NANTUCKET - Placing a surprise trump card on the table in the testy and complicated match over opening up ferry service from New Bedford, Steamship Authority governors announced yesterday that they will take immediate steps to buy the ferry Schamonchi, a private passenger ferry that operates between New Bedford and the Vineyard.

At a special meeting on Nantucket early yesterday afternoon, boat line governors said they will purchase a right of first refusal on the Schamonchi from the Tisbury Wharf Company in Vineyard Haven. The Tisbury Wharf is owned by Ralph M. Packer.

Buying Mr. Packer's right of first refusal gives the SSA the exclusive right to negotiate to buy the Schamonchi - and SSA governors made it clear yesterday that they intend to move at once.

"We hope to go forward to buy the boat and we intend to run the ferry and take over the route," declared Vineyard boat line governor and board chairman J.B. Riggs Parker. "I think this is an appropriate thing for us to do, and it is the right business decision to make," he added.

"From a strictly business standpoint, we have to do this," said Robert Murphy, the Vineyard member of the SSA financial advisory board.

"I'm interested in any good business decision that makes us money," said Nantucket SSA governor Grace Grossman.

The special meeting was held in the lobby of the boat line terminal on Nantucket. The announcement about the Schamonchi was the sole item of business on the agenda. Mr. Parker said at the outset that the meeting was held on Nantucket for the convenience of Mrs. Grace Grossman, who had to attend a funeral yesterday afternoon. Falmouth SSA governor Edward DeWitt was present and seated in the general public section at the meeting, but he did not participate because of a possible conflict of interest.

The regular monthly SSA meeting will be held next Thursday in Woods Hole.

Owned by the Cape Island Express Lines Inc., the Schamonchi has operated passenger service between New Bedford and the Vineyard for more than 20 years. It was known some time ago that company owner Janet Thompson had put the Schamonchi up for sale.

In recent weeks it has been widely reported in various business and political circles that Mrs. Thompson was nearing a sale agreement with Hy-Line Cruises, a prosperous Hyannis company that operates several ferry lines to the Vineyard and Nantucket, including a high-speed passenger ferry to Nantucket.

The announcement by the boat line yesterday that it now owns the exclusive right to buy the Schamonchi caught the owners of the Hy-Line by complete surprise.

"Wow," said Murray Scudder, one owner of the family-owned Hy-Line company. "Wow," he repeated when asked to comment on the SSA announcement.

Reached at his office in Hyannis after the SSA meeting, Mr. Scudder confirmed that his company had been negotiating to buy the Schamonchi. "That is very much true," he said, adding: "I probably at this point shouldn't comment until I have a chance to digest what you just told me. I really have to digest this. Holy cow," he said.

The announcement by the SSA signals a clear shift in position by the boat line on the subject of ferry service between New Bedford and the Islands, but Mr. Parker underscored yesterday that the decision is business pure and simple, with no veiled political message.

"When this opportunity arose, we realized we couldn't not do it," said Mr. Parker. "It's a business decision, it's not a political decision and it's not in response to anything else. It's a business decision for the future," he added.

The boat line has gotten a pounding from Boston politicians in the last two years as powerful lawmakers from New Bedford have pushed for a hostile takeover of the 40-year-old boat line. If the SSA buys the Schamonchi, it will mark the first time since 1960 for the boat line to operate its own ferry service out of New Bedford.

Yesterday Mr. Parker sketched the bare outlines of a changing business plan that includes an emphasis on more passenger service, reducing the number of cars traveling to the Islands and shifting the pressure for parking cars away from Woods Hole and Falmouth.

"Passengers of all types are the bread and butter of the Steamship Authority," Mr. Parker declared.

"We are looking at a change in our service model and this is a basic component of that changing service model and a new business plan," he said. Asked to elaborate on the new business plan, he declined.

"Not at this meeting, but this is within the context of that," Mr. Parker said.

Robert O'Brien, a member of the boat line board from Barnstable who has voice but no vote, expressed support for the business decision, but Mr. O'Brien also questioned the sportsmanship of his fellow board members in preempting Hy-Line on the Schamonchi purchase.

"From a financial standpoint I agree with this, but I have some very mixed feelings, because we know that the Hy-Line has been negotiating for the purchase of the Schamonchi," Mr. O'Brien said. "I am just a little concerned about the fallout," he added.

Mr. Parker's answer was blunt.

"The right of first refusal was there and anyone who wanted to buy the Schamonchi had to address it," he said. "I have not personally had any negotiations or conversations with Hy-Line or with Mrs. Thompson. But this is a route that exists and we feel we need to acquire that route," he added.

"We have a responsibility to the rate-payers and fiscally this makes sense," agreed Steve Tornovish, a member of the SSA financial advisory board.

The agreement to buy the right of refusal from Mr. Packer's company was approved by a 2-0 vote yesterday. The agreement includes:

* An initial payment of $10,000.

* Fees for the use of the Packer-owned Pier 44 wharf in Vineyard Haven for the duration of the lease (the lease runs for three more seasons) - whether or not the boat line decides to use the wharf.

* A commitment, if the SSA buys the ferry, to pay Mr. Packer 7.5 per cent of the first $1 million sale price plus 5 per cent of any sale price over $1 million.

Following the meeting yesterday, SSA general manager Armand Tiberio said he agrees with the decision.

"Yes, I think at this stage, with the information we have, it's a good business decision," Mr. Tiberio said. "It gives us an operational base [in New Bedford] and it allows us to give our customers a choice, and that's the key," he added.

Mr. Tiberio said if the sale goes forward, the SSA will operate the passenger service between New Bedford and the Vineyard next summer. He said the ferry carried 100,000 passengers on its summer run last year.

The Schamonchi is a 130-foot, 640-passenger ferry that first came into service in 1978. The ferry operates between a privately leased pier in New Bedford and Pier 44 in Vineyard Haven. The SSA has the power to license its competition, but the Schamonchi is a grandfathered carrier.

The Thompson family presence in New Bedford ferry service in fact dates back even earlier than the Schamonchi; from 1973 until 1978 the Thompsons operated the Manisee, a smaller, open-decked passenger ferry.

Schamonchi is a Gaelic word that means Spirit in the Sea.