IT'S USUALLY the engine that spoils an otherwise lovely sailboat charter, so we've learned to relax and ignore the bloody things.

It was the engine that clanged to a shuddering halt every other day in the Grenadines a few years ago and left us pleading with fix-it people on the radio when we should have been sailing.

It was the engine water pump that started spouting when we brought the charter yacht up the inland waterway last spring, and after that was fixed it was the alternator that quit and left us powerless and adrift on the Waccamaw River. Then the water pump quit again.

It was the starter that wouldn't start that had us hand-cranking the diesel on a Chesapeake charter last fall, and the battery that wouldn't batt that left us stuck at dockside early this spring, cursing the mechanical fates.

Okay, marine engines are fickle. Everyone who owns a sailboat either learns the whims of his auxiliary power plant or spends a lot of time cussing.

One engine wants its carburetor tickled; another wants to be started dry. One wants half choke, another none; one demands its throttle pumped, another wants it floored and yet another requires no throttle at all when hot, half- throttle when cold. These things you learn with time.

The problem, of course, is that if you rent someone else's boat (called "chartering") you don't learn the tricks until it's time to turn the wretched thing in.

So it was the other day when Joe, Jim-Bob and I scurried down to the dock in Annapolis on a wild, gray, windblown day to sail a sleek, 33-foot Scandinavian rocket called the BB-10. We'd arranged the one-day charter a week in advance and blind luck gave us a howling northwester to enjoy.

The yacht lay at the shoreside end of a dock full of oher yachts, leaving a narrow passage to open water to negotiate under power. We stowed the beer, rigged the sails and turned the key. The little diesel said "Rrrr, rrrr, rrr" slowly, and then said nothing at all.

Jim-Bob grabbed the hand-crank and cranked until his ears got red, but the diesel emitted only one trace of smoke on about the 15th crank, then clammed up entirely.

The money was paid, the stores laid in, a breeze promised to blow all day and we couldn't get out of the slip.

It was unqualified disaster, until someone came up with the idea of bringing a car across the beach and alongside the dock, running jumper cables to the boat battery and cranking the diesel to life. After a fashion it worked, and we were off on a glorious, all-day sail.

It wasn't till mid-afternoon I suggested we try the key and see if the engine would start again when we had to get back in.

"Don't touch that key," said Joe. "If it won't start I don't want to know. I'd just spend the rest of the day worrying."

So we blasted around the Severn River, went out in the Bay for one last hammering beat and turned for home.

Fifty yards out we turned the key and heard the familiar, dying bleat. "RRRrr, rrrrr, rr . . . "

Joe said he'd take the helm, and he hatched a plan. Sailing into a slip is tricky business, doubly so if you don't know the boat. There was a strong following breeze pushing down the passageway. One mistake and we could knock down a structure or two.

The scheme was to sail to the passageway entrance under mainsail alone, as slowly as possible, then drop the main and let momentum carry us the final 40 yards to the slip, where we'd turn hard right and stop however we could.

If the mainsail jammed and didn't come down, of course, we'd be blasting down the passageway under full sail, headed straight for concrete.

And there was a nice audience. On the left lay a restaurant full of people staring out a picture window; on the right was a line of boats full of bored yachties looking for something to laugh at.

But the sail came flapping down on cue, the boat floated in as if on a track, we grabbed the appropriate lines and in moments we were safely in the slip, swilling beer in the cabin and grinning.

Next time we won't even try the damned motor.

Spring is ideal for Chesapeake Bay sailboat charters because there's usually a breeze, so you don't need an engine.

The BB-10 we chartered through Scandinavian Yachts Ltd. is an absolute gem of a sailboat and at $95 a day the best chartering bargain I've seen on the Bay. Rates for plusher boats range up to $2,000 or more a weekend.

For a list of about 30 charter sailboat operators in the Annapolis area and the close-by Eastern Shore, write to Chesapeake Bay Yacht Charter Association, Box 4022, Annapolis MD 21403.

Other, less formal charter arrangements are advertised by individual boat-owners in the Sunday boat advertisements, which in the Post apper in the Sports section.

Close to home, Belle Haven Marina (768- 0018) rents 19-foot sailboats on the Potomac in Alexandria, as well as a 34-foot yacht with captain available for rent by the hour; and Washington Sailing Marina (548-9027) has 14- and 15-footers for rent by the hour.