I
ARCH1E
Snipbui1dig and Shipping R'cord, May 10, 1968Considerations of propefler layout from the
enginebuildr's point of view
TO YIJIY3E TIlE PrRFORMANCE of propellers,
the marine engineer uses different para-meters frorii the naval architect. Exaggerat-ing matters. one might say that the im&rine engineer considers the propeller as a
dynamometer. which unfortunately has no means of :idjustrnent. Whereas the naval architect takes the ship's speed as the basis for his calculations and judgment
of propeller performance, the marine
engineer bases his approach on propeller or engine revolutions. The reasons are
obvious. In actual service, the data
received from the ship is selcom reliable. The engineroom log-book,, however, gives some data which can he accepted as being true value, such as r.p.m., load indicator,
fuel consumption, pressures and tempera-tures; the relative accuracy fullowing the
xame sequence. The navigational data is, without any doubt, faithfully recorded, hut it is difficult to determine the actual ship's speed in the water under the dif-ferent service conditions.
Although the design and construction of the propeller does not lie within the
field of competence of the engincbuilder,
a strong interest exists, as only with a
properly laid-out propeller can the
pro-pulsion engine develop the specilied
out-put. Thc same applies to the supporting ancillary systems, such as fuel and
cool-ing circuits. etc.
Inadequate ambient conditions will lead
to increased wear and tear, higher
main-tenance costs and eventual breakdown of
machinery. T1e latter is the main
con-cern of the engine manufacturer as
unsatis-factory service results, from the owner's point of view, influence the engine-builder's reputation and can jcopardise
the future competitive position.
Special attention has been drawn to
propeller layout as the service results over
the last 5 to 10 years have shown that loss of rpm. is quite considerable. especially in large hulk carriers and
tankers with high block coefficients.
In the paper an effort is made to des-cribe the wishes and requirements
con-cerning propeller layout, strictly from the enginehuilders point of view, no new
propulsion or propeller theoriet are presented.
The ideal propeller working range The service conditions simplilìcd for a fixed propeller: N =output, n=r.p.m..
mean effective pressure (Pine), and the theoretical propeller law for the nominal
output and speed of the engine are shown
in Fig. 1. The range to the right of the
* A eynopis cf a paper of the same title read before the Institution of Engineers and Ship-builders in Scortand
propeller law with higher r.p.m. is the working range, which gives the best ser-vice results. The range to the left of the
theoretical propeller law represents work-ing conditions that should he avoided for
longer periods of time, unless increased
wear, tear and consequently higher
main-tcnance costs are taken into account.
These remarks are simply the opinion of the enginebuilder, but they do not give a complete picture as, additionally
a loss of power can occur when the
engine works in the range to the left of the theoretical propeller law. A diesel
engine shows accurately, by means of the position of the fuel pump rack, the
devel-oped torque, and it is common practice
to provide a limit stop on the fuel pump
regulating, gear to prevent excessive fuel injection, that is, mean effective pressures and ovcr-torquing.
Effect of increase of hull resistan'ce in service
When the propeller is designed to absorb the nominal rating, that is. maxi-mum continuous rating (N = 100%) at
nominal r.p.m. n= 100%) shown at point
A of Fig. I for trial conditions and fully loaded ship but with a clean new hull
and smooth weather; then it will be found that such a propeller is too heavy tor
actual service. Unavoidable increase of hull resistance by fouling and hull deter-ioration sviti cause a drop in r.p.m. along
the 100% Pme line (line ABl with
con-sequent loss of power. This loss of power,
which is in direct proportion to the drop
in r.p.m., occurs exactly when more power
Fig. I. Service conditions for a fixed propeller
and theoretical propeller law for nominal
engine speed and output
i)
Lab.
y. Scheepshoutzkimcie
Technische Hogsckoo(
Deift
651 J. A. Smit Sulzer Bros.is required to drive the ship. Although the engine is now producing less output,
and consuming less fuel, nevertheless the specific engine load is increased, thermo-dynamically and mechanically. In extreme
instances surging of the turbo-blowers may event occur. Due to the variations
in hull resistance, between the conditions
of the ship in ballast and fully loaded, as well as with a clean or dirty hull, the service point would wander between the points A and B. The yearly average would be approximately 96% output. or perhaps even less, hut at reduced r.p.m.
and consequently at higher specific engine load compared wills maximum continuous
ra tin g.
Recommended propeller design considerations
It is pointless to introduce a
recom-mended service output without specifying service conditions and propeller i'.p.m. lt
seems, again from the enginehuilder's point of view, preferable to make a
recommcndition concerning the propeller layout. The propeller should he designed
to absorb not more than 85 to 90% of
the maximum continuous engine rating. at nominal r.p.m. (,z= 100%), the ship being
fully loaded with a clean new hull and
trial conditions. With an increase of power
demand due to increased resistance, the service point will wander along the line C - A and, for adverse conditions, more
output is available without increased specific engine loading.
The margin of 85 to 90% is, o1 course,
dependent on the type and size of ship, trade, etc., and covers not only fouling
hut also a certain degree of hull
deteriora-tion. Without exact data and service
results, it is difficult to predetermine this
margin. The values given, however, are
a good avedige. based on results obtained
froua ships' logs and reports over the
past two years.
Realistic trial requirements
With a propeller designed according to these recommendations, it is dilTìcult, and sometimes even impossible, to fulfil some
of the trial COfl(litioflS stipulated in
present-day building contracts. For instance, it will not he possible to obtain full-load
out-put with the chip in ballast and clean hull without increasing the r.p.m. However, the
r.p.m. can he increased to about 106% of the nominal speed, which in most
instances will produce 100% output, except perhaps in fast cargo liners running acecptanc trials in light ballast.
In most present-day building contracts
a clause is found stipulating that the main propulsion unit must he l'un at full load during trials; tItis provides unnecessary
652 Shipbuilding and Shipping Record, May lO, 1968
complications. lt woild he preferable to
show during trials that at service speed
suilicient margin. iii output, torque and r.p.m., has been provided. This would pre-vent propellers heiiig designed to obtain propaganda trial speeds. which very often lead to the wrong conclusions concerning
charter and service speeds. In the case of
the diesel engine, it s not necessary during ship trials to give proof that full output
cari he produced, as most engines have been tested in the workshops. Even for
non-tested engines, reliable data can be
provided by the engine manufacturer as the engines are normally standard units out of a well-proven series.
Estimation of increase of resistance in service
To calculate the increase in hull resis-tance over the years and to analyse the totat resistance increase as a percentage
for fouling and bull deterioratïon, exact
data from the ship's records must be avail-able, including circumstantial information
such as weather conditions, hull treatment,
dry-docking intervals, etc. - Unfortunately, it is very seldom that enginchuilders are supplied with this type of information and then, generally speaking, only after
diffi-culties have occurred and the owner is
not satislied with the performance of the propulsion plant. The information which is available in thc form of continuous log extracts, or perhaps only sporadic
sum-maries, all shows the same tendency,
namely, that the increase in hull resistance is more severe with increasing sh'c of ships
and larger block coellicicnts. This
pheno-menon seems to continue to ships of
Fig. 2. Progressive increase of the required power, due to huit fouling Fig. 3. Progressive increase of the required power, due to huit fouling for a typical series cf 70000 ton d.w. tankers for a typical series of 12,200 ton d.w. cargo liners
sop 25000 24000 23 oca 22ooa 2 boa 20000 !G000- ,rj0 10 000-17ao
6000 -15 oca 13 oca 12 coo lic no
lOooø-j
/
'i'
\IIFI
72il I .4Conticiinus Rnr;e1"' 3 Non Rccnmmended Working Range
p --- 14 Months in Service
- - 13 Mopth n t'rvica -22Y.OcrIood at Nora (ng Sp.
-
8 Kontho inSprvico-U'/.Qi'erlood at NomEng Sp-
44Mvs(t:s In Ser vice - 214 Overload at Nom Eng Sp 2.4Monlhs in S,-v,c - j Y. I oad Reserve loadcd NcrkingRange during So Trials - (cad Reser veR500rnmeisded lorA ìpg Ronge during Sea -TrIa lo -17'/.Lead Reserve
t L
L t1. t
100 705 110 F5 ¡20
i5
approximately 90M00 tons d.w. For larger ships no information is available lo date,
mainly because ships of such large tonnage
hase not been sufficiently long in service.
To illustrate the phenomenon of hull
resistance increase, the diagram in Fig. 2 has been drawn showing the average
ser-vice results, main-engine output and ship's
speed on the basis of engine r.p.m., for a series of 70,000-ton d.w. tankers, trading
worldwide, equipped with a direct-drive diesel engine of 23,000 h.h.p. at 121 r.p.m.
During trials, with full load draught,
the propeller absorbed approximately 97%
of the maximum continitous rating
(M.C.R.) at nominal r.p.m. After only
two to three months, the propeller
absorbed tIme full M.C.R. at nominal
r.p.m. After seven months' service. 10%
overload M.C.R. was required to obtain nominal r.p.m. and after 13 months'
ser-vice 22'. overload would have been
required. Conditions actually became even
worse just before dry-docking, but this
data is not reliable. After dry docking,
conditions improved, but the service points nes'cr returned to the range to the right
ol the theoretical propeller law. lt proved impossible to run the engine with 10O:
Pme. During this period with constant
Pone or torque, a drop of about 11 r.p.m.
was experienced. During trials.. speeds
up to 175 knots were registered, the
engine producing 97% M.C.R at nominal
r.p.m.; with the same torque this speed dropped down to approximately 14'7 knots,
a difference of 16-5%.
The actual service speed at present does
not exceed 145 to 150 knots, which does not represent the excellent trial results.
11W 23 22 27 20 ¡9 Blip 15 eco l5000 14 eno 13 aso 12 000 lleoo lOoco 9000 i !
Working Range on Triat 5'! load Rsoerme Reconmd Working Por on Trials-1514 Load Res
Nora flisiput of 60D ¡Soca hP/122 hí'1 bOO'/. SOP 1bO'/, SlIP I
_.
.111
, T0'lo this particular instance, a propeller
cor-rection is necessary as the engine cannot
produce its full output, and the present
service conditions enlise constant over-loadinmg. With a propeller designed accord-ing to the original layout recommended
in the paper. the trial resulls would not have been so favourable. It would have
been necessary to increase the engine
speed to about 127 r.p.m. to obtain full
engine output, but the ships average
ser-vice speed would have been about one
half knot higher and the engine would
have been running under more favourable
conditions, although producing nsore horse power.
The above example is not an exception.
This tendency is apparent in a large
number of modern hulk carriers and
tankers. That these difficulties are not only
reserved for hulk carriers and tankers, is shown in Fig. 3.
This diagram shows the results of a
fast cargo liner of approximately 12.000 tons cl.w. equipped with a diesel engine of 15,000 b.h.p. intended for a service
speed nf about 20 knots. The basic
prob-leni here seems to have been slightly
optimistic speed prognosis.
Very little information concerning the
effect of hull deterioration has been
pub-lished, but (Fig. 4) shows the effect of
the deterioration in hull surface on a ship's
resistance for an 11,000-ton d.w. carga
ship. After five years, an increase of 17'.. in thrust for constant ships service
speed can he expected.
Effect of ship size on resistance increase
An effort has been made to prepare a
--j-i ,140.'--j-i° Uierlood after85
IS 14 0. '.1od of t 614 s' - .- 5140. '.'14c of 1er 5 414 Oeriooda (toi-314 A Cnnthuous Range 1 i7 IIí'nRccarendediç14m.agR,,rge 1 0014 8/tP g 's. ir, Ballast t5.nths ¿n Service at ¡'ca Pig S,aee.1
N Q so KN 78 77 16 15 153 105 110 115 120 125
dgram (Fig. 5) showing the hull resis-tance incrca\e as a function of ship's se
for bulk carriers and tankers over a period
of live years. The curve shows the ten-denc as noted from the different ships' records.
For .i ship of 70,000 tons d.w. the
increase in required engine output, due to
increased hull resistance, to obtain
cori-slant ship's service speed. is a little more than 40 of the original maximum
con-tinuus rating, again after a period of five years with regular dry-docking.
lt is very dangerous to extrapolate the curve lo ships of larger tonnage. Some
data, however, indicates that the increase
will not exceed 42 to 43 of M.C.R. The large step in the increase of hull
resistance cornes in the range from 20.000
to 70.000 tons d.w. lt is not intended to attempt to lind a physical law tor this phenomenon: in faet, it will hardly be
possible to find a theoretical explanation. The curve only gives a summary of service
results reidstered and should he regarded
with reserve.
Engine power for use in propellerdesign After the engine manufacturer has made recommendations concerning the propeller
layout, the question as to how tu cal-culate and design the propeller is often raised. This problem, however, does not lic within the field of the enginchuilder,
as it is related to ship design, guaranteed speeds and other factors. The engine-builder is only interested in the results, namely. hie power absorbed by tite
pro-peller in the different service conditions.
In a book by Dr. van Lammercn and others, the following paragraph appears
in the Section Ori Data required tor the
design of a screw ". "If the propelling machinery consists of a diesel motor, it is advisable to design the screw in such a way that the revolutions for the screw in the mean service condition do not fall below those for the corresponding
maxi-mum power as guaranteed by the
engine-builders, in this way the mean pressure for which the motor was designed need
not be exceeded with greater screw
load-irigs. and the motor will last longer. At
smaller screw loadings, for instance in the
trial condition, the screw will rLtn a fcw revolutions faster, which, however, does not present any difficulties to the motor. In designing the screw the revolutions given by the manufacturer of the motor are generally increased by - to 1% for
the service condition." For the ships in
question, lanze bulk-carriers and tankers,
this mart
even 5.
I,.Another important factor for the
pro-peller calculation is the speed estimate. A propeller designed for an optimistic
ship's speed will provide too great a load
in actual service. Opinions differ as to which horse posver should be used for the propeller calculation. It seems
obvious that the maximum continuous rating at nominal r.p.m. increased by a
certain percentage should be used. Such a propeller makes it possible to use the full output available in the propulsion
plant as opportunity or circumstances
20
g
a
4 5 8
Shipbuilding und Shipping Rerord, íay 10, 19a8 653
Fig. 4 Effect of the deterioration on huit
surface on ships resistance for a t 1,000 d.w.t.
cargo ship. Basis: constant ships service
speed
permit. Another approach is to use the
calculated service horse power at reduced
r.p.m.. again increased with an r.p.m.
percentage. Such an approach may give theoretically a slightly better propeller
efficiency but tends again to turn heavier
so that it will he more difficult to obtain l00' M.C.R. under all cireLimstanees.
Propeller correction
Propeller correction is a problem which
the engi nebui der encounters regularly.
The two methods of correction are
crop-ping of the blades and pitch variation.
[lie first is relatively simple to carry out
and is often even done in situ. A rule of thumb is that 15% diameter correc-tion gives approximately 1 io l'2% r.p.m. increase with constant torque. The limit
of blade cropping is approximately 10%
of diameter, which gives a maximum of 65 to 8-5% r.p.m. Needless to say, the propeller efficiency and cavitation characteristics arc influenced. This measure
only leads to effective results when the
pitch over the outer 50% of radius is constant. When the pitch is reduced at the outer 20% radius, then cropping has little or no effect.
The second method is to correct the
pitch. This modification can only be
car-ried out in a speciahised workshop and
is limited by the size of the propeller.
The latter must be removed, transported and evenly heated, which is a costly and time-consuming job. Itere again, the
correction limit is approximately 10% in
pitch. Generally speaking, a combination
of cropping tise hladcs and clianoing the pitch gives the desired effect; the
maxi-mum r.p.m. increase with constant torque
iii5 stsould iarr._..taken rather as that can be expected is 13 to 15%.
owevcr, if such a large correction is
necessary, it will be preferable to provide a new redesigned propeller.
Variable pitch propellers
With stich large variations in torque and output, the introduction of a variable pitch
propeller seems logical. From the
engine-builder's point of view this solution is
perfect, as proper pitch adjustment for varyitig service conditions is possible. Although today variable pitch propellers
have been designed and are being
manu-facturer! for outputs of 12,000 h.p. and
io
o C 20 30 40 50
L
Fig. 5 Approximate increase of required
engine output due to the hull fouling and
deterioration for tsnkers arid bulk carriers
after a 5 year service period and regular
dockings. Basis: constant ship's service speed
more, the application is not popular for ocean-going world-wide trading ships.
Capital investment and doubts of reliability may be the important factors.
flic cost of a variable-pitch propeller installation complete with shafting and
Sparc tail-shaft amounts to approximately
33 '-., of the cost of the main engine,
pared with approximately 8% for a corn-parable fixed propeller with shafting.
The advantage of a variable-pitch
propeller installation lies more in the
pos-sihihity of pitch adjustment, obtaining at all times maximum output and increased
ship speed, than in easier manuvrahihitv.
Automatic load control can be installed but the mails difficrilty of such a control
system is stability (freedom from huntingl.
Unless constant mtinuvring is required,
for instance for short-trip ferries, harbour tugs, and other special purposes, the quick m:Lnuvring capacity of 2-stroke diesel
engines, remote-controlled, will be found
to he more than sullicient.
To conclude, with a properly laid-out
propeller more satisfactory service results can be expected, with increased relial-iilitv
and reduced maintenance costs. This ruling is applicable to all types of
pro-pulsion equipment, diesel and steam alike,
althouoh the effects can be somewhat difierent. The most important factor
remain.s proper maintenance of the hull. which is unfortunately more and more
neglected with longer periods between dry-docking. etc. With the coming of even larger tankers, and possibly even longer periods between hull cleaning, the martains
in determining the installed horsepower
and for the propeller design must be
looked at with special attention.
Bibliography
(I) Notes and Coissrnents. Moto Ship, 1961, vol. 42, p. 141.
" Resistance Propulsion and Steering of
Ships '', by W. P. A. van L3mmcren,
L. Troo.t and J. G. Koning. Vol It,
p. 224. 11. Siam, Flaarlens.
Cost Relations of the Treatments of
Ship I lulls and the Fuel Consumption of
Ships ". by istrs. Drs. Il. J. Lagwceri-v.i n Kuyk. International Sliipbtiitrling
Progress, 1967, vol. 14, p. 292.
