I
L.
RA Station of the
Ministry of Techno]ogy
s
See note inside cover
SHIP REP. .100October 1967
NATIONAL PHYSICAL
LABORATORY
SHIP DIVISION
HYDRODYNAMIC DESIGN OF MERCHANT SHIPS FOR
HIGH SPEED OPERATION
by
A, Silverleaf and J, Dawson
Crown Copyright Reserved
Extracts from this report may be reproduced
provided the source is acknowledged.
Approved on behalf of Director, NPL by
Mr. J. A. H. Paffett, Superintendent of Ship Division
SUMMER MEETING IN GERMANY
l2m-16TH JuNr, 1966THE SCHIFFBAUTECHNISCHE GESELLSCHAFT E.V. Ti INSTITUTE OF MAItme ENGINEERS
THE INSTITUTION OF ENGINEERS AND SHIPBUEDERS IN SCOTLAND ThE NORTH EAST COAST iNSTITUTION OF ENGINEERS AND SHIPBUILDERS
THE ROYAL INSTITUTION OF NAVAL ARCHITECTS
HYDRODYNAMIC DESIGN OF MERCHANT SHIPS FOR HIGH SPEED OPERATION
By A. SILVERLEAF, B.Sc. (Member of Council),* and J. DAWSON, B.Sc. (Member)tRead in Munich on June 14, 1966, The Right Hon Viscount Simon, C.M.G. (President R.J.N.A.), in the Chair Summary
This paper discusses some of the hydrodynamic features of medium size and large merchant
ships intended to operate at speeds higher than those general today. The ships considered are bulk carriers, tankers, cargo liners and passenger vessels from about 400 ft. to 1,000 ft. in length with service speeds from just below 20 knots to above 30 knots and which may have
propelling powers up to about 100,000 h.p. on one or two shafts. Power requirements in calm water are considered and criteria in the form of a boundary speed and hydrodynamic efficiency factors are introduced. These criteria are applied to fine form cargo liners and large, full form
tankers and bulk carriers at relatively high speeds. Some effects on propulsive efficiency of
varying propeller diameter and rate of rotation are examined, and the possible advantages of
contra-rotating and ducted propellers are discussed.
Some of the factors which affect the performance of high speed ships in waves and their manoeuvrability and steering qualities are then described. These include freeboard
require-ments and the influence of ship length and speed on pitch and heave motions in specified sea
conditions. The effects of bulbous and ram bows on resistance in calm water and on
sea-keeping behaviour are also discussed.
Finally, possible future developments of unorthodox high speed merchant ships are briefly
considered. These include ships designed for super-critical operation, submarine tankers and
cargo ships, and very high speed displacement ships. Infroduction
During the past twenty years there have been many remarkable
changes in the size and composition of the merchant fleets of
the world, and in the sizes and service speeds of the ships which
form their largest groups. By about 1949 the total active world fleet, excluding the U.S. reserve merchant fleet, had replaced wartime losses and overtaken its total pre-war size of about
70 mithon gross register tons (or about 100 million deadweight tons). In the next fifteen years the active world fleet doubled in
size, and at the end of 1965 totalled about 160 million g.r.t. (or about 220 million tons dwt.). The composition of the fleet also changed considerably; the proportion of tankers and dry cargo ships increased substantially, and the average size of ships in
these dominant groups also increased markedly.
The service speeds of ships have also changed during the past twenty years, probably more than in any previous similar period.
As the size of the largest tankers has increased tenfold (from
about 20,000 to 200,000 tons dwt.), so the service speeds of the
fastest tankers have tended to rise slowly from about 14 knots to about 16 knots. On the other hand, the pre-war cargo tramp
has been steadily superseded by the cargo liner of today, which, Superintendent, Ship Division, National Physical Laboratory. f Head, Ship Design Branch, Ship Division, National Physical Laboratory.
167
although not very much larger than her predecessor, is very
much faster. Indeed many modern cargo liners, with service speeds above 20 knots, are among the fastest merchant ships
afloat on a basis of speed-length ratio, and have hull forms even
finer than those considered suitable for passenger liners and
ferries.
Many of these changes in the size and speed of ships have
been accompanied by changes in hull form proportions and shape
and by marked alterations in the characteristics of propellers.
Almost all of these have been natural developments and exten-sions of earlier practice, and there have been few abrupt breaks
in the steady evolution of ship forms and propulsion devices
in the continuing attempt to maintain and improve the standards of hydrodynamic efficiency. It is reasonable to suppose that
the size of many types of ship will continue to increase and that service speeds will rise further; indeed, speeds appreciably higher than those general today are likely for several important classes of ship. Will it be possible to satisfy shipowners' future demands for higher speeds without introducing quite novel hull forms and propulsion devices? To what extent can present design methods
provide good hydrodynamic performance if much faster ships become essential to maintain economic competitiveness? The
aim of this paper is to present some hydrodynamic data which will be of assistance in answering these and similar
direction, and to what extent, progress is both possible and
desirable.
The ships which dominate world merchant fleets are tankers,
bulk carriers and dry cargo liners from about 400 ft. to more than 1,000 ft. in length, and their present service speeds are, in general, from about 15 knots to less than 25 knots. This first
overall assessment of future ships is concerned primarily with ships of these classes having somewhat higher service speeds
from just below 20 knots to 30 knots and above. Power Requirements in Calm Water
Before considering in detail the hydrodynamic design features of high speed merchant ships, it is desirable to define what speeds are to be regarded as high, and to establish efficiency criteria by which to judge the performance standards of present and future ships.
Boundary Speed
The concept of "maximum economic speed from hydro-dynamic considerations" has often been used as a measure of high speed for a hull form, though generally without any clear or precise definition. Formulae of the type Tq = a - b CB,
whee Tq is the Taylor quotient or speed-length ratio V/,/L,
C is the block coefficient, and a and b are constants, have been much used to calculate the highest speed at which it is "wise"
to drive a hull of specified fullness; the original Alexander
formu1a(' and its later variations are well-known examples of
such formulae. During the past few years several attempts have
been made at N.P.L. to analyse available model resistance data to derive a more up to date relation of this kind, though not necessarily of this simple form. In these analyses the vague
concept of "maximum economic speed," which cannot be
defined in terms of hydrodynamic factors alone, was replaced by a rather more precise definition of a "boundary" speed. For
any given hull form, the Boundary Speed is defined as thatspeed below which the resistance coefficient does not vary greatly and above which it begins to increase rapidly. Although it has not
yet been possible to express this definition in exact mathematical
terms, it was found that, for most hull forms, this boundary
speed could be derived with reasonable precision from a curve of resistance coefficient in terms of speed coefficient (such as © or C, in terms of or Tg), as indicated in Fig. I.
An early analysis by Dawson, quoted by Hughes,2> of N.P.L.
model data for loaded single screw ocean-going vessels with normal bows, led to the relation Tq = 1-63 - 1-3 CB for the
boundary speed, similar in form to the original Alexander
RENSTA?&C! OCF,!CICNT
SPE,O COCrrIC!tnr
FIG. 1 DERIVATION OF BOUNDARY SPEED FROM TYPICAL RESISTANCE CURVES
OVt RON IVE N REGION
NORMAL
S PE E N S
relation Tq 2-08 - 2 CB. An independent analysis by Hughes(2) of N.P.L. model data showed that Dawson's relation
gave a reasonable indication of the limiting value of ® for a
given hull form up to speeds for which the mean wave resistance varies as the sixth power of the speed and can be defined by the
formula ©
= y although Hughes pointed out that ittended to underestimate the limiting speeds for finer forms.
A more recent analysis of N.P.L. data, including further results for forms both finer and fuller than those originally
available, has yielded a relation which endorses Hughes'
com-ment. The boundary speeds for about 100 representative
single-screw forms (excluding tugs and trawlers) and about 50
twin-screw forms (excluding ferries), selected for good performance,
covering a very wide range of values of block coefficient and length-beam and length-draught ratios, and including values
for two fine single-screw forms (CB 050 and CB 0-525) specially designed to provide information for this purpose, were carefully examined. Although other parameters are doubtless important
in determining the boundary speed, this analysis suggested that
block coefficient may be taken as the dominant parameter for
both single-screw and twin-screw forms having block coefficients from 050 to 0-86, and that a common simple relation involving
speed, length, and fullness only can be useful for preliminary
design purposes. Within the ranges of length-breadth and
length-draught ratios indicated on Fig. 2, an acceptable relation for the boundary speed is
T = l-7 - l4CB
. . . . (1)and this will be taken to indicate the "normal upper speed" for
a form of specified fullness. Speeds above this boundary value will be regarded as "high speeds" for the purposes of this paper;
ships which operate at speeds above their boundary speed will be defined as "overdriven." Fig. 2 illustrates relation (I) for the boundary speed in dimensional terms; it facilitates
deter-minatic'n of either boundary (maximum) speed, boundary
(maximum) fullness, or boundary (minimum) length separating
the "normal" and "overdriven" regions.
An alternative way of presenting relation (1) for the bourdary speed is shown in Fig. 3. This gives the value of the displacement-length ratio L/(0 -01 L)3 at any boundary speed Tq for different
values of the product of the length-breadth and length-draught ratios; greater values of displacement-length ratio correspond
to overdriven conditions. Typical values of the product
(L/B) (L/T) are below 80 for trawlers, around 120 for cargo
liners and tankers, and froni 160 to 250 for passenger liners.
Other ways of defining the boundary between normal and
overdriven regions have also been examined; one of these which
has been used by designers is a formula of the type proposed
by Posdunine as quoted by Baker,t3 giving the minimum
"economic" length for given speed and displacement as
L=24[V/(V+2)]2i.)I3
. . . (2)This formula has been found to have very limited validity. Hydrodynamic Efficiency Factor
The simplest and clearest measure of hydrodynamic efficiency
is the power needed to propel a specified displacement at a set
speed; the long established Admiralty Coefficient, and the Telfer
merit factor as modified by Saunders,4 are overall criteria of this kind. However, the required machinery power, or pro-peller delivered horse power, is influenced by the type of
machinery if this controls the propeller rate of rotation, since
this affects the attainable propeller open water efficiency.
Conse-quently, a better measure of the hydrodynamic efficiency of a
hull form and its appendages is obtained by eliminating propeller
V l(ÑOTS 3O 00 I0 C9 .0
FIG. 3.MAXIMUM DISPLACEMENTLENGTH RATIO AT HYDRODYNAMIC
BOUNDARY SPEED
which take account of the interaction effects between the hull (including any appendages) and the components of propulsive
efficiency. An efficiency criterion of this kind which gives an
overall assessment of the hyd.rodynamic design of the hull and pendages is a Hydrodynamic Efficiency Factor defined by
H '7D/'7o
(1 ±b)
It was found that, for speeds at or close to the boundary speed given by relation (1), the values of this hydrodynamic efficiency
factor H varied consistently with a speed-displacement ratio s
71-t 7R
(1 + b) (
(3)FIG. 2.BOUNDARY SPEED FOR SINGLE AND TWIN SCREW ShIPS
169
C8
such as or F for a very wide range of good quality hull
forms. This gives, as desired, a criterion involving only speed, displacement and power which is considered to provide a useful basis for assessing the quality of a hull form and for estimating power requirements. Although length is not explicitly involved
in this relation, it is necessary for direct comparisons to refer
the hull resistance coefficient © and appendage resistance
factor (1 + b) to a standard reference hull length this has been
taken as 400 ft. and the values of the factor H in Fig. 4 are for 400 ft. which can be expressed as
H400=
26 0-5 (g)
- Single screw for cg) = 1-2 to 26H400 = 238 0-5
- Twin screwfor (j') = l4 to 2-8
AR0) MATE SINcLE SCREW L/B64-7.7
RANGE TWf N SCREW65 8.5
23 -0.54 0.80 (4)Values of H for other ship lengths can be derived from the
correction factors also shown in Fig. 4, which are sufficiently
accurate for preliminary design and assessment purposes. Some
typical values of speed-displacement constants are given in
Table I.
For many hull forms for which propulsion experiments have
been made at N.P.L. at speeds up to about 20 per cent
beyond the boundary speed, values of H were calculated for speeds below and above the boundary speed. It was found
that the ratio H/HB, where H8 is the hydrodynarnic efficiency factor at the boundary speed and H that at any other speed, decreases steadily as the speed increases, and that this ratio
H/HB is generally independent of hull form and of the absolute
values of H8. This variation with speed is shown in Fig. 5, and provides a starting point for assessing the power require-30 r 08 V/it 06 04 0-3 0- S FI-L 0-2 0-IS .7- l-4 0.4 0-6 0-6
VALUES 0F '1460 UT = 7- .4 C0 SINSLE SCREW / H.00.260E ® f
/
TWIN SCREW N405 2. 30 - OS H -l-4 IS IN 20 2.2 2.4 2-6 2-5 30 % 3.5 4-0 4-5ments for "overdriven" ships intended to operate at speeds
appreciably above the boundary speed and higher than those
general today. The marked drop in the hydrodynamic efficiency factor H at high speeds is a first indication of the penalties which
such speeds impose; a further penalty is the inevitable drop in
propeller open efficiency, discussed later, also shown in Fig. 5. u
.4
Hull Resistance Coefficients
The values of the resistance coefficient © at the boundary
speed clearly vary with several form parameters, but it is of some 1.0
interest to note that a value of © 071 (Froude basis for length 400 ft. L) is a reasonable first approximation at all values of °
block coefficient. The values of H400 in Fig. 4 correspond broadly to © 0-71; if the resistance coefficient is known to have
a different value at the boundary speed, perhaps because of
'Z .5
V/v
FIG. 5.EFFECT OF SPEED ON 1-IYDRODYNAMIC EFFICIENCY AND POWER
H, P and j0 are values at any speed V
HB, PB and soB are values at boundary speed V
special features of the hull form, the value of H0 may be
estimated more accurately from Figs. 6 and 7.
FIG. 6.EFFECT OF CHANGES IN RESISTANCE COEFFICIENT ON HYDRO-DYNAMIC EFFICIENCY FACTOR AT BOUNDARY SPEED
Single screw ships
FIG. 7.-EFFECT OF CHANGES IN RESISTANCE COEFFICIENT ON HYORO-DYNAMIC EFFICIENCY FACTOR AT BOUNDARY SPEED
Twin screw ships
Ship Type V/z i F Largetankerorbulkcarrier
..
2-4 1-404
Coaster..
.. ....
2-6l5
O-45 Dry cargo .. .. ....
3-2 1-9 0-55 Refrigerated liner . . . -33
19
0-55 Trawler..
. . .. .. 3-822
065
Cargo liner .. .. .. 4-1 2-4065
Vehicle ferry..
.. .. 4-225
07
Passenger liner
..
....
45
26
O'75400 600 000 1000 200
t.
FIG. 4.HYDRODYNAMIC EFFICIENCY FACTOR AT BOUNDARY SPEED
TABLE I
TYPICAL VALUES OF SPEED-DISPLACEMENT CONSTANT AT THE BOUNDARY SPEED
0-4 OS F, 0-6 0.7 0-0
10
VARIATION 0F H WITs SHIP LENCTH
lOS 20 '2 10 2-T 5.0 I-B H400 I'S .4
It is of interest and practical design importance to know what
proportion of the total resistance is wave resistance; estimates
of this depend on the methods used to separate the total
measured model resistance into viscous and wave components and to convert these into ship values. The recent analysis by
Hughes(2 is considered to represent the most satisfactory method of separating these components, and this has been used to derive estimates of "good" values of the wave resistance coefficient
at the boundary speed. Fig. 8 gives the relation between this wave resistance component and the total resistance; although
this will vary slightly with shiplength, it shows that, at the
boundary speed, for full forms (CB 080) the wave resistance is about 20 per cent of the total, while for fine forms (CB 055)
the wave resistance is generally at least 40 per cent of the
total resistance. At overdriven speeds the proportion of the
total resistan due to wave-making is greater and increases
rapidly with speed.
Propeller Open Water Efficiency
For most ships, unless the screw diameter is severely restricted, the open-water efficiency of the propeller is largely independent of the overall hydrodynamic efficiency of the hull and appendages
as defined by the factor H. Limitations on the draught of a
ship or, for very large vessels, those imposed by propeller
manufacture may restrict the propeller diameter to considerably
0.6 C. $ 0.4 C.? o. t
o-FIG. 8.APPROXIMATE RELATION BETWEEN WAVE RESISTANCE AN]) TOTAL RESISTANCE AT HYDRODYNAMIC BOUNDARY SPEED
less than the optimum value; in other cases the propeller
open-water efficiency depends primarily on its rate of rotation and
on the required thrust and speed of advance, which, at a known ship speed, can be broadly related to resistance and hull fullness.
Examination of data from propulsion experiments at N.PL. in which the propeller diameter was close to the optimum value
TABLE II
EFFECT ON CALM WATER POWER OF HIGH SPEED OPERATION
171
has suggested that it is possible to provide a first estimate of
propeller open-water efficiency at the boundary speed in terms of block coefficient. Fig. 9 gives approximate values of these open-water efficiencies for propellers designed for N 120 rev.Jmin. at
L 400 ft. (or n,/ L = 40 for all values of n in rev./sec. and L in
feet); these values can be expressed as:
m (120) = 098 - 0-55 CB - Single screw
and (120) = o-90 - 0-33 CB - Twin screw
An indication of the drop in efficiency when the propeller rate of rotation is increased is given by the correction factor so that:
o (N) = o (120) + &o . . . (6)
The effect on these efficiencies of changes in ship speed above and below the boundary speed is shown in Fig. 5.
Preliminary Power Estinlates
The delivered horse power is readily given in terms of the
hydrodynamic efficiency factor H and the propeller open-water efficiency m by the relation
1 (1 + x)213
y3 . . (7)
427 0H
in which the load factor (1 + x) is the performance prediction
factor linking model and ship powers (as defined in Ref. 5), and z, V and H are the ship displacement, speed and hydrodynamic efficiency factor respectively. The possible significance and
usefulness of the criterion H for power estimates are not affected
by the method of extrapolation used to derive the resistance coefficient for the ship from that measured on the model. The absolute values of H will, of course, depend on the method by which the ship resistance coefficient is obtained, but power
estimates will not be affected because the performance prediction factor (1 + x) will also change correspondingly.
The effect on power requirements of increasing speed above the boundary speed can be readily estimated from the hydro-dynamic efficiency ratio H/H2 and the propeller open water
efficiency ratio O/OB in Fig. 5. These together give a power ratio P/P2, in which B and P are respectively the shaft powers at the boundary speed B and at any other speed V, in the form
P
y'3 (HB\
(8
'VB) 'ii) ')
This power ratio for single and twin screw forms is also shown
in Fig. 5 and typical values are given in Table II; these show that to increase speed 10 per cent above the boundary speed demands an increase in power of about 50 per cent, while to
provide a 20 per cent increase in speed the power must be
almost two and a half times that needed at the boundary speed. Higher Speeds for Cargo Liners and Tankers
Fine Forni Cargo Liners
The general criteria developed in the preceding sections can be used to examine some of the problems which may occur if
(5)
Speed ratio ..
..
....
Hydrodynamic efficiency factor ..
V/V2 (V/V2)3 H/HB
08
051 I 08 0-9073
1 Ø5 1-0 1-0 I Ø1l
1-33090
l-2 173074
Propeller open-water efficiency ratio -. o/oB Single 101 i-oiio
o-98 0-94Twin 1-01 1-01 1-0 Ø.99 0.97
Power ratio ..
..
. . P/PB Single o-47 0-68 1 -0 1 -51 2-480.8
0.7
(l2o)0.6
05
0'l
o
-0.I
-02
the service speeds of certain classes of ship are raised above those current today. The modern cargo liner is an outstanding
example of this trend, and for some time there has been a growing need for design data for hull forms suitable for this type of ship. To avoid excessively "overdriven" conditions these forms have
block coefficients less than 060, and for such fine shapes it is
often difficult to reconcile the conflicting demands of low power and adequate initial stability to provide a safe working margin. Available information has recently been surveyed and assessed
by Moor(6); this includes N.P.L. data from designs to meet specific requirements of owners and builders and from others
specially developed by N.P.L. as parent forms for two B.S.R.A. methodical series (CB 060 and CB 055) to provide information
on the effects of systematic changes in principal form
para-meters. However, recent experience has shown that data are
needed for even finer forms, and two further parent forms
(CB 0525 and CB 050) have been developed independently at Largely because of port limitations, many recent high speed
cargo liners have closely similar dimensions. It is thus possible
to suggest that a reasonable "basis" ship to represent an
important group in this class for the next decade or so will have
dimensions about 530 ft. 78 ft. breadth, and 30 ft. load
''o
ATTc= 17-I4 C
TWIN SCREW CB
- -.
SINGLE SCREW 170.98_0.55 CB (w) = (12o)+ .TS SSdraught. For such a ship the maximum block coefficient to avoid overdriving is about 059 at 20 knots falling to 050 at about 23 knots, and to even lower nominal values at higher
speeds. Since a block coefficient less than 050 is unlikely to
provide either sufficient cargo capacity or adequate stability, this value has been taken as a practical lower limit, and thus at all speeds above 23 knots it is not possible to avoid overdriven
conditions. Preliminary power calculations have been made
for both single and twin screw ships of these dimensions for service speeds up to 30 knots8 using results derived directly from N.P.L. experiments with a form of block coefficient 050.
In these calculations, summarized in Fig. 10, the hull resistance coefficients for speeds above the boundary speed (VB 23 knots)
are estimates for forms designed specifically for these higher speeds and are thus less than the values derived directly from the N.P.L. parent form. Although there are several important differences between single screw and twin screw hull forms,
including the effective hydrodynamic length and the possib.Llity of increasing the effective length of a twin scrcw form by having a transom stern, these have not been taken into account in these preliminary estimates, thus giving the same hull naked resistance
for both single and twin screw forms. However, the total
resistance of the twin screw forms is 10 per cent greater than that
05
06
0.7
08
09
CB 100 ISO 0O 250N (iev/mfri)
f I I I 20 40 60 80FIG. 9.-PROPELLER OPF.N WATER EFFICIENCY AT HYDRODYNAMIC
of the single screw hulls to allow for the drag of the shaft supports.
For both single and twin screw hulls it was considered
pos-sible to accommodate propellers of diameter 225 ft., and it
was found that the propulsive coefficients are greater for twin
screws than for a single screw, while the optimum propeller rate of rotation is significantly lower for twin screws; these differences are direct consequences of the lighter loading of each of the twin screws. The overall result, as shown in Fig. 10, is that the required
machinery powers are essentially the same for single or twin
screw propulsion. It is of interest to note that power estimates made using the simple method embodied in relation (7) and in Figs. 4, 5, and 9 over the speed range for which they are valid
give results which agree closely with those in Fig. 10.
It is possible to draw some tentative conclusions about future
trends from Fig. 10. A cargo liner of the size and fullness con-sidered here could have a service speed of 25 knots with about 35,000 shp installed; although this is a high power, it is not
excessive even by present standards. However, service speeds
above 26 knots appear unrealistic using machinery having present power-weight characteristics, and even marine gas
turbines would probably be impractical for the powers needed
to maintain 28 knots or more. The differences in optimum propeller rates of rotation may well also influence the choice of machinery for higher speed cargo liners; while direct drive diesel engines may be suitable for speeds up to 25 or 26 knots
140000 120000 100000 60000 HORSE POwER 60DOO 40000 2O00 SS TS + ESTIMATE FROM EQUATIOÑ (7) 200 SO (rev/ 111n) loo SO 15 ©SN OS
Appendage resistance coefficient
Performance prediction factor Service power allowance
Transmission efficiency
Propeller diameter Prçpeller, No. of blades
particularly if the power output of a single engine is raised
above present levels, there is clearly a field for other forms of
propelling machinery.
Since the high powers needed to maintain speeds of 25 knots
and above may make it economically impractical to operate
cargo liners of 530 ft. length at these speeds, some further
similar calculations have been made for larger ships having
dimensions 650 ft. 95 ft. breadth, and 38 ft. draught. For
such ships the boundary speed is about 254 knots at block coefficient 050, and about 244 knots at CB 0525.
Conse-quently, these calculations were made for speeds from 24 to 30 knots for CB O5O and CB 0525. The results, summarized in Fig. 11, show some remarkable and perhaps unexpected
features. At 24 knots the powers required for the larger 650 ft.
ships, with displacements approximately double those of the smaller ship, are about one-third greater than for the smaller
530 ft. ship. However, the difference narrows as speed increases,
until at 28 knots the powers are substantially the same for both sizes of ship, and at 30 knots the larger ships actually require almost 20 per cent less power than the smaller ship. These
rather surprising comparisons emphasize the severe power penalty imposed by raising speeds signifiòantly above the
boundary speed, and, of course, strongly suggest that, if cargo
liners are to have service speeds above 25 knots, they should be larger than the present typical vessels of this class.
4 4 Single screw
(l+b)
l0
Twin screw1l
(1 + x) 094 097(l+y)
12SI25
(T) 098 098 (ft.) 225 225 o 22 24 26 28SERVICE SPEED (rics)
FIG. 10.-PoWER ESTIMATES FOR HIGH SPEED CARGO LINERS
Power estimates for the larger 650 ft. ships made by the
simple method previously described again give values which agree closely with those in Fig. 11, which were derived quite
independently using detail model resistance and propulsion data. Full Form Tankers and Bulk Carriers
Although for most tankers and bulk carriers under
con-struction today there is no definite relation between size and
speed, it is reasonable to consider typical service speeds as about 15 knots for ships 500 ft. long and 20,000 tons dwt.,
increasing slowly to about 17 knots for mammoth vessels over 1,000 ft. in length and close to 200,000 tons dwt. As shown
in Fig. 12, these speeds are generally above the boundary speed
for full forms with block coefficient O80 for lengths up to
almost 800 ft., but below the boundary speed even for very full forms with CB 0-825 at lengths above 900 ft. Although hydrodynamic factors doubtless affect the service speeds adopted
by owners for tankers and bulk carriers, they do not appear to
be the decisive factor. It is therefore possible that service speeds
may be raised if other factors indicate that this is economically justifiable, and some estimates have been made of the
hydro-dynamic consequences of raising the speeds of large tankers up
to and beyond the boundary speed.
Fig. 12 shows typical breadths and draughts of full form
vessels built recently or at present under construction; although
the proportions of some ships are appreciably different, it is considered that these values are sufficiently representative to
be used for general power estimates. Since the boundary speed
80000 H05E POWER o00o 40000 e-20000
CB 050
©sN EQUATIOÑ (I)/
,
/
/
-rfor CB 080 is higher than that for C 0825, the estimates have been based on forms having CB 080, and the corresponding displacement and typical deadweight curves are also shown in
Fig. 12, the deadweight-displacement ratio varying with size of
ship according to information derived from shipbuilders' data.
The hull resistance coefficients used in the power estimates are
typical values for full forms with good performance when
overdriven.
It has been suggested that for very large tankers and bulk carriers the propeller diameter and rate of rotation can have a
strong effect on power requirements, particularly if manufacturing
limitations result in the use of a screw well below the optimum
diameter, or the diameter is less than the maximum which could be accommodated because of the need to match diameter to high
shaft revolutions. The maximum diameter of propeller from hydrodynamic considerations alone depends on draught aft when loaded and in ballast, on the need to have a cruiser stem
reasonably well immersed, and on the need to provide adequate tip clearances above the keel line and below the immersed cruiser stem. Detail examination of these factors suggests that, for ships longer than about 500 ft., the maximum ratio of propeller
diameter to mean load draught can generally be two-thirds. This gives a maximum permissible diameter from hydrodynamic
con-siderations of about 30 ft. for ships of approximately 800 ft. in length; since 30 ft. is presently regarded as the maximum
dia-meter of propeller which can conveniently be manufactured, this
could mean that larger ships are at present penalized to some
extent by having to fit propellers of restricted diameter.
CB 0525
+ E5TIMA1E FROM e-7 N / //
/ 0-5 28 30 140000 20000 100000 SS/
I-/ NFia. 11.POWER ESTIMATES FOR HIGH SPEED CARGO LINERS
Ship 650 ft. 95 ft. B mid. x 38 ft. T mld.
Single screw Twin screw
Appendage resistance coefficient
(l+b)
l0
l-1Performance prediction factor
(I +x)
0-89 094Service power allowance (1 +y) 125 125
Transmission efficiency ('i-r) O-98 O98
Propeller diameter (ft.) 275 275
Propeller, No. of blades 4 4
+
/
24 26 28 30 24 26
SERVICE SPEED (Inots)
so
10
o0000 200000-Dw -(tons) ,80000 -T 40
()
20 400 600 800 L (;t) o 000 1200power, transport efficiency increases steadily with ship size, even
though the service speed naturally drops as ship size increases.
The single curve of hydrodynamic transport efficiency in Fig. 14
can be taken as a starting point for initial estimates for a wide
range of large, full form ships, and as a basis for comparing the qualities of different designs the variation in transport efficiency
with speed shown here agrees with the power ratio values in Fig. 5 and Table II.
The curves of optimum propeller diameter for N I 10 rev/mm.
ISO in Fig. 13 show that propellers of diameter up to 30 ft. are
adequate even for the largest tankers for service speeds up to the
1Go boundary speed; this is above 19 knots for ships 1,100 ft. in
length, and is thus appreciably higher than present service speeds,
and would demand powers probably greater than could be
40 transmitted by a single screw ship. However, the results of the
B other power estimates for propellers free to run at optimum
o rates of rotation, summarized in Fig. 15, show that some improve-(ft) monts in propulsive efficiency are possible using larger propellers than those indicated in Fig. 13. Some tentative general con-100 clusions from these estimates are:
For propellers having diameters 30 ft. and above the
maximum attainable open water efficiency may be higher
than the best possible when N is fixed at 110 rev./min.,
but these large propellers must run at less than 90 rev./min. if appreciable gains are to be achieved.
The advantage in open-water efficiency decreases sharply as ship speed increases; the ratio of open-water efficiencies 4 falls by about 10 per cent for 3 knots increase in speed in
almost all cases.
The gain in propulsive efficiency with large, slow running
propellers is less than the gain in open-water efficiency, and in some cases very much less. This is due to the
very strong influence of propeller-hull interaction effects
which vary markedly with diameter-draught ratio.
Detail knowledge of this effect is necessary before the full consequences of any change in propeller diameter
and rate of rotation can be accurately assessed.
Any gain in propulsive efficiency also decreases as ship
speed increases.
(y) Gains of more than 10 per cent in propulsive efficiency appear likely only for the largest ships (L> 1,000 ft.,
D > 30 ft.), and then only if the propeller rate of rotation is reduced to about 70 rev./min.
(vi) If the speeds of very large tankers and bulk carriers are increased beyond their boundary speeds, there would
appear to be less advantage in departing from propellers suitable for present direct drive installations. Indeed, it might be possible to obtain some of the gains now often
considered dependent on increasing propeller diameter and reducing rate of rotation by shaping the afterbody to give favourable propeller-hull interaction effects at
smaller diameters and higher revolutions. Nevertheless,
although not significant in altering the basic power
requirements for higher speed tankers and bulk carriers, the differences due to present propeller restrictions are sufficiently important, even at current speeds, to justify
special efforts to develop methods of manufacturing and
handling larger propellers and of enabling them to run
more slowly than is customary today.
IO
¶8
V
6
12
FIG. 12.TANsRs AND BULK CARRIERS, TYPICAL DIMENSIONS AND
SPEEDS
To examine this point, estimates were therefore made for a series of full form ships up to 1,100 ft. in length for different
combinations of propeller diameter and rate of rotation. Since
most large tankers and bulk carriers have direct drive diesel
engines running at about 110 rev./min. an initial set of estimates
was made for this propeller rate of rotation, ignoring any pos-sible limitation on propeller diameter due to manufacturing
capacity. Further estimates were then made assuming that the
maximum diameter was limited to 25 ft., as was the case not long
ago, to 30 ft. as at present, to 35 ft. as may soon be possible,
and finally to 40 ft., accepting that such large propeller diameters might lead to optimum rates of rotation considerably lower than those of present direct drive installations. In those cases where
the ratio of propeller to draught was less than two-thirds,
approximate values of wake and thrust deduction fraction were estimated from the results of model propulsion experiments with screws of varying diameter-draught ratio; these values gave hull efficiencies which decreased significantly as propeller diameter
increased, principally because of the decrease in wake fraction
with increasing diameter-draught ratio.
The results of these power estimates are summarized in Figs. 13,
14, and 15 in forms considered useful for preliminary design purposes when assessing the hydrodynamic and other
conse-quences of increasing ship speeds. Fig. 13 gives the results of
the initial estimates for fixed propeller rate of rotation
N 110 rev/mm. The curves of constant speed and length give approximate values of the hydrodynamic transport efficiency
LW/dhp (or its reciprocal, the specific power dhpJL V) for speeds
from09 VBtoabove 11 Vforshipsoflength 800 ft. to 1,lø0ft.;
these demonstrate again that increasing speed above the boundary speed involves not only sharp increases in power, but a marked
drop in transport efficiency. This point is also illustrated by
the curves of constant power, which show that, for a given engine 175
Propulsion Devices for Cargo Liners and Tankers
It has frequently been suggested that higher propulsive
effi-ciencies can be obtained with a ducted propeller or with contra-rotating propellers than with a single orthodox propeller. These
suggestions were examined recently at
as part of a
feasibility study into the use of geared medium-speed diesel engines for cargo liners of the type considered in the previous
IZO too -/dbp GO 20 35 30 (ci) 25 20
CURVES OÊ CONSTANT
SPEED AND LENGTH
O 9 Ve
OPTIMUM OPELLER DIAMETER DR N ITO V"
CURVES OF COÑSTAN
POWER AÑO LEÑH
I Ve
V21
V/v
FIG. 14.-VARIATION OF HYDRODYNAMIC TRANSPORT EFFICIENCY WiTH
SPEED
section, and for tankers and bulk carriers up to about 750 ft.
in length. The results of this examination, and of subsequent
work at N.P.L., are summarized here.
T 00 80 /dhp GO 40
FIG. 13.HoRoDyNAMIc EFFICIENCY OF LARGE TANKERS AND BtYLK
CARRIERS L800-1,lOOft.: CB 080 V 15-21 knots: N 110 rev/mm. 13 l0 D
N
N N NN\ N N N N N OPTIMUM PROPELLER RATE OF ROTATION'N
S.N
00 NS. NN
N?-'__ N
03 S.'I0...N
','-- N BOo oO ---28--
_s ___9_Q_ -IS 19 v (kneEs) 20N
N
il loo N 800
FIG. 15.-EFFECT ON POWER OF C}IANGES IN PROPELLER DIAMETER AND RATE OF ROTATION
Contra-Rotating Propellers
The gains in propulsive efficiency possible by replacing a
single propeller by a coaxial pair of contra-rotating screws have
been demonstrated several times by the results of experiments and calculations published during the past fifty years. One of the clearest and earliest accounts of the principal effects is that by Luke,t10 while interest has recently been revived by work in the United States11 and elsewhere. The main conclusions of these investigations are:
There is little difference between the "open" efficiencies
of equivalent single and contra-rotating propellers
designed to absorb the same power at the same ship
speed and at about the same rate of rotation.
How-ever, as the screw loading increases there is a growing
advantage in favour of the contra-rotating pair, and
this can be appreciable for the conditions in which
higher speed cargo liners will operate.
The interaction effects between hull and propeller are generally more favourable for contra-rotating screws than for a single propeller. These effects can be
ex-pressed by the ratio of overall hydrodynamic propulsive efficiency (iD)to propeller open efficiency ();
improve-ments in D/iØ of more than 15 per cent have been reported, particularly for full form ships, though the
potential gain in propulsive efficiency D is generally little more than 5 per cent.
There is some evidence that, at the same rates of rotation,
the diameters of optimum contra-rotating propellers are slightly less than the equivalent single screw, and
this accounts for part of the improvement in interaction effects. There is no evidence that, for the same
dia-meters, the optimum rate of rotation for a
system is significantly different from that of the equi-valent single screw; consequently, the gains possible
with orthodox marine propellers by increasing diameter and reducing rate of rotation will also apply to contra-rotating screws.
Thus in many cases a contra-rotating propeller system on a
single shaft can have a higher overall propulsive efficiency than
an orthodox single propeller absorbing the same power at the
same ship speed. This is principally due to more favourable interaction effects between the propellers and the hull. These
differences in efficiency have not been large enough to induce any general use of contra-propellers for merchant ships, pre-sumably because of the mechanical problems and extra costs
involved. If these objections are removed, a close examination of contra-rotating propellers may be justified.
Ducted Propellers
For ship propellers operating at high loading coefficients, as
in tugs when towing or fishing vessels when trawling, the
advan-tages of enclosing the propeller in a duct which accelerates the inflow have been appreciated for many years. However, the
operating conditions of some large tankers and bulk carriers already appear to be in the range where ducted propellers may be useful and if service speeds are increased, this will be more
likely. There are three main possible advantages; for a given thrust the rotor of the ducted system may be smaller than the
conventional open propeller, although the overall diameter may
be the same; a substantial proportion of the total thrust can be
transmitted by the duct, thus reducing the steady and the
fluctuating forces on the rotor; the duct may reduce the non-uniformities in the inflow to the rotor, thus further reducing
the fluctuating forces which can cause shaft and hull vibration.
To determine whether these advantages can be achieved, and
whether the furtheradvantage of improved propulsive efficiency can also be obtained, experiments have been made at N.P.L. to
develop a ducted propeller system for typical single screw full
forms. These began with a relatively simple axisymmetric duct
added to a hull form for a 750 ft. bulk carrier (CBO8O), and were intended to give basic flow and performance data for use
in subsequent improved systems designed as an integrated unit.
Although this initial ducted propeller arrangement did not
demonstrate any clear advantage over a conventional open
screw, it indicated that a gain in propulsive efficiency should be possible for a larger ship in which the propeller loading
coeffi-cient would be greater, partly owing to the effect of diameter
restriction discussed earlier.
Further experiments were then made with a model of a typical
mammoth tanker about 1,000 ft. in length, again with block
coefficient 080, fitted with an improved ducted propeller system,'2 the overall duct diameter being about 275 ft. and that of the ducted rotor 23 ft. The propulsive coefficients obtained in these experiments are shown in Fig. 16; the values are about 10 per cent higher than those obtained with a model of a conventional open propeller of diameter about 25 ft. In
the loaded condition about 30 per cent of the total thrust is
carried by the duct, and thus it should be possible to reduce
the size of the propeller shaft as well as that of the rotor compared
with a conventional screw. These experiment results suggest
that serious consideration should be given to installing ducted propellers on very large tankers, particularly if their speeds are
raised above those usual now.
Seakeeping Qualities Performance in Waves
The likely performance of high speed ships in waves could be
a critical factor in their development, particularly since at
177 070
FROUDE AAL't5s LOADED CODlTION 060
FOR AVEP.AE TRIAL CONDIrIONS A
LOAD FACTOR O 085 SHOULD BE USED
4.s 150 155 160 185 170 175
V-
knotsFia. I 6.PRopulsioN ANALYSIS WITH DUCTED PROPELLER SYSTEM
present the master ofa ship frequently finds it necessary to reduce speed in heavy weather because pitch and heave motions become excessive. Recent studies of ship motions at N.P.L.'3 have
indicated that the ship characteristic which most influences
pitching motion is ship length, and that the effects of variations in speed and block coefficient are small, though heaving motion is influenced by ship speed as well as length. The calculations
and measurements (both model and full scale) on which these conclusions were based were generally for speeds below the boundary speed, and to examine their validity at higher speeds
a further series ofexperiments and calculations are being made. 14)
These are for the fine forms (CB 050, CB 0525) previously mentioned(7); experiments are being carried out in irregular head seas reproducing sea states Beaufort 5 and 7 (as defined by the British Towing Tank Panel) for ships about 550 ft. in
length at speeds up to 35 knots, while motion calculations have
been made for a range of ship lengths and speeds in the same
sea states.
The results of the motion calculations for CB 0525 are given
in Fig. 17. These have been derived entirely from theoretical
considerations without using any empirical data, and agree well
with the values directly derived from the model experiments
where direct comparisons are possible. These calculations thus probably give a good indication of the way in which ship length
and speed influence motions for the range of length and speed
appropriate to present and future high speed cargo liners. Generally, variations in speed from Tq o9 to 11 have little
effect, except possibly on pitch and relative bow motion, and the most important factor is ship length. In all cases increase
in ship length decreases motions and acceleration forward, in some instances very markedly. The broad conclusion from these calculations is that increasing the size of present cargo
liners will improve rather than worsen their seakeeping qualities
while increasing their speed may not adversely affect these
qualities. Similarly, increases in the speeds of large full form
tanicers will not tend to affect their behaviour in most sea
conditions.
Some present high speed cargo liners have experienced steering difficulties in strong following and quartering seas. The need
to keep the longitudinal centre of buoyancy aft of midships to reduce calm water resistance and powering leads to afterbody
sections which induce marked changes in the effective transverse
stability when in waves coming from astern or on the quarter. These stability changes can cause violent rolling and yawing which make course-keeping difficult, and these effects may be accentuated by attempts to maintain higher service speeds. It
is possible that the introduction of a transom stern may be
helpful in such circumstances, though increasing the initial
' 85 1-00 I IO LOAD FAOrOR (i
+ x)
080 ..0 802
o
RELATIVE BOW MOTION
SEAUFORT 7.5 50 25 -(de3) 500 600 700 L (ce) BEAUFORT S 09 F, 027 10 030 800 300 30 20 S173 25 20 15 2I3 IO (ii) e 3EAUF0T 7 ACCELERATION FORWARD HEAVE EAUF0T 5 500 600 700 L (Çi)
FIG. 17.EFFECTS OF SHIP LENGTH AND SPEED ON MOTTONS IN IRREGULAR HEAD SEAS metacentric height above the value of about 1 ft. customary
today may also be beneficial.
The proper power allowance to enable service schedules to be
maintained in the weather and sea conditions anticipated on
normal routes is important in any ship design. It is particularly
important in high speed ships because of the large powers
involved. In the power estimates made here the allowance for
average service conditions above the power needed for measured
mile trials has been arbitrarily taken as 25 per cent for cargo liners and 20 per cent for large tankers at all speeds. Some
recent cargo liners built abroadtt5 have installed powers which only allow a much smaller margin, but it is believed that these
have not always been able to maintain service schedules. Clearly
there is a strong need for information which will enable power
margins to be assessed more accurately, and model experiments and measurements at sea are now being made to help satisfy this need.
Freeboard Requirements
Calculations and measurements of relative bow motion, such
as those shown in Fig. 17, are valuable in assessing the
prob-ability of occurrence of wetness at the fore end of a ship.
Pre-dictions of this kind have been made at N.P.L.t16 for ships of varying fullness and length in typical irregular head waves
representing sea states Beaufort 5 to 9. These showed that, for a given probability of wetness, the necessary freeboardratio at the fore end decreases steadily as ship length increases; indeed,
for ship length above about 600 ft. the decrease in freeboard ratio is equivalent to a constant freeboard. Although these calculations were made for a relatively slow speed (Tq 060), it is believed that the general trend of the results will apply to
higher speeds. If so, then the adoption of higher service speeds
for either large tankers or large cargo liners need not involve a significant change in the proportions of the above-water form
forward.
Special Bow Shapes
Many unusual hull forms have been proposed during the past few years. Although most suggestions for special details in hull features have not justified the claims made for them when
sub-800 Oo os O4 03 y3 02 0.1 o
jected to critical examination, including carefully conducted
dispassionate model experiments, some novel ideas have
undoub-tedly proved very successful in laboratory conditions and on measured mile trials, although their performance in normal
service conditions is generally difficult to assess. A hull feature
which should have increasing value as ship speeds increase is
the bulbous bow, the general principles of which have long been
understood. A more recent innovation is a particular form of bulbous bow more correctly described as a ram bow; although
the flow mechanism by which it operates is not yet clearly
understood, it also may have increasing use as speeds rise. Bulbous Bows
Bulbous bows have been used for many years in high speed
ships in attempts to reduce resistance in the deep load condition,
and recently their popularity has increased considerably.
During the past twelve years about 60 sets of resistance
eperi-ments have been carried out at N.P.L. for hull forms (other than trawler forms) with and without bulbous bows where the direct effect of adding a bulb can be readily determined. These bulbs
were seldom simple additions to the parent form, but were usually associated either with a reduced waterline angle of entrance or a finer forward shoulder, or with both of these
changes. Although the bulbs varied somewhat in size, shape,
and position, the majority had a bulb area ratio about 5 per cent
with a ram area ratio usually 7 per cent to 74 per cent.
The results of these experiments have been examined0' to derive a broad indication from N.P.L. experience of the likely effect on calm water resistance of fitting a bulbous bow to a normal ship form. The principal purposes of this examination were to determine the conditions under which gains or losses are to be expected, the way in which these are related to the characteristics of the parent form, and whether these effects confirm the findings of theoretical analyses, particularly the
suggestion that greater gains are possible when the wave
resistance of the parent form is high. The first analysis con-centrated on the effect of the bulbous bow on total resistance. For each form the change in resistance after fitting . bulb was
related to the total resistance of the normal form at a series of speeds below and above the boundary speed.
Fig. 18 is a
to (ci) EEAUFORT 5 o T 0.9 F 027 03010
-I.---
033LOSS 5' c5 0-6 LOSS o, 0-5 0.1 0.7 0-6 0.5 1
FIG. 18.EFFEcT OF BULBOUS BOWS ON CALM WATER RESISTANCE
composite plot giving the average values derived iE this way; as anticipated, it shows that, in general, a bulbous bow gives a reduction in resistance at speeds above the boundary speed,
and an increase in resistance at speeds below this speed. Next,
the resistance change due to a bulbous bow at the boundary speed was compared with the total and wave resistances of the
parent form at that speed and with the "good" values in
Fig. 8; this indicated that appreciable gains at the boundary speed due to a bulbous bow occurred for forms for which the wave resistance coefficient was higher than the "good" values. A further analysis was based on calculated values of the wave resistance over the whole speed range for each form; this con-firmed that a bulbous bow is likely to be most effective if the wave resistance of the parent normal bow form is high, either
because the ship is "overdriven" or because the wave resistance
is greater than the lowest value attainable for its designed
operating condition. This clearly suggests that bulbous bows will be of increasing value in all classes of ship as speeds are increased above the boundary speeds. However, the problem of deciding whether to incorporate a bulbous bow in any parti-cular hull form should not be considered in isolation, but as part of the more general problem of designing a low resistance
hull form to suit the specified design conditions. Ram Bows
Recently ram or projecting bows have been incorporated in
many full form tankers and bulk carriers to obtain power
reductions in the ballast condition, and ships fitted with them have achieved excellent performances on measured mile trials,
closely confirming the predictions of gains based on the results
of model experiments. However, although these ram bows show clear advantages in the ballast condition, this is not so in the deep load condition, nor are the gains achieved in calm water maintained in heavy seas. Indeed, recent model
experi-ments at N.P.L. with models of full form ships in waves have
shown no difference between normal hull forms and those fitted with ram bows. Consequently, the decision whether a ram bow
should be fitted to a ship depends on the proportion of time at sea likely to be spent in the ballast condition, and also on the
distribution of weather conditions likely to be met over a fairly long period in service. It is therefore not easy to decide whether
to recommend fitting a ram bow or even a more conventional bulbous bow, and efforts are being made to establish criteria
to determine the design characteristics giving the highest hydro-dynamic efficiency for a ship throughout her service life.
e
179
Unorthodox High Speed Ships
Previous sections of this paper have been concerned with some of the hydrodynamic problems involved in raising the speeds of
typical present day ships by 20 or 30 per cent. It is clear
that even such relatively unspectacular increases in speed will only be achieved if shipowners consider it justifiable to install
propelling machinery with much higher powers than those fitted
today to almost all merchant ships other than passenger liners. Engineering developments may make such machinery both available and economically practical, and it is not impossible that within twenty years or so there will be marine propulsion machinery with very much higher outputs than feasible today, and with such improved power-weight and fuel consumption
characteristics as to make much higher ship speeds commercially attractive. Indeed, some owners have already made tentative
enquiries about ships with such high speeds that only completely
novel machinery could provide the necessary power outputs-and only completely novel propulsion devices could transform
these powers into propulsive thrust effectively. Ships for Supercritical Operation
Apart from hovercraft, hydrofoil ships and other relatively novel high speed marine craft, there are several possible forms of very high speed surface and sub-surface ships which could utilize extremely high powers effectively to reach speeds of 40 knots and above. In a general survey of such ships,t18)
Lewis compares the performance characteristics of long, slender
ships (with very low displacement-length ratios), ships with
very large bulbs at bow and stern, semi-submarines in which the main hull runs just below the surface and carries a small
super-structure above water on hydrofoil struts, and submarines
running either shallow or deeply submerged.
Lewis stresses that seakeeping qualities are a vital factor in assessing the possibilities of any surface or near-surface high
speed ship. One way of reducing motions in rough seas at
speeds above 40 knots is to design for "supercritical" operation
in which the period of encounter with the longest important wave is shorter than the natural pitching period of the ship. The slender hull with a large bulb at both bow and stern is a
potential supercritical ship, and it may well be that such unusual hull forms may be essential when sustained sea speeds of 40 knots
and above become realistic for ships about 600 ft. in length. In the meantime, power estimates for relatively conventional high speed submarines and surface ships may provide useful
approximate standards of comparison. Submarine Tankers and Cargo S/lips
A detailed comparison of the power requirements for surface
ships and submarine tankers and cargo ships was made about six years ago by Todd,t19 and its conclusions are broadly
con-firmed in a more recent study by Watts.t20) These indicate that,
for equal deadweight and speeds up to about 25 knots,
submarines of circular cross-section could be designed to havesubstantially the same power requirements in average service conditions as surface ships, largely because submarines could
operate immune from the effects of bad weather and would thus
need much lower service power allowances. However, such
submarine ships would have excessive draughts, and if these are
avoided by using elliptical sections, then the superiority of the
submarine disappears. In addition, submarine merchant ships
have obvious handling and rnanoeuvring difficulties. Very High Speed Displacement Ships
Although it is extremely unlikely that large, single hull dis-placement ships will be built to operate at very high speeds, it may be of some interest to estimate the general characteristics which such ships would have. As a starting point, tentative
power and weight estimates have been made for a series of very -o-00 -005 -004 o -003-s -o02 GAIN o -00l +0Ql -0-50 0-70 0-30 0-30 - V/,, -00 IO
fine form ships (CB 040) designed for service speeds from 30 to 60 knots, and the results of simple calculations for one of this series are given in Fig. 19. For a ship 400 ft. in length at
50 knots the speed-length ratio is 2 5; this value is considerably
higher than any likely boundary speed of the kind considered previously, and would require a hull form with very different characteristics from those used as a basis for the high-speed cargo liners for which estimates are given in Figs. 10 and Il.
A round bilge form with high prismatic coefficient and a transom
,40O n z 'I) n 2 -U coo 300 200 Q-00
04
0.2 o 25 2.0 d hp t.5 3000 WEl'!T 2000 (tons) f000 oall consumption of about 05 lb./hp/hour for a range of 1,000
miles, and machinery power-weight ratios of 400 and 200 hp/ton
have been assumed; about 400 hp/ton has been achieved for
marine gas turbine plants of 20,000 shp, but this high ratio may
not be possible for much higher powers. On the basis of these assumed figures, Fig. 19 shows that a useful payload between
500 tons and 1,000 tons might be possible at speeds about
50 knots. Although these deadweight and power figures are different from those for equivalent hovercraft and hydrofoil
ships,t21 they are sufficiently similar to provide a basis for
practical comparisons between these forms of high-speed marine craft.
Acknowledgment
The work described in this paper forms part of the research
programme of the National Physical Laboratory and is published
by permission of the Director of the Laboratory.
Symbols and Nomenclature
= Amplitude of significant acceleration. a, b= Constants in formula for boundary speed. (1 + b) = Appendage resistance coefficient.
1.0 B = Breadth of ship at load waterline.
CB = Block coefficient. 1,000 r
2 = Circle resistance constant,
non-chmen-sional if in consistent units.
©sN = Circle resistance constant for naked ship
at 15° C. (590 F.).
= Circle wave resistance constant. r
= Resistance coefficient, non-dimensional if
Py2 in consistent units.
dhp = Delivered horse power.
F y - if in consistent units. (Froude number)Speed-length constant, non-dimensional "
- Speed-displacement constant,
non-dimen-\/g V " sional if in consistent units.
g = Gravitational acceleration.
H = Hydrodynamic efficiency factor.
= 05834 = Speed-displacement constant L = Length of ship (generally in feet).
= Length between perpendiculars N = Ship propeller rate of rotation.
P = Horse power, in general.
r = Resistance, in general.
s113 = Amplitude of significant bow motion.
S = Wetted surface area. t = Thrust deduction fraction. T = Draught of ship.
Tg = Taylor quotient (as defined by Saunders) or speed-length ratio.
y = Speed, in general. V = Ship speed in knots.
= Boundary speed in knots.
w = Taylor wake fraction.
x = Overload fraction.
y = Service power allowance fraction. z113 = Amplitude of significant heave.
= Displacement of naked form (generally in tons S W).
30 40 SO
y (knos
GoFIG. 19.-VERY HIGH SPEED DISPLACEMENT sHIPS
400 ft. x 64 ft. B x 22 ft. T x 040 CB
6,400 tons SW.
stern would probably be suitable, and a systematic series of
experiments with such forms has been in progress at N.P.L. for
some time; data from this H.S.D. (high speed displacement)
series has been used for the power estimates in Fig. 19. Although
only approximate, these estimates show that extremely high powers would be needed to reach high speeds, even in calm
water; for 50 knots the power exceeds 350,000 dhpa quite
unrealistic value by present standardsand the hydrodynamic
transport efficiency ¿ V/dhp is less than I, compared with values
of over 70 for large tankers and between 15 and 20 for cargo
liners at their boundary speeds.
To give an indication of the possible useful payload or
dead-weight of such a ship, dead-weights of hull, machinery and fuel have
been estimated from information for smaller high-speed ships and from other sources. The hull weight has been taken at
over'1H
-= Change in ship resistance constant.
6'10 = Correction factor for open-water efficiency. = Quasi-propulsive coefficient.
-
= Hull efficiency.w
= Propeller open water efficiency. = Relative rotative efficiency.
p = Mass density of water. 6/3 = Amplitude of significant pitch.
y = Volume of displacement.
References
AYRE, A. L.: "Essential Aspects of Form and Proportions as Affecting Merchant Ship Resistance and a Method of Approximating E.H.P.," Discussion by F. H. Alexander,
Trans. N.E.C.1.E.S., 1927-28, Vol. 44, p. 186.
HUGHES, G.: "An Analysis of Ship Model Resistance into Viscous and Wave Components," TRANS. R.I.N.A., 1966. BAKER, G. S.: "Some Considerations in the Design of
High Speed Cargo Vessels," Trans. N.E. C.I.E.S., 1942-43, Vol. 59, p. 23.
SAUNDERS, H.: Hydrodynamics in Ship Design, I, 517. Standard Procedure for Resistance and Propulsion
Experi-ments with Ship Models, N.P.L. Ship Division Report
No. 10 (Revised), 1960.
MooR, D. I.: "Resistance, Propulsion and Motions of
High Speed Single Screw Cargo Liners," Trans. N.E.C.I.E.S., 1966.
DAWSON, J.: "Performance Data in Calm Water for Single
Screw Hull Forms with Block Coefficient 050 and 0525," N.P.L. Ship Division Tech. Memo. No. 130,
1966.
Wmm, G. P.: "Preliminary Power Calculations for Some High Speed Cargo Liners," N.P.L. Ship Division Tech.
Memo. No. 131, 1966.
SILVERLEAF, A., and ENGLISH, J. W.: "A Note on the
Hydrodynamic Efficiency of Propulsion Devices Suitable
for Use with Geared Diesel Engines," N.P.L. Ship
Division Tech. Memo. No. 129, 1965.
Professor H. B. Benford, B.S.E. (Member): The title greatly
understates the paper's contents; but I interpret this as a reflection on the authors' modesty rather than any imperialistic ideas they may hold concerning the realm of hydrodynamics. In addition to touching on several non-hydrodynamic matters, the authors give us much useful information on basically slow speed ships such as tankers. Their paper forms a compendium of useful
hull form and powering data, which can be combined with other
technical and economic factors in seeking an optimum design.
In this regard, I am particularly pleased that they make no claim that the most economic design can be determined by
hydrodynamic considerations alone.
With regards to bulk carriers, I should like to suggest a
simple approach to finding the most economic hull form and
speed. These ships generally find their cargo in unlimited
supplies and should therefore be made as large as possible. If
we assume that operating draught is the most severe restriction,
we should extend the hull proportions as far as practical based on that constraint. This may produce a beam-draught ratio of
30 and a length-depth ratio of 14. We then make the block coefficient as high as practical, usually in the range of 080 to
083. The final step is to treat designed sea speed parametrically,
seeking the most profitable ship by iteration. This is most
conveniently done by assuming arbitrary values of shaft
horse-power and estimating, for each, the speed, weights, costs,
DISCUSSION
LUKE, W. J.: "Further Experiments upon Wake and Thrust Deduction," TRANs. I.N.A., 1914.
HADLER, J. B., MORGAN, W. B., MEYERS, K. A.: "Advanced
Propeller Propulsion for High-Powered Single-Screw
Ships," Trans. S.N.A.M.E., 1964.
ENGLISH, J. W., GRANT, S., POULTON, K.: "Mammoth
Tanker Propulsion with a Ducted Propeller System:
Experiment Results," N.P.L. Ship Division Tech. Memo. 121, 1966.
EWING, J. A., GOODRICH, G. J.: "The Influence on Ship Motions of Different Wave Spectra and of Ship Length," TRANS. R.I.N.A., 1966.
GOODRICH, G. J.: "Comparison of Calculated and
Measured Motions in Waves for Single Screw Hull
Forms with Block Coefficient 050 and 0525," N.P.L.
Ship Division Tech. Memo. No. 132, 1966.
MooR, D. I., SILVERLEAF, A.: "A Comparison of the
Hydrodynamic Performance of Some Recent High Speed
Cargo Liners," N.P.L. Ship Division Tech. Memo. 57,
1964. Also Shipping World and Shipbuilder, September
1964.
GOODRICH, G. J.: "The Influence of Freeboard on Wetness,"
Fifth O.N.R. Symposium on Naval Hydrodynamics,
1964; also N.P.L. Ship Division Report No. 60, 1964. SILVERLEAF, A., DAwsoN, J.: "A Preliminary Assessment of
Bulbous Bows for Ships," N.P.L. Ship Division Tech. Memo. No. 50, 1964.
LEWIS, E. V.: "High-Speed Ships," Intnl. Science and
Technology, April 1963, 38.
TODD, F. H.: "Submarine Cargo Ships and Tankers,"
Third O.N.R. Symposium on Naval Hydrodynamics,
1960; also N.P.L. Ship Division Report No. 20, 1961. WATTS, B. R.: "The Commercial Operation of Cargo
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transport capability and all other factors leading to some
measure of profitability. The important thing to note here is that, as far as bulk carriers are concerned, optimal speed and
fullness of form bear no simple relationship such as that expressed in equation (1). Studies here show that optimal speed decreases
slightly as length of voyage increases and, as indicated by the
authors, becomes gradually higher with increases in size. The following table shows our estimate of the most economic speeds for ocean ore carriers. These are designed around the specified
operating draughts in the manner outlined above. Block coefficients of 080 are assumed throughout:
Operating Draught
(ft.)
Deadweight (long tons)
Optimal sea speed in knots 4,000 miles round
trip 14,000 milesround trip
20 8,000
