Waterjet-Hull Interaction
Author van Terwisga, Tom
Title Waterjet-Hull Interaction
Conference/Journal PhD-thesis, Delft University of Technology
Month April
Year 1996

The main objective of this work is to develop and validate tools for the analysis
of interaction effects in the powering characteristics of jet propelled vessels.
Despite our knowledge about the hull and the waterjet in isolated conditions, a lack
in knowledge with regard to the interference between hull system and jet system
seems to exist. Many discrepancies between computed and actually measured
power-speed relation of the prototype vessel are ascribed to interaction. Little
knowledge is available on the mechanisms and the magnitude of these effects
Misunderstandings in the field of jet propulsion are believed to often originate
from a lack of clear definitions of concepts. It is demonstrated in Chapter 1 that
a great deal of confusion can be found in the existing literature on definition and
description of jet-hull interaction. Hence, this work starts with a theoretical model
describing the complete waterjet-hull interaction. The effect of interaction on the
hull is expressed in a hull resistance increment. The effect of interaction on the jet
performance is expressed in a thrust deduction and so-called momentum and
energy interaction efficiencies. The latter efficiencies account for the change in
ingested momentum and energy flux due to the presence of the hull.
Although a rough procedure for model propulsion tests was provided by the ITTC
in 1987, this procedure was found to easily lead to large systematic errors,
rendering the results of the tests doubtful. In addition, the data reduction procedure
was based on an incomplete theoretical model. An improved experimental
procedure based on thrust calibration through bollard pull tests is developed,
together with a data reduction procedure that allows for quantification of the
interaction parameters.

Detailed computations and LDV measurements were made on the flow in the
intake and aftbody region. They give insight into the validity of assumptions made
in the experimental data reduction procedure. They show that a rectangular cross
section of the imaginary streamtube upstream of the intake with an effective width
of 1.3 times the geometric width, provides an adequate representation of the
ingested flow. They also indicate that the jet system's thrust deduction fraction is
not negligible in the speed range where the transom clears.
Computations were conducted with a potential flow code and a Savitsky method,
aimed at a direct computation of interaction. These computations did not show a
satisfactory agreement with the experimental results. An empirical prediction
model based on test results is recommended for preliminary power-speed
The present work provides a consistent set of definitions for a complete description
of both the powering characteristics of the isolated hull system and jet system, and
their interaction. An experimental procedure with a lower uncertainty level than
hitherto published in the open literature is proposed for their quantification. The
results of this work are hoped to contribute to a wider acceptation of the waterjet
system and to smoother contractual negotiations, as the final performance of the
hull-jet system is better predictable.

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