P1985-5
VOL.1
WORKSHOP ON DEVELOPMENTS
HULL FORM DESIGN
MARIN
22 - 24 OCTOBER 1985
WAGENINGEN, THE NETHERLANDS
PROCEEDINGS
WORKSHOP ON DEVELOPMENTS IN HULL FORM DESIGN
October 22 - 24,-1985 Wageningen, The Netherlands
PROCEEDINGS
VOLUME 1
Publication No.
785.
Maritime Research Institute Netherlands Wageningen, The Netherlands
WORKSHOP ON DEVELOPMENTS IN HULL FORM DESIGN PROCEEDINGS VOLUME 1
CONTENTS
Introduction
LARGE BLOCK HULL FORMS FOR DRY AND LIQUID BULK CARRIERS
Hydrodynamic Considerations in the Design of Full Hull Forms by J.J. Blok and J. Holtrop (MARIN)
Twin-Skeg Afterbodies Can Save Fuel by W.B. van Berlekom (SSPA)
HULL FORMS FOR CONTAINER VESSELS, RO-RO SHIPS AND FERRIES
Hydrodynamic Design of Container Vessels
by K. Takekuma (Nagasaki Experimental Tank, MHI)
On the Design of Modern Ferry Hull Forms IV
by J. Tikka (0y Wartsild) and J.D. van der Baan (MARIN)
DEVELOPMENTS IN AFT-BODY DESIGN
The Use of Non-Viscous Flow Calculations in Hull Form optimization/
Historical Development in Hull Form Optimization V
by A. Jonk (MARIN) and E. Vossnack (Nedlloyd)
Non-Symmetrical Aft-Bodies VI
by G. Collatz (HSVA)
HIGH-SPEED DISPLACEMENT SHIPS
NSMB-Systematic Series of High-Speed Displacement Ship Hull Forms VII by P. van Oossanen (MARIN) and J.B.M. Pieffers (RNN)
High-Speed Displacement Ships; Trends in Hull Form Design VIII
ADVANCED MARINE VEHICLES AND PLEASURE CRAFT
Advanced Marine Vehicles - A Review IX
by M.R. Bebar, C.G. Kennell, W.N. White (NAVSEA) and D.R. Lavis (Band, Lavis & Associates, Inc.)
(Semi-)Planing Pleasure Craft x
by D.E. Calkins (University of Washington)
Design of Racing Yachts XI
INTRODUCTION
The Workshop on Developments in Hull Form Design has been organized by The Maritime
Research Institute Netherlands (MARIN) with the aim to exchange information and ideas on the design of hull forms and associated problems. The emphasis of the workshop has
been on flexibility and promotion of discussions. To this end a number of invited
papers have been presented by specialists from industry, sister organizations and
MARIN. In addition, a large number of short contributions have been presented on recent developments. Five sessions were selected to cover the following specific subjects:
large block hull forms for dry and liquid bulk carriers hull forms for container vessels, ro-ro ships and ferries developments in aft-body design
high-speed displacement ships
advanced marine vehicles and pleasure craft.
Volume 1 of the Proceedings includes all invited papers, whereas Volume 2 includes
all short contributions and the discussions to both invited papers and short
SUMMARY
The present paper reviews a number of hydrodynamic aspects that have -or may
have- to be considered in the design of a single-screw ship form. The paper
discusses resistance and powering aspects viewed from the calm water angle and continues with discussion on wave effects, not only in relation to performance but to motions and loads as well. The paper draws on data obtained through numerous model experiments backed
up by full scale trials and computations. Although by no means exhaustive, it is felt that the major trends in present-day design of full ship forms are covered.
1. INTRODUCTION
In the design of a merchant ship two fundamental requirements are laid down concerning cargo capacity and speed. The cargo capacity determines the size of the ship, whereas the speed
dictates the form and propulsive power to be installed. Traditionally,
aspects of speed and power were treated from the calm water viewpoint solely. Still the powering aspect is very important, the more so, since developments in merchant ship forms have been extended over ranges where
the resistance and propulsion of single screw full ships have become
HYDRODYNAMIC CONSIDERATIONS IN THE DESIGN OF FULL HULL FORMS
J.J. Blok and J. Holtrop
(MARIN)
critical from a fuel economy point of
view:
extreme fullness,
low length-breadth ratios.
These conditions require that much attention is paid to a proper
assessment of the still water powering performance in order to design hull forms which are cheap to build and successful to operate.
There are a number of reasons why looking into the wave aspects is comparatively new in relation to the still water considerations. First, the complexity of the seaway and the response of a ship could not easily be described nor understood. Secondly, the appearance of the sea is so often calm, so why bother about waves if they do not attain sizeable
proportions except for very limited periods of time. Thirdly, improvements made in the seakeeping behaviour, e.g. a reduction of the pitch angle, could not easily be translated into a reduction of running costs in comparison to savings in resistance and propulsive power in still water. Nevertheless, the last decades have shown a growing interest in the influence of the seaway on the design and operation of the class of ships discussed here.
In this paper the calm water aspects are discussed in part A in relation to different fore and aftbody forms in combination with the effect of the main dimensions. In part B the wave effects are treated with respect to motions and accelerations, extreme effects as propeller emergence,
slamming etc., global sea loads, wave-added resistance and speed loss.
PART A - STILL WATER ASPECTS
2 HYDRODYNAMIC ASPECTS OF FULL FORMS Although the hull forms of ships with a great fullness can be considered as comparatively simple at first sight (a rectangular box with a rounded bow and sharp stern), the hydrodynamic still water phenomena involved are very complex. Some of these are less well understood than the hydrodynamic behaviour of slender hull forms, and, as a consequence, the prediction of the hydrodynamic performance can sometimes be rather difficult. Typical still water hydrodynamic aspects related to full hull forms
are:
large viscous form effects in the resistance
separation of flow
propeller hull interaction wave breaking
course keeping.
Although general guidelines and methods are available to take these specific aspects into consideration, it is commonly accepted, certainly for the larger ships in this class, that model experiments are necessary to assess the hydrodynamic behaviour. The conditions for model tests on full hull forms are in general slightly less favourable than in experiments with slender forms and therefore the risk of less accurate predictions is probably larger in tests for full
I . 2
forms.
Reasons are:
large scale factors and hence comparatively large scale effect corrections (resistance, wake), instability of the flaw on model scale with danger of separation, -low test speeds, laminar flow on
model scale.
Nevertheless, the traditional model experiments still give the most accurate results but it is believed that model testing can be made more effective and more accurate by introducing available hydrodynamic theory in the analysis of the results, and by adding supplementary
experiments in order to control the model test conditions in a better way.
Calm water resistance
The still water resistance is
generally subdivided into components of viscous and wave making origin. As to the wave components a distinction is generally made between the wave pattern and the wave breaking resistance. In relation to the resistance of viscous origin the magnitude of the wave components is comparatively small on full ships. The wave breaking resistance is found as a loss of momentum in the wake of the ship whereas the wave pattern resistance is related to the energy radiated from the ship as free surface gravity waves. The viscous resistance includes both the frictional component and that part of the pressure
resistance which is caused by the development of a boundary layer on the hull and the wake close to the centre
plane.
It is becoming more and more accepted, especially for full hull forms, that the viscous resistance of the hull is described by a three-dimensional method employing the form factor l+k.
According to this concept the viscous resistance of the ship is considered proportional to the resistance of a
flat plate of the same length and wetted area as that of the hull moving at the same speed.
According to this concept the total resistance is written as:
R= l'pV2S(CF(1+k)+CA)+Rw
Here CF is the coefficient of
frictional resistance of a smooth flat plate, S is the wetted area and CA is the model-ship correlation allowance. Rw is the sum of wave breaking and wave pattern resistance. The form factor l+k can be derived
from low-speed model resistance measurements provided the flow around
the model remains turbulent and the wave resistance vanishes according to
a certain speed dependent function. It has been shown in various correlation studies that the correlation allowance CA, which is supposed to reflect primarily the effect of the hull roughness, depends mainly on the ship length and a suitable hull roughness parameter.
Propulsion
As to the propulsion the classical subdivision into propulsion factors is generally used. In this way the
effective wake fraction, w, the thrust deduction, t, the relative-rotative efficiency,
and the propeller open water efficiency, no, are discerned. Just as the resistance components the propulsion factors of the ship are determined by model experiments in combination with corrections for scale effects. By definition:
PD = RV/(no (1-t)/(1-w)OR)
Scale effects are normally considered present in the propeller open-water efficiency due to a different entrance velocity V(1-w) and due to differences
in the propeller blade friction.
It is well known that more
specifically for full hull forms in ballast substantial scale effects are
present on the effective wake fraction w.-The magnitude of the wake scale effect, important for the accuracy of the rotation rate prediction, can then be rather uncertain, especially when on model scale the flow is close to
separation.
In modern extrapolation methods as e.g. the ITTC-1978 method, Ref. 1, the
scale effects are accounted for in a rational way.
Further improvements with respect to form factor determination, accuracy of rotation rate prediction and extension to complex propulsors is needed, however, Ref. 2.
3. CHARACTERISTICS OF FULL HULL FORMS The class of full hull forms discussed
here is characterized by the following
features:
-comparatively slow (Froude number
V/VgL below 0.2)
block coefficient larger than 0.75 midship section almost rectangular
long parallel midtody
moderately loaded propeller(s)
When related to the hydrodynamics aspects discussed the full hull forms have the following features:
for these ccmparatively low speeds the wave breaking and wave pattern resistance depend essentially on the entrance and the transition of the entrance into the parallel midbody,
Ref. 3;
-the parallel midbody and the run have no significant influence on the wave resistance, provided the length beam
ratio is sufficiently large (L/B>5); the propulsion factors depend
primarily on the shape of the run; the form factor expressing viscous form effects is substantially higher than 1.0 for high block ships and comparatively small modifications of the run can affect l+k significantly; 1+k decreases when the ship is
lengthened by an insertion of an addition to the parallel midbody.
Based on these and some other hydrodynamic considerations design concepts have been developed for full
hull forms.
In the design method developed at the Nagasaki Experimental Tank of MHI, Ref. 4, the bow form is designed to give low wave resistance and the aftbody is designed to give a good propulsive efficiency whereas the parallel midbody is designed to give the desired displacement. Essential for such a design procedure is that effects of main dimensions and other design variables such as the form coefficients and the speed are reflected in a correct manner in the design procedure. When this has been accomplished a proper choice of main dimensions and form coefficients can be made and an optimum balance between bow and afterbody fullness can be assessed. In fact, a hydrodynamic design model should include not only the resistance and propulsion. Also
the aspects of manoeuvrability, vibration and added resistance in a seaway should be considered. Moreover, the effect of local hull shape should be taken into account in the choice of the form coefficients and main
dimensions.
The influence of the main dimensions and fullness can be determined from results of model tests on methodical series, Refs. 5, 6, 7, 8, 9, 10. A
1.4
point of concern is that these series do not always cover the parameter ranges of modern very full ships. Another factor is that recent developments in hull forms are not always incorporated in the methodical series. A recent theoretical and experimental study of a modern systematic series was carried out by K.S. Min and S.K. Hong of Daewoo in close co-operation with MARIN, Ref.
11.
Another approach, followed by Holtrop and Mennen was to derive empirical expressions for the resistance
components and propulsion factors by a statistical analysis of random model test data and results of sea trials, Refs. 12, 13.
A gradual extension of the parameter ranges, also for high block ships, has resulted into a practical prediction
method.
Nevertheless, care should be taken to correlate the method to samples of actual trial results available to the
user of the method. A drawback of the statistical methods derived from random data is that the methods are in general not suitable for optimisation of form coefficients or local hull
form particulars. Anyhow the
statistical methods should be linked to experimental data in the parameter range of interest.
An interesting aspect of full hull forms in particular is the
simplification of the hull form by introducing flat and singlecurvature forms. For a historical review of form simplification see Ref. 14. According to MARIN experience, Ref. 15,
simplified single-screw forms of great fullness are almost equivalent to comparable normal hull forms. For twin-screw full forms optimised simplified forms can even be slightly
view. Probably, a reduction of three-dimensional vortex separation, which accounts for the viscous form effects on the resistance, causes the
favourable performance of the simplified forms.
4. FOREBODY FORM
From experience of model tests on very full hull forms and moderately full hull forms with LCB far forward of
II, it is known that beyond a
PP
certain value of forebody fullness the wave resistance and the viscous form
effects increase rapidly. At these extremely blunt bows not only the shape of the bow itself is relevant but also the smoothness of the forward shoulder is important.
Forward shoulder
In several cases of very full forms it was shown in model experiments that smoothening the forward shoulder significantly reduces the resistance.
In Fig. 1 two curves of sectional areas are shown both representing forebody shapes of the same character. Model experiments indicated that the fuller forebody experienced 12 to 15 per cent more resistance than the
forebody with the smoothened shoulder.
A. P.
Fig. 1
This observation suggests that the balance between the fore and aftbody fullness should be considered as a function of the block coefficient,
F.
local hull shape and the ratio between the main dimensions. The parameter which expressed this balance between forebody and afterbody fullness is LCB. In Section 6 the choice of LCB is discussed in more detail.
Bulbous bows
The bulbous bow on very full forms has been considered as an effective means to reduce the wave breaking
resistance, see e.g. Refs. 3 and 4.
Some practical figures for the design of a bulbous bow in relation to the operational conditions of forward draught and sea way can be found e.g.
in Ref. 16.
It is understood in general that a bulbous bow does not only affect the wave resistance but also the viscous effects. Flow measurements on ship models have shown that a bulb can
suppress the longitudinal outward rotating vortices shed from a conventional bow.
From experience and data in literature the following considerations can be used in the design of a bulbous bow on full forms:
the optimum bulb size increases with fore body fullness,
a deeply submerged bulb is not effective at the low speeds of full
forms,
blunt bulbous bows may cause severe resistance penalties when emerging from the water surface at light draughts forward,
a bulbous bow is optimal for only one operational forward draught and
Speed.
On full hull forms with a large forward draught variation essentially three solutions for the design of a bulb are of interest:
The bulb is optimised for the
effectiveness at larger draughts is
accepted.
Resistance penalties for surface protruding
bulbs
are then avoided. This is an attractive solution for ships with unpredictable forward draught variations.-The
bulb
is optimised for the loaded draught. Its benefit at this draught is supposed to be larger than the resistance increase at lighter draughts. For ships having one well defined loaded draught this is a good solution, provided incidentaloperation in ballast does not lead to excessive resistance penalties. -The shape of the bulb is optimized
for a well balanced pair of two forward draughts. In general this results into a
bulb
with rather large cross sectional area at the forward perpendicular at the loaded draught(10-12 per cent of the midship
section or even larger). The shape of the underwater form at the other draught forward (ballast) is then similar to that of a hull form without a bulbous bow. Due to the lengthening effect of the bulb the waterlines and diagonals below the ballast waterline can be made
straight and very slender and, as a consequence, the ship with bulb performs better than a ship of traditional form without a bulb.
Especially this last solution is now frequently applied because it offers the possibility to apply a bulb form close to optimum for the loaded draught in combination with good
resistance performance in ballast. An example of this kind of
bulb
is shown in Fig. 2.Experimental results at MARIN have shown that the effectiveness of a similar
bulb
is even better when the sections intersect the1.6
Fig. 2.
water surface vertically at the two draughts of interest. This has
resulted in a bulb form with two knuckle lines around the loaded waterline and very steep sections at the ballast waterline.
Results of pressure calculations based on potential flow theory indicated that further improvements could be achieved by raising the forefoot of
the
bulb.
An example of such abulb is
shown in Fig. 3.Fig. 3.
Cylindrical bows
On extremely full forebodies a cylindrical bow may be advantageous, especially at the full load draught. Results of methodical series on cylindrical bows reported by
Muntjewerf, Ref. 17, show that for the tested combination of L/B=6.5 and B/T=2.65 the cylindrical bow is very attractive for block coefficients
above 0.80 in the loaded condition. It was concluded further that larger bows were needed for optimum
performance with fuller hull forms for a given Froude number. In ballast the bow size would have to be appreciably
smaller than at the loaded draught. The resistance reduction due to the cylindrical bow are thought to be attributed both to an influence of the wave breaking and viscous form
effects. The effect of smoothening the forward shoulder by shifting
displacement to the bow is clearly
present.
A proper shaping of the transition of the buttocks in the forebody into the bottom lines is essential to avoid flow separation and vortex shedding at the forefoot of cylindrical bow forms. An interesting point is the
application of a bulb bow on a cylindrical bow. A bulb for the ballast draught may reduce the wave making generated by the blunt forefoot of the cylindrical bow. As an
alternative a large bow may be fitted to improve the resistance at the
loaded draught provided the ballast waterlines are straight and shaped in
such a way that a slender form is
obtained.
The added resistance of ships with a cylindrical bow in short waves (X<0.4L) is substantially higher than that of ships with a conventional bow.
Despite the successful application of cylindrical bows for some classes of ships there is some doubt if
cylindrical bows can universally be applied on all full forms. The small or even negative improvements at ballast indicate that on relatively shallow draught ships in the range B/T>5 the cylindrical bow will probably not be the best solution. On such ships V-type sections, with or
without a bulb should then be considered as more practical.
5. AFTERSODY FORM
The afterbody form of high block ships is essential as regards tha magnitude of the viscous pressure resistance and the propulsion factors. Not only the total efficiency is to be considered but also limitations as to propeller induced vibration should be obeyed. A practical aspect of aftbody design is that locally below the shaft axis enough space should be present to position the propulsive machinery as
far aft as possible.
In Fig. 4, derived from Ref. 18, the flow pattern recorded by means of tufts in a grid at the aft
perpendicular of three different afterbody forms is shown. These pictures clearly demonstrate the development of longitudinal vortices at either the bottom or the sides, dependent of the form. This
three-dimensional flow separation by vortices increases the viscous resistance and should therefore be avoided, if possible. The longitudinal vortices can, however, be advantageous for making the wake distribution more
uniform.
These considerations have led to several single-screw afterbody forms which are reviewed in brief.
Classical V-shaped afterbody
In Fig. 5 three single-screw afterbody plans are shown, Ref. 18. The V-shaped afterbody has favourable resistance and propulsion properties but on very full wide beam ships the danger is present of flow separation ahead of the propeller. Separation is induced by steep negative pressure gradients as a result of the bluff waterline and diagonal endings in the upper part of
54
4 3
Fig. 4
the propeller disc. This type of afterbody forms will in general show a very inhomogeneous wake distribution with a deep upper wake peak.
As a consequence, this type of afterbody hull form is extremely
suitable for slow full ship forms with a comparatively low power installed.
A limiting factor to apply a V-shaped afterbody can be the propeller induced vibration. 1 \
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in Fig. 5 are shown.
U-shaped afterbody
When the risk of signifiant propeller vibration excitation is present attempts should be made to homogenize the wake distribution. This can be done by applying a U-shaped afterbody with bulbous stern as shown in Figs. 5
and 5.
The wake pattern reveals the presence of two strong, inward rotating
vortices aside of the bulbous stern, which transfer momentum loss from the boundary layer to a wider area. As a result the wake peak which would have been present behind a V-shaped hull is now filled due to the rotation of the
flow.
When compared to the V-shaped
afterbody of the same fullness it is evident that the U-shaped afterbody with bulbous stern will induce more resistance due to the additional momentum loss associated with the
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azo V-STERN U-STERN BULBOUS STERN Fig. 6vortex separation. This additional momentum loss is reflected also in the effective wake fraction which is
larger behind a U-shaped hull than behind an afterbody with V-shaped
sections.
When the total propulsive power is considered it appears that the power required to propel a ship with a -U-shaped afterbody is only slightly
more than that required to propel the similar ship with V-shaped sections. Apparently, the additional resistance of the U-shaped afterbody is almost compensated by an increased propulsive efficiency. This increase of
propulsive efficiency is caused completely by the increased hull efficiency although this effect is neutralized somewhat by the higher thrust loading causing a (slightly) lower propeller efficiency.
Pram-type forms
Over the last decades, see e.g.
Ref. 19, several attempts were made to adopt flat, barge type forms of a
rather simple geometrical shape as an alternative for the more complex forms of seagoing ships. These ship forms
(examples are shown in Figs. 7 and 8)
are characterized by straight, flat buttocks. The propelling machinery is then located in a gondola. These hull forms have shown to perform
Fig. 7
excellently as far as the resistance is concerned. Further simplifications of these hull forms by introducing knuckle lines have proved to be feasible without affecting significantly the favourable
resistance properties, Ref. 15. The reason why these afterbody hull forms have a very low resistance is to be attributed to the reduction or even absence of three-dimensional
separation along trailing vortices. The boundary layer on the hull adheres to the hull above the propeller disc and the velocity defect in the propeller disc is very small, except for a local wake peak behind the gondola. As a consequence, these hull forms have a very low form factor and effective wake fraction. The gain in resistance in comparison with a conventionally shaped afterbody is lost again by the low propulsive efficiency. The low efficiency is caused primarily by the low wake fraction but also by an increase of the thrust deduction which often appears to be somewhat higher than that of a normally shaped hull form. The prospect of the application is three-fold:
Provided the gondola is slender a favourable wake distribution can be achieved to control the vibration excitation induced by the propeller. Because the flow on a pram-type stern is not so liable to separation a greater afterbody fullness can be allowed without an excessive increase of pressure resistance. Hence, a pram-type form can be applied with success on a ship with an extremely high fullness.
Thecomparatively flat afterbody
ensures a wide deck area in the after part of a hull which can easily be constructed by single-curvature forms (the gondola excepted).
Another practical aspect of the
1.10
pram-type form is the ratio between the displacement of the afterbody at full load and ballast draught. Because of the relatively small ballast
displacement, a pram-type form appears to be quite appropriate to be fitted with a slowly running large diameter propeller which can then be submerged comparatively easy. Due to the wide
flat afterbody the initial stability is comparatively high.
Propeller arrangement
The modern tendency is that the direct drive propulsive power is supplied at
a lower rotation rate than some years ago. This enables larger propellers to be fitted with a substantially lower thrust loading and lower
circumferential speed than previously. Because ships with these large
diameter propellers are less liable to propeller induced vibration, afterbody forms can be applied which show a rather inhomogeneous wake
distribution. Of course, this applies only to ships with a low to moderate propulsive. power. The practical
problem of 'choosing the hull form, the propeller particulars and the
clearances remains. Apparently, a compromise should be made between V-ness and U-ness to give acceptable
vibration excitation for high propulsive powers. Limiting factors for high powered ships can be found in
Ref. 20.
For the relatively low powers
currently applied these days a rather straightforward approach can be made:
For the given power-rotation rate combination the propeller diameter is optimized to give optimum propulsive efficiency in the "behind" condition.
This propeller is located in an open or closed aperture. A closed aperture (applicable on low powered ships) has the advantage that a thinner rudder with a lower drag can be applied.
The hull clearances are chosen such that vibration problems are
prevented.
Propeller design particulars (skew, tip-unloading, blade area, etc.) are adjusted to give acceptable vibration and cavitation performance.
About the hull clearances it is noted that the traditional ccncept of expressing minimum clearances as a fraction of the propeller diameter is doubtful in the situation where the diameter is a basic variable. If for a
certain available power the design rotation rate is reduced, a
corresponding optimum diamEter is found which is larger. This larger propeller requires smaller clearances than the original propeller because:
it has a reduced thrust loading it has a lower circumferential tip
speed
blade passage frequencies are
lower.
In several experimental studies, carried out about ten years agc in MARIN research programmes, it was concluded already that propeller diameter variation for a certain hull form should involve rather constant absolute clearances instead of maintaining clearances as a constant fraction of the diameter.
An interesting research topic would be to study the practical applicability of large diameter propellers with a
zero tip clearance. A practical solution will then be needed for the erosive cavitation which is likely to be generated in the slot between the propeller blade tips and the hull
plating.
Comparison of afterbody forms by model experiments
On several occasions the question has been put forward if differences in
propulsive power measured in the towing tank at the self-propulsion point of ship do really reflect
differences to be expected on the hull shape when V-shaped, U-shaped and pram-type afterbodies are compared. In order to discuss this point an
assessment should be given first of the various scale effects concerned on the viscous resistance and the
propulsion factors. Essentially two main approaches are of interest: A - Classical two-dimensional methods
with rotation rate correction for wake and propeller friction scale effects;
B - Modern form factor methods with scale effect correction on the entrance velocity and separate propeller friction corrections.
If by the classical method (A) tests on a U-shaped afterbody are compared to model test results of a pram-type form, a certain difference is found in resistance and propulsive power. However, the actual gain in resistance in favour of the pram type form will be smaller on the full scale than that predicted by the classical method. In the classical method the form effect
is ignored, which is substantially larger for the U-shaped afterbody. In Other words: The classical method exagarates the favourable resistance properties of the pram type afterbody
form. As regards the efficiency, however, the effect is opposite. The scale effect on the efficiency will be larger for the U-shaped afterbody form because it is to be expected that the wake scale effect correction will be much larger for the U-shaped form than
that for the pram-type form. In the two-dimensional classical
extrapolation technique with rotation rate correction these differences in wake scale effects are ignored and hence the classical methods now
favours the U-shaped form because the differences in scale effect on the wake (hull efficiency) are ignored. In the modern extrapolation methods the differences in scale effect
corrections are more rationally accounted for and it is expected therefore that differences between the propulsive performance of two hull forms is predicted with reasonable accuracy.
The accuracy can be improved if the following is considered:
-The differences in form factor should be determined accurately. If two afterbody forms are tested the form factor difference can be determined accurately from test runs at
relatively high speeds under the assumption that the wave resistance depends only on the forebody of full
block ships.
-The wake scale effect should be predicted for the two afterbody forms by means of a suitable method. It is noted here that the wake scaling formula incorporated in the ITTC-1978 method fails in some respects when pram-type forms are considered. The relatively low (model) effective wake
fractions in combination with relatively high thrust deduction factors result into a wake scale effect which is negative for the slender and moderately full hull forms and which is unconceivably low for pram-type afterbodies with a high block coefficient. Thus, the
ITTC-1978 wake scaling favours the pram-type form.
Little is known if differences in propulsive performance predicted by classical two-dimensional methods are realistic in all respect. When two afterbodies are compared the accuracy of the difference in predicted power will depend on the coherence between the form factor and the viscous wake
1.12
through the propeller. For
conventional single-screw propeller arrangements there is a strong correlation between l+k and wm and it is expected therefore that by using the two-dimensional classical
extrapolation technique reasonable predictions can be made using the same allowance factors. Of course, when the correlation between l+k en wm is absent (e.g. twin-screw ships, ships with appendages etc.) separate levels of the trial allowances have to be applied in order to make fair comparisons for different afterbody
forms.
6. THE BALANCE BETWEEN FORE- AND AFTERBODY FULLNESS
When the fullness and main dimensions are established a proper balance between fore and afterbody fullness is to be made. This balance is well expressed by the LCB. In several designs LCB can be chosen from a hydrodynamic viewpoint.
In the past several attempts were made to give a clear indication how the optimum LCB would vary with other particulars. Despite these efforts no definite guideline can be given, probably because the optimum LCB depends on the specific forms of fore-and afterbody concerned, the method of extrapolation of the model tests and the obvious fact that the optimisation of the resistance will yield an
optimum different from that obtained by optimising the propulsive power.
In order to give some tentative indications a re-analyses of model tests on a few series of LCB variations was made. These analyses revealed that for a rather
conventionally shaped form having a length-breadth ratio of about 6, a cylindrical bow and a block
coefficient of about 0.83 the optimum LCB depends slightly on the method of extrapolation and the optimization procedure:
Table 1
The figures in the table, obtained from model experiments, indicate that:
the three-dimensional method of extrapclation produces optimal LCB-values which are aft from those obtained by the two-dimensional method, especially at the higher
speeds
the LCB for optimum propulsion is a little forward from the optimum LCB for resistance
the optimum LCB shifts aft with increasing speed.
The analysis of this case and some others revealed further that the effect of LCB variation for high-block ships is not accurately represented in the power prediction method of Holtrop and Mennen, Refs. 12 and 13. In all cases too aft optimum LCB-values were indicated. A comparison of the
dependency of the resistance components and propulsion factors revealed that in the method of Holtrop and Mennen most tendencies as
published in e.g. Refs. 21, 22 and the discussion of Ref. 5 are correctly represented except for the influence on the thrust deduction. It is expected that when this method is adapted further in the future in this respect (rough) guidelines can be
given how optimum LCB is affected by the main particulars and form
coefficients. From several published series and MARIN experience a diagram was composed giving tentatively optimum LCB values for high block
ships, Fig. 9.
0.6
0.8
Fig. 9
This diagram applies to ships of ccnventional form. The effect of ship size, breadth-draught ratio and length-breadth ratio is not included. Neither can yet be given accurate information how LCB should be adjusted for non-cylindrical bow forms, forms with a bulb or pram-type sterns. From
non-systematic model tests on various
Table 2
2 3 4
OPTIMUM LCS (./.L FORw. OF y2 Lpp)
Optimal LCB in per cent 1, forward of I
V Optimisation procedure method of extrapolation Resistance propulsion 0.13 3.4 3.5 two-dimensional 0.13 3.3 3.4 three-dimensional 0.17 2.7 2.8 two-dimensional 0.17 2.0 2.4 three-dimensional
L
Imil
CYLINDRICAL BOWSWITHOUT BULB, U-TYPE AFTERBODY FORM
KUM
KENN
MEMO
MIN
MIN
Mmuraiii
Form deviation Shift of optimum
LOB with respect to Fig. 9
Bulb Non-cylindrical bow Pram-type afterbody Twin-screw Increase of B/T forward aft aft aft aft (le 08 08 078 07 0.9 09 cs
forms the tentative indications given in Table 2 can be used for departures from the form indicated in
Fig. 9.
7 EXAMPLE
In this section an example is shown of a full hull form in which several elements discussed in the previous sections could be incorporated. It
concerns a design of Companhia
Comercio e Navegacao (CCN), Brasil, of a 55,000 tdw tanker. MARIN
participated in this design by optimising the hull form from a resistance and propulsion point of view. The shipowner, Petrobras, ordered the first three ships of this type. Permission to release particulars of this design is greatfully acknowledged.
In the table below the main
particulars are given and in Fig. 10 the body plan and the curve of
sectional areas is shown for the loaded draught.
In this case the hull form was to be developed for the displacement, LCB and main dimensions, specified by CCN. The curve of sectional areas was drawn up in such a way that very smooth transitions between the parallel
1.14
Fig. 10
midbody and the entrance and run were obtained with the objective to reduce viscous form effects and shoulder waves to a minimum.
For the forebody form a relatively large bulb was chosen with a sectional area at the forward perpendicular of
12.5 per cent of the midship section. This bulb was designed such that it would be very effective at the loaded draught. In order to obtain favourable resistance properties when in ballast, the lower waterlines were made
straight and slender in combination with the raised forefoot.
As regards the afterbody V-shaped sections were chosen because of their
favourable resistance and propulsion properties.
The moderate propulsive power installed in combination with the specified rotation rate allowed this afterbody form to be used without the prospect of serious propeller induced vibration.
Although this design could have been improved somewhat further by iterative model experiments, it is believed that the present hull form is close to the optimum because the results of the model tests were very good, even better than the expectations based on experience with successful designs
from the past. Of course, full scale trials are needed to confirm the good results in the model experiments. Loaded Ballast L PP 208.00 208.00 m LWL 212.96 211.16 m B 32.20 32.20 m TA 11.00 5.50 m TF 11.00 8.50 m 60677 36588 m3 L/B 6.46 6.46 -B/T 2.93 4.60 -CB 0.824 0.780 -MCR 9570 hp Rot.rate 117 RPM D prop 5.90 m
PART B -
WAVE EFFECTS 8. SOME CONSIDERATIONSIn the past aspects of speed and power were treated from the calm water view-point because of the complexity of the
seaway, the comparatively low occurrence of big storms and the inability to translate wave effects into an increase in running costs. Although this line of thought has the attractiveness of any simple
reasoning, the picture has changed over the last decades. The seaway is complex, yet using spectral analysis techniques it has been possible to describe the main properties of a seaway with reasonable truthfulness. In addition, owing to the continuous effort of the meteorological societies the designers came to realize that, although truly big storms occur rather infrequently, the time period during which the sea is in a moderate state of confusion is anything but short. Finally, the designer has become accustomed to take a more
differentiated look at the importance of wave effects and seakeeping.
In the field of motions and their effects, an improvement of say 10 per cent in heaving amidships would seem of little interest, yet 10 per cent in pitch would be more important since it reduces the vertical acceleration all over the ship length by very nearly the same amount. This in turn does not seem to be much unless one realizes that -in some frequency ranges- the number of seasickness victims increases progressively with the motions and accelerations, so that a comparatively small improvement can make an inproportional difference, to the well-being of crew and passengers. An even more important consequence of ship motions are the extreme effects
like water on deck, slamming under the
bow, propeller out of the water. The probability of such events happening is an exponential function of the motions so that in some range a small change in motions yields a dramatic change in the probability that something unpleasent happens.
Another aspect is the loading induced by the waves on the ship's hull. As to the hull girder, the design procedure on basis of the trochoid wave has given way to the design procedure on basis of strip-theory in which the Smith effect and the inertial loads are taken into account. Using spectral analysis a design load can then be established for the hull girder loads which incorporates all probabilistic aspects of the service life of the
ship, thus leading to more rational (often lighter and cheaper) designs. A factor more easily translated into financial terms is the extra resis-tance a ship encounters on top of the calm water resistance. Major compo-nents are the wave added resistance and the wind resistance. In the design of a ship one can make a substantial gain in overall resistance if the dependency of extra resistance on hull properties is taken into account. It is not entirely inconceivable that in some time to come the design requirement for a merchant ship will be as shown in Fig. 11.
A factor that hampers the
implementation of seakeeping into general design in the fact that all statements lie in the probability domain and thus would seem to be difficult to get to grips with. This is fiction. For a given trade route and a long enough period to smoothen the probabilistics (say a year) the most probable average speed loss, or the average increase in fuel
At constant power Extreme effects Sea state At constant speed Extreme effects Sea state
Fig. 11. Sketchy design requirement for sustained speed in a seaway.
determined. Not every roundtrip will suffer from the weather by the same amount, but taken over a year the extra costs thus incurred will be more of a certainty than of a "maybe".
9. MOTION AND ACCELERATION LEVELS
General aspects
Motion levels and the associated accelerations depend to leading order on the main dimensions of the ship in relation to the characteristic
dimensions of the waves. Broadly speaking, a big ship will be less susceptible to the excitation by the sea than a small ship. However, due to resonance effects and synchronism with the excitation this need not always be the case, so we have to examine the matter more closely.
The present subject is confined to full ships which are in the majority
1.16
of cases also big ships with rather blunt shape.
The influence of wave height, wave period and heading
The characteristics of the excitation -the waves- have a very great
influence on the motional behaviour of
a ship.
In the theory of ship motions the linearity of the motions with the wave height is an accepted fact which greatly simplifies the design and
prediction procedure. A single figure may serve to illustrate just how well the motions and accelerations of a big
full ship even at shallow ballast draught complies with this linearity
"rule"; viz. Fig. 12.
Irregular waves 111,3 - 3 m sign. wave height Irregular waves Ry/2 7 m sign. wave height
Regular waves 24 3 m double amplitude
a Regular waves 24 6 m double amplitude
7
t
, 1 k , \,
, , .., 0 5 1.0 15Wave frequency in rad/s
Fig. 12. Vertical acceleration on the bow of a VLCC in ballast.
A well-known exception is the rolling of a ship which on account of its low damping exhibits large amplitudes and consequently more of non-linearity. Rolling can be greatly influenced by a
judicious selection of roll period. In cruise ships the roll period is of paramount importance for the well-being
Speed Power 0.06 0.04 . 0.02 8 8
2
of the passengers and a period of some 20 seconds is often chosen. The same applies to heavy load ships where
jerky rolls has to be avoided to keep the inertia loads down. In full ships transporting bulk commodities no such stringent requirements are laid down and the roll period is much lower, for a VLCC around 10 seconds in ballast and 13 seconds in full load. To avoid resonance with the waves by altering the roll period, quite drastic action would have to be taken. Narrowing the ship to bring the metacentric height down would result in a greater draught, or a longer ship, both very impractical in view of existing port facilities and underkeel clearances already stretched to the limit. So one keeps the roll period in the present range and accepts the fact that resonance in a seastate around 7 coming in abeam is bound to occur. Bilge keels are the expedient to reduce the roll angle and luckily on a full ship a long parallel bilge allows the fitting of a long bilge keel which is quite effective, as shown in Fig. 13 as to roll decay and Fig. 14 as to roll angles in a beam sea.
The wave period or the dominant wave length to ship length ratio is also an important parameter for vertical and
2 4
+
2 in degrees
Fig. 13. Roll decay experiments on a fully loaded VLCC at 15 knots.
Without bilge keels
ave length in m
Fig. 14. Forced rolling experiments in calm water on a fully loaded VLCC at 16 knots.
horizontal ship motions alike. As concerning heaving and pitching full and big ships operate in the short wave length range for most of the time. So as illustrated by Fig. 12 the response of the ship to the waves increases only if the dominant wave length get close to the ship length.
0 45 270 225 180
Wave direction in deg.
Fig. 15. Non-dimensional pitch angle as a function of wave heading for an 80,000 DWT tanker.
PITCH
200
100
The wave heading affects the effective ratio between ship length and wave
length. For very big ships of over 200 m in length, operating mostly in the short wave length range associated with moderate seaways, the effective wave length incrases if the ships heading changes from head seas to bow seas and the vertical ship motions are bound to increase. Fig. 15 shows what pitch angle does as a function of wave heading for a VLCC.
The influence of speed
The speed range of bulk carriers and tankers is in practice extremely limited. Nominal speed is usually around 15 knots, with slow down to 8 or 10 knots on account of bad weather. The slow steaming operation of some companies to keep the ships in operation at reduced transport capacity was limited to a speed of some 10 knots. From the engine viewpoint a lower speed was not practical. Within this narrow speed range the influence of speed on ship motions and accelerations is small. The encounter frequency effect is very weak and it is mainly the ship
length-to-wave length ratio that determines the behaviour.
The influence of ships main particulars
In the design of a full big ship one has some limited lattitude to vary the main dimensions and the block
coefficient. The influence of these parameters has been investigated in a systematic manner by Vossers et al /23/ for the series 60 models. The parameter that influences the seakeeping behaviour, more
specifically the vertical motions, to a great extent is the length-draught ratio, more than block coefficient or the others. As an illustration of this effect Fig. 16 shows the pitch angle
1.18
Fig. 16. Pitching amplitude as a function of length-draught ratio for
Fn=0.20 and Fn=0.25 for a series 60 ship with CB=0.70.
Ref. /23/.
as a function of length to draught ratio and the influence is certainly not marginal. In general the motion amplitudes decrease with the increase
1.5
1.0
0.5
Fig. 17. Pitch response.
Effect of ballast draught variation on pitch angle Draught I Tfore = 5.68 m Draught II : Tfore = 3.78 m Draught Tfore = 1.89 m Ref. /24/. I a DI DRAUGHT Fn=alas DRAUGHT --- DRAUGHT ..." ...-./.../ .4,---0/L 0.6 1.2 1.8 OIL 0025
-
---11.00 1,11 1/.1 Mr" NM 14,7s 1011. 1,e0 05 10 15of L/T ratio in the longer wave range and increase for the shorter waves. Ship length, of course, can effect the seakeeping behaviour but not for very big ships. Except for high seastates with 10 seconds dominant wave period or more, all other lower sea states contain wave components comparatively
short in relation to the ship length. A little variation of the ship length does only alter the range over which the waves can be considered as comparatively short. In keeping with the identified importance of the
length-draught ratio one can expect to find a difference in behaviour between full load and ballast for a bulker or a VLCC.
Some studies shed some light on this like Van Sluys and Flokstra /24/. Pitching appears to be affected by L/T ratio only in the very short and in the long wave range, as shown in Fig. 17. The difference on vertical
acceleration at the forebody is greater because of a greater difference in phase angles. As illustrated in Fig. 18.
Reducing the draught forward in ballast results in higher accele-rations forward. The order of
magnitude of the accelerations forward is 0.1 g single significant amplitude
in a seastate 7 seaway, so still
rather low.
A quantity that may loosely also be grouped under the heading of "main particulars" is the longitudinal gyradius. The gyradius can hardly be varied at will by the designer, yet some idea of its effect may be
helpful. A small gyradius reduces the pitch angle for most wave lengths, headings and for the practical speed range, as Fig. 19 Measured Calculated P.11,1 X, 021 ,or 01
Fig. 19. Pitch angle in head seas for CB=0.60 ship Ref. /25/.
taken from Swaan and Joosen /25/ illustrates, although the ship that was the subject of their investigation
15 DRAUGHT I a EL Fri .0.145 ----.---- DRAUGHT
--- DRAUGHT
7.A/111. ..--.c.,--,1>.,...-... ---4Zi
/
17 full ship. Draught I Tfore = 5.68 m Draught II : Tfore = 3.78 mDraught III: Tfore = 1.89 m Ref. /24/.
05 1.0 1.5
Fig. 18. Response of vertical acceleration at F.P. Effect of ballast draught variation on acceleration of a
0.6
3
had a CB=0.60 we do not think that for higher block ships the trend would
reserve.
The influence of form
The main particulars of the ship are often dictated by economic
considerations rather than
hydrodynamic ones. In the form of the ship the designer has more freedom to adapt the ship to the prevailing flow conditions. In seakeeping the shape of the forebody is more important than the shape of the aftbody. In most sea conditions the absolute and relative motions forward are twice or treble the motions on the aftbody and this has an bearing on anything that has to do with seakeeping. This leaves the aftbody design as the province of the propulsion "opti-miser" and allows the seakeeping analyst to concentrate on the forebody.
Forebody shapes of full ships can vary from the bluntest cylindrical bow to a comparatively pointed stem. Both extremes can be fitted with a bulbous bow and can have a bow flare of
usually moderate inclination. A number of investigations have addressed this topic from whose work we can obtain more major trends.
Swaan and Vossers /26/ have reported model experiments on a CB=0.70 series featuring a variation in forebody shape from U form to V form. Their results show that V-shaped sections in the forebody are favourable for the vertical ship motions. Apart from their much higher
C forward value they offer a wp
progressive increase of bouyancy and have the edge over U-shaped sections.
However, the greater part of the advantage lies in the wave frequency range where the wave length is longer than the ship length. For full and big
ships this range is of lesser
1.20 10 s 1.0 0.5 SPEED F,,,' 0.15 SPEED Fn. 0.25
THRUST INCREASE THRUST
V- BOW INCREASE
IN V-
BOW Amill
YIN
BOWmv...
U-BOWnu
....MI
-FA
p1=1
..A
HEAVEME
OW "pm
imv .0
u_BOW ral
BOW Ar.
r,
10 15Fig. 20. Influence of speed on the difference in behaviour between two models with different section shape in the forebody
for the loaded condition. Prismatic coefficient C=0.71 Wave direction =170°
Ref. /26/.
importance. In the more important short wave length range, the
difference is only marginal as can be seen in Fig. 20 reproduced from /26/.
The same applies to the absolute vertical motion fore and aft, which is a measure for the accelerations; the advantage of V forebody is lost if the waves become shorter than the ship
length.
In their study a variation of C, value has also been included, mainly effected through a proportional change in midship section coefficient, with some small difference in C that
wp
1 0 5 7.c 1.0 00 computations.
A recent study was done by Blok /28/ who conducted model experiments on a bulkcarrier fitted with four forebody forms; a cylindrical bow with vertical sides all up to the deck, and a cylindrical bow with bow flare, a conventional sharp bulbouw bow with bow flare and a bow with a small waterline entrance angle (pointed stem) and vertical sides all up to the
deck.
The results showed that as far as vertical ship motions heave and pitch are concerned the differences are small, yet the cylindrical bows produce lower motions perhaps more on account of their larger waterplane area coefficient than because of bow form proper. Fig. 22 is a typical
Bow NO. I
BOO No. I/I -
--BO. NO. 0 BOw No. IV
---2 Bow form WAVE DIRECTION a 0170° U.-BOW V- BOW THRUST INCREASE ,,,,,--, -",. Cp=0.76 Cop0.71 '('N / Cp= 0.76 ... Cp= 0.71 RELATIVE =0.71 BOW MOTION Cp= ....--1.. cCp Cp. 0.76 Cp= 076 a '
/\\\
, 0.71/MOTION AT FORE PERPENDICULAR
C.=0 71 16. Cp =0.76 ? Cp=0.71
/77-/
/
,..//
05 10 55 5/1..Fig. 22. Pitch response of a bulk-carrier for four different forebody forms, Ref. /28/.
1.0 1.5 1.0 1.5
x/L
Fig. 21. Influence of Cp value on behaviour in waves for models with different bow form. Speed Fn=0.20, Ref. /26/.
It shows in Fig. 21 that in waves from ahead for U-shaped sections the
higher
C value it to be preferred and for V-shaped sections the lower Cpvalues.
Ewing /27/ investigated the ship behaviour for a full ship (CB=0.70) using a strip theory computer program. He came to different conclusions than Swaan and Vossers to the extent than V sections forward would result in reduced motions and relative motions as well. Because the computation program does not take the above water hull into account we vest more faith in the model experiments than the
6 4
12
4 312
illustration and as shown the
differences in pitch angle are small.
Correlation to mathematical models Given the prevailing linear behaviour of ship motions in relation to the sea and the slender hull form, computation models, mostly based on strip theory, turn out to produce most satisfactory results. A factor that helps is the fact that ship motions depend on the global i.e. integrated hydrodynamic force and inaccuracies in the detailed distribution of the pressures and forces will in most cases still result in quite accurate force predictions.
Vertical ship motions, heave and pitch, are very well predicted even for very big blunt hull shapes like VLCC's. Horizontal motions like sway and yaw are also well predicted, even in oblique wave directions. Fig. 23 shown an example for an LNG carrier taken from Blok /29/. Rolling is a
research topic in its own right and
Measured
-0- F
0.44n 0.17
0.5
0.5
Fig. 23. Sway response in stern quartering waves (450) for a fully loaded LNG carrier, Ref. /29/.
the problems encountered are not restricted to full ships.
1.22
10. EXTREME EFFECTS
General aspects
Under this heading we usually group those effects that do not exhibit a gradual detonation by a function of sea state, but which only show up when a certain motion (velocity, acceleration) amplitude is exceeded. If such happens, the effect is usually quite severe and no ship master is prepared to sustain them for a long
period.
The most well-known effects are: green water on the (fore) deck, bow out of the water,
bow slamming into the water, propeller (tip) coming out of the
water.
These effects can and do occur for any ship type, though not in all cases. It is extremely unlikely that a fully laden VLCC will pull its forefoot out of the water, but water on deck is very likely to occur. For specific designs and operational conditions additional effects have to be included.
The mentioned extreme effects are all associated with the relative motion of the water surface relative to the ship's hull and are critically dependent on the exceedance of a certain treshold value (draught freeboard, submergence of the propeller tip). If the relative motion exceeds the treshold value some of these effects occur, and although these things cannot be entirely avoided, their consequences should be alleviated as much as possible. This kind of problems incorporate some interesting
features.
If the relative motion remains smaller than the treshold value
(draught or else) nothing at all
occurs, //../ ,,,--/
<
../.,-/
/
P
160
uo
re
80
40
if the relative motion just exceeds the treshold value, something very important happens, if the relative motion increases by a further say 10 per cent the magnitude of the effect and the probability of occurrence shows a dramatic increase.
So, if the ship is on the verge of encountering some extreme effect a reduction in motion levels of may be only a few per cent can make a world of difference.
If it cannot possibly be avoided that some extreme effect occurs some time, its impact (literally and
figuratively speaking) can be mitigated through changes in local hull form and design, for instance bow flare angle, propeller tip submergence, shape of the forefoot.
The influence of wave height, period and heading
The main influence of the wave properties is on the ship motions through which the relative motions are affeccted. In keeping with the foregoing considerations it will be clear that comparatively minor
V/r /rt.?,
111115111
1110/1111111121111111
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mwmgrammum
IIVIAIMEN111
NIMEMEIMIN
'
Wave helyticar. ui (sec-1)changes in the sea can have great consequences for extreme effects. Just to show how much the effect is,
some examples are given.
Figure 24, reproduced from Vermeer /30/ shows the results of model experiments on a bulkcarrier in which the amount of water shipped over the bows was measured using the
catch-tank technique. Experiments were done in regular waves from ahead at constant model speed and the total measured quantity of water was
divided by the number of wave
encounters to arrive at the ordinate value: water shipped per cycle of bow motion. As shown, a small decrease in wave height resulted in an increase of the amount of water out of proportion.
Also shown in this figure is the importance of wave period. Usually ship motions respond appreciably to the waves only in a restricted range of wave frequencies. Since shipping of water occurs only when the motion exceeds some treshold value the frequency bandwidth for that effect is even much narrower than for the motion. This is clearly shown in the
figure. Only wave frequencies between 0.42 and 0.54 (or wave length between 211 m and 349 m) would produce water over the bow.
Keeping in mind the great effect wave heading has on ship motions it is easy to accept the great effect it has on any of the extreme effects. Course change (and speed change) are the only measures a ship master can take to reduce detrimental effects, apart from ballasting_
Fig. 25 shows the propability of propeller emergence for an 80,000 tdw tanker at full load for some wave headings. It shows that stern
quartering comes out highest on account of the largest relative motions at the stern.
0-04 0.5 0.6
Wave frequency
Fig. 24. Amount of water per cycle shipped over the bow of a bulk-carrier, Ref. /30/.
100 75 50 25 Level 1 Speed of tanker: 16 kn. 5 m 8 s + 270 deg. heading 225 deg. heading 180 deg. heading 45 deg. heading 0 deg. heading
Fig. 25. Probability of propeller emergence of an 80,000 DWT tanker in various wave headings.
The influence of speed
Although the influence of speed on motions for a full big ship is not
Summer draught Winter draught 'Average period of relative motion 0 5.0 10
Fig. 26. 200,000 DWT VLCC (fully loaded). Frequency of shipping water at
F.P.
1.24
great, it will make more difference on the probability aspects of extreme effects. Yet the bigger the ship the
smaller the Froude numbers and the lesser the effect is. For a 200,000 dwt VLCC the effect of speed on the frequency of shipping water at the bow is shown in Fig. 26, taken from Blok /31/.
,8
speed of Canker. 16 kn. weee heading, .0 deg.Fig. 27. Probability of propeller emergence for an 80,000 DWT tanker in various sea states from ahead.
Another example is shown in Fig. 27 for an 80,000 ton tdw tanker where the effect of a speed change from 16 to 12 knots on the propability of propeller emergence in various seaways from ahead are shown.
The influence of ship main particulars
There would be little point in investigating the dependency of extreme effects on the main
particulars proper, since the ship motions and the treshold value like draught or free board are more important and constitute the link between them. It does make sense to realize that as changes in main particulars change the ship motions to a sizeable extent they are bound to also change the extreme effects by the same token Sheer ship size has much to do with it. The only ship 1NOIAN OCEAN
V
,,,-701 Service speed -Service speed ,.., ...'V
/
-/-/
ir-70% ,,,/--Service 70% Service speed Service speed Service speed speed 5 e 11 sec. . 119 Speed of canker: 12 kn. gave heading: ISO deg. 50 100 50 100 7, 12 5 1n metres 50 100DO
type that has seen a 50 times
magnification in deadweight capacity is the oil tanker so this may serve as an interesting example. As the ships got bigger and bigger the free board in relation to the length did not keep pace. On the other hand big ships are much less excited by the waves and much more steady. An interesting example is shown in Fig. 28 where for four tanker sizes ship motion computations have been carried out and the propability of shipping water over the bow is illustrated. Clearly the bigger ships are much less susceptable to wave action and have a much lower probability level. A different ocean area does not alter the relative position of the ships in the diagram. It is also easily
Crrn draught
--- Winter draught -1Freenoard increased _._J by 1 metre
Fig. 28. Probability of shipping water at ordinate 20 at 700 of service speed. Significant wave height 8
metres.
understood that an increase in free board (treshold value) has a
considerable influence.
The influence of hull form
Here we have to make a dt,stinction between hull form above and below water. Changes in below water hull will change the ship motions and
through them the extreme effects of the ship can be significantly
affected. Changes in above hull shape have only a marginal effect on the ship motions.
For the same underwater hull and the same ship motions the extreme effects can only be influenced through a change in above water hull shape
details. More specifically, green water on deck can be reduced by proper
20
Fig. 29. Influence of flare on motions and thrust increase. Speed Fn=0.20; 2b/L=0.02, Ref. /26/.
01711007 FLARE WiTM EXTREME FLARE
WAVE DIRECTION a x130' wAVE DIRECTION a :170'
mri
PITCHI
ELI
....
PELAT VE OW MOTION ... NIA.
.1
I OR ANT ICREASEii
, /111
, , % % , \ 4\
,
\
\
/NOLAN OCEAN\
\
\
C., ._._ 77---.7ATTm\
\
ATLANTIC OCEAN\
=
200 300 100Ship length in metres
0.5
design of the bow flare and an increase of free board, and slamming loads do depend on the local form of the forefoot. An example of the extent of the differences to be obtained is shown in Fig. 29 taken from Swaan and Vossers /26/ in which the influence of bow flare on
vertical motions and on relative motion at the bow is shown. As shown, ship motions show little difference. Yet, the relative motion forward, in particular in bow seas, was higher for the extreme flare case than for the normal flare. Apparently the extreme flare throws the water up and to the side thereby increasing the relative motion. The increased relative motion of the extreme flare bow does not necessarily induce more water on deck. Model experiments show that a properly designed flare will impart so much momentum to the water that it is thrown aside rather than
on deck.
Correlation to mathematical models To correlate experiments to
mathematical models in the field of extreme effects is mostly carried out in the probability domain. On basis of ship motion computations the relative motion between wave and hull is calculated and the probability of exceeding the freeboard or the draught can be evaluated using the exponential function law following the Rayleigh distribution.
Correlations on probability are extremely sensitive to the accuracy of the computed relative motion as one can imagine.
Even more difficult to compute are extreme effects expressed in
magnitude rather than probabilistics. Studies in this field are extremely scarce. An example is the study reported by Vermeer /30/ who
1.26 800 600 40C 200 0
investigated the average amount of water that would be shipped over the bow. He devised a computation scheme based on strip theory and compared
y(in3) h.-4 2.5rn o expchrm.of
cad -ulaCron
04 05 06
Wave frequency
Fig. 30. Amount of water shipped over the bow of bulk-carrier, Ref. /8/.
to model experiments. Fig. 30 gives a typical result of amount of water plotted on a basis of wave frequency for constant wave height and ship speed. Although considerable discrepancy exists between
measurement and computation the trend and the qualitative correspondence is encouraging.
11. GLOBAL SEA LOADS
General aspects
The global loads on the hull girder of the ship, like bending moments and
shearing forces, consist of two major components, external wave loads producing a space and timewise variation of the buoyancy load meant to support the ship and the inertial load caused by the motions of the ship as a whole. Thus, global loads depend greatly on the relative motion of the water, more specifically the distribution of relative motion along
the hull.
The greatest vertical bending moment generally occurs amidships. Although this moment decreases towards the bow
and the stern, the section modules decrease less rapidly, so that as a consequence the highest material stresses occur amidships. This is the case for wave length of the same order as the ship length. For waves much storter than ship length the greatest bending moment does not occur amidships. However, the overall magnitude of the moment is then much reduced. Global loads on the hull girder go associated with a typical period of occurrence, the encounter period with the waves and if this coincides with a structural natural period hull girder resonance will
occur.
In the following some effects of global hull girder loads are related, structural resonance effects like
"springing" and "whipping" are left out of the picture. Springing
received due attention in the 1970's. Stresses due to springing are not great but may affect the fatigue
life. However, now that ships are not growing in size anymore and the ship service speed is being reduced on tankers and long container ships. Springing as a problem area is
reduced.
"Whipping" excited by bow or bow flare slamming remains of interest though, but is so intimately related with extreme effects like slamming that the master will take avoidance
action.
The influence of wave height, period and heading
With a fair degree of approximation the moment and shearing forces in the hull girder of a ship can be treated as linear with the wave amplitude, thus following the trend set by the vertical ship motions heave and
pitch.
An early investigation of this
linearity was due to Dalzell /32/ who showed that for a destroyer hull even in steep head waves and even at considerable speed the loads would show a linear trend, viz. Fig. 31.
.0 016 .0014 .0012 00 10 -.7 .0008 .0006 .0004 -.0002 SAG .0002 -.0004 --.0006 .0008 .0012 .0012 .0014 -.00 16-m001:1. 2170 0497801:E2 610001. HEADING' 180° 11640 5E.A5 APPROX.R100E1. SPEED:v/.47.. 0.12 to 0.14 "i0 M7 -.001
Fig. 31. Trends of bending moment with wave steepness, Ref. /32/.
Much the same for a very big VLCC at ballast draught. We find that in an irregular seaway response amplitude operators obtained through spectral analysis all very neatly coincide with data points obtained from regular wave tests. And all that for sea states ranging from 3 to 8 as shown in Fig. 32.
So also in the field of hull girder loads spectral analysis is a very powerful method for analysis and prediction. 0'2 .06 Ve 10 2 14 6/ .1. 50 0.75 \Ixo las .001 - HOG
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