Wayne.B wrote:


From a teaching standpoint, I think it might be more clear, and to a
wider audience, if you referred to the concept as "directed thrust",
i.e., using the rudder to direct the prop thrust to port or starboard.
It means the same thing but to me it's more intuitive.

G Thanks .... as I say, "thrust vectoring" may need work.

I think the most difficult concept to visualize is "prop walk". It's
not instantly clear what is generating the side force, or in which
direction. Prop walk is important to understand because it seems to
cause much of the consternation when backing a single screw. I like
to think of it as a jet of water created between the prop tips and the
hull, but I'm not sure if that's an accurate visualization or not. If
true, it would imply that deeply mounted props, away from the hull,
should generate less "walk" but I'm not sure if that is valid.

I consider prop walk to be due to the angle/pitch of the blade, pulling
the prop to one side, on the downward stroke and pushing to the same
side on the upward stroke (rather simplistic), which really has nothing
to due with shaft angle (look at many trawler types and ships - little
shaft angle but significant walk, plus the props are generally away from
the hull).... again, this is MY way of explaining it.

It seems to be a consensus among experienced twin screw captains that
leaving the rudders amidship is good practice for most maneuvers.
There are exceptions of course but having the rudders amidship leads
to more predictable response in my experience.

Comments?

G This last paragraph is probably my main reason for trying to find a
"simplistic" yet effective way of discussing this.
First off, how you handle a particular twin screw boat will depend on a
number of factors:
1. the boat - length, breadth, hull shape, windage
2. the props - inboard turning or outboard turning
3. the conditions - wind and current
4. the operator - what works for one person, does not always work for
another.
5. the rudders - large, small, old fashion, semi balanced, balanced.

I consider myself to be a fairly experienced twin screw boat handler,
but contrary to the above, I normally will start off using rudders and
will continue to do so, until and unless I find them of no value.
Now, this works for me - it's not written in stone, but my main feeling
is that I want the rudders to be a familiar option, if a REAL need
should arise, i.e., I don't have to give their use a second thought.

Again, thanks for your comments.

otn