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5.7 Form (Eddie-Making) Resistance
Previously, we made an assumption that the friction resistance
coefficient of a ship (or a model) is the same as that of a smooth
flat plate with the same length (Re) & wetted surface area;
namely, the friction resistance of a ship is the same as that of a
flat plate with the same length and wetted surface area. In
generally, this assumption is approximately correct. However, a
careful investigation has shown that there are differences between
the friction resistance of a ship and that of a plate with the same
length & wetted surface. Usually, the friction resistance of a
curved surface object is greater than that of a flat plate with
same length & wetted surface. Their difference is called the
form resistance or form drag.
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The form drag consists of 3 parts.Eddy-making Resistance; the
curvature causes the pressure change along the ship. Due to the
viscosity, the pressure change will cause the flow separation from
the surface, & generate eddies. Energy is fed into eddies, and
the resulting resistance is called eddy-making resistance. Main
contribution to the form resistance is made by eddy-making
resistance. For a low speed ship, it is important to avoid the
abrupt change of the hull in order to minimize the eddy-making
resistance.
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2. The curvature of a ship (or a model) will change the local
velocity along the ship. Since the path along a streamline from bow
to stern is longer on a shaped body than on a flat plate, the
average velocity along a ship > V. Thus
3 Interaction between viscous & wave-making resistances,
which is very complicated. It is a research topic in Marine
Hydrodynamic and ship-model test. The increase or decrease of
resistance due to the interaction are classified into form drag.
Sometimes, some items may be directly classified into wave-making
resistance.
It is understood now that why the difference between the total
resistance coeff. & frictional resistance coeff. is called the
residual coefficient,
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5.8 Air or Wind Resistance
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5.9 Appendage ResistanceUsually, the model resistance test gives
the resistance of the naked hull (without appendages). Appendages,
such as bilge keels, rudder and bossings (open shafts and struts),
will result in additional resistance, aka appendage resistance.It
is usually added to the naked hull resistance, about 10 15% of the
latter as listed in the following table. Appendage resistance of a
multiple-screw (propeller) ship is larger that that of a
single-screw ship. The upper limit for V/(L0.5)= 0.7 seems to be
higher. Ship typeSpeed/length ratio0.701.01.6Large fast
quadruple-screw ships10-16%10-16%Small fast twin-screw
ships20-30%17-15%10-15%Small medium V twin-screw
ships12-30%10-23%Large medium V twin-screw ships8-14%8-14%All
single-screw ships2-5%2-5%
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5.10 Computing the naked hull resistance according to its model
test results The model resistance test follows the Froude #
similarity.
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Ex. 1 Computation of Resistance & EHP
Ship Dimensions 390 x 54 x 23 (LWL x B x T)CB = 0.69, VS = 12
knots, SS = 29,621 ft2 , sail is S.W. Its model Lm = 15 , sail in
F.W. t = 67.5 F, Rtm = 4.4 lb at corresponding velocity, find Rts ,
& EHP.
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Ex. 9.1 Computation of Resistance & EHP (see textbook
p160-161)Ship Dimensions 140 x 19 x 8.5 m (LWL x B x T)CB = 0.65,
VS = 15 knots, SS = 3,300 m2 , sail is S.W. Its model Lm = 4.9 m ,
measured , sail in F.W. at corresponding velocity of VS . Find RTS
and EHP at VS = 15 knots
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Problems of predicting the resistance of ships based on model
tests (Summary)
It is assumed that the frictional resistance coeff. of a ship
(or model) is equal to that of a flat plate at the same Re #.
However, there is difference between the friction resistance of a
ship (curved surface) & the friction resistance of a flat plate
is form resistance as described in section 5.7. CR = CT CF ,
includes wave-making & form resistances, not only wave
resistance. That is why CR is called residue resistance
coefficient. It is noted that a model test follows the Froude
similarity. The form drag depends on viscosity or Re # and does not
obey the Froude Law. Therefore CRS is not exactly equal to CRm
.
These problems result in errors in determining ship resistance
from its model test.
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5.11 Methods of Presenting Model Resistance ResultsIt is
desirable that there is a standard method of presenting model
resistance data. However, so far it has not been reached.
Users want the original data. (speed, resistance, water
temperature, method of turbulence stimulation, cross sectional
area) The user can convert them to any desired form.
The data in the past were not presented in non-dimensional
form.
Introduced the following are a few methods commonly used in
presenting Model Resistance data.
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CT ~ Re or CT ~ Fr
circle K & circle C system, they are non-dimensional
.
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At a low speed, , is almost independent of . When increase in
speed, , increases with Dimensional Form of circle C & circle
K
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5.12 Relation between Hull Form & Resistance Choice of Ship
Dimensions p165-169The owner usually specifies that the new ship
shall carry a certain deadweight (How much cargo can be loaded) at
a particular speed, and the designer estimates the probable
displacement and principle dimensions. Displacement = cargo weight
(dead weight) + self weight Length Cost, scantling, manning,
docking, navigations. longer L reduces wave-making resistance at
high speed. Draft increase draft will decrease resistance, reduces
scantling, but is restricted by the water depth of harbor or
channel & stability.Breadth important to have adequate
stability. Increase in B may decrease L (smaller Fr, smaller wetted
surface) thus reduces the cost but results in the increase in
wave-making resistance. Also is limited by the width of canals.
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Choice of Form CoefficientsThe most important form coefficient
may be the block coeff., or prismatic coeff. A larger CB, results
in larger wave-making &form resistance.Block or prismatic
coeff. should be reduced as the speed of a ship increases so that
in designing a ship there is a limit of fullness to be observed for
a given speed. A formula of the type, called the economical block
coefficient has often been used.
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Definition of trial, service, & sustained speed
Before an owner receives a newly built or renovated ship, there
is a trail sail for the ship. Trial speed is the required speed
when the newly built ship takes a trial sail. Service speed is the
required speed for the ship is service. Usually a service speed is
smaller than the trial speed.Sustained speed lies very close to
that at which the resistance coeff. curve begins to rise steeply;
i.e., to the speed at which the power begins to increase rapidly
than V3.
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5.13 Series Experiments & Model Resistance Data Sheets
Series ExperimentsA series of models is a set of models in which
the principal characteristics are changed in a systematic manner.
The purposes of having resistance test of a series of models
are:
A series of tests can be made to ascertain the best form of the
ship to give minimum resistance & this would involve tests run
with various alterations to some basic form.
The data from the tests of series models can be used to estimate
the resistance & EHP of a ship
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Well-known series models:
Taylors Standard Series: starting from a single parent ship
Series 64. For naval ship.
Series 60. Began 1948 with ATTC Cooperation and is published in
1963, (TMP Report 1712).
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Model Resistance Data Sheet, SNAME.
This valuable sheet was issued by SNAME Project 2 of
Hydrodynamics Sub-Committee of SNAME. Model and Expanded Resistance
Data Sheets, available from Society.
About 200 ships, their model test results were obtained in
various towing tanks and all types of ships were included, which is
different from the Series Experiment. The sheet gives: 1.) all
principal form coeff., 2.) basic model data 3.) results are
presented in
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Estimation of EHP from Series Resistance Results
The series forms a very suitable basis for making estimate of
power (EHP), particularly in the early stage of a design (concept
design).
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It is important to use the units of variables consistently.