How fast is fast enough for crowded Bay Area waterways?
By Wes Starratt, P.E.
Published: August, 2001
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.
