CP propellers begin to catch on in the North American tug world
It had to happen, of course. Nothing ever stays the same in life, and now we know that is also true of Crowley Maritimeâ€™s famous Invader-class tugboats.
Crowley has already contracted with Bollinger Shipyards for construction of a pair of its new Ocean-class tugs that are nothing if not modern looking. People are saying that these 10,000-hp tugs could, if successful, become the eventual replacement class for the 136-foot, 7,200-hp Invaders, which are already more than 30 years old.
The new Ocean-class tugs, while sleek and streamlined above the waterline, will also be up to date below the water, and here we are referring to the propellers. While the Invader tugs have conventional fixed-pitch propellers, the Ocean-class tugs will have controllable-pitch (CP) propellers in nozzles.
That may seem surprising for a tugboat that will most likely earn its keep towing heavy objects at steady speeds over thousands of miles of open ocean, but CP props seem to be the propulsion gear of the future for many types of tugboats.
We noted that a substantial portion of tugs featured in the 2010 edition of American Tugboat Review (PM #137) are outfitted with CP propellers, including one azimuthing stern drive (ASD) tractor tug and three heavy tugs designed to push barges with articulated tug-barge (ATB) coupler systems.
While CP props are fairly common among European tugs and workboats, they are somewhat unusual on North American tugs, though hardly new technology.
CP props were first introduced in the early 20th century and were readily adopted for boats as well as propeller-driven aircraft.
U.S. and Canadian tugs with European influence in the design process tend to have CP props, particularly those of Canada. An example is the new 5,300-hp ATB tug Victorious, built last year in China for Canadaâ€™s McAsphalt Industries. This new tug is paired with a 70,000-barrel oil barge, which it will always push, moving between the Great Lakes and the Atlantic Coast. The tug is set up with MaK engines, Taacke reduction gears and Rolls-Royce CP propellers without nozzles. She is the second ATB tug operated by McAsphalt with CP props.
â€œThe controllable-pitch propellers seem to be the choice, at least for us,â€ said Andy Mitchell, general manager of Provmar Fuels, an Upper Lakes Group subsidiary. â€œOur captains prefer the infinite control on propulsion that they get from this equipment. With a conventional setup and reversing gear, it is hard to â€˜creepâ€™ the vessel ahead or astern, and since our vessels are always transiting locks in the St. Lawrence Seaway system, the CP option seems to make for a better fit.â€
We recently visited the new 5,400-hp, 128-foot ASD tug Independence in Boston where addiction to CP props is running unchecked. During a morningâ€™s cruise aboard â€œIndie,â€ skipper Chuck Deloring explained why he loves to control the pitch of the four blades on those Aquamaster 102-inch-diameter propellers.
â€œIt reminds me of a diesel electric system, since we are often able to run the tug at a constant rpm on the engines,â€ he said. â€œWhen we are maneuvering or out on station I can go up to 80 percent pitch without having to increase power.â€
Independence, which includes reversing-gear systems in its powertrain, can adjust its propellers to 100 percent forward pitch and up to 30 percent in reverse pitch, according to the captain. While cruising along at 10.6 knots in calm sea conditions, the captain had his tugâ€™s engine operating at 1,500 rpm with roughly 80 percent pitch on the props. In that configuration, the tug was burning 51 gallons per hour for each engine. Thatâ€™s not bad considering that with everything burning, including generators and boilers, the tug can consume more than 300 gallons per hour. A computer system can be set into several different modes from a full-manual system to a firefighting mode, sharing power between engines, fire pumps and propellers, and a combination mode in which the computer matches optimum pitch to rpm when underway for best speed and fuel burn.
The original intent of installing CP props on Independence was to enable full FiFi-1 firefighting capability without having to install dedicated diesel engines just for the two fire pumps. With clutchable fire pumps shaft-driven from the front end of each engine, the CP props would allow power to be diverted to run the pumps that might otherwise have been going to fixed-pitch propellers. By reducing pitch on the propeller blades, the propellers could get enough thrust for underway operations while allowing the engines to divert up to 1,000 hp each to the fire pumps.
Here is the savings from that scenario: A typical FiFi-1 auxiliary firefighting engine might be in the 600 to 1,000-hp range, such as the Caterpillar 3412 diesel. These engines weigh about 5,000 pounds by themselves, and a pair of them (one for each fire pump) takes up a large amount of engine room space. And they are hardly ever used, except for practice, drills and photo ops. While use of a PTO from each main engine to power the fire pumps may not save a lot in outfitting costs, it certainly has these other advantages of space and weight savings.
The way it has turned out, however, the CP props have been shown to be advantageous in so many other ways, and the tug crews are loving them, even though they hardly ever use the fire pumps.
Bill Skinner, the engineering-minded marine superintendent of Boston Towing & Transportation (which operates Independence) has a long list of advantages for CP props.
Here are just a few of them:
â€¢ Operating at constant engine rpm helps to reduce emissions (no more puffs of black smoke at every throttle change), and to optimize engine efficiency and fuel savings.
â€¢ Constant rpmâ€™s are ideal for operating a shaft generator.
â€¢ CP props are great for holding station or operating at dead slow speeds, either ahead or reverse.
â€¢ CP props are ideal for sharing engine power among power systems, such as fire pumps.
â€¢ CP props make it practical and efficient to operate on just one engine or propeller.
â€¢ CP props provide a faster and smoother means of shifting into reverse mode, particularly when compared to turning ASD units around 180°. In some conventional installations they can save on the need for reversing gears.
Skinner said he feels that the emissions advantage alone will be enough to tip the design scales in the direction of CP props for many new tugs to be built in the near future.
The real, every-day advantage of CP props, however, stems from the fact that the engines and drivetrain on most workboats and tugs are not doing the same kind of propulsive work at all times. Even a tug that is constantly pushing a barge might be faced up to a loaded barge one day and an empty one the next. Each type of work could be better done by a propeller with different pitch.
A fishing trawler might be racing out to its chosen grounds one day and then dragging nets or setting traps the next. A sport fishing boat might have its propellers fixed for high speeds in order to get out there as fast as possible, but for the hours it spends trolling, those props would be inappropriately pitched. A conventional tug might be towing a barge down the coast one day and then futzing around in port moving barges and ships the next. On the other hand, a globe-girdling bulk cargo freighter might be a good example of the type of vessel that would probably not benefit from installation of a CP propeller at the end of its single shaft.
OSG Vision, first put into service this year, is an example of a very large tugboat seemingly designed for a single purpose, but on a day-to-day basis, it is faced with a wide range of operating situations. Vision, for the moment the largest ATB unit operating in this part of the world, is the first of two 12,100-hp tugs engaged in crude oil lightering on Delaware Bay. It is matched with a 342,000-barrel barge, constantly moving oil from huge tankers anchored in Delaware Bay to refineries further upstream near Philadelphia.
Naval architect Robert Hill of Ocean Tug & Barge Engineering, said OSG Vision is the first ATB tug he has designed with CP propellers. â€œThis type of propeller is very good when you have a changing load,â€ he said. â€œYou can always tune the propeller to exactly what the barge is requiring. Whereas, with a fixed-pitch wheel, it is designed for one condition only, and anywhere off that condition you are going to be a little bit less than optimal efficiency.â€
It may be safe to say that most (not all) tugs in the United States with CP props today are of the tractor-tug variety.
Crowley Maritime operates three oil-spill response ASD tugs with CP props (Alert, Aware, Attentive) with its fleet of ship-assist tugs in Alaska, while Foss Maritime outfitted its cycloidal-drive tugs Wedell Foss and Henry Foss with auxiliary ASD-CP configurations in the past few years.
McAllister Towing has a pair of converted Navy YTBs with single ASDs with CP props supplied by Schottel (Donal G. McAllister and Kaleen McAllister).
In addition, Edison Chouest Offshore operates a fleet of CP true tractors (forward-mounted z-drives) for the U.S. Navy in Kings Bay, Ga., Mayport, Fla., and San Diego, Calif. Edison Chouest also recently built eight 6,000-plus-hp scaled-up versions of these boats for LNG terminals on the Sabine River, located on the Texas-Louisiana border.
The Canadian company Ocean Group, based in Quebec, has been using CP props on some of its most recently built ASD tugs for ship-assist work.
And then we are back to the next generation of ocean-towing tugs for Crowley Maritime. These tugs, with full towing gear and double-drum waterfall-style towing winches, will surely do their share of conventional long-distance ocean towing of heavy objects. But as with the Invader class, they could also be involved in any number of other type assignments where the flat-out-full-towing-pitch propeller might not be the best choice.
Jonathan Parrott, president of Jensen Maritime Consultants, a Crowley design affiliate, said the many advantages of CP props in todayâ€™s economic environment easily outweigh the risk of mechanical problems related to the complex design and construction of the propellers themselves.
â€œThe advantage of the CP prop is that it makes your boat more efficient throughout its operating range, and if all goes well, the fuel you save from being more efficient throughout that range offsets any maintenance issues,â€ said Parrott, whose firm created the design for Crowleyâ€™s new Ocean-class.
CP propellers are â€œa dime a dozenâ€ in Europe, said Parrott. They put them on every type of vessel, he noted. And it may not be long before something similar is happening to North American tugboats.