Background Information
Weight
Classification Systems
On a vessel, in accordance
with Archimedes Principle the weight of all the
components, crew, effects, and other loads is equal to the weight of
the volume of water that the vessel displaces. In order to be
able track weight growth, as well as assist in cost estimating, and
predicting the estimated weights for new designs, all the individual
components on a ship are categorized into groups based on the function
of that component. In the US for modern naval vessels, a system
called the Extended Ship Work Breakdown Structure (ESWBS)
is currently used. ESWBS is an extension of the
previously used Ship Work Breakdown Structure (SWBS)
and for what I am considering here they are pretty much the same.
Prior to the mid 1960s (or
so) though, a different system, called the Bureau of Ships
Consolidated Index (BSCI) was used.
Although this system is in general similar to the SWBS/ESWBS
system, there are some differences between how some specific items are
categorized.
Additionally, overseas
there are other, nationally based systems, including a system used in
the UK based on their Naval Engineering Standards (NES).
As with the BSCI system this system is in general
similar to SWBS/ESWBS, however here are some differences
between how some weights are categorized.
For the most part I
believe that most of the data I have collected is based on either the SWBS/ESWBS
or the BSCI system (depending on the date that the
vessels were designed/built) however, I am pretty certain that a couple
of the data points I have collected are in the UK NES
system. At some point in time I hope to go back and clarify which
data points are based on which system, but for now I believe that there
is currently a mixture of the different systems represented.
Overall, all three systems
tend to break ship weights into the following categories:
 Group 100
 Structural weights
 Group 200
 Propulsion system weights
 Group 300
 Electrical system weights
 Group 400
 Command, Communications, Computers, Controls, Intelligence,
Surveillance, & Radars (C4ISR)
 Group 500
 Auxiliary systems
 Group 600
 Outfit & Furnishings
 Group 700
 Weapon systems
 Group F 
Loads (which is identified as Group 800 in the NES system  I believe)
 Group M 
Margins
In general, groups 100 to
700 add up to give the vessel's basic light ship weight,
though for early stage designs a number of margins (from Group M) are
often added to address the uncertainty in early stage design numbers.
Also, if ballast is required it is often added in as well.
Light ship
weight plus loads (such as crew & their
effects, fuel, stores, and munitions, etc) add up to give the vessel's full
load displacement.
Finally, since it is
realized that over the life of a vessel it will probably grow in
weight, due to added systems, ship alterations, and other various
additions, typically a service life allowance is also
included to reflect the allowable expected growth of the vessel.
As such, the full load displacement of the vessel when
new (at delivery) plus its service life allowance would
be equal to the vessel's full load displacement at the end of
its service life. I believe that for nonUS vessels some
of the terminology will be different (ie that may not call all the
weights, margins, and allowances exactly the same) but I believe, in
general, the overall concepts are similar.
Resistance &
Powering
Resistance
Resistance
Components  As a ship moves through the water it
creates a drag force, or resistance. In general this resistance
consists of several major parts, and some additional minor
components. Specifically, there is;
 the frictional
resistance generated by the contact of the hull's surface with the
water,
 a wavemaking
resistance generated by the hull as it pushes the water aside as it
passes,
 a wind resistance
caused by the above water portion of the vessel as it interacts with
the air,
 the resistance of the
appendages on the hull below the water, including such things as
rudders, propeller shafts and brackets, as well as potentially other
items like bilge keels, sonar domes, stabilizers, bow bulbs, and stern
flaps,
 added resistance
in waves, and
 other items such
as wave breaking resistance, spray resistance, and an added resistance
caused by the control surfaces (such as the rudders) as the ship makes
minor steering c orrections in an effort to maintain its course and
added resistance due to hull roughness and fouling.
For simplicity, it is not
uncommon to combine some of these components together.
Specifically, wavemaking, wave breaking, and spray resistance (where
applicable) are often rolled into a single category called "Residuary"
resistance. In order to estimate the resistance of a new design it is
necessary to make a reasonable estimate of the components of resistance.
Resistance Estimating Techniques
 If data exists on a similar ship it is sometimes possible to use that
data for estimating the resistance of a new design. However, if
the new design is significantly different from the existing design this
may not be fully adequate. Additionally, over the years work has
been ongoing in trying to develop mathematical means of estimating a
ship's resistance by modeling the ship's hull in a computer and using
Computational Fluid Dynamics (CFD) methods. However, this is
somewhat time consuming and not well suited to early stage estimation,
where full details of a ship's hullform may not yet be fully worked
out. Similarly, it is possible to build a scale model of a design
and measure its resistance in a tow tank, however, this is also costly
and time consuming and typically is not done till later in a design
when more details on its hullform have been set. As such, several
method's have been developed over the years to estimate a ship's
resistance either from data on other similar ships and/or concepts that
have been developed previously or through the use of model test data on
Systematic Series of similar notional hullforms.
Systematic Series
 In general a Systematic Series is a series of similar hullforms which
have been developed and model tested. These models typically have
certain major characteristics varied over the family of hulls, such as
block coefficient, LCB location, and L/B ratio, etc. The model
test resistance data is collected and analyzed and presented in such a
way as to allow a user to interpolate an estimate of the resistance of
his vessel, at its given block coefficient, LCB location, and L/B
ratio, etc from the range of data provided. Some of the more
common typical Systematic Series that have been developed include;
 The Taylor Standard
Series 
 Series 60 
Methodical Experiments With Models Of SingleScrew Merchant Ships
 Series 62 
 Series 64 
 Series 65 
 The BSRA Series 
 The SSPA Series 
 The National Physical
Laboratory (NPL) Series 
 The Marwood and
Bailey 
 The Webb Trawler
Series 
 The NTUA Series 
 The Naval Acadmey
Hydrodynamics Lab Series 
 etc
One
potential problem with using Systematic Series
though, is that they are only useful over a limited range of vessel
parameters investigated and may not cover the hullform shape or other
hullform parameters of the design that you are interested in. In
order to account for this it is sometimes possible to combine the use a
Systematic Series with data on existing vessels. I believe that
in the US Navy a method like this was used for many years, where
restance data on existing vessels or designs were compared to the
estimated resistance for those vessels as determined by the Taylor
Standard Series . The ratio of actual resistance to
estimated resistance was then plotted over a range of speeds to develop
a "Worm Curve" factor for that vessel. If you then
had a ship some what similar to an existing design, but for which some
parameter, or group of parameters, were different (such as block
coefficient or LCB, etc) you could then use the Systematic Series
to make a preliminary estimate of resistance but then multiply those
results by the Worm Curve factor for the similar
ship to adjust the results to account for differences between your
design and the hullform of the vessels in the Systematic Series.
Mathematical Regression Methods
 In more recent years alot of effort has been made into using
mathematical regression methods to condense this data down into
relatively simple mathematical terms to ease in the estimation of early
stage resistance estimation. These methods easily lend themselves to
use in spreadsheets or simple BASIC or FORTRAN programs.
Additionally, more recently some individuals and research
establishments have also made an effort at combining the data from
several Systematic Series together sometimes including
data for other specific designs or vessels for which model test or full
scale data was available. One of the most well known of this type
approach is the Netherlands Shipbuidling Model Basin's (NSMB)
mathematical method, which is also known by the name of its two
principal authors Holtrop & Mennen.
The NSMB/Holtrop & Mennen method is very powerful
and useful because it incorporates data from a wide range of hullform
types, but it is not always clear whether in doing so, if it is as
accurate for a given hull type as a method that only addresses that
specific hull type. For this reason, on this site right now I have
decided to make use of data developed by Sui Fung specifically for
transom hull ships representative of most typical modern frigate and
destroyer type hulls, instead.
Resistance Scaling
 Because many resistance estimating routines are based in part on
model tests, it is important to understand how model test resistance is
scaled when trying to estimate the full scale resistance of a ship.
For starters, it was discovered long ago that the wave pattern that a
ship generates as it moves through the water is impacted greatly by the
speed that the vessel is going in relation to the length of the
vessel. If a vessel is moving at a speed where the the waves
generated result in the bow and stern resting on peaks of the waves
then the ship will be at near even trim which is good for minimizing
resistance. However, if a vessel is moving at a speed where the
bow is at a peak but the stern is in the trough of a wave then the ship
will be operating at a greater than normal trim and the ship's
resistance will be adversely impacted. As such, the humps and
hollows in the waves generated by the ship as it moves through the
water can result in reltive increases and decreases or "humps"
and "hollows" in the ship's resistance curve,
as shown below.
Here the
Red curve shows the
estimated total resistance while the two
major components of total resistance, the residuary
and frictional resistance are shown in Blue and Orange
respectively. As can be seen in this figure the frictional
resistance increases smoothly as speed increases, but there are humps
(as denoted by the Purple
arrows) and hollows (as denoted by the Green
arrows) in the residuary resistance curves, which also are
apparent in the overall total resistance curve.
The term Froude Number (Fn)
has been defined as a means of reporting a ship's speed in relation to
its length in a nondimensional fashion. Specifically;
Fn = v /
sqrt (g * L)
Where;
v =
the vessel's speed
L = the vessels length
g = the acceleration due to
gravity
(all in consistant units)
In general the wave pattern generated by a
model at a given Froude number will be the same as the
wave pattern generated by the full scale ship at that same Froude
number. As such, wavemaking resistance is said to follow Froude
scaling rules. Additionally Model Scale Ratio is
defined as the ratio of the length of the fullscale ship in comparison
to the length of the model and is sometimes called l.
A s such the actual speed that a model will be operating at to give the
same Froude number as a fullscale ship will be equal to
the speed of the fullscale ship divided by the square root of the Model
Scale Ratio (eg Vm = Vs / sqrt(l)).
However, because of the viscous properties of water it is not possible
to scale all of a ship's resistance directly from a scale model, as the
frictional components of resistance do not follow the same Froude
scaling rules. For frictional resistance a different
nondimensionalization of the vessel's speed is more significant.
This is called the Reynold's number (Rn)
and is is defined as;
Rn = v L / n
Where;
v =
the vessel's speed
L = the vessel's length
n =
the kinematic visosity of water
(all in consistant units)
Based on the size and wetted surface of the model, an estimate of the
frictional (or in some cases viscous) resistance of the model is made
based on a standard formulation. In the US many early model tests
and systematic series used a formulation put forward by the American
Towing Tank Conference (ATTC) based on
observations of measured frictional resistance of flat plates.
Here (I believe) an equation for a nondimensional frictional
resistance coefficient (Cf) was derived as;
0.242*sqrt(Cf) = log10 (Rn *Cf)
Because the frictional resistance of a vessel
is a function of the vessel's Reynold's number,
Frictional Resistance is said to follow Reynold's number scaling
and if a model were run at a the same Reynold's number
as the full scale ship then they would have the same
nondimensionalized Frictional Resistance coefficients, however the Froude
number's would be different and hence the wave patterns
developed by the model and full scale ship would be different, which is
why the Frictional resistance and residuary resistance components are
treated separately.
Later in 1957 the International Towing Tank Conference (ITTC)
agreed on a newer formulation for calculating a Frictional Resistance
coefficient as follows;
Cf = 0.075/(log10 (Rn)  2)^{2}
I believe that is this Frictional Resistance Coefficient that is used
by most (though not all) of the Systematic Series
identified above.
More recently the ITTC put forward a revised methodology
in 1978 where, instead of just considering frictional resistance based
on data for flat plates, an effort was made to account for the form of
the ship, as well. In this 1978 formulation a vessel's viscous
drag was defined as being equal to the the vessel's frictional
resistance times a (1+k) term to account for the impact
of hull curvature and form. I believe that the NSMB/Holtrop
& Mennen equations are based on this newer methodology.
As such, when scaling model test data for conventional displacement
hulls it is typical to measure the total resistance and convert it into
terms of a nondimensional frictional resistance coefficient. For
each data point collected the calculated frictional resistance
coefficient at that speed is then subtracted from the total resistance
coefficient giving a "residuary" resistance coefficient which more or
less includes all the other factors like wave making, wave breaking,
and wave spray (if significant), etc. This is the portion of the
model test data that gets scaled to full scale by Froude scaling.
An estimate of the frictional resistance of the fullscale ship is then
added onto the residuary resistance estimate (using a similar method as
used for estimating the frictional resistance of the model), and then
additions are also made for any other additional components (such as
wind and appendage resistance).
Because
most models do not include all of the "above water" components of a
ship, the wind resistance of a ship is something that is typically
added in later based on approximation equations or if available wind
tunnel data for the ship. However, depending on the towing tank
and their standard procedures, sometimes there may be a correction to
the modelscale data to account for the wind resistance of the limited
portion of the above water hull that was included in the model tests.
Additionally, sometimes models are tested
with small scale appendages (especially for fixed components, like
bulbs etc). However, since of scaling issues with items like
rudders, fins, and bilge keels, etc, it is not uncommon that these are
not included on the model (for resistance estimating) and that an
approximation of their effects are considered later based on equations
derived for the specific component type or simply by adding in an
allowance based on previous experience.
Finally, typically an additional Correlation Allowance,
based on the size of the full scale ship and its likely surface
roughness, is typically also added in.
Appendage Drag
 There are several methods of estimating appendage drag for a
vessel. The book "Principles of Naval Architecture" (Ref B1)
gives information on a couple of these methods.
Specifically, the book
provides several equations relating the resistance of certain specific
appendage types to the geometry of these items. This includes
equations specifically for;
 Bilge Keels
 Control Surfaces
(such as rudders, shaft brackets, and stabilizer fins, etc)
 Shafts &
Bossings, and
 Skegs
An alternate method to
using these type of shape specific equations is instead to estimate the
area of the appendages and multiply the area for each by an Effective
Form Factor of the appendages (k_{2}). From
this a total Effective Form Factor for the ship is calculated
taking into the account the surface area and k_{2} of
each of the appendages using the ITTC 1978 viscous resistance
curves. This type method is incorporated into the NSMB/H&M
resistance estimation equations.
A third method is one
outlined in the US Navy's Design Data Sheet 0511 (DDS 0511) entitled
"Prediction of SmoothWater Powering Performance for
SurfaceDisplacement Ships" (Ref B2). In this method, curve fits
through data on exisitng vessels are provided to allow the user to make
an initial estimate of appendage r esistance for a given ship.
For early stage desgin,
the first two type methods noted above can be a little cumbersome in
that they require the user to estimate the size of all the appendages.
As such, for early stage design I have made use of the method outline
in DDS 0511.
Wind
Resistance & Still Air Drag  As a ship moves through the
water it encounters resistance to this motion not only from the water
it is moving through, but also the air that the portion of the ship
above the waterline comes in contact with. This air resistance can be
considered in three ways;
 you can consider
only the still air drag generated solely from the ship's forward motion
(condition 1),
 you can consider
the still air drag generated from the ship's forward motion plus a
certain amount of head wind acting on the front of the vessel
(condition 2), or
 you can consider the
total air resistance of the ship taking into account any existing wind
and the relative motion of the ship (condition 3).
In the first case, still
air drag is the resistance that is generated by the ship assuming that
there is no wind blowing other than the wind the ship is generating
itself as it moves forward. Thus in this case the speed of the
self generated wind is equal to the speed of the ship, and it can be
considered acting only on the frontal area of the ship.
In the second case you
simply add a set amount of wind speed to the ship's speed, and assume
that this acts on the frontal area of the vessel. The final case
however, is more complex and takes into account the relative direction
of the wind to the ship as the ship moves through the water and acts on
both the frontal and side areas of the ship. It would be
important to calculate for estimating actual vessel performance, but
for design purposes, either assuming only still air drag, or still air
drag plus a certain amount of head wind is (I believe) most typical.
The book "Principles of
Naval Architecture" (Ref B1) gives several different methods for
estimating Wind Resistance & Still Air Drag.
These are typically of the form;
Raa = coefficient * ½ r * A_{T} V^{2}
Where;
 Raa = The
total added air resistance
 coefficient
= an empirically derived coefficient
 r
= the density of the air
 A_{T}
= the frontal area of the ship above the waterline
 V = the
total wind velocity (for condition 1 this would be equal to the forward
speed of the ship, but in condition 2 this would include both the speed
of the ship and any additional head wind)
Ref B1 notes that in 1943
RADM D.W. Taylor derived a simplifaction to the above equation for
ordinary ships where;
Raa = 0.783 * ½ * B^{2} V_{R}^{2}
Where;
 Raa = The
total added air resistance
 B = the
Beam of the ship (in meters)
 V_{R}
= the apparent relative wind velocity (for condition 1 this would be
equal to the forward speed of the ship, but in condition 2 this would
include both the speed of the ship and any additional head wind) (in
meters per second)
This version of the
equation is convenient for early stage design use as it doesn't require
an estimate of frontal area, which may not be fully known in early
stage design.
Powering
Once the resistance of a ship is estimated it is then necessary to make
an estimate of the powering requirements for the ship. In very
basic terms if you multiply the resistance of a vessel times its speed
(and make any necessary corrections for units) you get what is called
the vessel's Effective Power requirement (or EHP
for Effective Horsepower). If there were no efficiency losses or
need for margins an engine capable of producing an amount of power
equal to a ship's EHP would be able to propel the ship
at the speed for which the EHP was calculated. However,
there are many different efficiencies and margins that must be
considered which drive the total installed power requirement for a
vessel up to a value sometimes approaching twice the value of the
calculated EHP at design speed.
Hull Form
Effects
One of the first things that must be considered in determining full
power requirements is the effect of the hull on the water around
it. In addition to the drag already considered in the resistance
estimate, there are other factors that must be considered.
Amongst these are three terms called;
 wake fraction
 thrust deduction, and
 relative rotative efficiency
Wake Fraction
 as a ship moves through the water, the viscosity of the water will
result in a layer of the water near the vessel being dragged more or
less along with the ship. This results in the flow into a ship's
propellers being overall typically a little less than the speed that
the ship is traveling at, and depending on the level of detail that you
are going into for your resistance and powering estimates it may become
necessary to try and estimate this. For early stage design this
is trypically done using curve fits or rules of thumb based on data on
similar ships.
Thrust Deduction 
similar to wake fraction, there is also a term called the thrust
deduction factor. In simple terms, the total thrust that a
propeller (or group of propellers) must produce to propel a ship tends
to be a little more than the total resistance of the ship. This
differnce is sometimes called an augment of resistance or reduction in
thrust available at the propeller. According to the book
"Resistance and Propulsion of Ships" (Ref B3) it is caused by a number
of factors including the propeller accelerating the water into the
stern which can cause an increase in frictional resistance, influences
of the potentialvelocity field in which the propeller operates and
possible influences that the propeller may have on the stern wave
pattern of the ship. For our purposes its enough just to know
that the total thrust required to be produced by the propellers is
going to be a little more than the estimated resistance, and like the
wake fraction, in early stage design it is usually estimated by means
of using curve fits or rules of thumb based on data on similar ships.
Relative
Rotative Efficiency 
Propeller Efficiency 
Shaft
and Mechanical Efficiency 
