Capt. Troy J. HotardJul 2, 2014 01:10 PM
Florida Marine Transporters figures out how to make a proven towboat even better
Photos by Brian Gauvin
Capt. Troy J. Hotard is the 56th 90-foot towboat Florida Marine has purchased from Eastern Shipbuilding.
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Capt. Troy J. Hotard, a 90-foot, 3,000-hp towboat built by Eastern Shipbuilding for Florida Marine Transporters (FMT), represents a new chapter in a long-running story.
The boat, which went into service in early February, is the 56th 90-footer of the same basic design built for FMT by Eastern. The first was delivered in 2006. FMT has described this series of boats as “the largest single towboat build contract in history.”
Originally designed for smaller waterways, the boats can operate on big rivers like the Mississippi.
Yet Hotard, named after one of the company’s port captains, also represents the continuing evolution of this tried-and-true design. It is the first of the 90-footers to be powered by twin 1,500-hp Tier 3 engines. And it benefits from a hull that was redesigned a few years back, when FMT decided to move to more powerful engines.
The original 25 boats were powered by twin 1,200-hp engines, for a total output of 2,400 hp. Then FMT boosted the power to 2,600 hp. When Eastern was building hull No. 40 in early 2010, FMT realized that the 1,300-hp Caterpillar engines it had been using would no longer be available as the engine maker updated its lineup to conform to changing emission control standards.
Capt. Mario Zepeda in the wheelhouse.
The new Cat 3512 engines coming onto the market would have higher power ratings. Jeff Brumfield, FMT’s senior manager of boat construction, understood that before the company could install the more powerful Cat engines in his 90-footers, some design changes would have to be made.
Instead of operating at 1,200 rpm, the new engines would be running at 1,600 rpm. That would require a change in the gearbox ratio to 6.04, up from 5.04, as well as a revision in the size and pitch of the propellers. But most importantly, the hull form would have to be tweaked.
“We needed to change the hull form to use that horsepower,” he said. “We knew we’d end up at 3,000…. We were handling 2,600 fine. We knew we were at the edge of the envelope.”
The new boat was named after one of Florida Marine’s port captains.
An increase in power, he explained, can induce turbulence and cavitation. “There are all sorts of issues if the hull isn’t ready for the power. If you’re right on the edge of the envelope, a couple hundred RPMs induces things you don’t want.”
However, FMT did not want to radically alter the design of a boat that had proven to be such a reliable workhorse. The solution was to redesign just the stern. The job was given to Gilbert Associates Inc., a naval architecture and marine engineering firm based in Hingham, Mass.
Over the life of the long contract, Eastern Shipbuilding has figured out how to build the boats efficiently, so Gilbert Associates was told to accomplish its goals with a minimum of changes to the hull.
“We worked within the confines of the existing structure,” said John W. Gilbert, president of Gilbert Associates. “We weren’t allowed to go too far forward.”
The primary change was to increase the size of the “tunnels,” the indentations in the hull that aid the flow of water to the props.
The tunnels had to be enlarged to accommodate the larger horsepower and propellers in the newer boats and to improve the flow of water to the props.
The new design also addressed the separation of flanking rudder positions due to the larger propellers and tunnels.
The original tunnels “wouldn’t have allowed a larger propeller to turn in that tunnel,” Gilbert said.
“You’re trying to open them (tunnels) up as much as you can. You don’t want to starve the propellers of water,” Gilbert said.
The more powerful engines and larger propellers also created potential vibration problems, compounded by the fact that the boats often operate in shallow water, where the forces created by turbulence can be transferred to the hull.
“The boat can undergo tremendous amounts of stress,” Gilbert said, strong enough to cause structural damage. And the vibrations can also mean higher noise and stress levels for the crew. By allowing the boats to make efficient use of the larger engines, the design changes also contributed to crew comfort.
“All river towboats suffer some amount of vibratory loads presented from bottom influence, especially in very shallow depths,” Gilbert said. “Proper structural continuity, tunnel form for water flow and other design measures to appendages are made to reduce the vibrations as much as possible.”