Unlubberizing the Single Screw
Part I, Natural and Mechanical Forces


Single screw inboards suffer from an inaccurate reputation. Some
boaters labor under the impression that controlling a single engine
powerboat, particularly in close quarter situations, is an arcane art
or esoteric science. Many a mind's eye must envision a ceremony
during which a capricious King Neptune instantly bestows the secret
knowledge required to tame a single screw boat to some randomly
selected group of old salts, and totally prohibits the rest of the us
from ever really catching on. It isn't uncommon to meet boaters who
feel that a twin engine boat is absolutely essential, and some of that
set would be nearly helpless if required to maneuver with one of the
engines out of commission. Still others would only consider a single
engine powerboat if there were bow thrusters, stern thrusters, (or
possibly both) installed.

Boaters with twin engines can often "power through" situations in a
fashion that would be impossible for a vessel with a single engine to
duplicate. Even so, understanding the art and science of controlling a
single screw inboard will enable any boater to more completely
appreciate some of the variables that will effect the handling of
nearly all types of powerboats.

Can a book, a video, or a magazine article teach somebody how to
maneuver and dock a specific, single screw boat? In reality, no. Every
boat behaves somewhat differently, and even the same boat will
encounter a different mix of variables to sort out in nearly all close
quarter situations. The encouraging fact is that while boats behave
somewhat differently, there are common principles that govern the
successful operation of single screw inboards. Becoming adept with a
single screw boat will depend on developing an understanding of the
common principles and following up with some intelligent practice.
(Practice only makes perfect if one practices perfectly). We will exam
some of the basic principles and describe how they can work to the
advantage of a boater operating a single engine inboard boat.

Galileo and Isaac Newton observed principles that apply when handling a
single engine inboard boat. Among them, "A body in motion or at rest
will remain in motion or at rest unless acted upon by an outside
force," and "For every action there is an equal and opposite
reaction." Motion, forces, actions, and reactions; four simple words
that define the practical parameters of any discussion of handling a
single screw.


Natural Forces

Every boat operates in an unstable environment. The superstructure of a
vessel is exposed to air that refuses to stand still and can at times
be completely unpredictable. When properly harnessed and in moderate
amounts, wind can be a boater's best friend. Wind can quickly become
a boater's worst nightmare as well, and unless the air is dead calm
it will always either assist or impede any boat maneuvering in close
quarters.

Some boats carry more "windage" than others. Stacking a couple of
decks above the waterline, adding a flying bridge, (and perhaps
erecting a four-sided sail, a.k.a. "enclosure" around said
flybridge) can create a boat with a lot of accommodation and impressive
design- but the skipper of such a vessel will learn almost immediately
that even a moderate wind will dramatically effect his or her docking
decisions. Boats built with less freeboard and superstructure, (such as
an express cruiser or a runabout) will conversely be somewhat less
susceptible to the vagaries of wind.

It is usually true that the shallower the draft of any vessel, the
greater the effect of wind upon the hull. It takes less pressure to
push a boat through air than it does through water, so the greater the
ratio of draft to exposed superstructure the more pronounced the
"braking" influence of the water can be. Boats with substantially
less draft and more freeboard in the forward sections, such as many
trawlers with a semi-displacement hull, may be prone to "losing the
bow" in a severe crosswind while the stern is somewhat more resistant
to being pushed around.

While wind is a factor that must be considered when maneuvering at
close quarters, the movement of the water itself (current) will
frequently create more challenges for a boater than a gentle or
moderate wind. A two or three knot wind might not even cause a boat to
tug lightly against its moorings, but a two or three knot current will
sweep everything floating therein along at the same speed "unless
acted upon by another force". When a boat is made fast to a dock or
float or is riding at anchor, the tension of the mooring line or anchor
rode offsets the pull of the current and allows a vessel to remain in
place. When underway, powerboats rely on forces generated by mechanical
propulsion to equalize or overcome the effects of current.

One of the specific challenges with current is that it is often much
less visible than wind and can be easily underestimated. It isn't
unusual to encounter enough current to confound a docking effort, and
in situations where one does not initially expect it. If the boat seems
to have a mind of its own and wants to behave awkwardly on a relatively
calm day, unexpected current is commonly the problem. (Experience and
observation will confirm that the dreaded variety known as "Dag
nabbed current!" certainly gets the blame for a high percentage
flubbed dockings.)

Not only must we deal with the individual challenges of wind and
current when handling a single engine inboard, but the two forces will
unite in a seemingly endless variety of scurrilous combinations that
will prevent many dull moments from intruding into perpetually
interesting docking exercises. Dealing with a combination of wind and
current while trying to dock a boat may not be entirely unlike trying
to maintain balance on an icy treadmill while simultaneously running in
place and dancing the hokey-pokey. Wind and current may work together
to push a boat away or slam a boat against a dock, or they may work at
cross purposes, and in some cases one of the natural environmental
forces may declare a holiday and leave all the bedeviling to the other.
Effective handling of a single screw powerboat requires that the
skipper be well aware of the presence of wind or current, understand
the effects of these forces on his or her specific boat, and know how
to counter or manage the natural forces with the available mechanical
devices and techniques.


Mechanical Forces

It can be accurately observed that maneuvering a single screw
powerboat, or any boat for that matter, is an exercise in the creation
and management of high and low pressures. The primary tools for
overcoming (or harnessing and controlling) the natural forces of wind
and current available to a single screw boater are all dedicated to the
generation and administration of high and low pressures.

The most important tool available to a single screw boater is the
propeller, including by extension the throttle and gearshift controls
that dictate its speed and direction of rotation.
When the propeller is activated in forward gear, the rotating blades
create a high-pressure area immediately aft of the prop and a
low-pressure area directly ahead of it. It is this pressure
differential that causes a boat to move forward when the forward gear
is engaged. The flow of water across the propeller is known as the
"suction current" as it is being drawn into the prop and the
"discharge current" once beyond the propeller.

Propellers also create pressure differentials on either side. As the
blades propeller turn, a zone of higher pressure forms on the
"descending side" (where the blades travel away from the hull and
toward the bottom of the arc) than on the ascending side. This unequal
side pressure actually pushes the stern to one side, even while the
boat is supposedly being steered in a straight line. Unless a boater
compensates with a continuous series of small corrections with the helm
a vessel would travel in a large circle as a result of the difference
in side pressures surrounding the prop.

Propellers are said to be "right handed" or "left handed"
depending upon whether they turn clockwise or counter-clockwise when in
forward gear and viewed from astern. A prop that turns clockwise is
known as a right-handed propeller, and is more commonly encountered
than the counter-clockwise turning left-handed variety. Whether a
propeller is right or left-handed will have a profound impact on the
strategies and techniques adopted by single screw boaters.

Some mariners refer to a propeller as a "wheel", and this image can
be very useful when envisioning the effects of propeller side thrust
while operating a single screw. Imagine that instead of a propeller, a
boat were fitted with a wheel and tire on the end of the prop shaft and
sitting on the hard rather than floating in water. A right handed wheel
would turn clockwise in forward gear and carry the stern of the boat to
starboard, while a left handed wheel turning counter-clockwise would
carry the stern of the boat to port.
Single screw boats steer around a "pivot point", commonly about 1/3
of the way aft from the stem. Anything that sets the stern to either
port or starboard will set the bow in the opposite direction, but the
bow is pivoting on a much shorter radius than the stern.

The management of high and low pressures against the rudder allows us
to steer a single engine powerboat. Water must be passing across the
rudder in order for the rudder to have any effect. The passage of water
can be generated by the discharge current from a propeller, or by a
vessel simply moving through the water with headway or sternway.
When the rudder is amidships, the discharge current from the prop or
current from headway flows equally along both sides of the blade and
the stern is not forced to port or starboard.

When the wheel is turned to port, the rudder rotates to place high
pressure against the port side of the blade. The discharge current and
the current generated by headway move the stern of the boat to
starboard, changing the heading of the bow (beyond the pivot point) to
port. The reverse principle is true, of course, when steering to
starboard. One of he first and most important concepts that new boaters
need to appreciate is that the boat is not steered by the bow, but
rather by the stern. Due to unequal radii to the pivot point, any
change in heading observed at the bow must be created by a far greater
amount of movement astern. One of the popular comparisons suggests that
maneuvering a boat is much like steering a supermarket shopping cart
with fixed front wheels and pivoting wheels in back, but a shopping
cart will pivot around the "inside" front wheel while a boat is
spinning on a point about 1/3 of the way back from the stem.

If handling a single screw powerboat is really at all similar to
pushing a shopping cart, it would have to be a shopping cart in a
grocery store with narrow aisles, slippery sloping floors, and one with
the roof torn off just a few moments earlier by a tornado.

We have identified the natural forces that we must consider when
operating a single screw inboard powerboat and taken inventory of the
primary mechanical tools we have to harness and/or overcome those
forces. In the next installment we will begin solving the mystery of
when and how to apply the mechanical forces in order to cause a single
screw powerboat to react as desired; most of the time. We will also
examine how to recover and when to start over when the best-laid plans
come akimbo and several dozen dock-walkers are being entertained by a
particularly lubberly and clumsy performance. (Please don't ask me
how I know).