Page 44 - Theory and Design of Air Cushion Craft

28 Introduction to hovercraft

1.27) were procured under a design competition and completed in 1971. The speeds
achieved by each craft were 70 knots and 90.3 knots, respectively. A ship-to-ship
guided missile was successfully launched from the SES 100B, and hit its target (Fig.
1.27), as part of the trials. Based on this success the US Navy proposed the 3K-SES
in 1974. It was planned to construct an air cushion guided missile destroyer weighing
3000 tonnes and with a speed of 80 knots. Further, a mini aircraft carrier would be
completed on the basis of the 3K-SES. A design competition was held between Bell
Aerospace and Rohr Marine Industries, won by Rohr. In order to complete this devel-
opment, new work shops, facilities for testing high speed water jet propulsion systems,
lift fans, skirts etc. and new carrier borne weapon systems would be formed in Rohr
Marine Industrial Corporation on the west coast of America. The plan was techni-
cally demanding, and the SES was power intensive, to reach the 80 knot goal.
In 1974, the fuel crisis hit the Western world. Policy changed overnight to one of
extreme energy consciousness, so that the 100 knot Navy appeared the wrong direction
to be developing. The 3K-SES programme was therefore cancelled. It was only in the
mid 1990s that vehicle carrying commercial ferries began to use this technology. It was
disappointing at the time that the 3K-SES plan was cancelled, though fuel consumption
was not the only challenge faced by the 3K-SES. Further reasons included the following.

Technical risks
High frequency vibration could occur to a flexible skirt at the craft speed of 80 knots,
and so produce very high accelerations (more than 500 G on certain skirt compo-
nents). In addition, heat generation at prototype skirt tips at the time seriously
affected their life, reducing it to a limited period of operation.
The high power propulsion systems on both craft were novel: SES 100A had vari-
able geometry ducting water jets, while SES 100B had semi-submerged supercavitat-
ing propellers. Water jets for commercial applications have developed greatly since
then, based partly on that experience.
There were also a series of technical problems with respect to seakeeping quality,
ride control systems, high power transmission gear boxes and fire resistance of marine
aluminium alloy structures, which had to be solved during the 3K-SES programme
itself. The high power also led to a limited range, only just sufficient for the mission,
which was not fully cleared through the Defense Department at the time.
Novel materials and systems
The material, equipment, weapon systems etc. which were in use on other ships of the
fleet would have had to be abandoned for the 3K-SES, and new equipment, material
and weapons with aviation type would have had to have been adopted and so lead to
new construction methods. This would not have helped the Navy maintenance system.
US Naval administration concluded that very high speed craft would lead to a series
of problems not only on some ship materials and equipment, but also with some ship
performance parameters, for instance high drag peaks, low range and large speed loss
of craft in waves etc. This arose from the choice of a low cushion length/beam ratio,
and thin sidewall configuration.
Model tank and small scale prototype tests at DTNSRDC had already indicated
that high L/B could have advantages. For this reason, the US naval administration
considered that the second generation of SES should be craft with a high cushion