Calculating passenger limits
Calculating passenger limits
The Coast Guard, under pressure from the National Transportation Safety Board following the fatal capsize of a Baltimore pontoon boat, began two years ago to look at some of the calculations it makes in determining the passenger capacity of boats.
The agency had assumed since the 1960s that the average passenger weighed 140 pounds, while the Federal Aviation Administration uses 174 pounds as its estimate. The October capsize of the Ethan Allen on Lake George again focused attention on the Coast Guard, which originally had rated that Dyer 40 for 50 passengers when it was launched in the 1960s.
There is more to the calculation of a boat’s safe passenger capacity than the assumed weight of passengers, however. First, the use of the boat — whether for recreational or commercial operation — dictates how capacity is determined.
Commercial boats like the Ethan Allen must be subjected to one of two tests. The simpler of the two costs less to perform but is also supposed to be more conservative than the second, more technical capacity test. Under Coast Guard regulations, a small-business owner who cannot afford to hire a naval architect can conduct the simple test, called the “passenger heel criteria,” for Coast Guard observers, according to recently retired Coast Guard Capt. Ray Petow, who was in charge of the agency’s Office of Design and Engineering Standards. With the boat moored to a pier, weight is loaded on deck. The weight could be concrete blocks or barrels of water.
“Normally, the owner is going to say, ‘I need to carry X number of passengers,’” Petow says. So the weight, simulating the weight of passengers calculated at 160 pounds each for partially protected waters and 140 pounds for rivers and lakes, is brought aboard and placed along the boat’s centerline. Then the boat’s freeboard is measured.
Next, the barrels or blocks are moved halfway across the deck toward the rail. “The vessel then heels. As long as half of the freeboard remains above the water, it passes,” Petow says. “If more than half remains, the owner can say, ‘OK, I want to put more passengers on,’ ” he continues. “It still has plenty of reserve buoyancy to keep the vessel upright.”
The total weight on board is divided by 160 pounds or 140 pounds, depending on the boat’s intended use, Petow explains. Those numbers now are under review following the March 2004 capsize of the water taxi Lady D in Baltimore Harbor, with the deaths of five passengers. In March 2006 the NTSB, in a report on its investigation of the Lady D, said the Coast Guard certified the pontoon boat to carry too many people “as a result of an inappropriate stability test on the vessel to which it was granted sister status.” The Coast Guard’s “regulatory stability test standards on which the Lady D’s passenger allowance was based used an inaccurate average passenger weight,” the NTSB found. The Lady D had been subjected to the simpler test described above.
The second test to determine the passenger capacity of a commercial boat is a more involved, “mathematically rigorous” process, Petow says. “It requires a naval architect and gives you a more accurate portrayal of the vessel’s stability characteristics,” he says.
In the case of recreational boats, Coast Guard regulations require that all monohulls smaller than 20 feet be tested for passenger capacity. Boatbuilders are allowed to self-certify, according to Tom Marhevko, vice president of engineering standards for the National Marine Manufacturers Association.
The first step is fairly simple. The builder can fill the boat with water and then measure the weight of that water, or the same thing can be done by computer analysis. Now the process becomes more complicated.
The second step involves subtracting the dry weight of the boat and all of its machinery from the weight of the water as determined in the first step. As a safety factor, that figure is divided by 5, then 32 is added to that. Lastly, that figure is divided by 141 to “roughly approximate one person’s weight,” according to Marhevko.
If, for example, the weight of the water the boat holds is 3,000 pounds, and the boat’s dry weight plus machinery is 340 pounds, you would subtract the second number from the first and get 2,660 pounds. Divide this by 5 to get 532 pounds, then add 32 pounds. That leaves 564 pounds as the maximum weight the boat can be certified to hold, or four persons after dividing by 141.
“In the NMMA certification program, we verify these [builder] calculations,” he says. “They send us all these, and we download them and recalculate, and make sure we verify that all these calculations are correct.”
The Coast Guard requires no passenger capacity ratings for boats larger than 20 feet. However, if the builder wants an American Boat and Yacht Council rating on a boat between 20 and 26 feet, the same test must be conducted, according to Marhevko. Over 26 feet, there are no calculations except for pontoon boats, which have their own computations, he says.