"Chuck Gould" wrote in
ups.com:

Unlubberizing the Single Screw, Part II


In last month's installment, we examined the natural forces that will
influence the handling and maneuvering of a single screw inboard
powerboat and the mechanical forces that we can generate to compliment
or counteract these environmental variables.
Now we will examine some of the specific, situational considerations
commonly encountered as well as some of the techniques that have proven
useful to generations of single screw boat handlers.


The Most Important Mechanical Consideration


Handling a specific, single screw boat would be impossible to do
(properly) without remaining aware of the "handing" of the prop. Is
the propeller is a left-hand prop (turning counterclockwise when viewed
from astern with the vessel in forward gear) or a right hand prop
(turning clockwise when viewed from astern with the vessel in forward
gear)? The handing of the prop is one of the few fixed factors in a
complex equation that must be solved to maneuver a single screw in
close quarters. Wind and current change speed as well as direction. The
rudder can be turned to starboard, port, or left amidships. The
throttle can be increased or reduced, and the gears can be shifted into
forward or reverse.
Draft and windage will not change, nor will the handing of the prop. In
practice, a boater will always need to make decisions based directly
upon the handing of the prop, while there will occasionally be those
lucky circumstances in which windage and draft are virtually
non-factors.

Novice boaters, or experienced skippers called upon to operate a boat
with which they may not be entirely familiar, may wonder whether a
specific boat has a left hand or right hand prop. It wouldn't be
advisable to get behind the prop to watch it turn when engaged, and
particularly if single handed it's not always possible to leave the
wheel to open the engine room hatch and examine the direction the prop
shaft is turning when underway.
In some cases, but not all, the rotation of the prop is marked near the
output shaft of a gearbox and can be determined when performing a
routine engine room inspection prior to startup. In some cases engines
are actually "counter rotating" to drive left hand props, but in
most situations change of direction occurs in the gearbox so any marks
on the flywheel housing of an engine should be considered unreliable
indicators of prop handing. Fortunately, there is a reliable way to
evaluate prop handing that doesn't require any marking on the gearbox
or observations of a propshaft.

Shifting into reverse, particularly with no headway, will move the
stern to either port or starboard. It can help to have the rudder
amidships, but if the boat is not making way rudder position will be of
little consequence when first shifting into reverse from idle.
The single prop directs the prop discharge current away from the rudder
in reverse. The prop is shaped to be most efficient when in forward
gear, so when in reverse the side pressure of the prop tends to exert a
greater influence than the low pressure prop suction current passing
with little effect around both sides of the rudder. In some cases, and
depending upon the diameter and pitch of the prop, the movement of the
stern to port or starboard can be immediate and dramatic. In the worst
cases "prop walk" can be ridiculously extreme, and only slightly
less desirable would be a situation where the prop walk to port or
starboard was barely noticeable. A moderate amount of prop walk is a
useful tool when maneuvering a single screw inboard: a vessel that
steers by moving the stern has greater flexibility and will be easier
to handle with more options for moving the stern rather than fewer.

The majority of single screw powerboats use right hand props and will
back to port. The concept of visualizing the prop as a wheel on the
ground to understand how side pressure pushes the stern to one side or
the other (discussed in the last issue) is equally applicable whether
in forward or reverse. A single screw inboard with a left hand prop
will back to starboard. Vessels with the helm on the starboard side
will generally find docking more easily accomplished with a left handed
prop than a right handed prop, but the common disadvantage of the right
hand prop can be overcome with an adaptive technique.

Knowledge of the handing of a boat's propeller and an awareness of
what the stern will do when the vessel is in reverse gear is critical
to successfully handling a single screw inboard.


Backing and filling:

One of the common misperceptions associated with single screw inboards
is that they will always be awkward to turn in tight quarters. It would
be almost impossible for a skilled single screw operator to turn a boat
as tightly as an equally skilled operator would with twin engines, and
for good reason. Twin engine boats tend to "pivot" farther aft, and
with the advantage of one prop in forward gear and the other in reverse
(as well as the judicious adjustment of throttles) it isn't unusual
to turn a twin screw boat practically within its own length. Single
screw boats can often be turned within just a little greater space,
(perhaps 125% of the boat's LOA) but will require the employment of
some specific skills and strategies rather than engaging a second
engine. The general process of making a tight turn from a dead stop in
a single screw inboard boat is sometimes referred to as "casting",
but more commonly called "backing and filling".

Single screw inboards will back and fill more efficiently in one
direction than in the other, for reasons that are very apparent when
considering any single screw's tendency to back to one direction.
Right hand props that back to port will make a tighter turn to
starboard, and the reverse will be true of left hand props.

To back and fill in a tight circle from a dead stop, first put the helm
hard over in the direction you want to turn. To make the tightest
circle with a right hand prop, put the wheel to starboard.

With the helm hard over, engage forward gear and throttle ahead just
enough to get the stern moving to port. Once the stern is moving and
before making any serious headway, shift to reverse gear and turn the
wheel hard aport. The prop walk will continue moving the stern to port,
and the rudder turned to port will capitalize on any slight amount of
sternway that may be generated with the shift into reverse. Before
making any serious sternway, take the wheel hard astarboard engage
forward gear again. Momentum will continue to carry the stern through
its circle during those moments when the gear is neutral. Continue this
process until the vessel has turned almost through the number of
degrees desired, remembering to begin preparing to power off on the new
heading just prior to achieving it.

A preliminary precaution: The back and fill maneuver is always subject
to the external forces of wind and current. A persistent wind or
current will move the backing and filling vessel from the location of
its originally intended circle while the maneuver is being conducted.
Be sure there is enough room "downwind" or "downstream" before
beginning a tight turn.

Good article...... naturally I don't agree 100% with everything but the
only item really worth mentioning is that on 999 out of 1,000 boats I
never waste time shifting the rudder in the above maneuver as you will
never (if properly done) have enough sternway to have rudder effect.

otn


Goin