On Tue, 22 Apr 2008 11:58:25 -0700 (PDT), Bob
wrote:

On Apr 22, 9:04*am, (Richard Casady)
wrote:
On Tue, 22 Apr 2008 09:22:08 -0400, "Roger Long"

Big ships all have direct drive engines. If the engine in running the
prop is turning. You stop it and restart it turning the other way for
reverse.

Casady


Hello Casady:

Well, what can I say. Im going to cosider anything big as 500GRT.

They make 500 ton dump trucks. Strip mining equipment. A big size of
truck but very small for a ship. Panamax boxboats are big. There is a
freighter with a 500 ton crane. Maybe for loading those big trucks. Or
locomotives.

What are some the engines Ive seen on these boats.

Of course there are the old air start reverse diesels ya might be
thinking about. If your going forward and want to go revers then ya
got to kill the engine and restate the engine in reverse. makes for
interesting manouvering at the dock.
That's all there is on really large ships. All modern cargo ships with
any size to them are single shaft diesel with direct drivel. They
haven't built anything else in years.
Lets try steam as in the SS Ocean Phonenix: boil water and run steam
through turbin. Lots of ways to boil water.
They quit building steam ships some time ago. They use quite a bit
more fuel than diesel, and steam ships cost significantly more to run.
HOw about EMD...... (Electric Motor Division) There, like the railroad
locomotives, the engine runs a generator that then supplies
electiricty to an electric motor which then pushes the boat. Just
manage the electric motor.

Less efficient than direct drive. Checked the price of oil lately?

NOw there are a couple ways to do this all.

1) Z drives can swivel 360 degress so no need for a marine grear
(transmission)
2) varriable pitch props. There ya can just increase/decreas shaft
turns and fiddle with pitch contorll.
A small proportion of the tonnage out there. Cargo ships don't use
them.
I have heard of variable pitch props on some US warships, but, except
aircraft carriers, warships aren't all that big.
Some tugs have odd ball propulsion systems
Some cruise ships have diesel electric, and at least one has Z-drives.

So me thinks there are a lot of options you may not have considered.
But thats okay, ship propullsion systems is an interesting subject
changing annually.

I think ya best stay talking about ur recreational yachts and leave
the big stuff to thoese who have actually been on one.

That's seems an overreaction to posting one small fact.

Casady