In message <uy53f.2$25.88@news.uchicago.edu>,

In article <5TIvSTDhHOTDFwoJ@baesystems.com>, Richard Herring
<junk@[127.0.0.1]> writes:

In message <434C0808.F1A91399@hate.spam.net>, Uncle Al
<UncleAl0@hate.spam.net> writes

Ian Macmillan wrote:

A dinghy is fitted with a long horizontal cylinder with an open end below
the waterline at the stern. The cylinder has a piston that reciprocates
to alternately suck in and expel water.

The dinghy is propelled forward.

Does anyone have a problem with this?

All the best
Ian Macmillan

1) Hey stooopid, "Gedankenexperiment."

I think he knows that. It's a joke. J.O.K.E. danken -> dinghy.

You're losing it.

2) Hey stooopid, homogeniety of time and Noether's theorem.

If you mean conservation of momentum, say so. Invoking Noether doesn't
impress, particularly when you get the wrong symmetry. Time <-> energy,
translation <-> momentum.

Suck in water from all directions, throw it out in one direction.
Reaction, momentum conserved, forward motion. Noether wouldn't even
blink.

3) Hey stooopid, if the Navy doesn't do it then it is too stupid for
words.

And you think they don't?

They probably don't, since the idea is not very efficient. With few
one directional flaps you can convert it into "suck water from mostly
forward direction, throw it out in backward direction. Already much
better.

True, provided you can add those flaps. I was thinking more of the
low-tech process called "underwater swimming".