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Contributors (In alphabetical order) -
Robert D. Horsefield, P.E.,
Chief Naval Architect (Editor)
Jonathan G. Parrott, P.E., Director of Engineering
Craig A. Pomeroy, P.E., Senior Naval architect
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In today's economic climate, many vessel operators
need to optimize vessel speed and powering characteristics. The benefits
can be measured in voyage days saved, fuel saved, and increases in the number
of voyages possible. There are three main ways to have a vessel's speed-power
characteristics determined:
Computer Modeling by Parametric Analysis
Computer Modeling by Computational Fluid Dynamics
Model Testing
None of these methods are prohibitively expensive,
and when compared with the possible economic benefits the cost may be considered
minor. All of the methods described above may be used in the following ways:
Optimization of the hull form
Evaluation of proposed hull modifications
Optimization of propeller design
We had a recent occasion to study the effect of
modifications to an existing hull in a model basin. In this instance the
testing showed insignificant improvement, and the owner decided not to perform
the hull modifications. Without the study, the owner might have proceeded
with an expensive modification that had minimal economic benefit. By having
the foresight to study first instead of build first, the owner didn't invest
in a modification with an extended payback period.
This method of determining speed & powering
characteristics takes hull characteristics as input, and calculates hull
resistance & powering performance. Since this method is based on statistical
manipulation of a large number of model tests and sea trials, it can be reasonably
accurate if the hull form being analyzed falls within the scope of the database.
Accuracy of the results falls when hull characteristics are outside of this
scope.
Parametric Analysis is less expensive than other
evaluation techniques, but the results are not as accurate either. Your Naval
Architect can help you determine if Parametric Analysis is adequate for your
needs. It is not unusual to first study a hull in the computer using Parametric
Analysis to get the basic shape right, then use other prediction methods
to validate the design.
The accuracy of Parametric Analysis can be greatly
improved by aligning the results with hard data, such as model tests or sea
trial data. This means comparing a similar vessel's known speed-power characteristics
with its calculated speed-power characteristics, and applying the corrections
to the calculated speed-power characteristics of the vessel of interest.
For evaluating proposed modifications to existing
vessels, the accuracy can be improved by first comparing the calculated speed-power
characteristics of the unaltered vessel with existing sea trials data. The
corrections thus generated can then be applied to the calculated speed-power
characteristics of the altered vessel.
Computer Modeling by Parametric Analysis is not
particularly useful for evaluating subtle hull changes. For example, some
hull designs have particularly sharp "shoulders" (the transition between
the bow and the midbody), sharp chines, and immersed transoms. Parametric
Analysis could significantly underestimate the benefits of altering these
configurations. For such cases, your Naval Architect may recommend other
methods instead.
III.
Computer Modeling by Computational Fluid Dynamics (CFD)
In basic terms, this method models a vessel's
surfaces and its water environment in a computer to calculate speed & powering
characteristics. Both accuracy and cost for this analysis fall between the
Computer Modeling by Parametric Analysis and Model Testing.
This type of analysis is typically performed by
specialized firms. If this method is right for your needs, your Naval Architect
will liaison between you and the CFD provider to ensure that your needs are
met, and present the results in an understandable format.
Computer Modeling by Computational Fluid Dynamics,
unlike the Parametric Analysis described above, is quite good at evaluating
subtle changes. For example, this method could be quite effective in determining
the optimum shape for a bulbous bow. A bulb can be altered in the computer
until an optimum shape is determined. Doing the same thing in Model Testing
would mean physical construction and testing of a model with several different
bulbs, a far slower and more expensive process.
This method utilizes a scale model of a vessel,
which is attached to specialized measuring equipment and dragged down a water
channel. The resistance of the model is then scaled up to full vessel size.
This method produces the most accurate results possible short of sea trials,
and is also typically the most expensive. Larger models will produce more
accurate results, with the maximum size limited by the model tank's capability.
The photo show below shows a typical resistance test. Model tanks can also
perform self-propelled model tests and seakeeping tests.
This type of analysis is typically performed by
specialized firms. If this method is right for your needs, your Naval Architect
will liaison between you and the model tank to ensure that your needs are
met, and present the results in an understandable format.
Subtle changes in hull form can be modeled by
means of interchangeable components, such as bulbous bows. If a large number
of such changes need to be analyzed, consider using Computer Modeling by
Computational Fluid Dynamics instead.
