Giant New Ferries Experience Bad Vibrations -- $52,000 Per Ship To Remove Shaking `As Bad As The Old Kalakala'
Seattle Times Staff Reporter
Two of the state's three new $80 million ferries were put into service with a jarring design flaw, even though an identical problem plagued older ships in the system.
The giant ferries vibrate so badly that the tiles on the passenger-deck ceiling shake.
As one tooth-rattled passenger put it, "it feels as bad as the old Kalakala," the 1930s-era vessel that was famous for its jittery hull.
State ferry officials said the rattle will be removed on the Tacoma starting next week as part of a scheduled maintenance visit to an Everett shipyard. The Wenatchee will be sent in for the same fix in June.
The cost of correcting the vibration will be $52,000 for each ferry.
Stan Stumbo, chief design engineer of the Washington State Ferries, said the new ferries are otherwise trouble-free.
But Stumbo said the vibration flaw should have been caught when the boats were designed.
And the state must pay the bill for the repair because the problem originated with the boats' design, not with shipyard construction.
The new ferries, known as the Mark IIs, were designed using the blueprints for the state's 28-year-old jumbo ferries as a starting point. The slightly smaller jumbo ferries vibrate, too, for the same reason.
"We missed that one," said Stumbo, adding that the Mark II design is otherwise a great improvement over the old Jumbos.
The vibration has never been fixed on the jumbos, which carry 206 cars vs. the Mark IIs' 218. The rattle reportedly is more severe on the Mark IIs and has generated more passenger complaints, according to ferry-system spokeswoman Susan Harris-Huether.
The irritating vibration became apparent as soon as the Tacoma was launched for sea trial about 18 months ago. Stumbo, sensing a public-relations problem and a maintenance headache, took time finding a solution, which involves tuning the new boat as if it were a troublesome new violin.
The vibration stems from a problem with the boat's harmonics. Stumbo and his crew clamped sensors to various parts of the boat, looking for pieces that might vibrate at a certain frequency. They were searching for a beam that was the right length so that it would set up an empathic vibration when it "heard" the pulse of the propellers.
The propellers have four blades, each of which emits a pulse of sound as it passes beneath the hull - 11 times a second when the boat is fully under way. Stumbo figured there was a beam somewhere that was tuned, like a violin string, to that frequency.
Passenger complaints indicated that the vibration increased as the ferry turned, likely because the force of the propeller pulse increases during a turn.
Ferry engineers found the culprit beams underneath the main passenger deck.
Work was done to correct the problem before the ferry Puyallup, the third Mark II in the class, was launched from the Todd Shipyard. Crews installed reinforcing beams on the Puyallup, to shorten the harmonics of the lengthwise beams, and disrupt the vibrations.
Stumbo said passengers should notice a difference when the Puyallup replaces the Tacoma on the Bainbridge run next week.
"It's like you get a new car and there's a rattle in the dashboard at certain speeds," he said.
"If you press the finger on a certain spot it stops. We are doing the same thing (with the reinforcing beam)."
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