I wasn’t sure what to expect. The huge new 425 hp 5.6L V-8 Yamaha was mounted on a Sportsman Masters 267 Bay Boat, which seemed to be a bit of a mismatch. At the dock the boat sat a little stern-heavy, owing to the heft of the outboard, which looked disproportionally large from the dock. Viewed from the helm, however, it looked like it belonged there thanks to its narrow forward profile. Soon enough, a shove of the drive-by-wire throttle caused the big V-8 to roar to life.
After our test boat reached plane, with a fair bit of bowrise, in four seconds, the velocity quickly picked up steam: GPS showed we reached 30 mph in 6.2 seconds. After I trimmed it out, top speed reached 61.2 mph, nearly 10 mph quicker than this boat’s top speed with Yamaha’s F300. I was a bit confused, since the presentation we just heard indicated this outboard was designed for large offshore boats, but here it was, performing well on a bay boat.
Yamaha isn’t afraid to shock people with its bold designs, and the XTO Offshore (aka the XTO OS) is no exception. Mercury and Seven Marine have come out with high-horsepower outboards, but the 425 hp Yamaha is the most powerful non-supercharged outboard in the industry, and it’s big, weighing in at nearly 1,000 pounds for the 35-inch shaft and 952 pounds for the 25-inch shaft. While this seems to be at odds with the current lighter-is-better trend set by engines such as Yamaha’s own V MAX SHO 250 that weighs 505 pounds, the mission is different. From the get-go this motor — like its big-boy predecessor, the F350 — was designed to push a new generation of outboard-powered boats that is now stretching past the 50-foot mark, as well as future boat designs. And while power-to-weight is a familiar yardstick for measuring outboards, Yamaha’s primary mission is reliability followed closely by performance in all parameters, whether going fast, slow or in reverse.
While it’s a huge engine, Yamaha designers cleverly packaged it so it still has a 28½-inch mounting center, which, amazingly, matches the F250, so it’s possible to repower on almost any flat transom that can handle 425 hp. Its slim forward profile allows it to tilt into most splashwells, and its increased 73-degree tilt range can raise its lower unit 5.4 inches higher than the F350, which should put it clear of the water — great news for owners who leave their boat in salt water. And the tilt pump shuts off when it reaches its maximum rather than continuing to run like on all other outboards.
The XTO OS is the first four-stroke to feature direct injection. Using a three-stage system with five fuel pumps (three electrical, two mechanical), this high-pressure system injects fuel directly into the combustion chamber at a staggering 2900 psi for extreme atomization. This far outpaces Yamaha’s discontinued HPDI two-strokes, which operated at only 1100 psi. Like high-performance sports car engines, the XTO OS uses an industry leading 12.2:1 compression ratio to help create its 425 hp, but it can run on 89 octane gas. High compression can lead to overheating, but it is averted by the cooling effect of the fine direct injection fuel mist.
Raw horsepower isn’t nearly as important as thrust, however, when it comes to moving mega-center console boats such as the Grady- White 456, a 45-footer with a 22,500-pound hull. To move more water, the XTO OS can swing up to a 171/8-inch prop, which is massive compared to a 15-inch prop an F300 might use. For context, visualize the difference between a large pizza and a medium.
To harness all the power, torque and abuse offshore outboards encounter, the XTO OS has a massive set of hardened gears and shafts, which contribute to its weight but add to its durability and longevity. The offshore bracket and wide-span powerhead mounting system work in conjunction with its long-span mounting system to minimize vibration and provide a secure platform.
Another element that adds to its overall weight is the integrated first-of-its-kind electrical power steering system. It’s a real time saver for people who are used to rigging hydraulic steering systems and their attendant pumps, reservoirs and endless yards of hydraulic lines that must be run from the helm to the engines. Another advantage is that it only draws electrical power when the driver is turning it, unlike hydraulic pumps, which continually draw power, so its huge 90-amp alternator won’t be taxed as it keeps the batteries charged to provide power for the electronics, livewell, stereo, radar and more. The lack of hoses also makes the installation look really clean.
One of the reasons outboards don’t do so well in reverse is that the exhaust runs through the hub and creates turbulent, aerated water the prop must somehow gnaw its way through. The XTO OS features an innovative exhaust routing system that directs exhaust gases above the cavitation plate at less than 2500 rpm, which gives the prop clean water to spin through. It’s no gimmick. In reverse this motor romps, which is great news for anglers who back down while catching big fish.
Aboard a Grady-White 456 with quad XTO OS outboards — 1,700 hp for the math-challenged — the driver buried them and we rocketed rearward. No doubt this is one of the reasons Yamaha went to great lengths to make the seven-piece cowling watertight, with all but the top panel, which opens easily when it’s time to check or add oil, dogged down by fasteners. In another nod to maintenance, owners can change the gear oil while the boat is in the water.
Despite a rigged weight of 35,454 pounds, the Grady-White Canyon 456 reached a top speed of 58 mph. The only downside I noted was the noise. I measured 96 dBA at wide-open throttle, which was a little surprising given that Yamaha outboards are noted for their quietude.
While we thought the F350 would be for big boats only, it wasn’t long before we saw them on virtually every kind of boat, and that will probably be the case with the XTO OS. In addition to large offshore boats, I expect to see them on 30-something pontoon boats and midsize center consoles that might usually have twin 225s. The price starts at $44,750, so it’s not inexpensive, but compared to the Seven Marine (now owned by Volvo Penta) 527’s price tag of $70K or to twin Yamaha F225s ($46,820), it looks more affordable.
Content extracted from http://www.boatingworld.com/engines/big-bad-and-beautiful-yamaha-xto-offshore-425/