A man in Annapolis knows so much about sailboats that last year 800 prospective new-boat buyers paid $15 or $20 for his expert opinion on which boats are good and, more significantly, which aren't.

His name is John Potter Jr., and he views sailboats the same way Ralph Nader does cars. He surely fills a need. The occasional guarded piece in a boating magazine expected, it's rare to have an authoritative voice raised against anything coming out of a boat plant. The products usually extolled with the fervency one associates with the vending of automobiles.

But Potter claims his experience and expertise give him the right to proclaim that some new sailboats costing thousands of dollars won't sail well, won't last long and may even respond to the first strong breeze by capsizing.

Potter's stance is based on 20 years experience in designing, constructing and maintaining boats, plus 50,000 miles of high-seas sailing. Because he also has done considerable marine survey work for potential purchasers of used boats, he reckoned that that effort could be logically extended into a service for people groping with the uncertainties of buying a new product.

He went to many of the local boat dealers and talked them into letting him examine their stock. Once aboard a boat he spent as much as two hours picking it apart without actually taking it apart.

After looking he began writing, reducing his findings to a three-page report. Although he was specific about what he didn't like, he went to some pains to tell a new owner what he could and should do to make things better.

Not a single boat that Potter pronounced fair or foul cost less than $4,000. Many of the bigger boats were priced in the mid-$50,000s. Potter had no trouble finding people who, for a fraction of that amount, would buy his considered opinion. For a boat that didn't exceed 30 feet, his report cost $15. And if it did exceed it, the bill only came to $5 more.

Potter reasoned he could reach an even wider audience by abbreviating the reports and selling them in book form.

The work is called "Annual Yacht Survey Reports." It covers 90 boats, consists of 209 pages within a 10 inch by 11 1/2 inch loose-leaf binder and costs $30. The individual reports in the book are not quite as comprehensive as the single reports that sell for $15 or $20, but they go right to the point.

He says this about the Seafarer 22: "(There are) design weaknesses which can affect the water-tightness of the boat . . . Several through-hull fittings are not equipped with seacocks or proper shut-off valves . . . The mast should be stepped on the keel. This one is deck-stepped and the chain-plate attachments are not very substantial - they lack even a backing plate . . . There is not standard mainsheet traveller, making proper sail trim difficult . . ."

It also should be noted Potter is not stingy with his praise of good sturyd merchandise.

Serious shoppers at the upcoming Annapolis Boat Show - it will have a four-day run starting Thursday - might find the Potter product useful. Potter will be in the Independent Yacht Survey Co. booth and he says the book will be on sale as a boat-show special for $24. If the boat you're interested in isn't in the book, he'll inspect that boat after the show.

Copies of the book also may be purchased from Independent Yacht Survey Co., P.O. Box 32086 Washington, D. C. 20007. The $30 includes postage.