(First appearing in Professional Boatbuilder Magazine)
The art of positioning a propeller underneath a boat hull is not
a new one. Designers and naval architects have been grappling
with every aspect of the propulsion-by-propeller problem for
generations, and the result has been the evolution of a well
known set of standard and efficient solutions.
When a new and promising solution to a very old problem appears,
it's usually made possible by advancements in some other related
technology - material science or control and instrumentation
devices, for example.
But in the case of surface-piercing propellers, there's really no
new technology involved at all. Simply the re-arrangement of all
the traditional elements of a propulsion system into a different
configuration. A few features may be borrowed from the drive-
line and hydraulics fields, but these technologies are also as
old as the hills. The implication is that we've been wrong - or
at least quite a distance away from optimum - for an awful long
time. So it is with understandable skepticism that the idea of
using surface-piercing propellers on more-or-less conventional
small craft is greeted by the boatbuilding community.
What is a surface-piercing propeller, anyway? Simply stated, a
surface-piercing propeller (or surface propeller) is a propeller
that is positioned so that when the vessel is underway the
waterline passes right through the propeller's hub. This is
usually accomplished by extending the propeller shaft out through
the transom of the vessel, and locating the propeller some
distance aft of the transom in the relatively flat water surface
that flows out from the transom's bottom edge. (The exception
being single-shaft catamarans, where the propeller hub intersects
the undisturbed waterline.) In the case of articulated surface
drive systems, the propeller shaft is driven through a double
universal joint inside an oil-tight ball joint, allowing the
shaft to rotate athwart ships for steering and to trim up and down
for control of propeller submergence. Fixed-shaft surface drives
can use conventional shafts and stern tube bearings, but require
rudders. In many racing applications, outboards and outdrives
can be positioned sufficiently high on the vessel for the
propellers to operate in a surface-piercing mode.
The important operating feature is that each propeller blade is
out of the water for half of each revolution. And here is
another reason for skepticism. Surely a propeller blade is more
efficient if it operates continuously in the smoothest possible
flow, rather than splashing through the water surface twice with
each revolution. But nature can play tricks on our intuition.
Sometimes an unsteady process is actually more efficient than its
continuous counterpart.
Why use a surface propeller?
A summary of the principal reasons for the high performance of
surface propeller systems relative to conventional installations
follows.
Propeller Efficiency: Traditional propeller design and selection
is almost always an exercise in trading off diameter against
several other performance-limiting parameters. Basic momentum
theory tells us that for a given speed and thrust, the larger the
propeller, the higher the efficiency. While there are
exceptions, most notably the effects of frictional resistance on
large, slow-turning propellers, it is generally borne out in
practice that a larger propeller with a sufficiently deep gear
ratio will be more efficient than a small one.
A number of design considerations conspire to limit the maximum
feasible propeller diameter to something considerably smaller
than the optimal size. These include blade tip clearance from
the hull, maximum vessel draft, shaft angle, and engine location.
While this may at times make life easy for the designer - the
propeller diameter specified is simply the maximum that fits - it
can also result in a considerable sacrifice of propulsive
efficiency. And if these geometric limits on propeller diameter
are exceeded, the result can be excessive vibration and damage
due to low tip clearances, or a steep shaft angle with severe
loss of efficiency and additional parasitic drag, or deep
navigational draft that restricts operation or requires a
protective keel and its associated drag. In many cases, the best
design solution is to live with a mix of all of the above
problems to some degree.
The surface-piercing propeller frees the designer from these
limitations. There is virtually no limit to the size of
propeller that will work. The designer is able to use a much
deeper reduction ratio, and a larger, lightly-loaded, and more
efficient propeller.
Cavitation: When a submerged propeller blade cavitates, the
pressure on part of the blade becomes so low that a near vacuum
is formed. This happens more easily than one might think -
atmospheric pressure is only 14.7 psi, not a very big number
considering the size of a typical propeller and the thrust it is
required to produce. If the suction on the low-pressure side of
the propeller blade dips below ambient pressure - atmospheric
plus hydrostatic head - then a vacuum cavity forms. (To be
strictly correct, there is water vapor in the cavity, and the
pressure is not a true vacuum, but equal to the vapor pressure of
the water.)
When these vacuum cavities collapse, water impacts on the blade
surface with a local pressure singularity - that is, a point with
theoretically infinite velocity and pressure. The effect can
approximate that of hitting the blade with a hammer on each
revolution. Cavitation is a major source of propeller damage,
vibration, noise, and loss of performance. And although high-
speed propellers are often designed to operate in a fully-
cavitating (supercavitating) mode, problems associated with
cavitation are frequently a limiting factor in propeller design
and selection.
The surface propeller effectively eliminates cavitation by
replacing it with ventilation. With each stroke, the propeller
blade brings a bubble of air into what would otherwise be the
vacuum cavity region. The water ram effect that occurs when a
vacuum cavity collapses is suppressed, because the air entrained
in the cavity compresses as the cavity shrinks in size. Although
the flow over a superventilating propeller blade bears a
superficial resemblance to that over a supercavitating blade,
most of the vibration, surface erosion, and underwater noise are
absent.
In theory there is a slight performance penalty for allowing
surface air into the low-pressure cavities. Instead of near-zero
pressure on the forward side of the blades, now there is 14.7 psi
pushing backwards. But in practice, this effect is not
significant considering the total thrust pressures involved in
high-speed propellers.
Note that cavitation can also be associated with sudden loss of
thrust and high propeller slip, often caused by a sharp maneuver
or resistance increase. This can still occur with surface
propellers, although the propeller is ventilating rather than
cavitating and the result is not as damaging.
Appendage Drag: Exposed shafts, struts, and propeller hubs all
contribute to parasitic drag. Inclined the exposed shafts not
only produces form and frictional drag, but there is also induced
drag associated with the minus-effect lift caused by their
rotation. There is a surprising amount of power loss resulting
from the friction of the shaft rotating in the water flow. In
fact, for conventional installations a net performance increase
can often be realized by enclosing submerged shafts in non-
rotating shrouds, despite the increase in diameter.
Surface propellers virtually eliminate drag from all of these
sources, as the only surfaces to contact the water are the
propeller blades and a skeg or rudder.
Variable Geometry: When a surface propeller is used in
conjunction with an articulated drive system, the vessel operator
then has the ability to adjust propeller submergence underway.
This has roughly the same effect as varying the diameter of a
fully submerged propeller, and allows for considerable tolerance
in selecting propellers - or it allows one propeller to match a
range of vessel operating conditions. This capability is
somewhat analogous to adjusting pitch on a controllable pitch
propeller.
When the articulated drive is used for steering, the result can
be exceptionally good high-speed maneuvering characteristics. On
single-shaft applications, drive steering can also be used to
compensate for propeller-induced side force, without resorting to
an excessively large rudder or skeg.
Shallow Draft: This is the characteristic that motivates many
designers to investigate surface propeller propulsion in the
first place. The vessel's navigational draft can be as low as
half a propeller diameter. Compared with other options for
shallow water propulsion - most notably water-jets - surface
propellers enjoy a very significant efficiency advantage. This
advantage is most dramatic for low-speed applications, but is
still present throughout the performance spectrum.
In the case of articulated drives, the propellers can be trimmed
up until just the tips are submerged for intermittent operation
in very shallow water, including beaching. Sometimes the design
allows the propellers to trim sufficiently above the baseline so
that the vessel can "dry out" with the props well clear of the
bottom.
These are the intrinsic performance advantages of surface
propellers. Other desirable characteristics include flexibility
in machinery arrangement, ease of maintenance and repair, and
simplified installation. In some applications involving hybrid
propulsion systems, such as the combination of diesel cruise
engines with a gas turbine sprint engine, the ability to retract
one set of propellers completely clear of the water when not in
use is an overriding consideration.
Selecting a Surface Propulsion System:
Having elected to investigate the surface propulsion option, the
builder or designer is faced with a series of major decisions and
a very limited amount of reliable data. First is the issue of
fixed versus articulated. As outlined above, articulated drives
have the advantage of variable propeller submergence, high speed
maneuverability, and extreme shallow draft capabilities. Fixed
systems, on the other hand, are less expensive to buy and maintain,
do not require the hydraulic cylinders and associated pumps,
control devices, and high pressure plumbing. Further more, fixed
systems are often designed to work with conventional solid shafts
and stern tubes, rather than the more complex universal-joint
drivelines found in articulated systems. It should also be noted
that articulated surface drives should not be relied upon to control
vessel trim angle. Trimming the drive up and down will have only a
small effect on vessel running trim, and separate trim tabs or other
devices may still be desirable.
Very frequently, the nature of the vessel or operating conditions
will dictate the fixed/articulated decision. Some multi-hulls,
for example, have very narrow transoms that practically rule out
an articulated system unless some alternative attachment points
for the hydraulic steering and trim cylinders can be found. But
in cases where variable trim is required for shallow draft or
propeller retraction, a fixed system is clearly not viable.
In most cases, both the fixed and articulated options can be made
to work, and the maximum performance possible with each should be
comparable (although there have been applications in which
variation of propeller submergence is necessary to pass through
certain transitional speeds). Personal preference, relationship
and proximity to dealers and distributors, and the existence of
successful vessels with similar propulsion systems will probably
govern this choice.
At this level, it is important to establish a relationship with
the surface drive dealer or manufacturer's representative. For
fixed surface drives, the Levi and Simplicity Drive Units are the
most popular worldwide. A handful of other fixed drive manufactures
compete in certain areas. For articulated surface drives, the
Arneson Surface Drive is the dominant product, thanks to the
"universal joint inside a ball joint" configuration patented by
Howard Arneson.
Get the drive vendor involved as early as possible in the design
process. But remember to carefully evaluate the advice and
predictions made by non-technical sales reps. They want to make
sales, and are understandably prone to exaggeration at times.
Sometimes the most valuable service that the salespeople can
provide s a reference to a successful project similar to yours.
Naval Architecture has traditionally relied heavily upon
improving previous work. And while there may at times be a fine
line between plagiarism and "design evolution," it certainly
behooves the responsible designer to acquire full knowledge of
the current state of the art.
A number of designers and builders have succumbed to the
temptation to engineer their own fixed surface drive. Results
have usually been less than satisfactory, for a variety of
reasons. Probably the most common is placement of the propeller
much too close to the transom. Another pitfall is propeller
design. Without the support of a propeller or drive vendor
experienced with surface propulsion, the propeller performance is
an unknown variable. And finally, the self-engineered system is
difficult to fine tune. Modifications to propeller and drive
geometry in the course of "dialing in" the system can be time
consuming and expensive.
Propeller Selection: Surface propellers are usually associated
with the stainless steel "cleaver" style common to race boat
applications. These propellers have straight trailing edges,
razor-sharp leading edges, and sometimes as many as eight blades.
Probably because the roots of surface propulsion technology are
so firmly imbedded in the race boat world, it's no surprise that
the popular perception is that all surface propellers are
cleavers. Yet the vast majority of surface propellers being sold
today have round-tipped blades, are made of bronze (or NiBrAl),
and have only three or four blades. In fact, at first glance
there is very little to distinguish them from conventional, fully
submerged props.
What distinguishes a surface propeller from an underwater design?
The pressure face of the blade is always concave, the leading
edge is relatively sharp with a narrow entry angle, and the hub
and blade root are built to withstand heavy eccentric and
alternating loads. There is major incentive to keep the blade
section thin (it's the strength of the steel blades that really
gives cleavers the edge at high speeds and loading). Nearly all
successful designs have moderate to heavy trailing edge cupping.
Propeller selection begins with an estimate of required thrust at
the design speed. This is usually based on one of several
computational methods, but can also be generated from empirical
formulas or, if available, trial data from nearly similar
vessels. Then a preliminary gear ratio and diameter is chosen,
adjusting both until slip and pitch/diameter ratio are optimal
and the required thrust is generated. This will generally result
in a non-standard reduction ratio, so the remainder of the
process involves adjusting diameter and pitch to fit the
available drive train hardware. This is, of course, a somewhat
simplified description of a "design spiral." Usually the initial
design conditions will be modified in the course of the analysis,
and there are numerous other considerations such as number of
blades, propeller submergence, drive train structural
limitations, and vessel trim. Note that unlike propeller
selection for a large proportion of conventional applications,
diameter remains a variable parameter throughout the entire
process.
The drive or propeller vendor is usually eager to perform these
calculations for you, and in some cases can supply you with a
computer program that will enable you to play with various
options on your own.
Problems:There can be problems with surface propulsion systems,
although some of these difficulties stem from other factors not
inherently associated with this type of propeller operation.
Vibration: One of the amazing features of surface propulsion is
its smoothness at high speed, due mainly to the suppression of
cavitation. This is contrary to intuition, and must be
experienced to be fully believed. However, some installations
have experienced serious vibration problems. In most cases this
is due to improper design or alignment of the shafting between
the gearbox and drive input shaft. When double universal joint
drivelines are required, as is the case with articulated systems,
it is especially important to plan the driveline geometry so that
operating angles of the two joints are approximately equal and
within accepted tolerances. This is because a universal joint
does not transmit rotational velocity evenly, causing angular
acceleration and deceleration twice with each shaft revolution.
As a general guideline, joint angles should not exceed six
degrees per joint, and the difference between the two joint
angles should be less than one-half degree. This allows the
angular accelerations produced by one joint to be almost exactly
compensated by the other joint. (Depending on the orientation of
the universal joint yokes, the joint angles can be opposite with
driveline flanges parallel, or they can both angle in the same
direction for a net total shaft angle change of up to twelve
degrees.
The less common vibration problems that are not driveline-related
can almost always be solved by using propellers with a larger
number of blades, although there is some cost penalty involved.
Backing Performance: Surface propulsion has a reputation for very
poor performance in reverse. A certain amount of this reputation
is based on the fact that until very recently, nearly all surface
propeller installations were on very high-speed vessels using
"cleaver" style propellers. These propellers, due to the thick
trailing edges, concave pressure face, and often heavy trailing
edge cupping, are notoriously poor performers in reverse. And
this is true whether they are used as surface propellers or as
cavitating fully submerged propellers.
However, there is an occasional problem with backing performance
of surface propulsion systems, regardless of propeller style.
Part of the slipstream of the propellers is directed right into
the vessel's transom, with an obvious loss of net astern thrust.
Side curtains (hull side extensions aft of the transom) can
seriously aggravate this coition. In fact, there has been at
least one installation in which the vessel was actually propelled
forward when the propellers were turning backwards at certain
speeds. The aft overhang and side curtains combined to work like
the reversing bucket on a water-jet, except that in this case
reverse thrust was being "reversed" to forward thrust!
Fortunately there is an easy fix. The addition of baffle plates
between the transom and the propeller that direct the slipstream
down and forward (the plates are dry when the vessel is operating
ahead at speed) has proved extremely effective. This wedge is part
of the drive housing on Simplicity Drives. But for the
majority of applications, no such hardware is required to provide
adequate, although not outstanding, performance in reverse.
Transitional Speeds:
Most planing hull designs, especially moderately low-powered or
heavy designs, are subject to problems getting through "hump"
speed. High vessel resistance at pre-planing speeds, high
propeller slip, and reduced engine torque output at less than
full RPM can sometimes combine to make it impossible to reach
design speed, even though the vessel may be perfectly capable of
operating at design speed once it gets there. The boat that
"can't get out of the hole" is a phenomenon that should be quite
familiar to many designers and builders. With surface propulsion
systems there is an additional factor which may make the
situation worse - the propeller is designed to operate with only
half of the blade area immersed. But a low speeds, before the
transom aerates or "dry's out," the propeller must operate fully
submerged. Not only is the submerged area doubled, but the top
half is operating in very strong wake turbulence right behind the
transom. The result is that it takes much more torque to spin the
propeller at a given RPM, and sometimes the engine is not capable
of providing the torque necessary to turn the propeller fast
enough to get the boat up to the speed which allows the transom
to aerate and unload the top half of the propeller.
To reduce this potential problem, various methods of aerating the
top half of the propeller have been employed. The Levi and
Simplicity Drives, for example, directs engine exhaust into
the water in front of the propeller. On some installations,
passive "aeration pipes" leading from above the static waterline
to the forward side of the propeller have been effective. When
the lower surface of the aft overhang is below the static waterline,
it is sometimes advisable to leave cut-outs through the overhang to
let air get to the propellers. With articulated drives, maximum up
trim can sometimes reduce propeller submergence sufficiently to
achieve required RPM for take-off power.
Fortunately, these measures are not required for the vast
majority of applications. However, designers and builders should
be particularly diligent in checking power and thrust margins
over the entire speed range, and also be aware of the possibly
disastrous consequences of producing a vessel that is seriously
overweight.
The Future:
The ability to use large diameters and deep reduction ratios is a
capability that is just beginning to be exploited. Surface
propellers have long been accepted for racing applications, where
minimizing appendage drag and cavitation are the major
motivations. In recent years, an increasing number of high-speed
yachts and patrol boats have been propelled by surface
propellers, and some of these applications have been
spectacularly successful. But the use of surface propulsion for
relatively heavy and slow vessels is new. A major obstacle to
overcome is the first-cost of the large propeller and power
transmission equipment capable of handling the higher torques
associated with the deep reduction ratios. Life-cycle economics,
however, especially for commercial vessels with heavy duty
cycles, can be extremely attractive.
We should also look for major evolution in propeller design. The
fact that there is no performance penalty for large hub diameter
opens the door to new versions of controllable pitch, counter-
rotation, and other exotic variations. Propeller blade design is
one area where material science may be the controlling
technology, as propeller builders experiment with composite blade
materials.
But from the builder's point of view, one of the major
attractions of surface propulsion is the fact that it does not
require any sophisticated or exotic new technology. If anything,
the installation of a surface drive is a simplification over
conventional shafts and struts. It is simply a re-arrangement of
the familiar parts - with significant value added.
SEVEN DESIGN RULES FOR SURFACE-PROPELLED VESSELS
1) Make sure the hull form is appropriate for the intended speed
range. Semi-planing or low speed planing designs with a high
degree of bottom warping (deadrise angle that continues to
flatten aft of maximum beam) or keel rocker (curved buttocks aft
of maximum beam) will be very poor performers if pushed beyond
their intended speeds. Sometimes rocker or warp is included in a
hull form because it is believed to improve propulsive
efficiency, by increasing the wake fraction (slowing down the
water relative to the hull) in way of the conventional propeller
location. There will be no benefit if the hull is to be
propelled by surface propellers located outside of this wake
field. Avoid flow obstructions, such as water pick-ups or trim
tabs, directly upstream of any part of the propeller disk.
2) Be realistic with weight and center of gravity estimates.
Nearly all boats weigh more than the designer and builder would
like them to weigh, and this is by far the single most common
cause of failure to meet anticipated trial speed. Surface drive
vendors will generally be delighted to estimate vessel
performance for you, but they need accurate data.
3) Use the optimum reduction ratio. It is tempting to save cost
by using a shallow reduction ratio and smaller, faster turning,
and generally less expensive propellers. At a higher RPM, the
same power produces less torque, thereby also reducing the cost
of the drive drivelines, and gearbox. This obviates one of the
major advantages of surface propulsion, and there are many
examples of applications which fail to perform satisfactorily
because of insufficient reduction ratio in the interest of first-
cost economy.
4) Don't neglect trim control. If the design requires trim tabs
with underwater propellers, it may require them with surface
drives as well. There will be a net vertical force from the
surface propellers, depending on a number of parameters including
deadrise angle and direction of propeller rotation. Occasionally
a vessel that trims well with conventional underwater propellers
and no trim tabs is excessively bow-down with surface propellers,
and the only satisfactory fix is a center of gravity move. Ask
the drive or propeller vendor for assistance in estimating what
effect the drive will have on vessel trim. (Trim tabs should not
be positioned in front of the propeller disk, however.)
5) Leave enough space for the engine! Although engine placement
and installation is greatly simplified with most surface drives,
there is still a certain amount of length required for the drive
input shaft, driveline (which usually includes universal and slip
joints), and gearbox. There are also some geometric limitations
on what a double universal joint driveline can and cannot do.
Working with the various vendors early in the design process
could avoid a serious problem later on.
6) Design the transom to conform to the requirements of the
drive. The proper transom angle will eliminate the need for
wedges, and in the case of articulated drives, clear space for
hydraulic cylinder attachment brackets is essential. Flat
transoms allow the most straightforward installations.
7) Protect the propeller, but use side curtains sparingly. A
surface propeller can be a very substantial hazard to anything or
anybody that falls off the stern of a vessel underway. The bears
a striking resemblance to a giant food processor! Nearly all
recreational designs include an aft cockpit extension, deck
extension, or "swim step" that overhangs the propellers, and even
military designs use a pipe-and-canvass overhang to protect
personnel. Sidewalls, however, should be used with care. They
may have an adverse effect on backing performance.
Author's Bio:
Paul Kamen is a naval architect with degrees from Webb Institute
of Naval Architecture and the University of California at
Berkeley. Formerly the Applications Naval Architect for Arneson
Marine, he is now an independent consultant specializing in
surface propulsion.
He can be reached in Berkeley, California at 510-540-7968 or via email
at pk@well.com
