Sabotage by Fletcher Knebel (Doubleday: $16.95; 300 pp.)

Mitch Donahey, the hero of Fletcher Knebel’s new novel (described by its dust jacket as a “tale of international intrigue, suspense, and romance on the high seas”), is a shrewd oil tanker broker with two problems. One is a relentless nightmare prompted by his participation in a Vietnam incident not unlike the one that transpired at My Lai. The other is his infatuation with a mysterious beauty who sailed out of his life after a weekend of monumental passion. “Their embrace lasted for long minutes,” Knebel writes of the initial encounter, “two bodies clinging together while waves broke against the seawall, receded across the sand and surged back in rhythmic flood.” (If the imagery is too subtle, not to worry. The author also provides descriptions that are more anatomically to the point.)

As it turns out, the lady, Mona Harkinson, is a mildly feminist sea captain whose tempest-tossed supertanker spews a multimillion- dollar oil slick along the Delaware shore. She’s being hounded by her bosses and the press, not to mention the FBI. If Mitch wants any more breakers to hit the old seawall, he’s going to have to prove that the beached ship was the result of . . . sabotage.

This is an acceptable-enough premise for a romantic thriller, I suppose. What’s annoying is that Knebel, an old pro at the suspense game (he co-authored “Seven Days in May” and soloed on more recently acclaimed works like “Night of Camp David” and “Vanished”) has submerged the hard-edged aspects of his yarn in favor of turgid, soft-core romance writing.

The best sections of the book are those that occur early on, before Mitch and Mona have rekindled their affair. Knebel is at the top of his form in describing the wheeling and dealing of a wily tanker broker or the way danger multiplies when a ship starts to flounder and die.

Once the lovers are reunited, however, Knebel concentrates on their coupling. Secondary characters are murdered, sinister plots are hatched by the villain, Sarge, and his Yakuza pals (the author takes advantage of one of Mark and Mona’s infrequent sleep breaks to explain the involvement of the Japanese crime lords) and fortunes are made and lost somewhere far from their bedroom window and Knebel’s primary concern.

Finally, hero and heroine leave their hotel room long enough to be captured by the Yakuza and held for ransom. There is no excitement or suspense in any of this. Nor is there any doubt that truth and justice will prevail. The fates of gangsters and good guys are as predictable as the tide. The surprise is that while huffing and puffing to keep Mitch and Mona afloat in their concupiscent bliss, Knebel let the rest of his novel get hopelessly hung up on the shoals of their athletic but otherwise ordinary romance.