Page 46 - Theory and Design of Air Cushion Craft

30 Introduction to hovercraft

and it was re-named as SES-200. As a result of the modifications, the craft drag was
reduced at cruising speed, and the economy and seakeeping quality of such SES with
and thickened sidewalls was improved significantly. The craft speed on calm
high LJB C
water was about 30 knots but the speed loss less than 20% in a sea state of Beaufort
4. In these conditions the craft captain would have to throttle back the governor so as
to reduce the engine revolutions, or change the course, in order to avoid the extreme
slamming motions and shipping of water. The BH-110 has good seakeeping: it can
maintain a speed of 28 knots in calm seas, 16 knots in head seas of 8 ft and 25 knots
in following seas of 12 to 14 ft, respectively.
Three production BH-110 craft in service with the US Coastguard during the 1980s
have been operated for up to 181 consecutive days and nights. The Coastguard con-
cluded that maintenance labour was equal or less than that on conventional coastal
patrol vessels, and also realised that the crews had a good rest during a three day
patrol operation.
Features of third generation SES craft are as follows:
1. A fair performance at low/medium speed, and low peak drag as well as increased
range;
2. Good seakeeping capability in cushion borne operation due to its raised wetted
deck, which was similar to a catamaran;
3. Thanks to the craft ride control system (RCS), the cushion pressure could be kept
almost constant, arising from regulating both the air inlet and outlet control valves,
so as to reduce the vertical motion of craft in waves. The RCS had been mounted
on the XR-1D, SES 100A and SES-200, and a large number of tests had been car-
ried out which validated the excellent effect of these systems. Vertical acceleration
could be reduced by 50%, 30% and 25% at sea states of 1 to 2, 3 and 4, respectively;
4. The pitch angle of SES-100 at full speed in head seas was decreased, as shown in Table
1.6. It was found that the pitch motion of the craft was less than the required pitch
motion for landing helicopters (less than 3°). It is probably safe to assume that the
helicopters could be landed safely on SES-200 weighing 200t at less than sea-state 4;
5. Thanks to the medium speed of the craft, the wear rate of skirt bow segments tip
improved to between 1500 and 3000 hours, whereas the life of the bow skirt might be
reduced to 300-700 hours at operational speeds of 40-60 knots. In addition, the main-
tenance cost was reduced further due to adopting a skirt design which could be
replaced while the craft was moored on water, and was found to be lower than the
main engine maintenance cost which was relatively low due to the use of diesel engines.
The US Navy were encouraged by the success of tests carried out on the SES-200
craft, and later worked on the development of two applications of such craft, the
Mine Countermeasures SES and the medium sized Patrol SES.

Table 1.6 The pitch motion of SES-200 at full speed and
in head sea
Sea State Pitch angle (single amplitude)
1 < 0.2 degrees
2 <0.9
3 <2.2
3.5 2.5