I need to know how the speed of steam engines (specifically marine
turbines, but hey, whatever) was controlled. To be precise: when a
steam-driven vehicle had to slow down in a hurry, was it normal practice to
waste pressure by venting it to atmosphere?

Or if not by venting it, then how?

Glasseyed --

The active venting of steam into the atmosphere is rarely done in
steam engine designs because doing so loses substantial energy. 
Instead, steam engines generally use a condenser to recycle and cool
water vapor -- particularly in marine engines where colder water is
readily available to cool the steam.

However, two things contribute to the belief that people expect steam
engines to emit steam.  The first is that is more-compact railroad
engines that don't have the benefit of water cooling, steam IS vented
to the atmosphere.  This is both a well-diagrammed indication of how a
RAILROAD steam engine works -- and there's an excellent bibliography
at the end:
How Stuff Works
"How Steam Engines Work"
http://travel.howstuffworks.com/steam.htm

It is important to have a closed-loop system within the steam engine
to conserve energy but also to make sure that distilled water is used.
 There are simply too many minerals and impurities in river water or
ocean water that would damage the complex tubing of a boiler:
Elton Engineering books
"Samuel Hall" (undated)
http://www.polybiblio.com/elton/5205.html

For a very complete description of a modern steam engine, the American
Society of Mechanical Engineers has written a detailed description of
the engines powering the S.S. Badger, a car ferry that crosses Lake
Michigan (Manitowoc to Ludington)"
ASME
"S.S. Badger Engines and Boilers" (Sept. 9, 1996)
http://www.asme.org/history/brochures/h191.pdf

I'd mentioned that there was a second reason that people BELIEVE they
see steam coming out of the funnels of ocean liners -- it's because
larger liners used steam whistles until late in the 20th century. 
They often had a steam whistle in each of the stacks or funnels -- so
observers would "see" steam venting from the funnels when the ship was
calling attention to itself:
North Shore News (Vancouver, BC)
"Rotterdam V Sails Her Last Voyage" (Sept. 8, 1997)
http://www.nsnews.com/issue/w090897/rotterda.html

As Engineguy-ga has correctly noted, the slowing of ships is done by
the marine transmission.  The steam boiler can remain fully stoked
while the propellers or screws are completely stopped.

About.com's history of steam engines.  Beware the popup ads on this
page, which otherwise is very complete on the development of steam
engine technology, even having a separate category for steamboats:
http://inventors.about.com/library/inventors/blsteamengine.htm

Google search strategy:
I cheated a little on this one, as I was already doing research on the
SS Badger when I ran across the interesting article on its engines.
But further research was possible using:
"steam engine" + condenser
"ocean liners" + steam + whistle

Best regards,

Omnivorous-GA

---

Clarification of Answer by omnivorous-ga on 22 Aug 2003 10:15 PDT

Glasseyed --

I don't know if this will help but there are instances (beyond use of
a steam whistle) where steam is release en masse and intentionally. If
you read reports of ships that have been holed -- with water rising in
the engine room -- there's a need to shut down one or more boilers to
prevent a catastrophic explosion.

Examination of the Titanic sinking; or using a Google search strategy
like the following would give you some accounts to work with --
boilers + sinking + steam

And thank you very much for the extra sum on the Answer!

Best regards,

Omnivorous-GA

Thanks, omnivorous. (Sigh) It would have made a striking visual image,
but I'm glad I checked. Maybe I'll have a chorus line from the Folies
Bergere dancing on the railing instead. ;)

Hi, glasseyed,

I don't know anything about steam engines, but I would guess that in
an application like a marine turbine, the engine would be connected to
some kind of transmission that could be disengaged/switched into
reverse/etc.

Good luck,
smudgy.

Thanks, smudgy! That would be my guess, but I'm hoping it isn't true
(oops, I've given away the answer I'm hoping for; still, I need to
confirm or deny the speculation).

I want it for visual effect in a story I'm writing: a steam yacht
blowing off great clouds of steam to slow down quickly.

It would be most likely for a steam yacht to use a steam engine rather
than a steam turbine. A steam turbine uses very high pressure steam to
spin a wheel at high speed. A steam engine, as used in steam
locomotives and smaller marine applications, uses pistons operating in
cyclinders to convert steam energy into mechanical work. In these
devices, the steam inlet was often throttled by a valve that could be
opened and closed to restrict the flow of steam into the cylinders.
The cloud of steam often associated with a stopping locomotive occurs
when the pressure relief valve opens to prevent the steam reservoir or
boiler from bursting from over-pressure.

And thank you, engineguy! I appreciate all this interest.

Actually, Charles Parsons first unveiled the steam turbine on his
"Turbinia," a pencil of a boat only 100' long by 9' wide. Her cruising
range was probably something like 50 miles, but damn, she was fast.

You're right: most steam yachts were piston driven. But my fictional
yacht is 280' overall, and displaces 800 tons (very light for her
length), so fitting her with turbines isn't a problem. Now if only
she'd look suitably picturesque and dramatic slowing down...

There is a much simplier way to slow down.  Just stop letting steam
from the boiler into the turbine.  If the pressure in the boiler rises
too high, then you would have to let it out, but otherwise, just wait
until it is needed.  The energy is in the boiler ready to use, as
little or as much as you need.  Mostly, you would just burn more or
less fuel to raise or lower the pressure.  A simple pressure valve is
the answer to your question......