On Wed, 22 Sep 2004 18:34:04 GMT, otnmbrd
wrote:

Frequently, I'll read a post discussing prop rotation, where the writer
says that he has twin screw, counter rotating props, which leads to
confusion, as this really doesn't tell you what you need to know.
Ignoring engine rotation ..... In the past, some setups used a left hand
and right hand engine to create "counter rotation", but to my knowledge
this is rare nowadays.
When looking at the prop rotation, you are always looking at the props
from astern of them looking forward.
When going ahead, a RH prop will turn clockwise and a LH prop will turn
counterclockwise.
In twin screw applications there are two possible configurations
.....(there's a third and fourth, but these are not worth mentioning G)

"inboard turning" - the stbd shaft has a LH prop and the port shaft
has a RH prop.
"outboard turning" - the stbd shaft has a RH prop and the port shaft has
a LH prop.

Outboard turning is the most frequently seen application. The reason, is
handling, as, for the most part the same boat with "inboard turning"
will handle totally differently from one with "outboard turning" props.

Outboard turning: The individual props, complement the offset thrust of
props with prop walk, i.e., when going ahead on the stbd shaft alone the
offset thrust tends to turn the boat to port while at the same time,
propwalk will pull the stern to stbd, increasing turning rate. Going
astern, the offset thrust/pull and propwalk work together to pull the
stern to port.
The advantages to this are fairly obvious ..... you can use engines
alone to steer both ahead and astern, and to increase turning rate you
can simply add throttle and basically leave rudders "midship" (G not a
practice I agree with in all cases).
Naturally, the degree of ease that a boat with this configuration will
turn or twist, or walk, will depend on other factors as well (prop
spacing, rudder size, hull configuration, etc.).

Inboard turning: Totally different (G normally)! If your one of those
who never learned how and when to use rudders when maneuvering your twin
screw, outboard turning, propped boat, standby, you may be in trouble.
With this configuration, the offset thrust is countered by the propwalk.
What happens when you put the stbd shaft in gear, ahead or astern? You
go straight. Add throttle, what happens? You go straight. Try to twin
screw? You sit there and throw water ahead and astern. Solution? G
That's why they put those rudders back there and you best start learning
to use them under maneuvering conditions.

Now, before someone with a boat with inboard turning props jumps in, in
anger, the degree of how straight your boat will go or how hard it is to
twin screw without rudders, will vary, from what I describe above, to
something just below a poor handling twin screw with outboard turning props,
BTW, there ARE advantages to inboard turning props.

Great info.

Thanks - I'm about to make the foray into counter rotating inboards so
that was an education.

Later,

Tom

The above is basicly correct... but not the full story.
The navy tend to put props on the other way around because props turning
inwards towards each other at the top is more efficient
This is done on the basis that navy ships have tugs etc to assist manouvers
in port.. so they trade the manouverability for better efficiency at speed.
When all is said and done, you will have to fit the props to suit the
engines/gearboxes as fitted to the boat unless you wanna swap them round

Add to this, most Navy surface combatants (not sure about carriers, though
doubt it) use variable pitch props, which throws in a whole other set of
handling characteristics .... some can do a credable twin screw

Shen

Subject: Prop Question
From: Short Wave Sportfishing

Great info.

Thanks - I'm about to make the foray into counter rotating inboards so
that was an education.

Later,

Tom

Just remember, the term "counter rotating" when applied to twin screw boats, is
basically meaningless, as 99.9% of all twin screw employ them. (props, that is.
I'm not that sure about engines ..... seems like an unnecessary expense)
What IS important, is to know "inboard" or "outboard" turning PROPS.

Shen