LIST OF PAPERS ON StAMMING AND
HYDRODYNAMIC IMPACT ON SHIP STRUCTURE (JAPAN)
SLAMMING AND IMPACT WORKING PANEL JAPAN SHIP STRUCTURES COMMITTEE
AUGUST 1966
ARCFÈF
Lab.
y. S
Techrisde osdoo
DeUÏi SLAMMING
1 -1 On the Correlation between Impact on Bottom of Forebody
and Whipping
Y. Watanabe, Jour. Soc. of Naval Architects of West Japan, No. 32, 1966
This paper deals with the characteristics of two kinds of impact
induced between the wave surface and the bottom of forehody, which are
classified such as inclined or horizontal impact to the wave profile. From the obtained results, although the pressure induced due to
horizontal impact is beyond twice as large as that due to inclined impact, the vibration induced in the former is smaller than in the latter. In
case of the horizontal impact, therefore, the effect to the vibration is small, but on the contrary, by the slamming phenomenon damages are developed on the bottom of forebody; on the other hand, in case of the inclined impact the whipping phenomenon is induced by the large
vibration motion rather than developing damages on the bottom. The
name of so-called slamming, therefore, should be given in case of the
horizontal impact rather than the inclined impact. This definition is the
same as already given by Weinbium.
1 -2 On the Determination of the Initiation of Slamming, and Its
Relation to the Draft of For ebody
Y. Watanabe, Jour. Soc. of Naval Architects of West Japan,
No. 32, 1966
In this paper a method of calculation of the maximum slamming pressure induced on the bottom of forebody
is explained. For the
calculation used were several curves of variables, which were alreadydetermined by other researchers under the assumption of 0. 80. Following the H Wagner' s theory about the wedge section, the mean value of impulsive pressure will result
TTJ
The ratio, therefore, between and the static pressure at full load draft, n is given such as
where g: gravitational acceleration d: full load draft
Y: relative velocity between bottom and wave surface at
the point of impact
inclined angle of wedge
It is the object of this paper to determine first and 7Jf, so that the magnitude of n may become maximum under the severest
condition which the ship would encounter in her life, and secondly under
the ballast condition to obtain the variation of n corresponding to the
change of draft of forehody.
L
As an example the classification of n is shown vher
-. 15
(L: ship length) such as:no damages develop when
n <,
small damages develop when n 5 to 10,
large damages develop when n> 10
The n -value increases always corresponding to the ship speed, and
if we put ' L as the distance from the point of impact
to the midship,
the position of the maximum n exists about ¿' 0. .5 to 0. 30, from
which
the damage therefore developes around this position and n becomes 15 to 16 at the severest condition.
For the different magnitudes of variables from the above case the different result will he obtained, and hence the Ship Structure Committee of West Japan is now pursuing to analyse such cases, and hence curves covering many cases will be published in future.
1 -3 On the Experimental Results of Ship
Motion, Longitudinal Bending Moment and Slamming
Pressure
T. Nagai, M. Kadota, T. Fukuda and K. Dokai, Jour. Soc. of Naval Architects of Japan, No. ] ¿0, 1966
Studies on the ship behaviour among waves have recently improved so much by the statistical approach, so that we might foresee some quantitative results about the ship behaviour even at the stage of initial
design. Sofaras our knowledge is concerned, the data about
the destroyer
which are available at the initial design stage, are considered very few. iii order to examine the motion of destroyers in rough seas and guess previously some motidn of them, the sea tests which covering
different sea states were carried out using two sister ships.
The items measured are pitching and rolling angles, acceleration,
heaving motion, hull
girder stress, local stress
on the bottom paneland slamming pressure on the bottom of forebody; and all of these
recorded data are used for statistical as well as spectral analyses by applying time series procedure.
s
Theoretical calculations, on the other hand, under the cross flow hypose sis arc pursued in order to obtain the response amplitude operators of pitching angle, heaving motion, bending moment and
relative motion, etc. Then the power spectra of them are also calculated
by estimating wave spectra of the experimental water areas.
Comparing with both calculated and analysed results, we obtained
the facts which distributions of variation in pitching angle, heaving motion
and hull girder stress etc. follow to Rayleight s law, allowing from 67% to 90% confidence limit and slamming pressure to truncated
exponential distribution; and also from the power spectra the pitching angle, heaving motion and acceleration show good coincidences from the practical view-point, but on the contrary bending moments some
discrepancies especially at the point of maximun magnitudes.
Number of occurrences of slamming impact and the distribution of slamming pressure are theoretically calculated under several
assumptions about the threshold velocity and the bottom shape, and
their results are compared with recorded data showing some discrepancies
for number of occurrences, but for the distribution of slamming
pressure good coincidences.
1-4 0n the Transient Transverse Strength of a Torpedo Boat during Slamming
T. Nagai, K. Ohtaka and H. Seki, Jour. Soc. of Naval Architects of West Japan, Vol. 31, 1966
The calculation of transient transverse strength during slamming is one of the important parts of the design of torpedo boats, but there has been no reasonable method of calculation available to its design
yet. In order to determine a method with reasonable approximation, the sea tests arranged for measuring transverse stresses which will produce
during slamming at Frs. 7, 15 and 18 were carried out in rough seas last year by using a torpedo boat having principal dimensions such as 30m X
8. 5m X 3. 4m and displacement 108. 6 tons, constructed
by aluminum alloy ANPSO (Al-Mg-Mn Alloy).
In this paper the data of transient transverse stresses during
slamming are presented together with the distribution
of stress
repeti-tions, and their results
are discussed in detail. Environmentalconditions such as wave heights and wave lengths,
accelerations,
longitu-dinal stresses and bottom pressures are also explained.
The damage developed on the bottom shell panels is finally discussed.
The fact that after tests in initially flat bottom panels permanent deformations developed, distributing continuously between the first and
second side girders, and extending from Fr. 12 through Fr. 16 concludes that the maximum stress should have developed near Fr. 15. However,
when the speed of boat increased, the position of the maximum
stress
Frs. 15 and 18 are reasonably considered chosen for measurement of
transverse stresses.
The speed of boat was changed from 19 to 35 knots during tests.As the conventional method of transiént transverse strength used so far is based upon so-called two-dimentional analyses, no reasonable results exist between both experiments and calculations. Judging from the experimentally obtained data, it is recognized that the
three-dimensional analyses must be applied with reasonable approximation to
its calculation, because the external forces are transmitted to bottom
longitudinals upon slamming immediately.
Even if in rough seas, no immersion process of bow section of a boat occurs when the wave lengths of ortcoming wave crests are short as in the se tests, there are no forces therefore developed by rapidly varying buoyancy forces due to immersion into oncoming wave crests. And in
such a case, the forces resulting from impact of bottom shell panels of
the boat with the water surface become predominant. This pressure
impact is called slamming loading, and leads so often to extremely heavy external loads restricted to the bottom plating. Therefore they can result in large plastic deformation of the bottom plates. The fact
that in an initially flat bottom panel the permanent deformations
developed on the application of repetitive slam loads during tests gives us necessary information of the solution to bottom shell plate due to repetitive slam loads. Deformations developed on the dished bottom
panels are shown and briefly discussed.
l-5 On the Transverse Strength of A Torpedo Boad
T. Nagai, Jour. Soc. of Naval Architects of Japan,
No. 119, 1966
Data of the transient transverse stresses during slamming obtained from sea tests of a torpedo boat showed that the transverse stresses were larger than the longitudinal stresses and, therefore, the transverse
strength had as great significance as the longitudinal strength. Sofaras
the authort s knowledge is concerned, there seems to be few papers about the transverse strength of torpedo boats.
In this paper the box model with V-shape bottom constructed by alumi-num alloy is considered as the model of a torpedo boat. This box model
above explained is composed of two neighbouring transverse bulkheads, side shells and bottom shells.
Under the action of the equally distributed statical pressure on the
bottom, the distributions of induced transverse stresses and moments are discussed in detail.
As the basic idea of numerical calculation has already been initiated by Nishimura, we followed his idea by introducing several concepts
suitable to torpedo boats. All of the numerical calculations were pursued by the digital computer.
-4-At the stage of initial design we are used to use the conventional method such as the two - dimensional analyses in order to get the rough
estimation of strength. As this analyses can not be, however, used,
otherwise the reactions due to the external load or pressure are known, the comparison between such a method applied to the box model as the three-dimensional analyses and the two-dimensional analyses
was tarried
out in order to determine those unknownreactions. After determined, by
substituting those determined reactions into the two-dimensional analyses, this method may be applied again at the stage of initial design.By using the measured data of the transverse stresses and the bottom
slam pressure, we have some relation among the equivalent statical
pressure, measured maximum impulsive pressure and boat' s speeds due to the comparison between the numerically determined stresses by the
three-dimensional analyses and test results.
l-6 On the Transient Response of a Beam with Spring.Cushion due to Impulsive Load
T. Nagai, K. Ohtaka and M. Oji, Jour. Soc. of Naval Architects of West Japan, No. 33, 1966
Several investigations on the response of ship in rough seas due to
slamming were already carried out. Some f them give us fruitful results about slamming phenomena as shown in references (1), (2) and (3).
In all of those papers authors considered the ship as a beam, hut actually
the external pressure induced by slamming acts directly upon the bottom
shell and secondly transmit to frames, longitudinals and so on, and
finally to the whole body of ship. Especially in such ships as torpedo
boats the bottom shell structure seems to construct the different system of local vibration from the system of the main hull structure, so that the response of ship due to slamming may be considered different from that obtained from the assumption of direct action of external force to the
ship. 'n this paper we are concerned with the response of beam having
variable section under the action of impulsive load induced through the motion of another system such as the spring cushion attached to the beam.
Theoretical analyses are carried out by using the analog computer, of
which the degree of approximation is checked in comparison withthe analytical solution in case of uniform beam.
As a practical example the solution obtained above is applied for determination of the transient response of a torpedo boat, considering the bottom shell structure as the spring cushion of the beam and using the obtained data by sea tests. Some noticeable conclusions are given as follows.
(1) The higher frequency components of the main hull than the frequency of bottom shell structure will be absorbed, and those of the former nearly equal to that of the latter will be magnified. On the contrary, when the frequency of the former is much lower than the
frequency of the latter, the mode components will become scarcely variable. In case of torpedo boats, as the frequency of bottom shell
structure is not so higher than that of main hull structure, the effect
due to the bottom shell structure on the main hull structure is not considered negligible in some cases.
(2) The maximum moment amidships due to symmetrical impulsive load about the center becomes higher about 20% in the case when
considering the effect due to spring cushion of the bottom shell structure than the case without its effect.
¿ HYDRODYNAMIC IMPACT ON SHIP STRUCTURE
2-1 Elastic Response of High Tensile Steel Plates Developed by Impact on Water Surface
T. Nagai, Jour. Soc. of Naval Architects of Japan, No. 120, 1966
The reason of the damage which would often appear on the bottom shell of forebody is simply because of the large impulsive pressure acted thereon, and when the angle between the bottom surface and the
surface of oncoming waves becomes nearly zero the water pressure is
generally known to become very high.
In order to get the fundamental phenomena on the response of the bottom shell panel having large aspect ratio, the author is now concerned
with the basic research of elastic response of high tensile steel plates
developed by impact on water surface, from the view-points of both theory and experiment.
The large elastic response of the long rectangular plate due to
impulsive water pressure is first solved theoretically by taking the effect
of stretching of the plate into consideration, but neglecting effects of higher frequency vibration. Introducing the measured data of impulsive water pressure into the theoretical formula above obtained, the calculated maximum elastic deflections at the center of the plate are compared with those of experiment, indicating good coincidences between them, and
both results are further compared with those by theoretical results
carried out by Greenspon, which show expanding discrepancies between
experimental results corresponding to the pressure increment.
The small elastic response of the stiffened plate is then solved theoretically by considering effects of higher frequency vibration, and the maximum elastic deflections obtained by introducing the measured
data of impulsive water pressure into the formula are also compared with experimental results, which show good coincidences between them.
¿-2 On the Results of Damages of Cylindrical Shells due to Underwater Explosion - First Report
T. Nagai, Jour. Soc. of Naval Architects of Japan, No. 117, 1965
The phenomena of underwater explosion and the damage mechanism due to explosion have already been investigated quite well by many
researchers.
It seems, however, to the author not yet being well discussed both the quant itative thea surement of wate r pre s sure due to effect of
expansion of successive gas globes and damage amounts.
In order to make one part of these unknowns above explained clear, the experiment of cylindrical shells due to underwater explosion has been carried out, and these cylindrical shells were made of mild steel or high strength steel and stiffened by inner frames with different span
lengths. After hanging the cylindrical shells by wire ropes at the depth
of 30 meters under the water on the about 1, 000 meters depth sea, the explosion tests were pursued by blasting the TNT spherical explosives
from either the bottom or side shell, yielding to the small and large
damages on shells. This paper is mainly concerned with small local damages happened between frames, and hence the large ones were disregarded.
Comparing both results of experiments and calculations on water
pressures and displacements, the semi-empirical formulas of Lime lag
among peak pressures due to gas globes and the amounts of maximum concavities of damages on the cylindrical shells were determined, and good coincidences were obtained between calculations and experimental results of the large cylindrical models of 5. 5 meters diameter made of mild steel, which were already carried out about 20 years ago.
2 -3 On the Results of Damages of Cylindrical Shells due to Underwater Explosion - Second Report
T. Nagai, Jour. Soc. of Naval Architects of Japan No. 119, 1966
Following the preceding paper, the author now discuss the effects due to the depth of water to the damages of cylindrical shells, because it seems to him not being well discussed yet both the quantitative
measurement of water pressures due to effect of expansion of successive gas globes and damage amounts occurred by explosion in the deep sea.
The cylindrical shells used were made of high strength steel and
After hanging the cylindrical shells by wire ropes at the depth of about 30, 149 or 300 meters under the water on the about 2,000 meters depth sea, the explosion tests were pursued by blasting TNT spherical explosives from the bottom, yielding to the small damages on shells.
This paper is mainly concerned with small local damages occurred between frames, and hence the large ones in case of buckling of frames were disregarded.
Comparing both results of experiments and calculations on water
pressures and displacements, the semi-empirical formulas of time
lag among peak pressures due to gas globes and the amounts of maximum concavities of damages on the cylindrical shells were determined.
The formula of maximum concavities was fufther compared with the experimental formula which ws developed quite recently in the USA in case of variable depth of water, showing good coincidences between both results up to the so-called safe depth of water.
The formula of maximum concavities which is available to the mild
steel material was also determined and compared with the experimental
results of the cylindrical models carried out by changing the depth of
water, showing reasonable coincidences between both calculated and
experimental results.
