Folks, I've been following the debate about efficiency of ships versus
aircraft with interest.
Just my 2p worth -
Fuel - Jet engines do run very nicely on hydrogen (better than they do on
kerosene), however the low density of the fuel and the need for cryogenic
storage if liquefied presents several technical and operational
difficulties, though nothing that can't be overcome. Although fuel tanks
become bulky, they need not be especially heavy (remember the fuel is very
light, and there are lightweight insulating materials available now), and if
integrated into the design from the start need not have too great a drag
penalty. One study back in the 1970's (after the oil prices jumped following
the Yom Kippur war and everyone panicked about security of supply) showed
how a Boeing 747 could have the 'hump' extended back the whole length of the
aircraft, this part being the liquid hydrogen tank. More recent designs
propose using the 'flying wing' concept as used on a smaller scale on the
current B2 bomber.
The concept has been recently tested on a modified Tupolev Tu154 in Russia,
and is particularly well suited to future hypersonic transports using
'scramjet' propulsion at very high altitudes (the fuel can be used to help
cool the airframe and engines). Not sure what the effect of dumping exhaust
water vapour in the stratosphere would be though! In other words if the
future 'hydrogen economy' does evolve, prospects for jet transport are not
gloomy - provided of course that the hydrogen is produced in a clean manner.
On the timescale needed for evolution of new-build hydrogen powered
airliners (c.35 years from now?), commercial nuclear fusion power might
actually be available - does that make the hydrogen economy more tenable?
Regarding lifespan of airliners, it's not necessarily true that ships last
longer than aircraft. A typical 747 seems to last about 20-25 years with the
first customer airline, soldiering on in the second-hand market until being
scrapped when the aircraft's fatigue life limit is reached or no-more
buyers/leasing opportunities can be found for an old aircraft - remember too
that the high quality alloys used in aircraft have a 'scrap' value, plus
there's a healthy re-sale on used engines. Early series-100 747's entered
service from 1970 and many are still in service, retirement seems more an
issue of operating economics rather than lack of airframe life.
Older airliners have less efficient engines and they tend to be set up for
three-crew operation (two pilots plus flight engineer) so few commercial
airlines are willing to pay the extra costs (hence you often see 60's/70's
vintage aircraft bought up cheaply by air forces for conversion to tankers
or troop transports, such as the RAF's VC-10 and Tristar fleet).
Fuel efficiency issues have severely curtailed the operational life of the
3-engine short/medium range 727 several years ahead of age-related
retirement, many of those still in service are used by UPS, Fed Ex where low
purchase cost seems to compensate for high running cost. Rule changes that
allow twin-engine operations over long trans-ocean routes pretty much killed
off the market for large three-engined jets (Tristar, DC-10, MD-11), but 4
engine types remain popular with longer trans-ocean routes with heavy
passenger or freight loads (Boeing 747, Airbus A340, new A380 double-decker)
Aircraft life can be extended through major rebuilds, replacement of main
wing spar and so on - hence the continued life of B52 bombers build back in
the early 1960's, and C5 transports built in the early 70's. Engines can be
replaced by more quieter, more fuel efficient models (e.g. DC-8 freighters,
built 1960's, still in widespread service, stretched and re-engined, KC135
tankers dating from late 50's onward, now being re-engined).
Some aircraft can have very long lives - small numbers of military Canberras
and Hunters are still serving at 50 years old, B52s are in their 40's, C47
transports date back to the 1940s. Even in Civilian use there are many
airliners from the 1950s and 60s still in daily service, particularly in
Africa and South America. The helicopter that runs the daily service to the
Isles of Scilly entered service in 1964.
In comparison the economic life of a merchant ship would seem to be about 30
years, after that it's cheaper to buy new-build than to refurbish,
especially when state aid is available to subsidise ship-building
industries. Changes in regulations (e.g. the requirement for double-hulled
tankers, or better safety for roll on/roll off ferries in Europe) also
encourage fairly rapid turnover of ships, or at least their transfer to less
strict flags and avoidance of EU/US ports.
As an example of a long-lived ship, our research ship Discovery was launched
in 1962 but after a hull survey found her to be in good shape, Discovery was
comprehensively refitted and lengthened by 11 metres in 1990-92, extending
her life to a projected retirement of 2010-15 so she'll have about 50 years
of service. Pretty good value to the tax-payer, but far from typical.
Discovery was solidly built to naval rather than civilian standards. In
comparison the 1984 RRS Charles Darwin is about to be replaced after only 21
years of service, her thin hull being more typical of average merchant ship
quality. (Darwin was built during the Thatcher era, when cheapest bidder won
the contract, and it shows!)
Big military ships such as Nimitz-class aircraft carriers are built to last
50+ years, but civilian operators couldn't afford to build at that level of
quality.
Sorry to have rambled on for such a long time - in summary I can't assume
that ships last longer than aircraft. As others have also said, most
merchant ships burn low grades of heavy fuel oil and have engines that are
far from the state of the art units fitted to prestigious cruise ships such
as Queen Mary 2. Fast ferries might be able to compete to a limited extent
with air travel, if they went where passengers want to go, but as soon as
you try and get a ship to move fast you need an awful lot of fuel oil.
Finally here's a couple of links I spotted earlier that might be of
interest: http://www.almc.army.mil/alog/issues/MayJun00/MS492.htm
http://ergosphere.blogspot.com/2005/06/post-oil-airliners.html
Regards
Steve
--
Stephen Hall
Room 256/27
Ocean Circulation & Climate
National Oceanography Centre, Southampton
Empress Dock, Southampton SO14 3ZH
www.noc.soton.ac.uk
Tel +44 (0)23 80 596435
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