Interceptors appear to limit ferry wakes
June 11, 2008 · Updated 10:59 AM
"Bolting an aluminum bar interceptor to the squared back of a fast ferry is proving to be a cheap, viable method of reducing wake wash through the shallow waters of Rich Passage.The interceptor installed on the Chinook reduced wake wash by approximately 25 percent, according to findings by Dakota Creek Industries, Inc., the company which built the Chinook and Snohomish passenger ferries. Dakota Creek Industries is working with Kitsap Transit to find fast ferry alternatives to reduce wake wash.In shallow water, wake height from the Chinook was reduced to nine inches and wave energy was further reduced, both falling under the Washington State Ferries standard. The interceptor increases the range of water depths through which the ferries can run at full speed and still meet wake energy criteria, said indepedent consultant Stan Stumbo.A graph comparing several vessels indicates that wakes measured from the Chinook would fall below Washington State Ferries' standard criteria for wake energy as it approaches 40 knots.In fact, slowing ferry speeds through Rich Passage might have contributed to higher wake effects, according to Dakota Creek Industries.Advanced Multi-Hull Designs, an Australian design and manufacturing company working with Dakota Creek, outlined the shallow water effect, a phenomenon in which vessels designed like the Chinook and Snohomish actually create greater wake wash by running slowly in shallow water. The vessels would do better and produce less wake wash by transiting Rich Passage at 37 to 40 knots, rather than the 30 to 34 knots practiced by WSF, according to a Dakota Creek Industries document. The worst speed in traveling through the shallow waters of Rich Passage for Chinook Class vessels is 30 knots.The interceptor could also prove to be the most inexpensive option. This was not a high-ticket item. We used material that was in the shop, said WSF spokeswoman Pat Patterson.Cliff Hill, project engineer for Washington State Ferries, said they got a special deal on the Chinook's installation. The cost for creating the aluminum bar and bolting it onto the vessel was $2,500, though Dakota Creek Industries, the company that did the work, didn't charge Washington State Ferries for dry-docking.Installing the Snohomish ran a little higher - $6,940 - but WSF paid for dry-docking and for some other maintenance work.The interceptor on the Snohomish was installed within a month of the Chinook, after wake energy results showed a trend toward less wake wash.A State Environmental Policy Act team is still gathering information on shorelines through Rich Passage while a lawsuit against Washington State Ferries still is pending. There's a whole lot of data, but no conclusions, Patterson said.But Stumbo said he can tell by looking that interceptors are effective, even more than Issaquah-class ferries that currently slow down through Rich Passage. Now, you watch one of our fast ferries installed with the interceptor and you watch wash come on shore, at least to me, the wash is considerably less, he said."