Small Propeller Cup: A Proposed Geometry
Standard and a New Performance Model
Donald M. MacPherson
VP Technical Director
HydroComp, Inc.
ABSTRACT
A widely used technique to correct and enhance vessel performance is propeller “cup” – a
curvature applied to the trailing edge of a propeller blade. Although cupping has become a
regular procedure in small propeller shops, it is typically applied without any systematic rules or
quantifiable understanding of its effect on performance. The traditional definitions of cup (e.g.,
light, heavy) vary greatly from one company to the next, and even from one project to the next
within the same company. It is the goal of this paper to provide a consistent definition of cup
geometry for practical use in industry, as well as a new performance model that can be used in
propeller calculations.
INTRODUCTION Custom propellers clearly offer the best
hydrodynamic solution to the problem of excess
Recent evolutionary changes in marine vehicle cavitation as they can be designed specifically for the
design and engine performance have resulted in an engine, gear and vessel. Custom propellers are not
increasing number of heavily-loaded small propellers. without their shortcomings, however. They are
Vessels are continuously pushed faster, and engines of typically effective only within a narrow design range,
a given size and weight have become more powerful to are costly to manufacture and repair, and require
accommodate this increased demand for performance. special design capabilities. Also, of course a custom
Unfortunately, all too often the propeller – the central propeller can only replace a poorly performing
element in performance – is not considered during early propeller – one cannot realistically redesign and
design stages. existing propeller after-the-fact. Propeller cup, while
Such an oversight results in stern geometries which not as hydrodynamically elegant as a proper custom
do not allow for adequate propeller diameter and design, can be applied by any competent propeller shop
clearance. The ultimate result is very high thrust to help resolve poor performance due to excess
loadings leading to noise, vibration and loss of cavitation. It can also be applied to a new propeller of
performance due to excess cavitation. Control of conventional design.
cavitation has become the focus of many of the efforts
of commercial propeller manufacturers and specialists WHAT IS PROPELLER CUP?
in the field.
One can always consider a different type of Propeller cup is simply the deformation of a
propulsor (e.g., waterjet or surface drive), but if fully propeller’s trailing edge toward the pressure face
submerged propellers are required, there are a few ways (Figure 1). Providing a measure of camber to the blade,
to mitigate the problem of excess cavitation: it changes the pressure distribution along the blade’s
chord length – adding lift toward the trailing edge.
1. Use cavitation to your advantage (e.g., super
cavitating propellers).
2. Prepare a custom design including features such as X
high skew or camber (also known as progressive
pitch). 30 deg
3. Apply cup to the propeller in conjunction with an
appropriate change in pitch.
Figure 1 – Example of propeller cup
Copyright © 1997 HydroComp, Inc. All rights reserved. Presented at the Propellers/Shafting '97 Symposium, SNAME, Sep 1997.
1
How is this change in lift distribution useful in A PROPOSED STANDARD DEFINITION OF
controlling cavitation? Typically you will find a peak in CUP GEOMETRY
the lift distribution somewhere on the leading half of
the blade. Cavitation occurs in this region when the To most members of the general marine
local lift is greater than the vapor pressure of the water community, cup comes in two flavors – light and
– causing it to vaporize or “boil”. (This vaporization heavy. Unfortunately, one person’s light cup is another
creates the vapor “cavity” which gives cavitation its person’s heavy cup. These trailing edge deformations
name.) Cavitation can be controlled if the peak lift can are very small – typically less than 10 mm (3/8 inch)
be reduced below the vapor pressure, while still for the heaviest of cups on most work boat and motor
generating the necessary total lift. yacht propellers – so differences on such a small scale
Here is where cup comes in. By adding lift away are easy to appreciate. It is also easy to see why a
from the peak via cupping, the entire lift can be reduced systematic definition of cup is so important.
by a reduction in pitch. The more we need to reduce Some propeller manufacturers have proprietary
pitch to lower the peak lift, the more cup we need to systems of rating cup. Cup gauges classed as A, B, C,
add to compensate for the lost thrust. See Figure 2 etc. document the amount of cup which was used and
below for a descriptive comparison of this effect. insure that a consistent cup is applied to all blades. Still,
a quantifiable system is needed to determine
performance changes.
One systematic treatment of the shape and amount
of cup was developed by the U.S. Navy’s small craft
Higher suction peak = more cavitation
group (Hankley, 1983) (Denny, 1989). Hankley and
Denny used the terms Light, Medium and Heavy to
describe the extend of trailing edge drop based on the
maximum thickness of the blade. These terms proved to
Higher pitch without cup be much more aggressive than commercial
terminology. (These terms correlate well to a
Same suction/pressure area (lift) = same thrust percentage of diameter for typical commercial
Lower suction peak = less cavitation propellers. The Light cup is typically about 0.5% of
diameter, Medium is 1% and Heavy is 1.5%. Thus, the
US Navy Heavy cup on a 36 inch propeller suggests
more than one-half inch of deflection!) These terms are
Lower pitch with cup
also subjective and are not “measurable”.
This author proposes a standard definition loosely
based on the Hankley/Denny definition described
Figure 2 – Performance comparison above. The goal is to remove any subjective terms like
Heavy and Light. We propose that cup be defined by:
The history of propeller cupping is not clear, but has 1. The trailing edge drop in millimeters. (Dimension
been in use for decades. Unlike progressive pitch X in Figure 1.) For example, a propeller might
propellers, cupping is not generally part of the original have a 3 mm cup, another a 7 mm cup. No more
propeller design or manufacture. It is indeed an Light and Heavy.
“aftermarket” performance boost applied in the best of 2. Cup curvature is to be an arc with a radius 7.5
propeller shop tradition – heat it and beat it. Of course, times the drop, giving an extent of curvature of 30
some shops are more elegant than others about the degrees.
precision and tolerance of their application of cup.
The lack of consistency in cup geometry is perhaps This now gives us a measurable definition with
the greatest individual roadblock to a systematic which we will build a performance model to determine
understanding of the effect of cup on performance. No how cup does affect thrust and torque.
two organizations use the same definition of cup, and it
is common to find differences in the application of cup A NEW PROPELLER CUP PERFORMANCE
even within the same shop. We can never develop a MODEL
methodical understanding of the effects of cup without
a consistent definition of the geometry. Before considering a new model of cup
performance, a review of current views is in order.
Starting with the public at large, this author has
2
witnessed a seriously misguided sense about cup. To from Hwang and HydroComp sea trial data. Based on
quote a few published comments: analysis of model test results and comparisons to full
scale trial results, it was found that the value of the
• “cupping the prop acts like increased pitch geometric pitch had little effect on the increase in pitch.
(approximately 1 inch)” Only the amount of cup deformation influenced the
• “select blades with 1 inch or 5 percent less pitch effective pitch.
than a similar uncupped blade” The new performance model is:
• “they serve no useful function on most vessels
operating under 30 knots” PEFF = PGEO + 21(XCUP)
• “add cup and your engine will lose about 200
RPM” where, PEFF = effective pitch
PGEO = geometric (uncupped) face pitch
All of the above may be true for individual XCUP = trailing edge deformation (drop)
circumstances, but as universal statements of fact they
fall short. These statements assume that all cupping is For example, the 250 mm propeller from Hwang
of the same size and has the same effect on all had an average cup of 1.85 mm. The effective increase
propellers. Of course, this is not the case. What is in pitch would then be approximately 39 mm. A
useful about the above statements is the notion of summary of geometric pitch, calculated effective pitch
“effective” pitch. (from the above relationship) and as-tested equivalent
In other words, a propeller with cup acts as if it pitch (from corresponding Kt/Kq curves) is shown
were a propeller with a somewhat higher pitch. (See below.
Figure 2 where the cupped propeller of a lower pitch
generates the same thrust as an uncupped propeller of a PGEO/D PGEO PEFF PEQUIV
higher pitch.) The use of effective pitch makes a great 0.8 200 239 240 (0.96 P/D)
deal of sense as it allows us to use conventional 1.0 250 289 285 (1.14 P/D)
propeller performance curves for the analysis of cupped 1.2 300 339 340 (1.36 P/D)
propellers. 1.4 350 389 390 (1.56 P/D)
Rather than stating that “cup was worth one inch of
pitch”, Denny offered a thoughtful analysis of effective Table 1 – Corresponding effective pitch
pitch. For their Light, Medium and Heavy cups, the
authors prepared a chart of effective pitch vs. geometric At no point was the calculated effective pitch more
pitch, and Kt curves for a variety of 3-bladed Gawn- than 2% from the tested equivalent pitch, and there was
style propellers. Unfortunately, no correlation for very good correlation with both Kt and Kq. So, to find
torque (Kq) was presented and the authors’ definition the necessary cup, first determine the pitch increase
of geometric pitch included the cup deformation, needed for performance and then divide the pitch
making the correlation difficult to evaluate. This has increase by 21.
lead this author and others in industry to the conclusion There is one final point to remember when using
that the charts significantly over-estimate the effect of this performance model. As you can see, effective pitch
cup. (Full scale analysis of a number of motor yachts can increase substantially with cup. One must be sure to
by this author also supports this conclusion.) Finally, check that the effective pitch does not exceed the range
three magnitudes of cup are inadequate to be useful to of P/D ratio in the data set of the Kt/Kq formula. For
the industry at large. A finer distinction is required (i.e., example, the above propeller with a geometric pitch of
something less than Light and between Light and 400 (1.6 P/D) is within the upper limit of the Gawn-
Medium). AEW equations (Blount, 1981), but the effective pitch
Hwang (Hwang, 1995) reached many of these of 439 mm (1.76 P/D) is outside of the range and the
same conclusions in their presentation of model tests extrapolated results may be unreliable.
and effective pitch for a 3-bladed Gawn-style propeller.
They intentionally used a propeller geometry and Cavitation and effective pitch
Medium cup definition following that of Denny. The
published results included both Kt and Kq curves over How is cavitation evaluated for a cupped
a range of P/D ratios, and show an increase in pitch to propeller? The simple answer is that while performance
be significantly less than that of Denny. corresponds to a propeller with a higher effective pitch,
The following simple performance model attempts the levels of cavitation correspond to the geometric
to eliminate the subjective terms of Light, Medium and pitch of the propeller ahead of the cup.
Heavy, and follows the more modest pitch increases
3
The traditional criteria for cavitation are all (90%R), with a smooth transition to no cup at both
empirically-derived functions of blade pressure extents. (One note of interest: a few companies have
(MacPherson, 1991). They were developed over time to had some success in varying the cup distribution,
represent relationships between amounts of cavitation typically with greater cup near the tip, in an attempt to
and “average” blade pressure for conventional fine-tune performance at various speeds.)
(uncupped) propellers. To use these criteria with
cupped propellers, it is necessary to calculate two CONCLUSION
different thrust values – a performance thrust at the
effective pitch and then a theoretical “cavitation thrust” The simple geometric definition and performance
calculated at the geometric pitch. Average blade model described above are intended to bring some
pressure, subsequent levels of cavitation and thrust loss sense of consistency to the community of propeller
are then derived from this “cavitation thrust”. manufacturers, after-market propeller shops, naval
architects and other marine professionals interested in
MEASUREMENT OF CUP cupped propeller performance. An earlier generation of
the performance model implemented in a commercial
Knowing how much cup to apply is only half of software package (HydroComp, 1996) has been used
the battle. The other half is actually getting the proper successfully by dozens of marine professionals for
cup onto the propeller. Using the geometric definition numerous new and repowered vessels.
of cup corresponding to Hankley and Denny, this Work is continuing to improve the performance
author suggests the use of a propeller cup gauge similar model by segregating thrust and torque, by evaluating
to that shown in Figure 3. the effect of other propeller parameters (e.g., mean
Each gauge is labeled for the amount of trailing width ratio) and by refining the relationship with new
edge deformation (drop) in millimeters (e.g., 5 mm). It data from model tests and sea trials, particularly for 4-
would have a step of the proper dimension (e.g., 5 mm) and 5-bladed propellers of very high blade area.
to measure the drop, and a radius of 7.5 times the drop
(e.g., 37.5 mm) to measure the curvature. Two marks of REFERENCES
0 and 30 degrees would be scored on the radius to show
the extent of curvature. A typical propeller shop would Blount, D.L. and Hubble, E.N., “Sizing Segmental
have gauges in 1 mm increments up to 10 mm or so. Section Commercially Available Propellers for Small
Craft”, SNAME Propeller Symposium, 1981.
25.0 25.00
5.0
Denny, S.B., Puckette, L.T., Hubble, E.N., Smith, S.K.
0 and Najarian, R.F., “A New Usable Propeller Series”,
30 Marine Technology, Vol. 26, No. 3, July 1989.
HydroComp
5mm Cup Gauge R37.5
Hwang JL, Tsai JF and Li CY, “Cupped Propeller Test
and Analysis”, Ship Technology Research, Vol. 32,
90.0 1995.
Hankley, D.W. and Denny, S.B., “Performance
Characteristics for a Series of Commercially Available
Propellers for Small Craft”, SNAME, San Diego
Section, Feb 1983.
87.5
HydroComp, Inc., HydroComp PropExpert™ program
and documentation, 1996.
Figure 3 – Cup gauge MacPherson, D.M., “Reliable Propeller Selection for
Work Boats and Pleasure Craft: Techniques Using a
Consistency of pitch and cup is very important, so Personal Computer”, SNAME Power Boat Symposium,
industry practice is to apply the same amount of cup 1991.
across the blade. Typically, the required cup would be
fit from the mid-radius (40%-50%R) to near the tip
4
DISCUSSION
Dudley Dawson, P.E., Member propeller that is slightly underpitched, but provided
with moderate cup, the problems of under- or over-
I’d like to thank the author for this significant displacement (cavitation, inability to attain full engine
contribution to the field of small craft propeller RPM, inability to attain full power loading or speed)
technology. As a designer specializing in power vessels can often be addressed by adjustments in cup rather
up to about 60 meters in length, I am well aware of the than having to repitch the propellers.
current uncertainties and problems in specifying Although both the title and the text of the paper
propellers with an unquantified light, medium or heavy indicate that the proposed measurement system and
cup. The proposed measurement system will address effective pitch methodology are for small craft and
these problems, but only if propeller manufacturers and small propellers, no quantification of “small” is given.
distributors put into place a system of implementing It would be of benefit if the author could indicate
and verifying cup dimensions. It is up to naval recommended lower and upper limits based on the trial
architects and marine engineers to use the system data used and his verification of the methodology.
regularly in their specifications and insist that suppliers Finally, there is one item of practical concern. The
adhere to it. Once it has been given a fair trial and any author proposes that the radius of the cup vary directly
bugs worked out of the system, then an ABYC or SAE as a percentage of the drop (or deflection), resulting in
standard formalizing the measurement system would be a discrete radius for each value of drop, and larger radii
in order. for larger drops (heavier cups). Many of the propeller
In addition to those cited in the paper, there are shops working on small propellers are indeed of the
several other reasons that the specification of cup may “heat it and beat it” variety with a minimal investment
be desirable. One of the unfortunate trends in modern in equipment. Often, current practice is for a piece of
power vessel design and construction is a general round steel bar stock to be used as a rough mandrel to
increase in both weight and power for a vessel of given cup the trailing edge, and light vs. heavy cup is thus a
physical dimensions. While weight and power have matter of varying the extent rather than the radius of the
increased, indicating additional propeller blade area, curved portion. Also, when an existing heavy cup is
the depth of water has not increased in the Caribbean modified to a light cup, it is often done by bending the
for yachts nor in the Gulf of Mexico for crewboats nor trailing edge back toward the flat, so a lighter cup may
in rivers for workboats and ferries. Some commercial end up with more radius than a heavy one. The author’s
vessels have addressed this problem by installing comments on technical considerations of the two
additional propellers (4 or more) of the same limited differing systems would be appreciated, along with his
diameter. Although there are a few yachts with 3 thoughts on whether it would be possible to use a
engines and propellers, most have been limited to 2 discrete number of mandrel radii (say, 2 or 3) to cover
engines and propellers by owner demands. For this the range from 1 to 10 millimeters of cup without
market, the use of hull-bottom propeller tunnels with significant loss of effectiveness.
larger diameter, high-area ratio propellers is finding
increased acceptance (Dawson, 1997). In many cases, Additional reference
this is sufficient to provide the necessary propeller
blade area without increased draft but in some extreme Dawson, D.A., “Faster, Farther, and More Fuel-
cases, the specification of cup in a new installation is Efficient”, Professional BoatBuilder, No. 44,
necessary to avoid or abate cavitation that cannot be December/January 1997.
designed out by other means.
There are also sound economic reasons for
specifying cup in a new installation. It is generally Author’s Closure
much less expensive to include cup in a manufacturer’s
stock propeller than to specify a custom propeller with
cambered and skewed blade patterns. Often, the Many thanks to Mr. Dawson for his real-world
theoretical difference in performance is quite small, and perspective on the use of cupped propellers. They are a
in full-scale tests, unmeasurable. Also, one of the most valuable addition to the paper.
common uncertainties between the preliminary design Regarding acceptance of the measurement system
and complete vessel stages is the full-load displacement by some regulating body (e.g., ABYC, SAE), any
of the vessel as built, and it is for this condition that system will have to stand on its own merits – and this
most propellers should be specified. By specifying a proposed system is no exception. It is my hope that this
5
will not be the industry’s final effort on this topic, but
will lead to additional interest, research and practical
development.
The term “small” is meant nothing more than to
provide some scope for the topic. As the fundamental
research was conducted and trial data evaluated on
propellers germane to small vessels (i.e., diameters
under about 32”), the model should not be extrapolated
to large propellers. Fortunately since cupping is
typically a “small” propeller practice, extrapolation to
larger diameters is moot.
Using a smaller mandrel to extend the cup onto the
blade so as to create an “effective” radius of cup would
depend on the skill of the craftsman, I suppose.
Unfortunately, I have no data to support or dismiss this
from a performance standpoint. I would not endorse
altering the proposed geometry, however, simply
because it is the way that things are done today.
Cupping is indeed a “performance tweak”, and it is not
unreasonable to suggest that if propeller builders and
vendors want to develop this expertise, then they
should have the proper tools – just as they do now with
a range of proper pitch blocks or even digital
measurement devices, for example.
My thanks again to Dudley Dawson for his
comments and to Peter Lapp of the Bird-Johnson
Company for this technical review.
