Puerto Rico Marine Management Inc., operator of one of the most important steamship lines at the port of Baltimore, is considering schedule changes that could divert a substantial amount of cargo to other ports.

PRMMI operates Navieras de Puerto Rico, which is owned by the Puerto Rican government. The company, which is among the top six lines at the port, is one of four that uses the state's new high-tech Seagirt Marine Terminal near Dundalk.

Under the current schedule, a Navieras ship leaves Baltimore each Saturday and sails directly to San Juan, Puerto Rico, arriving there Tuesdays. Under the schedule being considered, ships would leave Seagirt on Sundays and go to Puerto Rico by way of Jacksonville, Fla. Because of the extra stop, the ships would not arrive in Puerto Rico until Friday and, in most cases, cargo would not be available to customers until after the weekend.

Roberta Greening, a spokeswoman for PRMMI, said yesterday that the company had not made a final decision on the schedule change. She confirmed, however, that routing the Baltimore ships by way of Jacksonville was one option.

Louis Kistner, spokesman for SCM Chemicals, said the changes could double the time it takes to get cargo to a customer in Puerto Rico. This would force his company to consider alternatives, he said.