Magnum 80By George L. Petrie
Big diesels. Arneson drives, and surface-piercing props compose a gear-head’s symphony.
The day began, appropriately, in the muscle boat Mecca known as Thunderboat Row, at the end of Miami's 188th Street, home to Magnum Marine and several other well-known go-fast builders. Having been summoned on short notice, all I knew was that the yacht was 80 feet long, was powered by twin 1,800-hp DDC-MTUs turning surface-piercing props through Arneson surface drives, and had a top speed of more than 50 knots.
From that scant description, my imagination conjured up the image of a pencil-thin hull, painted like a discotheque, blaring a cacophony, throwing a roostertail tall enough to blot out the skyline. My preconception was wrong on all counts. Magnum is a more refined breed, a well-toned body in a custom-tailored tux. That is, except for the roostertail.
What I saw alongside Magnum's dock was a gleaming boat that captured my eye with subtle contours rather than flashy paint. Italian design firm Pininfarina introduced only minimal styling accents to accentuate the vast foredeck, like a pair of wedge-shape ports to admit light and air to the spaces below and a pair of angular scoops flanking the cockpit that direct fresh air into the engine room.
I could also see that this yacht was wide--a hefty 20 feet of beam that would offer plenty of room for accommodations below and spacious seating areas in the cockpit. Indeed, by my count, there's open-air seating for at least 20, with room for at least four to stretch out on a large sunpad at the stern. Beneath that sunpad is a big garage for PWCs or a tender.
Magnum plant manager Bob Adamo explained the reason for doing the test on short notice was that on the following day the yacht would be loaded onto a ship bound for Italy, where her custom interior would be installed. I took that news as a stroke of good fortune because it would allow me an unobstructed inspection of the hull's interior structure and wiring and piping systems. A close-up look at the hull-to-deck joint showed it to be bonded, fiberglassed, and secured with bolts every few inches. Attaching the stiffeners to the hull, secondary bonds were smooth, with a generous overlap, to minimize the chances of delamination as the structure works in a seaway. All through-hulls were bronze, grounded and with a proper seacock. And all wire runs were neatly bundled, wrapped, and secured to prevent chafing.
Driving an 80-foot yacht at high speed through heavy seas can put tremendous strain on her structure, but the details of her laminate schedule convinced me that the Magnum could take a punch. Hand-laid biaxial and triaxial hull laminates form the inner and outer skins, bonded to a 1 1/2-inch-thick foam core. Interior structural bulkheads are also cored with 1 1/2-inch foam, while joiner bulkheads are cored with 1/2-inch honeycomb to cut weight.
Though her accommodations area was just a shell, the engine room was complete, so I was able to check her machinery layout and accessibility. Flanking the centerline, DDC-MTU diesels were mounted on massive steel engine beds that were through-bolted to pairs of longitudinal stringers that gave new meaning to the term backbone of the hull. I judged the inboard stringers to be about four feet deep and the outboard pair to be at least two feet.
Access was good on both sides and above the main engines. Beneath the entry hatch, a centerline platform offers nearly six feet of headroom, with about three feet of open space between the engines. Located outboard to either side, the Lugger gensets were easily accessible, as were the Cruisair units mounted on shelves above them. Circuit breakers and genset controls were conveniently located alongside the steps down into the engine space. For normal maintenance, the layout seemed fine. But what if an engine needed a major repair? Though there's more than two feet of clearance above the engines, addition of a large access hatch (normally sealed) above each engine would allow for much easier handling of heavy components.
Speaking of handling, conditions offshore were perfect for testing this yacht. Seas were running three to five as we left the inlet at Fort Lauderdale, building another foot or so by the time we were offshore of Miami. Cruising at a steady 45 mph, the Magnum took the ocean in stride, barely budging in waves that seemed like ripples lapping at her bow. It took the sight of a 60-foot convertible laboring out into the Miami ship channel to make me realize that we were really in some heavy stuff.
Once inside the smoother waters of nearby Biscayne Bay, we broke out the test gear to measure speed, fuel rates, and sound levels. It was then that I realized just how quiet the Magnum really is. While the Arneson drives and surface-piercing props do create a bit of a stir, the engines exhale through huge mufflers that keep sound levels in the low 80-dB range (65 is normal conversation) at all but WOT, and most of that seems to be wind and propeller noise. At one point, when idling in neutral, the engines were so quiet I was sure they'd stalled.
Revving the big diesels to full rpm, we measured a top speed of nearly 52 mph. Adamo explained that the yacht had been idle at dockside for several weeks, with no bottom paint, and had grown a sizeable "beard" on her hull. He figured the fuzzy bottom cut 2 or 3 mph off her top speed. I countered that the hull was still shy by a couple of thousand pounds of interior weight, so our numbers were nevertheless likely to be a good indicator of her all-up speed.
Not to minimize the Magnum's impressive performance, but it was the Arneson surface drives that were the real showstopper. Mounted at the base of the transom, the twin drive units extend straight back about five feet, with heavily pitched, gnarly-looking surface-piercing props at the end of each shaft. While the units turn and trim up or down somewhat like a stern drive, they have no bevel gears so they can carry the huge torque of those 1,800-hp diesels. And because the only parts of the drive system that are actually in the water at higher speeds are a small fin and the bottom half of each propeller, Arneson drives produce much less drag than conventional inboards or even stern drives.
But the most striking thing about our surface drives was that roostertail. Like eggbeaters on steroids, they throw a spray that looks to be 100 feet high and at least four times that long. A fitting counterpoint to the Magnum 80's understated style, the cascading wake makes an indelible visual statement.
With the props trimmed so near the surface, I felt like they might be prone to aeration in a fast, sharp turn. But a modest adjustment promptly addressed my concern. The trick is to trim the drives down to maintain prop bite as the boat banks into a turn. And the same tactic works while accelerating out of the hole, to give the surface-piercers a good grip. Then, as the diesels spool up, raising the drives lets the hull come up onto an easy plane. Properly done, there's very little bow rise, important because the foredeck stretches about 50 feet forward of the helm.
Although my field of vision was not hampered by bow rise, I did have one minor gripe. While the Magnum's low, steeply angled windshield complements her profile, it fails to fulfill the customary function, acting instead to divert wind directly into one's face, whether seated or standing at the helm. On the plus side, with her generous freeboard and beam there was never a drop of spray to dampen the spirits. Or maybe all the spray just got sucked to the stern and shot out behind in that fabulous roostertail.
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This article originally appeared in the June 2001 issue of Power & Motoryacht magazine.