Shallow draft offers ultra-thin-water performance, but can have stability risks
A recent warning about steering and stability issues with the recreational fishing skiffs called Texas flats boats should also serve as a caution for designers and buyers of small shallow-draft workboats.
The July 2012 death of Kali Gorzell focused attention on the danger, and led to a joint state and Coast Guard investigation of how the San Antonio, Texas, teen died. Brian Goodwin, technical director with the American Boat and Yacht Council, ran some underway tests, and felt the alarming tendency of a flats boat to spin out on hard turns.
With a measured gravity force of 1G, “I weigh 200 lbs., so that was a side force of 300 lbs.,” Goodwin told an audience at the Multi-Agency Craft Conference in Baltimore, last week.
Goodwin and fellow investigator Michael Morabito of the U.S. Naval Academy worked on the investigation with CED Technologies, Annapolis, Md., the Coast Guard, and Texas Parks and Wildlife Department. They called the phenomenon “end swapping” – a sudden 180-degree spin on center when Goodwin applied a standard ABYC quick-turn test, attempting a 90-degree turn in a half-second with no throttle adjustment.
If Goodwin could feel that force at the helm, imagine what the physics were like at the bow and stern. That happened to Gorzell, who was thrown from the boat and struck by the propeller as the craft spun around. Her parents reached out to Texas wildlife officials, who knew of other accidents and convinced the Coast Guard to investigate.
CED conducted extensive tests of an 18’ Rockport skiff with a 90-hp Suzuki outboard, the same type of boat involved in the Gorzell accident, and another flats boat, starting with radio-controlled trials until they understood the forces. The company reported that “it would be difficult to envision how either of the two hull forms could be safely used for recreational boating.”
Goodwin said the weakness is in the design of some Texas boats, with a deeper forefoot at the bow tapering back to a flat bottom and propeller tunnel at the stern. When turning, operators could hear the propeller pulling air through the tunnel as the stern lifted. Adding skegs to the bottom made turns more stable, he said.
“A lot of this (testing) is not being done in the recreational industry,” Goodwin said. But workboat buyers who operate in shallow water should also examine stability as defined by the ABYC standards, he told the MACC audience. “How many of you have that in your specs?,” he asked.