Article
scanned from PassageMaker magazine
Sept/Oct
2002 – Pages 82 to 90
Semi-Displacement Boats: An
Alluring Compromise
by Charles Neville
In recent years heavy-displacement trawlers have become icons of
free-spirit invoking images of foreign ports and exotic adventures. For many,
however, the 8-knot lifestyle is something they cannot (or do not want to)
reconcile with other aspects of their lives, such as houses, businesses, kids,
grandkids, etc.
By the same token, for many (especially ex-sailboaters) the need for speed
isn't in their genes. For them, the thought of a high-speed boat doesn't suit.
This quandary has made the middle range of semi-displacement boats more and more
popular, especially with boaters not really interested in crossing oceans.
Semi-displacement boats are half-breeds: vessels that can move faster than
their displacement brethren but cannot quite match the performance of speedy
planing boats of equal length. The allure for owners is obvious. In return for
sacrificing some of the economy and offshore sea characteristics of displacement
boats, semi-displacement versions seem to reward owners
with that extra bit of speed to get home or to a safe port more quickly.
Many consider this a valuable tradeoff. The combination of features has
value, and the marketplace, never one to miss an opportunity, has responded by
offering these potential buyers an ever-expanding array of new semi-displacement
cruisers.
Do semi-displacement boats really offer the best of both worlds, or the
worst? Some insist that these craft provide little of the performance of planing
hulls with less economy of displacement ones. The truth probably lies somewhere
in between. The fact remains that semi-displacement cruisers will continue to be
an attractive option for many boaters. A better understanding may help to
demystify the breed.
It is easy to get confused when discussing these boats because even those
in the trade can't agree what to call them. Terms used include "fast
displacement," "semi-displacement," "semi-planing," or
the old favorite of technical journals, "pre-planing." All of these
are largely interchangeable. I'll stick to "semi-displacement."
Speed Is Key
Semi-displacement is less a boat type than a speed range. All boats can be
positioned on a straight line that represents an endless range of speeds from
the slowest to the fastest. The position each boat occupies generally is a
function of its hull shape and power. Because the whole range is seamless,
comprehending the proper place of semi-displacement boats requires a good
understanding of how they fit in the continuum between displacement and planing
boats.
This concept may be more easily understood if we temporarily set aside the
concept of hull speed for heavy cruisers. The notion of a speed-length ratio
suggests that displacement-speedboats cannot be powered beyond 1.34 times the
square root of a boat's LWL (load waterline length.) It is the speed at which
the hull makes a wave as long as its waterline.
But this notion is not completely true. Destroyers, for example, routinely
are powered to speed-length ratios of 1.6 to 1.8, though at speed their motion
in a seaway reportedly can be daunting. Other efficient displacement hulls also
can be pushed beyond the supposed limit.
You can even ignore the efficiencies of form in the following extreme
example. At least in theory, the bulkiest of tugboats could be propelled to
reach planing speeds (stability and practicality aside). It largely is a
function of the power installed. Clearly, we don't do so with
tugs because of the absurd amount of power required, the
lack of available space to put that power, significant control and stability
issues and, most important, the obvious insanity of doing so.
Strictly speaking, then, a hull speed of 1.34 is not an impenetrable
barrier. Like the sound barrier, it is one founded to a great extent on
economics and practicality. Designers have come to understand a need to adapt
hulls in various ways to minimize, or take advantage of, the way a boat and the
water must interact as speed increases.
What's Happening?
At the lowest of speeds all of a boat's weight is supported by buoyancy of
the water. The power needed to move a boat forward generally is that required to
overcome friction. Most of the friction to be overcome is caused by the
disturbance of the water rather than between a boat and the water. A boat rather
successfully pulls a thin layer of water along with it. That initial layer,
however, tries with decreasing success, to bring additional layers of water
along for the ride, resulting in friction.
As the speed slowly rises, other things begin to happen. With more speed,
the whole boat is pulled or sucked down almost unnoticeably into the hole in the
water it has created. The primary cause of this is water entering at the bow,
racing under the curved hull and returning up to its original position near the
water's surface.
Like an upside-down airplane wing, this action produces an area of reduced
pressure under the hull, causing it to be pulled down ever so slightly. A
low-pressure area created by the propeller that also increases downward suction
near the stern exacerbates this downward pull.
To move forward, the boat has to push the water out of the way. As a
result, a small pattern of waves is formed along the length of the boat. Their
length becomes longer and the number (along the hull) fewer as speed increases.
The length of these waves between any two crests is directly proportional to the
speed the boat is traveling: the relationship to speed-length ratio. Another
series of waves also is generated by the vessel's stern.
With an increase in speed, the boat is pulled down more and more while at
the same time waves generated along the length of the hull become longer and
longer. At some indeterminate speed the bow-generated wave will synchronize with
the stern-generated wave in a way that the bow wave will be at its highest point
while the stern wave is at its lowest. The
bow wave will neutralize the stern wave and the vessel will be operating
at its highest efficiency. A typical displacement hull will meet this condition
when traveling at a speed-length ratio of approximately 1.0.
As speed continues to increase, waves generated become an increasing
problem with more and more of the boat's propulsion power siphoned off,
producing ever-larger waves. The hull is sucked down still farther. This
combination of events causes the boat to experience trim changes as the bow
rises and the stern gradually falls or squats. If the hull form is of a purely
displacement type, the boat appears challenged with an unlikely task of trying
to drag itself up over its own ever-larger bow wave. Generally, this point is
assumed to occur at the speed-length ratio of about 1.34, the so-called hull
speed.
Other Limits
Beyond this, many elements conspire to limit ever-increasing speeds.
Trying to overcome these with brute force makes the condition even worse. The
stern wave is left behind leaving the vessel's aft end largely unsupported. As a
result, the stern squats dramatically as the bow rises uncomfortably.
Allowed to continue unabated, the power required would become outrageous.
The boat's safety and stability also could be jeopardized because of the greatly
exaggerated trim and violent wave pattern created. Something needs to change.
More power and less weight might make the boat go somewhat faster. But what
really is called for is a re-evaluation of the hull shape. The boat needs to be
optimized to go over the next speed hurdle and move into the semi-displacement
range.
To accomplish this, the combination of friction and ever-increasing
wave-making resistance must be overcome while still allowing water to flow
efficiently around the hull. The most effective method of doing so is to
rearrange the underwater hull shape in a way that reverses (and gradually
eliminates) that downward pull we have been talking about. That downward pull is
lift (as in an airplane wing). Up to now it has been pulling in a negative
direction.
Lift, however, has two major components. The first involves shape (the
camber of an airplane wing).
The second is angle of attack, or the angle that the fixed shape makes
with the oncoming flow of air or, in the case of a boat, water. If we can get
this so-called dynamic lift to work in our favor, the boat gradually will begin
to pull back up vertically with less friction, leaving less boat in the water to
create waves.
Working Favorably
How do we make the angle of attack work favorably? The bow comes up as the
angle of attack increases. To make that change in trim generate vertical lift,
the stern also must be shaped to resist being pulled down in the water and to
assist in inducing lift. In simple terms, the stern must be broader. At the same
time, however, the stern should not be too deep.
We still, after all, want the water to close back in around the boat as it
moves forward. Making this happen most likely will require the center of gravity
to be located farther aft than might be expected for a boat operating at purely
slower speeds. Keeping the boat lighter also becomes increasingly important.
If the shape is successful, here is what happens. Instead of continuing to
be sucked down farther, the boat will begin to rise as the lift generated
overcomes effects of the downward tendencies previously in play. Properly done,
the stern rises faster than the bow and the extreme trim angle begins to reduce.
The stern wave gradually is left behind. Speed increases, lift increases, and
slowly the boat pulls itself back to the same vertical position it occupied at
low-speed conditions. In this situation, hydrostatic forces and dynamic lift
share support of the boat equally. The approximate speed-length ratio where this
condition occurs is around 2.0.
Now, let me admit that beyond this point, where the boat begins to rise
above its original static-trim position, some books will refer to the boat as
planing or at least, "pre-planing." More accurately, the boat is in a
partial planing state. At the risk of criticism, and to avoid clouding the
message, I'm going to continue to call this a semi-displacement regime because
hydrostatic forces remain in play.
Continuing To Rise
A suitable boat with enough power will continue to rise beyond this point
of equilibrium. Downward forces diminish as lift continues to dominate. If a
boat has sufficient lifting or planing surface, it will continue to rise out of
the water until it is supported entirely by the dynamic forces of lift skimming
over the water's surface. The point of this transition is at speed-length ratios
of approximately 2.5 to 3-0. The transom would be running dry leaving little
stern wave and the chines would be running clear of the water below.
Both friction and the hull-induced waves would be minimized because so
little of the boat is in the water. The boat is operating in a full planing
condition. This point often is called "hump speed," referring to the
pronounced hump shown in a classic speed vs. horsepower performance graph. For a
brief period the boat is capable of increasing its speed with little additional
power. As it comes "on plane," it is not uncommon for a slight power
reduction to be possible and still keep the boat on top.
For the true planing hull, additional speed still is an option. The
transom is likely deeper and the run of the chines and bottom surface is close
to level with the water surface as it runs aft. Deep-V sections also often are
used to improve performance in a seaway. Speeds above this are limited only by
the amount of available power, and the stability and control requirements
necessary to keep the boat from becoming an uncontrollable projectile.
So, you say, that's a nice snapshot of a day in the life of a boat, but
how does it help to identify the semi-displacement boat you might be
considering? Unfortunately, it's not easy. The difficulty is compounded by at
least two other simple details.
First, there are technical definitions of a semi-displacement boat, and
second, are the overall characteristics of just about any planing boat.
Broadly speaking, you can define a boat operating in the semi-displacement
speed range as one that is partly supported by both
hydrostatic and dynamic forces (that is, lift). Unfortunately, in the real
world, identifying one that is doing so is virtually impossible when looking at
or even riding on it.
The second accepted definition of a semi-displacement hull is related to
the rise and fall of the boat as speed increases (waves not withstanding). This
definition is more a benchmark relating to how a boat reacts to its interaction
with the water around it. It is most easily understood by picturing a simple
graph noting the position of the center of gravity (CG) of the boat as it
travels through the range of speeds. The CG is simply the combined center of all
of the vertical and horizontal weights aboard. Think of it as the balance point
of an unequally loaded teeter-totter.
From speed zero, the CG is sucked down by hydrodynamic forces, discussed
above, to a point that approximates a speed-length ratio of 1.34. (Figure 1.) At
that point, lift joins the equation. The CG and the vessel begin to rise until
hydrodynamic and lift forces are approximately equal, at a speed-length ratio of
about 2.0. At this point, lift overpowers hydrodynamic forces and the boat rises
out of the water as speed and power increase. This process continues until at a
speed-length ratio of around 2.5 to 3.0. Here the boat lifts out of the water
with virtually all of the support for the
vessel provided by dynamic forces. Unfortunately, this definition also is
virtually impossible to perceive or easily predict.
Planing boats can operate at all of the speeds discussed. Clearly,
however, their performance, efficiencies and sea-keeping characteristics degrade
quickly when they operate below full planing speeds. Often their sea-keeping
characteristics also are extremely uncomfortable at high speeds. Still, this
ability to operate at all speeds allows planing hulls to masquerade as boats
with slower speed potentials.
The Ugly Truth
Although many types of boats can operate at a range of speeds, the
differing efficiencies of displacement, semi-displacement and planing hulls can
be seen easily in a graph of horsepower required as a function of speed-length
ratios for the three typically designed hull types (Figure 2). At lowest speeds,
the performance of all the candidates is similar. But this changes rapidly. Up
to a speed-length ratio of 1.0, the displacement boat clearly is the champion
for efficiency, with planing hulls bringing up the rear.
At slightly above a speed-length ratio of approximately 1.2, everything
changes again. The displacement-boat curve begins to reach for the sky leaving
semi-displacement boats to show the best efficiency. At some point above 2.0,
however, planing boats begin to come into their own. Beyond speed-length ratios
of 2.5 to 3.0, only true planing shapes and raw power control the limit.
So here's the ugly truth. There are three primary ways of achieving
semi-displacement
speeds on the water. The first is to load up a displacement hull with
excessive power. This technique usually is obvious because of huge wakes and
radical trims these boats generally produce.
The second, and most difficult technique is to design a boat specific to
the semi-displacement task. Often this means developing a relatively narrow,
light hull with a fine bow. Such geometry results in a boat with less interior
volume than others of a similar size, making it harder to sell.
The third method, and unfortunately the most common, is to take a typical
planing hull shape, not worry too much if it gets significantly overweight, and
pack in enough power to push it to whatever speed one believes customers will
demand.
Of course, the purist in all of us would lean toward a true
semi-displacement hull shape. Remember, however, that a good boat must be more
than just a quality performer technically. Safety aside, the boat must do what
the owners need it to do. In addition to a hull shape selected for speed, the
boat must live up reasonably to an owner's arrangement requirements and
equipment demands. As such, purist values aside, there is a definite place in
the market for broad overweight planing boats that do semi-displacement speeds
adequately.
Tradeoffs
The broader discussion, however, still is important to help folks focus on
differences and tradeoffs to be considered by comparing all available options. I
hope this explanation of the differences will help clear up two of the most
obvious points of confusion that exist about speed ranges and boat types.
Buyer No. 1 decides he wants a typical 45-foot (LWL) displacement boat
capable of 12-15 knots. Assuming a practical hull speed of 1.34, this
means a true 9 knots. Even at an optimistic speed-length ratio of 1.42, it's
still only about 9.5 knots. What this buyer really wants is a
semi-displacement-speed boat.
A second buyer falls victim to the confusion generated when a boat is
offered with a variety of engines for different speeds. Again, for a 45-footer
this could be a 9-knot boat that's also available in a 12-knot version. The
speed potential reveals that this is at least a semi-displacement boat that also
is masquerading as a displacement type.
Similarly, this 45-footer could be offered as a 15-knot boat with
horsepower options available for 20-knot speeds or beyond. Here, the speed
potential clearly points to a planing hull that also is being offered in an
underpowered version. Project all of the available possibilities, and one easily
could suppose that both examples may feature the same full planing hull simply
with smaller or larger power options designed to entice all available buyers.
Hulls are designed to match a speed potential. Whether an engine suitable
of meeting that potential is provided is a secondary consideration, and one
hopefully to be made by a reputable builder and an informed owner. The important
thing to understand is the implication suggested when various engine
combinations are offered. Also, remember the dominant factor that weight plays
in determining whether or not the boat can achieve the speed goals you have set.
No one boat does a broad range of speeds well. A hull designed to go fast
likely will be uncomfortable and inefficient at lower speeds. One designed to go
slowly usually is terrible if pressed beyond its optimum. In that sense, a hull
designed for the middle-speed range may be the right choice if those speeds best
fit your needs. In choosing a boat, be sure you understand the compromises so
the decision is an informed one.
Colin Mudie, designer, philosopher and author of the 1977 publication Power
Yachts, expressed these thoughts with the dryness that only an Englishman
would bring to the topic. He referred to the semi-displacement range as:
"an unfortunate region full of hazards and disappointments for boat
owners." In defending his placement of the topic within his book, he added:
"We will include them with the displacement group as it seems more polite
to consider them as rather overpowered-displacement vessels than as planing
boats which could not quite make it."
Sidebar article
Untangling The Specifics: Speed-Length Ratio
The speed of a yacht or ship is governed and controlled largely by a
vessel's relationship with the viscous body through which it travels: water.
Vessels are available in a variety of sizes, but water always has the same
physical properties. For that reason, the speed relationship of different-sized
vessels cannot be compared directly. The efficiencies of a 300-foot vessel at a
certain speed are entirely different than those of a 30-footer operating at the
same speed.
Consider a 300-foot freighter cruising along at 20 knots in the scenic
channel in Lake St. Claire, north of Detroit. Looking closely at the scene,
ignoring some minimal froth, the bow wave generated would appear negligible. The
stern wave also would show little fuss even at close range.
Now imagine our 30-foot cruiser running right alongside at the same speed.
The smaller boat would be churning up a mess of water as it struggled to keep up
with the metal giant. The bow would be high and the wake would be substantial.
One of these vessels appears to be out for an afternoon stroll, while the other
seems to be running for all it's worth.
Since the late 1800s, methods have been developed to help us understand
and evaluate differences between various-sized boats. These generally are
related to the interaction between the waves created and a boat's waterline
length. This relationship, again, stems from the predictable way waves are
generated along any boat or ship of a given length.
The most common format for quantifying this relationship among yachts is
called the speed-length ratio (s-l ratio). It is popular for a number of
reasons, not the least being the fact that it's easy to understand and easy to
determine with any calculator equipped with a square-root key.
The equation for this method is:
s-l ratio = V / sqrt(LWL),
where V = speed in knots
LWL = load waterline length.
Armed with the variables of boat speed and length, you can calculate the
speed-length ratio yourself. By comparing that speed-length ratio with generally
accepted numbers you get a pretty good idea of the speed category in which a
given boat is operating.
Here is a list of the
speed-length ratios for various hull types:
·
Full displacement: 0 to 1.34.
·
Semi-displacement: 1.35 to
approximately 2.4 to 2.9.
·
Full planing: Approximately
2.5 or 3.0 and above.
(Numbers are inexact because of variations in performance of different
hulls.)
By rearranging the above equation, you also can easily figure speed at any
given speed-length ratio: V (speed in knots) = (s-1 ratio) x sqrt(LWL).
Some other notes that might help:
Chine Or No Chine: Semi-displacement boats can function well with either rounded hull or
hard chines. Boats with rounded bilge sections, like lobster-boat hulls, often
reach into the planing region if kept light enough. Yacht versions, however,
likely are heavier and are more apt to top out at semi-displacement speeds. They
may not be carrying fish, but they are carrying much larger deckhouses and extra
equipment.
The rounded-hull shape can offer a softer ride in heavy seas than one with
hard chines. On the other hand, the hard-chine boat likely will be more
maneuverable and faster, pound for pound. Remember, though, hard chines deliver
a stiffer ride.
Bow Shape Forward: To allow a boat to break through oncoming waves, a sharper V-shaped bow
is more efficient. Above-the-water flared sections and other elements such as
spray chines will help a boat shed water. Without this help, water coming aboard
can make these boats wet at higher speeds.
Waterplane Shape Aft: The shape of the boat aft (say within the last third of its length)
becomes critically important when shooting for semi-displacement speeds. Imagine
looking down at the outline of the hole in the water created by the boat. This
is called the shape of the waterplane.
The waterplane for a semi-displacement boat would be more wedge-shaped,
but tucked in a bit at the stern. The transom width (at the water surface) might
be about 85 percent of the overall waterline beam. The same aft shape for a
displacement hull would be closer to that of a watermelon, or the same fruit
shape with just a bit of the aft end cut off. For a planing cruiser, the midship
beam would be the same, running straight all the way to the transom.
Weighty Issues: Weight is not the entire answer, but it is a big factor in the equation.
Semi-displacement boats must be kept light to reach higher speeds efficiently.
This is especially important if a round-bilged hull form is chosen. Weight is
the most common reason that many boats with a clear planing shape get relegated
to semi-displacement speeds or below.
Our benchmark to compare weight between boats of differing sizes is the
displacement-length ratio (d-l ratio). The formula is:
Displacement / 2240
(LWL / 100)^3
Displacement-speed trawlers generally have d-1 ratios of 300 and above.
Semi-displacement speedboats are between 225 and 300. Planing boats need to be
the lightest with displacement-length ratios typically between 225 and 220.
Sea Keeping And Boat Strength: Boats can be powered to hit a desired speed, but if the shape is
inappropriate for sea conditions, the boat would be impractical in service day
to day. Flat-bottomed boats may be exceptional performers in dead-calm
conditions, but crew comfort in choppy seas may force the boat to travel well
below its potential, if regularly used in rough service. These same rough
conditions also can put serious slamming loads on the hull and structure. Loads
also rise rapidly as speed increases because hydrodynamic loading is related to
a boat's velocity squared, not just the velocity itself. All of these loads
demand complex structures when low weight also is an important necessity.