It might not be a torpedo, but it sounds like a shot across the bow.

The city should find secure parking for prospective passengers before seeking release of a key federal grant for a new water ferry service between Kalaeloa to Aloha Tower, a federal transportation official has warned.

The lack of parking at the Kalaeloa pier "will severely limit the ability of the ferry service to appeal to a broad segment of the commuting public," Federal Transit Administration regional director Leslie Rogers said yesterday in a letter to city officials.

The city has been scrambling to lease space in shopping center parking lots nearby, but has so far been unsuccessful. Space adjacent to the pier is not currently suitable for commuter parking, officials said Thursday.

The plan relies on a $5 million FTA grant that would pay for the ferries, and the city hopes to begin service in July.

Rogers did not explicitly say that the money would be withheld if parking is not secured, but his letter was the first clear indication that the grant could be delayed or in jeopardy.

The city had no immediate response yesterday, but officials said earlier that they knew of the FTA's concerns and were working to address them.

The plan's main thrust is to link the ferry's terminals with new shuttle bus routes serving Wai'anae, Makakilo, Kapolei, Waikiki and the University of Hawai'i-Manoa.

But Rogers said that may only attract commuters who already take other buses, rather than persuade drivers to give up their cars.

That would provide "a very minimal benefit in relation to cost," he wrote. City Council members had raised similar concerns in recent weeks, but had generally supported the project.

The plan calls for ferries to carry up to 447 passengers total in each direction every weekday. The city had indicated it would consider the plan successful if ridership increased each month during the first six months.

But Rogers said there should be a more precise way to measure how effective the service would be.

The $5 million grant would pay to run the ferries for up to one year, making three trips in each direction per day. The city would spend an additional $1 million to run the shuttle buses.

Officials have stressed that they have no illusions the ferries would solve the island's traffic woes alone. But they would give some commuters an important new option.

A single city bus fare — adults pay $2 for a one-way trip — would be charged for the ferry and bus rides at each end. Monthly and annual bus passes also would be honored on the ferry.

The system would be the third attempt in recent years to link Leeward O'ahu with Downtown Honolulu by ferry. Earlier efforts suffered from low ridership and ended when federal grants ran out.

Reach Johnny Brannon at jbrannon@honoluluadvertiser.com.