Two old Chesapeake salts don't agree on much, but they do agree on the proper-size boat for fishing and exploring the Bay.

"Seventeen feet," says Salt A.

"Seventeen foot," says Salt B.

Moreover, they say a 17-footer is not only the smallest boat they'd feel comfortable with on the Bay, it's also the biggest.

A principal pleasure of cruising the Chesapeake lies in exploring the little tributaries that feed it, which are often as wild as they were when Captain John Smith came calling in 1608.

But those creeks are generally protected at the mouth by shallow sandbars. A big boat that draws more than 11/2 feet can't get over the bar and thus is forever banished to boring big water.

So you want a shoal-draft boat.

On the other hand, the Chesapeake can be a mean and nasty place, summer or winter, and anyone who has weathered an afternoon line squall here wants no part of halfway-seaworthy vessels.

A properly designed 17, operated by a reasonable soul who listens to the forecasts, can withstand all but the worst Bay weather. There isn't a 15-footer made that comes close.

Armed with this incontestable truth, Boatman went a-looking and found him a proper 17. Then he went back to the Salts Two and demanded advice on proper power to send him on his way.

But here the Salts differed.

"Hang a monster on the back and you won't regret it," said Salt A, recommending the maximum horsepower for which the boat is rated, 135.

"Go light," said Salt B. "Who wants a great hunk of metal on the transom, weighing you down? You'll save money buying it and get better efficiency with a 70."

Boatman lost his macho stance when his beard turned gray, and talk of saving money hits him hard. He ran right out and bought the 70.

And learned to regret it.

It's not a matter of comfort, he found. Adequate power is a matter of survival. Boatman discovered this when he carried three hearty hunters and camping gear across five miles of open Bay on a brisk November morning. When a headwind sprang up, he eased the throttle forward, then forward some more.

Before long, the hammer was all the way down and the sleek 17 was wallowing in the trough, unable to stay on plane. Mountainous three-foot seas assaulted the transom and more than a little water washed over. A hunter was dispatched to the stern to bail, but it was unclear whether his work offset the effects of the increased weight aft.

That was one of several hair-raising voyages Boatman made before he broke down and traded the 70 in for a 115; he has not regretted the change, though his pocketbook protests the expensive lesson. The 115 puts the boat quickly on plane, its proper cruising attitude, and keeps it there regardless of conditions.

Nor is the bigger motor necessarily a bigger gas-guzzler. The 70 burned up to eight gallons per hour when it was flat out, as it often had to be. The 115 burns 61/2 gallons an hour at cruising speed.

Lest others go through the same trial by error, one of the major power manufacturers offers some guidelines on proper powering of small craft.

Dick Snyder, chief of hydrodynamics for Mercury, reports that his extensive testing of hulls and motors indicates that the maximum rating a manufacturer stamps on a power boat is generally conservative, and he urges boat buyers to go with that size motor.

The ratings are based on a simple formula you can figure out yourself: Multiply the length of the boat by its width at the transom, double that number and then subtract 90. The number you get is the rated horsepower for your boat.

As a general rule of thumb, says Snyder, most small boats fall into these size-to- power categories: a 20-footer needs about 200 horsepower; an 18- to 19-footer needs 150; a 17-footer, 90 to 115; a 15-footer, 50 to 70. A 13-foot boat can take anywhere from a 10 to a 40, with the bigger engines reserved for boats with remote controls and steering wheels.

But Snyder is a firm believer, as is Boatman now, that underpowering is at least as dangerous with a planing-type hull as overpowering. Snyder believes most marine accidents are the result of bad judgment or error by the operator, usually at high speed.

Boatman's advice? Hang a monster on the back, within reason, and you won't regret it..