We had already hit 72.5 mph on an open, flat spot of the Intracoastal Waterway in North Miami Beach, throttling back only when we ran out of room; we were closing in on a bridge pretty fast. "Let's go outside now," said Michael Stocker, Magnum's factory captain who was driving the company's new 51. "After all, the only reason to have a Magnum is to go out on the ocean." A few minutes later we were heading out Baker's Haulover Inlet, making a sweeping left turn and simply eating up the miles in the Atlantic Ocean, powering ahead ever faster on the big, beautiful, headturning performance machine. As Stocker nailed the throttles, the boat's huge twin C32 Acert Cat diesels spooled up quickly, their combined 3,700 hp producing a true adrenalin rush of power, the Arneson surface drives churning up a foaming white rooster tail in our wake.

When we hit 50 or so, I thought that the midrange acceleration was phenomenal as it pushed me back in my seat. But the surge kept going and going until the GPS topped out at an even 74 mph and Stocker backed off a bit. This, I thought, was an awesome display of power in an opulent cruising boat, the newest entry from a brand recognized the world over for its lines, its performance and its star power. At 74 mph, the 51 is the fastest big Magnum yet. It's one thing to go fast in a stripped-out race boat (I know — I've driven a 46-foot Fountain 162 mph); it's quite another to go fast in one of the most luxurious boats in the world, one that virtually announces its international sophistication and undeniable sex appeal. A Magnum, in fact, is not for the shy or introverted. Indeed, the boat drew approving waves and smiles from everyone we passed on other vessels on our way from the Magnum factory on the old Thunderboat Row of Northeast 188th Street in North Miami Beach out to the ocean.

But the Magnum's performance went well beyond pure power and speed. Out on the ocean in two-foot seas, Stocker put the wheel all the way over at 57 mph, and the big 52,000-pound Magnum simply carved a beautiful turn; when we straightened out, the GPS read 48 mph and we were ready to take off again, the boat totally under control. One of the most impressive things about the Magnum is that it's solid — solid as can be. "We always put quality first," Katrin Theodoli, the president of the company, had told me back in her secondfloor office at the factory. And that focus showed on the water. Even at wide-open throttle, even during a highspeed turn, we didn't hear a squeak or a rattle or a thump. We heard the engines, the sound of the water going by and the air noise, but we didn't hear the boat. At one point on this beautiful early-winter morning, under a blue sky dotted with just a few puffs of clouds, we jumped the wake of a 60-foot Hatteras to see what would happen. Actually, nothing happened. The Magnum cut through the wake at about 60 mph and just kept going — no pounding, no rattles, no problems at all. "With this boat, you can cruise fast, say 60 mph if you want to, and be perfectly comfortable," Stocker said. "Or you can settle in at 40 and just cruise all day."

Magnum has been making fast boats on 188th Street ever since the late and legendary Don Aronow, "the King of Thunderboat Row," started the company in 1966 to build a 28-footer he wanted to race in the world championships (he won).

Over the years, the company has morphed from building small race boats to constructing large, elegant performance yachts — most now in the 50-, 70- and 80-foot range, although a model for a new 100-footer is in Theodoli's office and the prototype of a 27 is on the factory floor. As it grew, the company's clientele also expanded well beyond the boundaries of Miami Beach, or even the United States, to include Europe and the Middle East, anyone, in fact, with enough money to be able to afford one of the boats and who wanted to summer on the Med. "More than 80 percent of our boats now end up in the Med," Theodoli said. A brilliant (she speaks six languages), beautiful, Swiss-born former champion skier and Italian marchesa, Theodoli has been president of the company since her husband died 20 years ago. This boat, she said, was built for a client in the Middle East who summers in the Med; he wanted a Magnum for its speed, its lines and its quality. Actually, the new Magnum 51 isn't the client's primary boat; it's going to be a tender on his megayacht. He had to reinforce the surface of the garage of his yacht to hold the Magnum's weight (each 12-cylinder Cat weighs about 7,000 pounds). The big engines were necessary because the owner wanted the boat to go at least 70 mph, he wanted to drive it himself, and he wanted a 300-mile range so he could operate independently of the yacht for a full day's cruising. He also ordered the color, which Magnum describes as persimmon, defined as a red or orange plum-like fruit. "The client loved this color," Theodoli said. "It's hard to spray metallic paint, but we sprayed it in Imron and wet-sanded it and polished it to a mirror finish." The result is unique and stunning. Over the years, Magnum has developed a client list that epitomizes the dual nature of the boat's appeal: On the one hand, you have the jet set, who want all the über-luxe nature of the brand; on the other, you have the military, which wants the proven performance, the hulls that last, the engines and drives that meet the most stringent specifications. Indeed, Magnum owners have included the king of Sweden, the king of Spain, the emir of Kuwait, the crown prince of Qatar, the former king of Greece, the Agnelli (Fiat) family of Italy and Silvio Berlusconi, the current prime minister (and media baron) there. People treasure their Magnums; Christina Onassis left her ruby-red Magnum to her daughter, Athena. The company will go to great lengths to develop its international clientele. At one point, Theodoli herself delivered a new boat to the prime minister of Gabon in Africa. She also learned Japanese when a Tokyo businessman bought a boat and invited her to the launch ceremony there. In the United States, Lenny Kravitz has one, but then other owners include the U.S. Navy, the U.S. Coast Guard and U.S. Customs.

All Magnums have the classic high-performance hull with 24-degree deadrise at the transom. They all also now have Arneson surface-piercing drives that deliver greater speed (because of less drag) and better fuel economy, translating into longer range. The 51 has ASD 14s coupled to nibral five-blade, surface-piercing props. The driver trims the drives with rocker switches just above the Twin Disc throttles to the right of the wheel. Matching rocker switches to the left of the wheel are for trim tabs, but they generally would be used only for lateral stability. Standing at the wheel, you look out over the sweeping, low wraparound windshield to the long, gently sloping foredeck that seems to go on forever. (The decks are drawn by Luiz de Basto, the Brazilian designer.) There are no side or bow rails; they would spoil the looks. Magnum hulls are hand-laminated and vacuumbagged; the bottoms are solid glass, generally two inches thick. The interiors, custom for each boat, are made of aircraft-type Nomex composites to reduce weight (the interior of the 51 was not yet finished). Magnum's goal is to keep the boat stiffer and the deck and interiors lighter for stability and speed. Some manufacturers take weight out of the hulls; in Magnum, weight, stiffness and mass are crucial for the comfortable ride.

All this translates into a fast ride from Miami to the Bahamas for lunch, a swim, a dive, a walk on a white, powdery-sand beach, and then back in time to change for dinner, arriving on one of the most beautiful boats in the world. Think St. Tropez and Costa Smeralda; Newport and Nantucket; Long Beach and Catalina. "With a Magnum, you can leave the world behind," Theodoli said. "You're in a totally different world. You can go relatively fast in total comfort and then come back home. It's a different experience."

Magnum has come a long way from Don Aronow's first 28-footer. Easily bored, he sold the company after a few years to a conglomerate; it lost money and he bought it back for much less than it paid him for it. Meanwhile, Ted Theodoli, an Italian marchese, had embarked on an international advertising career in New York and Milan. He too got bored and turned to selling boats on the side. When Aronow made a fast 35-footer, Theodoli sold it to Europeans who wanted to explore the Med. The Europeans soon wanted bigger boats, while Aronow wanted out. In l976 Ted and Katrin Theodoli (they met on a boat) bought Magnum from Aronow for $1.5 million. Aronow moved a few doors down 188th Street and then started Cigarette. For his part, Ted Theodoli introduced the Magnum 53, which launched the company onto the forefront of high performance luxury cruising boats. For years, Ted was the face of the company, showing the boats and visiting stars and royalty around the world, while Katrin worked behind the scenes on the business side. He died suddenly in 1990, and lots of people told Katrin to sell the company.

Since then, of course, Katrin Theodoli has introduced a series of ever-larger, ever-more-luxurious, ever-faster models, and she has become the successful face of the company, spending summers in the Med, showing off new Magnums. In Florida, she water-skis every morning — behind a Magnum. "I have a passion for what I do," she said, "for the sea and for boats. We just want to be the best."