Page 36 - Theory and Design of Air Cushion Craft

20 Introduction to hovercraft

HM-5 series, of which two craft were built for service in Hong Kong. Development
beyond this point proved difficult, and it took Bell-Halter and Brodrene Aa to move
the concept forward in the direction of Air Cushion catamarans in the early 1980s.
Hovermarine nevertheless continued to have commercial success with variants of its
HM-2. A further trend which began in this period was the transfer from designers,
mainly in the UK, to licencees, in Australia and the USA. More recently the API.88-
400 (Fig. 16.12) construction has been carried out in Canada, and the ABS M10 (Fig.
16.7(b)) has been constructed under licence in Sweden.
The main drive through the 1980s on the technical side was to improve overall
service reliability, economy, seakeeping quality, habitability and maintainability.
Additionally there was a drive to maintain commercial competition, as catamaran and
hydrofoil manufacturers also began to target this market. In the UK some of the mea-
sures taken to improve competitive ability in the commercial market were as follows:
1. Replace aviation engines with lightweight marine diesels, and use marine hull mat-
erials and ship construction technology in place of aviation methods, so cutting
down the cost of craft;
2. Improve the configuration of skirts (for instance, adopting the responsive skirt
with low natural frequency) to enhance seakeeing quality and assist item 3, below;
3. Improve the lift and propulsion system to enhance economy and reduce fuel
consumption;
4. Improve the internal outfit of cabins and other measures to reduce internal noise
level and improve the craft habitability.
Consequently, features of second generation British ACV/SES were:

1. Procurement and operation costs reduced to less than 50% of first generation craft;
2. Maintenance costs significantly reduced;
3. Much reduced noise level, both internal and external to craft;
4. ACV/SES transport efficiency enhanced greatly, as shown in Table 1.5.

While the specific weight of a diesel engine is much higher than a gas turbine, by intro-
ducing a series of overall design measures such as responsive skirts, low bag to cush-
ion pressure ratio, lift systems with smaller cushion flow rate etc., main engine power
output could be reduced from 74 to 29-37 kW/tonne. For this reason the British ACV
API-88 (Fig. 1.20 (a)) was very competitive as a ferry compared to conventional ships
when it entered the market. Later, utility versions such as the API 88-300 also proved
very successful. See Fig. 1.20 (b).

Table 1.5 The reduction of power consumption per ton-knot of British ACV over time

Date of Craft Engine Structure Total power/(payload, speed)
Construction kW/(tonne.knot)
1960 SR.N1 Aircraft, Aluminium, 2.35
piston riveted
1965 SR.N5 Gas turbine „ 1.83
1970 BH.7 Gas turbine „ 1.25
1975 SR.N4 Gas turbine „ 0.74
1980 SR.N4 Mk3 Gas turbine 0.51
1983 API. 88 Air cooled Aluminium, 0.59
diesel welded