Speed at Sea - Application to Ship Design
The
OPTIONS
Design Exploration System has been applied to the design of
low resistance ship hullforms.
The designs are based on typical naval frigates
and were produced using gradient descent optimization methods and a statistically based
hullform resistance code.
UK RN Type 22 Batch 2 Frigate at Speed
Naval frigates are required to operate at high speeds
while remaining efficient at all times. Designers are therefore very
concerned with the hullform resistance of competing designs.
Simplified Hull-form Model
A simplified representation of each candidate hullform must be produced
for analysis using the resistance model. The analysis is based on a
statistical interpretation of a large data-base of results from previous
tests on model ships.
Variation of Resistance with Speed
Hullform resistance rises rapidly with speed and it is clear that
at high speeds efficient shapes must be chosen to limit the engine power
needed in a design.
Variation of Resistance with Shape
The hullform of a ship is a highly complex shape but its principal dimensions
may be varied by distorting the basic form. This can be achieved by varying the beam
to draught ratio (B/T) and length to displacement ratio (Mcirc).
Changes in these quantities have a profound affect on the resistance of
a ship at a given speed.
Ship Design System
Ship concept design must, however, consider many more aspects than just
hullform resistance.
In the work presented here,
account is taken of the required internal space for the crew and stores,
fuel capacity, roll stability, strength, etc.
All these aspects can have knock on effects on resistance,
given that the designer
must produce a concept capable of deploying the required payload and crew
with specified fuel range and stores endurance.
This leads to each resistance evaluation being underpinned by an extended
sequence of related computations.
Optimization of Hullform
Application of a gradient descent optimizer from within the OPTIONS package
allows the hullform to be rapidly modified to achieve minimum resistance
at 30 knots. Here, an active constraint is placed on the design by the
need for adequate hull girder strength, which effectively limits the length
to hull depth ratio of the design.
Optimized Design
The final, optimized design is considerably longer and thinner than the base ship with
a resistance that has nearly halved compared to the starting point, even
though the overall displacement has increased.
Rather shorter and fatter designs result if the maximum design speed is
reduced, since at such speeds wavemaking resistance is considerably lessened and the need
for such a fine form diminishes.
Full references for papers describing this work and a number of related subjects may be found
on the home page of Prof. A.J. Keane (http://www.soton.ac.uk/~ajk/welcome.html ).
This article may be found at
http://www.soton.ac.uk/~ajk/ship/welcome.html
