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The prediction o ship resistance

can be accomplished in a variety

o ways, rom statistical parametric

algorithms to computational CFD methods

to model testing. Te choice o which is

best to use or a particular task depends on

the nature o the task and the benefit-to-

cost measure o the method.

Parametric methods are the technique

o choice or analysis o entire physical

systems, but their data set does not ully

describe the shape o the hull. Model

testing and CFD rely on ull hull surace

geometry, o course, but at significantly

higher inancial cost with specialised

computational and man-power resources.

his cost is without question justiied

or many projects, but or some tasks,

computational methods using longitudinal

distributions o hull data can offer very

high benefit-to-cost and be successully

used or the prediction o ship resistance.

Tese techniques can take many orms and

go by different names, such as thin-ship

theory or linear wave-making, but all aresome orm o wave theory analysis.

Familiar names such as Havelock, Mauro

and Michell are associated with these

techniques. HydroComp has developed

its latest resistance prediction code based

on comparable wave theory analysis but,

with a unique twist the company says.

Unlike other methods, HydroComps

Prismatic Wave Drag uses distribution o

the sectional area (leading to the name

prismatic) rather than waterline cuts o

the hull surace. In other words, it treats

the distribution o the immersed volume

rather than the wetted surace. According

to Donald MacPherson, HydroComps

technical director, using immersed volume

as the basis or this new code is appropriate

or two reasons.

We eel that definition o the hull by

surace points is not in keeping with the

order o a problem that can be reliably

solved with these types o wave theory

analyses. Small changes in the surace do

not dependably show a corresponding

eect in resistance except on a macro

scale. Further, one o the important

characteristics with all o our products

is consistent behaviour, and analysis by

sectional area gives users that expected

HydroComp usability.Te detailed hull description that comes

with surace offsets is cancelled by the

practical matter o how irregular changes

in offsets, or things like tunnel thrusters

or propeller pockets, cause signiicant

inaccuracies in the calculations. Te only

response available is to alter the geometry

so that the data or the calculation is no

longer the real geometric representation

o the hull.

MacPherson also explained that

prediction o wave-making drag alone is

insufficient or a complete ship resistance

prediction. Accurate prediction o

wave-making drag is significant only as

a component o the correct prediction

o total resistance. We have engaged in

extensive in-house R&D or the viscous

components o rictional drag and orm

actor, and also into the correlation

unctions that are needed to make our

Prismatic Wave Drag analysis reliable or

a very broad range o non-planing vessels.

We are very pleased with the results o our

validation studies, and are excited about

putting this new capability in the hands o

our customers.

A variety o code-specific correlation

unctions have been developed, including

consideration o the influence o viscouseffects (such as an effective body length

to better model flow line convergence),

boundary layer thickness, transom

immersion, dynamic trim, and low-speed

pressure drag. A proprietary algorithm

was developed to estimate the proper

suppression o transverse wave amplitude.

he underlying viscous prediction

employs the Grigson CF riction line

with Reynolds number based on effective

viscous length. Form actor is predicted

using a proprietary algorithm that is

specific to the Grigson CF oundation.

Enabling the correlation unctions is by

selecting a HydroComp-defined Standard

set or individually as a Custom definition.

he Prismatic Wave Drag module is

one o a collection o latest modules that

will be available as part o HydroComps

new Premium license. Upgrading to the

Premium license will allow users to access

scripting capability so the Prismatic Wave

Drag calculation, which can be shelled as

a solver rom third-party sofware such as

CFD or simulation tools. NA

Feature 1 |CFD & HYDRODYNAMICS

The Naval Architect July/August 2014

HydroComp has developed its latest Prismatic Wave Drag module or the

prediction o ship resistance

Cutting the drag

The Prismatic Wave Drag module will allow users to have detailed hull descriptions for

tunnel thrusters and propeller pockets