it somehow affixed to
the boat, it would stop the speed boat cold.

Can we find something else to agrue about now?



Rodney Myrvaagnes wrote:
You are using "asymptote" in a metaphoric, rather than mathematical
sense.



On Tue, 31 Aug 2004 13:31:46 -0400, DSK wrote:


Looks like it to me. It's just much farther to the right on the graph


Rodney Myrvaagnes wrote:

In neither case does it get vertical, as an asymptote would.

1- an asymptote doesn't have to be vertical (or horizontal) on the graph

2- if the power/speed curve does not go vertical (or approach it very
very very closely) then you're saying that the boat can reach infinite
speed. This is impossible, nyet?


In the case of planing boats, the slope of the curve doesn't even
increase everywhere, buty goes over a hump at the onset of planing.

Depends on the boat. Some don't have much of a hump at all.

"Hump" is kind of a misnomer IMHO... what happens is that the boat's
power/speed curve trends increasingly upward as marginal power increases
faster than speed, then flattens out again as it starts planing. It's
not a hump, more of a plateau or shelf.


But ordinary medium-to-light-displacement sailboats zip right past
hull speed when the wind rises.

Do they reach infinite speed?

You're right about fast boats zipping right past hull speed like it's
not there... that's why I always say that "hull speed" is not a hard
limit... also you have to consider the speed-length ratio (or Froude
number if you prefer) is not the same for all boats. Two different boats
(say, a J-35 and an Island Packet 35) should not really have the same
"hull speed" even if their LWL is exactly the same.

The power/speed curve of all boats... power, sail, diplacement, planing,
mulithull, whatever... trends toward vertical as the speed increases.
For some boats it's way to the right, at impressively high speeds. But
it's there!

Fresh Breezes- Doug King



Rodney Myrvaagnes NYC J36 Gjo/a

"Religious wisdom is to wisdom as military music is to music."

--
Dan Best - (707) 431-1662, Healdsburg, CA 95448
B-2/75 1977-1979
Tayana 37 #192, "Tricia Jean"
http://rangerbest.home.comcast.net/TriciaJean.JPG