ST. LAWRENCE SEAWAY BOTTLENECK. Blocked Great Lakes shipping shows 54-year-old system may be wearing out

Gazing down from the deck of a ship more than two football fields long, a deckhand leans against a railing. Far below him, sitting on a chair suspended by ropes, another crewman carefully paints fresh lettering and new water marks on the sloping hull. It's been 10 days at anchor.

Some 65 ships -- grain carriers, ore carriers, and freighters -- are stranded along the St. Lawrence Seaway. The focal point is Lock No. 7 on the 26-mile Welland canal, which links Lake Ontario and Lake Erie. Repair crews worked around the clock this week and last to brace with steel beams a 125-foot section of concrete wall that collapsed Oct. 14. Repairs could take weeks.

These are long days for crews doing make-work. But they are even longer for shippers who depend on the 2,342 mile seaway system to link the Great Lakes with the ocean. This should be the highpoint of their year. Instead, the wait gets worse as the cost of sitting grows at $10,000 to $20,000 a day per ship.

As they anticipate the day the Welland Canal reopens, shipowners must also now weigh the costs and possibility of shipping their grain or goods by rail or some other means. In Moscow, Soviet economic planners are toe tapping, awaiting Canadian grain worth $5 million.

Costs are already rising. But for the United States and Canada, the long term loss could be worse if shippers lose confidence in the seaway system. Some shipowners saw profits drain away last November when 165 ships were stranded by a lift bridge that refused to lift for 18 days.