Breaking
The Speed Envelope For Passenger Ferries
How
fast is fast enough for crowded Bay Area waterways?
By
Wes Starratt, P.E.
Speed has been the essential
element in getting commuters out of their automobiles and into the
ferry. It has certainly been successful, as we have seen, with the
ridership of Bay Area ferries growing to match the capacity of the
existing vessels to carry passengers, and with feeder buses now needed
in Vallejo to carry excess passengers. After all, why not take a ferry,
when you can get from Vallejo, Larkspur, or Alameda to your office in
downtown San Francisco, with a beautiful ride on the Bay, in the same
time or less time than it takes in your automobile on the crowded
freeways? Why not indeed?
But what if the ferries went
even faster … would they attract more riders? What would it mean if
the Bay Area’s longest ferry run from Vallejo to San Francisco were
reduced from 50 minutes to less than 30 minutes? Maybe there would not
be enough time for passengers to have their coffee or check the
Chronicle? Maybe the current speed is adequate for safe operations in
crowded waters such as San Francisco Bay.
But, other factors need to be
considered in the development of San Francisco Bay’s proposed regional
ferry system. For example:
What if ferries could operate in
the shallow waters of the bay without requiring costly dredging …
wouldn’t that mean that ferry service could be established to many
more locations around the bay?
And, what if ferries could
operate without creating environmentally damaging wake … wouldn’t
that mean that ferry service would be welcome in more Bay Area
communities?
The San Francisco Bay Area Water
Transit Authority (WTA), looking into these and other questions, has
established a Technical Advisory Committee comprised of more than 50
highly qualified local leaders and added marine-consultant Mary Culnane
to its staff.
Tom Bertken, chief executive
officer of WTA, explains: "We are not promoting higher-speed
ferries, but are keeping an active eye on developments. Proven ferry
designs are in service on the bay, and are performing well. Their speed
of some 36 knots (40+ miles) per hour is adequate for the bay-area
services that we envision. We’re not looking for 50 to 60 knot (57 to
69 mile) per hour boats, nor is any sailor on the bay. There is a safety
factor involved!
"WTA is not in the
development business, but is actively watching developments, especially
for ferries with hull designs that would lessen a boat’s wake, which
could be a major problem on the bay. We are also looking at vessels that
are able to operate in shallow bay waters without requiring extensive
dredging, which can be both costly and detrimental to the
environment."
Further Design Developments!
Some years ago, we learned how
to weld aluminum and developed light-weight aluminum-hull boats that
ride higher in the water and require less draft than older steel hulls.
We also discovered that a twin-hull vessel can move at faster speeds
then the older single hulls. Thus was born the high-speed aluminum
catamaran ferries that have sparked today’s ferry revolution, with
boats that are now carrying commuters and other passengers throughout
the bay and in many other parts of the world.
Not satisfied with those
breakthroughs, engineers have continued to question and push the speed
envelop. Various conflicting groups have gotten into the act, and
confusing terms such as SES (Surface Effect Ship), MES (Multiple Effect
Ship), and Air Cushioned Vessels have arisen. But, all of them involve
getting a ship farther out of the water — since it can move more
easily through air with less resistance than through water. That lifting
is achieved by hull designs that provide a dynamic lift to the ship as
it moves through water, by the use of cushions or pockets of air to
raise the ship, and by the use of strong materials that are even lighter
than aluminum.
Engineers have asked, "What
if a boat could be lifted completely out of the water and ride on a
cushion of air?" It would take less force to move the boat through
air; so, its speed could be increased and its fuel consumption
decreased. Thus was born the "hover craft," with air blown
into an area under a boat that is surrounded on all sides by curtain
seals. But, there were problems. It took almost as much energy to
maintain air pressure in the cushion under the boat as it took to move
the boat forward. Also, there was constant wear and tear on the curtain
seals, with resulting heavy replacement costs.
The "Surface Effect
Ship" or SES
Another idea was needed, and the
U.S. Navy came to the fore with a vessel having a single
air-cushion between parallel side hulls. The air-cushion was maintained
by the side hulls and flexible curtain seals fore and aft. Some 80
percent of the boat’s displacement was carried by the air-cushion,
which needed only 20 percent of the air flow required by hovercraft. The
design was called the "Surface Effect Ship" or
"SES." The Navy invested heavily in this technology through
the mid-1980s, but there were problems with the flexible seals. Also,
the front seal was not effective in heavy seas, and the vessel provided
a rough ride.
In the San Francisco Bay, the
U.S. Coast Guard put three SES boats in operation for search-and-rescue
operations in the 1980s. However, because of "high maintenance
costs," their operation proved to be costly, and they are no longer
in use.
Other ssConcepts Investigated
Ship designers, looking at the
design of the twin-hull catamaran, thought, "What if we designed
each of the two hulls with a cavity in the bottom and pumped air into
those two cavities?" That would lift the hulls in the water, thus
reducing the required draft for the vessel and decreasing the resistance
of the water. The air cushions would largely support the boat, making it
possible to increase the speed and decrease fuel consumption. At the
same time, curtain seals would be completely eliminated.
Enter a number of groups in
Florida including Don Burg and his firm Air Ride Craft, Inc. A
mechanical and aeronautical engineer with experience as a project
engineer for Pratt & Whitney’s rocket engines, Burg has been
working since the 1980s to develop the design for the SeaCoaster, which
he calls an "Air-Assisted Catamaran." He built a 65-foot
SeaCoaster demonstration boat of relatively heavy fiberglass. Because of
its weight and relatively low-powered diesel engines, the boat’s top
speed is only about 40 knots. But, with the use of lighter-weight
composites and greater engine power, Burg claims that future vessels
could attain speeds of 50 or more knots. Furthermore, the power required
to drive the boat and power the fans that maintain the air cushion is
reported to be only about 60 percent of that of a conventional
catamaran.
After being transformed in to a
149-passenger ferry, the demonstration SeaCoaster was sold to Island
Express Boat Lines, Ltd., of Sandusky, Ohio. It was renamed the Rocket
II, and has been in use for excursion service to islands in Lake Erie
since 1999, operating at speeds of 32 to 36 knots "as a fuel saving
measure." Island Express is pleased with the operation of the boat,
which reportedly provides a "comfortable ride" and produces
"low wake". So, the group has requested Air Ride Craft to
design a larger and faster ferry. In addition, backers of the Sandusky,
Ohio, service say that they are exploring possible high-speed ferry
service between Miami and Key West using air-assisted catamarans.
A Strong Material Lighter
than Aluminum?
Ship designers have also been
saying, "why not make catamaran hulls of strong light weight
composites laminated onto foam?" The result would be a light-weight
vessel with a twin-hull design that could partially ride on air cushions
and require a very low draft. Such a vessel could operate in shallow
water at high speeds and create very little wake.
Fiberglass is not new to sail
boats, surfboards, and other types of pleasure craft. It is tough and
easy to mold into multiple shapes. But equally strong, yet lighter
weight materials are now available thanks to the demand for such
materials by the military and by the aircraft industries. There are now
whole families of strong synthetic fibers and carbon fibers that can be
embedded in resin and epoxy to form new generations of high-strength,
light-weight composites. These high-cost composites, which are at least
40 percent lighter than aluminum, are now being used extensively in
aircraft.
A hull of this type was moved to
Royal Crown Yachts’ shipyard at Tampa Bay where it has been modified
and is currently having a superstructure added to become the "PurrSeaverance",
the first boat for Fast Cat Ferries. After numerous delays,
launching of the 100-ft long, 149-passenger boat is anticipated in time
to inaugurate revenue service from Tampa Bay to Key West by Labor Day.
The PurrSeaverance
Some interesting facts reported
by the owners of Fast Cat Ferries:
l The hull of the boat is made
of Dupont’s Kevlar cloth, PVC foam core, and aircraft epoxy.
l It will be powered with four
Caterpillar, 1100 hp diesel engines, while the fan blowers will be
powered by twin Caterpillar 315 hp, one each per hull. The air cushion
will support 85% of the boat’s weight.
l Propulsion will be achieved by
advance surface-piercing propeller design by Pulse Systems, Inc., since
water jet propulsion is not compatible with the boat’s air cushion
design.
l The boat is expected to have a
shallow draft, reportedly requiring only 3.5 feet of water at rest and
14 inches at speed. The owner claims that it will have a wake of only
12-inches or less at normal operating speed of 50 to 60 knots.
Fast Cat Ferries plans two round
trips per day for the 220-mile Tampa Bay to Key West
run, which will require three
hours in each direction. Kent Rogers of Fast Cat Ferries states that the
firm plans to introduce as many as three additional ferries in the next
several years. "Our ship will have the distinction of being the
world’s fastest ferry to date, with a top speed in excess of 60
knots," claims Rogers.
But 60 knots may be fine for
long-run ocean trips; however, we must ask if such speeds are
appropriate in the crowded waters of San Francisco Bay. Using the proven
technology of aluminum catamarans, fast and comfortable service is now
being provided to Bay Area commuters. Undoubtedly improvements can be
made, but they will be focused largely on boats capable of operating in
the Bay Area’s shallow waters without extensive dredging and boats
that produce a low wake that is less damaging to the environment.
The jury will be out for a while
watching for developments of this new breed of ultra high-speed
ferries.