Heavyweight  contender

Yamaha introduces a 425-hp V-8 outboard for larger offshore boats
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Yamaha designed the XTO Offshore to push boats that weigh more than 30,000 pounds.

Yamaha designed the XTO Offshore to push boats that weigh more than 30,000 pounds.

There’s an old sports adage that says a good big man will always beat a good little man. Yamaha appears to be taking that approach with its new 425-XTO Offshore outboard, introduced in May at its test facility in Bridgeport, Ala. Thirty-eight journalists from 29 media outlets were on hand for the debut of the engine, which features a 5.6-liter V-8 4-stroke powerhead, a non-hydraulic integrated electrical steering system and an alternator that produces 72 amps at 1,500 rpm.

Yamaha Marine Group president Ben Speciale said engines must keep pace with ever-bigger outboard-powered boats. In 2002, a 30-footer was considered big. In 2007, as outboard boats reached 36 feet, Yamaha introduced its 350-hp engine.

“The 350 taught us that outboard boats could get a lot bigger in this industry,” Speciale says.

Development of the 425-XTO Offshore started with a target propeller size that Yamaha engineers wanted to spin. The diameters of XTO OS propellers range from 16 inches to 17 1/8 inches, which is significantly larger than the props used for multi-engine F350 installations. One of Yamaha’s three-blade XTO OS props weighs 28 pounds.

“What we want to do is spin a bigger prop, and to spin a bigger prop you need a bigger prop shaft, diameter and gears, and much more displacement in the engine to give you the torque,” Speciale says. “We’re pushing boats that weigh 30,000 pounds plus. The 350 was the same way. The engine’s kind of heavy if you look at a boat of today, but if you look at a boat of tomorrow, it’s not heavy at all.”

Yamaha’s product manager for outboards, Ry Landry, explained: “When you start thinking about building a different motor, you really have to think about how people boat. It’s not about the motor, it’s about the boat and motor as a package.”

Companies are building bigger boats and consumers are buying them. Grady-White’s Canyon 456 weighs 35,454 pounds with quad XTO Offshores, while Jupiter’s 43 SF checks in at 26,797 pounds with the same power setup.

Twin XTO Offshores have more than enough power to give a driver control over the Grady-White Canyon 336.

Twin XTO Offshores have more than enough power to give a driver control over the Grady-White Canyon 336.

 The XTO Offshore is available in 25-, 30- and 35-inch shaft lengths with weights of 952, 977 and 999 pounds, respectively. Doing the math, dividing 952 pounds by 425 hp gives you 2.24 pounds per horsepower. Yamaha’s F350 has a power-to-weight ratio of 2.18 pounds per horsepower. The XTO Offshore makes its peak power at 5,500 rpm. (Yamaha doesn’t release torque specs.)

“It’s easy to make horsepower,” Speciale says. “Durability is difficult.”

I drove four boats with the XTO Offshore in a variety of configurations. A Jupiter 43 SF center console rigged with quad engines ran at a top speed of 62 mph at 6,000 rpm and needed a typical amount of time to plane from a hard start. The boat had a conventional V-bottom design with 24 degrees of deadrise at the stern.

Next, I ran a twin-step 39-foot Contender 39ST with three of the motors to a top speed of just over 71 mph at 5,900 rpm. That boat had strong midrange punch when I nailed the digital throttle from cruising speed.

A single-engine Sportsman Masters 267 Bay Boat offered the purest seat-of-the-pants evaluation of the motor. It planed in a heartbeat and sprinted to a top speed of 61.2 mph in seconds.

Finally, a 33-foot Grady-White Canyon center console that tipped the scales at 13,194 pounds with twin XTO Offshores jumped on plane in seconds with hardly any bow rise.

The ability to swing large-diameter propellers translates into strong bites during turns. Even on the stepped-bottom Contender, the rpm never dropped in turns that were tighter than most captains would make at 60 mph. With the conventional V-bottom designs, I could turn hard over and the props held steady without slipping.

Another design innovation of the XTO Offshore is the location of exhaust ports above the outboard’s anti-ventilation plate. When you’re running at rpm of less than 2,500, the exhaust exits through these ports, not through the prop hub. This setup means the props are biting into denser, clean water, which makes them more effective.

Yamaha says the XTO Offshore has 300 percent more reverse thrust than the F350. All the boats I ran were responsive in reverse and had plenty of torque to back down on any fish. The only criticisms I and other journalists had were that the motors were louder than I’m used to with typical 4-strokes, but only above 4,500 rpm on some of the boats.

The motor is a totally new design from the propshaft to the powerhead.

The motor is a totally new design from the propshaft to the powerhead.

Feature Filled

The XTO Offshore mounts on the same 28½-inch centers as the 350-hp model with an identical bolt pattern. The new engine is designed to install on a flat transom. The mounting bracket has a wide span for better control of the steering angle, and improved vertical strength to accommodate the motor’s mass. The midsection was engineered to absorb shock loads and has rubber dampeners the size of soda cans.

Displacement of the 60-degree V-8 is 5,559 cubic centimeters/5.6 liters, and bore and stroke is 96 millimeters by 96 millimeters. The stainless-steel crankshaft weighs 66 pounds. Oil capacity is 8.2 quarts, which might seem like a lot, but it’s in the name of long-range durability. The same goes for the double-overhead camshaft design.

The XTO offshore has direct fuel injection and five fuel pumps: three electrical and two mechanical. An electronic control unit manages the variable fuel pressure, which can get as high as 2,900 psi. A 12.2:1 compression ratio is the highest among current outboards, and Yamaha says it’s possible thanks to the cooling effects of direct injection. The spark plugs are iridium, and the engine runs on 89 octane fuel.

Yamaha uses plasma fusion liners, rather than steel, in the cylinders. The material is thinner, lighter and harder than steel, a combination that should contribute to increased displacement without increasing powerhead size.

To ensure that the outboard runs at proper temperatures, it has two thermostats per cylinder bank. One pair helps to regulate engine temperature, and the other two maintain oil temperature.

yamaha-seminar

For durability, a self-tensioning chain immersed in an oil bath connects dual overhead camshafts on each cylinder bank. Single-sprocket belt engagement allows for a narrower, more compact profile. Bucket-type valve lifters eliminate the need for shims and are carbon-coated for reduced friction. A dual-chamber oil pump ensures lubrication.

Exhaust intake tracks enter the cylinder head from both sides of the powerhead, giving the gases a more direct route to the lower unit for improved flow. The exhaust outlets on the gearcase are positioned above the propeller, a design that Yamaha says gives the engine a 300 percent increase in reverse thrust compared with its 350-hp engine because the prop gets a harder bite in clean water.

To meet the electrical demands of large boats, the XTO Offshore makes 90 amps gross and 58 amps net at 650 rpm, and 72 amps at 1,500. Because making that power creates heat in the coils, Yamaha designed the motor with a forced air cooling system for the alternator.

A big motor that turns a big propeller needs a robust gearcase, and the XTO Offshore’s lower unit is 6 inches in diameter. By comparison, the F350’s gearcase is 5¼ inches in diameter, and the F300 has a 4¾-inch lower unit.

The bottom propeller is the new XTO Offshore wheel. It weighs 28 pounds.

The bottom propeller is the new XTO Offshore wheel. It weighs 28 pounds.

The XTO Offshore’s gearcase was designed for hydrodynamic efficiency. “If we can do a better job of making this slippier in the water, it produces huge benefits across the performance curve,” Landry says.

The lower unit has four water intakes and a two-stage water pump that has a rubber impeller and a steel impeller to keep cooling water flowing through the engine. Helical-cut gears are hardened for strength, and there are 25 teeth on the ring gear and 14 on the pinion for a 1.75:1 gear ratio. The prop shaft has a 30-millimeter diameter at the splines and 37 millimeters at the bearing supports. Currently, Yamaha has three-blade props available for the motors; four-blade models are expected to be available in the future.

The XTO Offshore’s integrated non-hydraulic steering uses an electric motor in the motor’s tilt tube, and there is a manual override if there’s an electrical problem. The amount of steering friction can be adjusted and, for safety, the steering system won’t make fast lock-to-lock turns at higher speeds. Steering angle is 31 degrees, and the motor communicates with Yamaha’s Helm Master system, which has been upgraded with new software for the engine, and with a CL7 display with a 7-inch screen.

“It’s not just a generation change of the power plant,” Speciale says. “It’s a generation change of integration.”

yamaha-steering-cutaway

Because bigger boats are not easy to take out of the water for service, the motor’s lower-unit gear lube can be changed with the boat in the water, and the engine can be tilted higher on the transom than the Yamaha 350. Maximum tilt-up range is 73 degrees, and there’s 5.4 inches of increased clearance beneath the nose of the torpedo on the gearcase and the water. The XTO Offshore’s rigging tube is 25 percent larger than on previous models and offers 3¼ inches more clearance at the outboard well when tilted up.

A dual system provides two methods for flushing the outboard: one routed through the rigging tube, and a conventional front-mounted hose connection. Yamaha says a multipiece cowling allows easier access to the powerhead and midsection. The XTO Offshore is available in white, gray or comes unpainted.

Retail prices for gray motors start at $44,250 for the 25-inch shaft in right-hand rotation and $45,185 in left-hand rotation. The 30-inch shaft with right-hand rotation retails for $45,000, and the left-hand version has a sticker price of $45,935. Finally, stepping up to the 35–inch shaft costs $46,120 for the right-hand rotation and $47,060 for the left-hand model. The XTO Offshores carry five-year warranties.

This article originally appeared in the July 2018 issue.

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