Nestling in the same category with sea cocks, bilge pumps and limber hole is the lowly boat trailer: It's one of those unromantic boating accessories that everyone ignores until, suddenly, a day afloat is ruined because the thing doesn't work any more.

Recognizing the importance of boat trailers, the U.S. Coast Guard has produced a pamphlet to help skippers choose among the myriad sizes and styles available. The full text is available free, from the Coast Guard Auxiliary National Supply Center, Watson Industrial Park, Crestwood, Missouri 63126.

Here are some highlights of the Coast Guard study:

All boat trailers must now be classified by the manufacturers according to their load-carrying ability. The four classes range from trailers capable of carrying more than 5,000 pounds. When selecting a trailer, the entire weight of boat, gear and trailer should be added together. The Coast Guard recommends that if this totdal figure is within 15 percent of the maximum of one class, but buyer should select a trailer in the next larger class.

Towing safety hingers on a proper match of the ball on the two vehicle to the trailer coupler. A trailer dealer should provide buyers with a table showing the correct ball sizes and coupler shank diameters for all trailer weights.

Safety chains provide an emergency backup to the towing ball. They need to be crossed beneath the shank to catch if it the ball breaks, and hold it off the road surface until you can bring the rig to a stop.

Boats are designed to float in the water, where their total weight is distributed over the entire hull. Trailers concentrate the boat's weight on a few support points. The more support points, the less risk of hull damage.

Ropes and loose cinch straps make inadequate tie-downs. Strong tie-downs are needed at both bow and stern, anchored to hooks or loops welded onto the trailer.

Trailer brakes should be triggered both by brakes on the towing vehicle and by an emergency system in the event the towing system fails.

Most boat-trailer breakdowns stem from tire failures and wheel-bearing failures. Such breakdowns usually are traceable to poor maintenance by the owner. Tire pressure and wheel-bearing lubrication ought to be checked before each trip. Trailers designed for use in salt water should have special wheel hubs to protect the bearings from corrosion.

Towing vehicles need a lot of power and weight. Even most full-size passenger cars have trouble pulling a heavy trailer and boat, and your brakes, transmission, and cooling and suspension systems may need beefing up to prevent breakdowns and expensive repairs. In many cases, heavy-duty, wide tires will prevent a towing vehicle from spinning its wheels on a launching ramp at low time when trying to retrieve a boat.

You can ease some of the strain on the towing vehicle by installing a load-equalizing hitch, to spread the load more evenly and prevent excessive wear on the rear axle.