September 12, 2019
By Florida Sportsman Editor
The amazing new Yamaha XTO 425
People who work and play on boats have always liked going fast—that will never change. It's wired into our DNA to enjoy the adrenaline rush we get when we push ourselves beyond our perceived limits.
But speed is no longer the only top priority. People want to own a reliable motor that delivers thrust, speed, efficiency, dependability, and operational convenience. And thanks to Yamaha's most recent innovation, the XTO 425, we can enjoy all of these luxuries and more.
Why does this 5.6L naturally aspirated beast make so much sense in the current outboard power market? For an easy answer to this question, you need to dive first, not into the subject of how much power is at your disposal; rather from where that power originates. As outboard powered vessel designs race north of sixty feet in length with no end in sight, the need for an efficient means of propulsion presents an entirely new set of challenges if you expect to stick with an outboard mounted power source. Taking a 30,000-pound boat hull from a standing position to plane and up to cruising speed is no small undertaking.
Some engine manufacturers in search of big horsepower in a smaller package have elected to rely for the most part; on the use of a turbo or supercharger system to force/inject air into the engine's combustion chamber as a way to create boost. At the mid to upper range of the power band, this technology will effectively build lots of horsepower. Here's the catch; a turbo depends on exhaust gas to spin up that power, and these engines don't produce sufficient air to realize their full potential until their RPM range reaches the point necessary to surpass the lag commonly experienced with a turbo engine. This is challenging when you require loads of raw power to swing a set of 17-inch propellers from a dead stop and build momentum quickly. It's here in the low end of the power band that a naturally aspirated engine like the XTO shines.
This is also where understanding the “why” gets a little technical. Without complicating things unnecessarily, a naturally aspirated engine, unlike its turbo counterpart, doesn't experience this low RPM hesitation. At 425-horsepower, the XTO is exceptional in the fact that this horsepower is immediate. This is partially the result of an industry leading 12.2:1 compression ratio. For those unfamiliar with how this translates into more power, it's pretty simple; the discrepancy between the volume of a cylinder (combustion chamber) when its piston is fully extended vs. fully withdrawn is its ratio. The larger the number, the larger the area for combustion.
This explains why the XTO 425 is so substantial in every way and why it weighs nearly 1000-pounds. Bigger cylinders require a bigger powerhead. The larger cylinders allow for a more significant combustion event that results in more energy being created. Here's the rub, if too much heat is created prematurely, you can experience engine “knock” or detonation inside of the combustion chamber; a common problem experienced when engineers have tried to push these compression limits. This irregular combustion event is not only inefficient; it's damaging and potentially fatal to your engine. A common workaround for most manufacturers to minimize knocking is to burn very high-octane gas and to add a “knock sensor” to the engine's ECU which dials back output. Yamaha alone has found a way to push these non-turbo engines to their limits of efficiency creating maximum horsepower without the need for high octane fuel (89-octane is fine) by the use of the industry's only four-stroke Direct Injection design. By injecting the fuel at 2900 psi for extreme cooling and atomization directly into the cylinder head at a precise moment at the beginning of the induction stroke, outbursts are almost completely avoided, and boost and power are consistently realized throughout the entire RPM range.
The advancements in technology aren't limited to achieving horsepower and low-end torque on this workhorse either. Another performance liability associated with outboard engines and their placement on a super-sized mega-console can be maneuverability. Considering the outer extreme location of your props, relative to the boat's center of gravity; and the relationship of thrust to the effects of wind and current, it's easy to see why many outboard operators in this size class struggle in close quarter situations. Through the use of a unique thrust enhancing exhaust routing system, Yamaha has given the 425 XTO a leg up, effectively increasing the propellers ability to grip clean, undisturbed water for a more efficient and immediate response when power is demanded. Routing exhaust gas through ports located above the engine's anti-ventilation plate at RPMs below 2500 offers up to 300% greater thrust in reverse than the F350 and this effect is immediately apparent while spinning a large boat into a slip or backing down on a hot fish.
A move to take close-quarters maneuverability to the next level has been underfoot in engineering circles for some time and the concept of electronically controlled joystick piloting has been widely explored and offered with mixed effectiveness in the industry. In a move to bring control under various fishing and docking scenarios in line with the results enjoyed on smaller craft through the use of GPS enabled electric trolling motors, Yamaha has capitalized on the effectiveness of the XTO's clean exhaust channel to maximize this effect.
A limiting factor with earlier versions of this technology has been the poor performance due in part to an engine's inability to have any significant influence on maneuvering without adding excessive RPMs to throttles. The resulting high revving and compounding turbulence requires excessive shifting, which is both hard on an engine and gear case and inadequate in its impression on a vessels attitude. In utilizing the clean water exhaust routing of the XTO in conjunction with Yamaha's Helm Master® controls, multiple settings can be selected to effectively perform various functions that are useful both at the dock as well as in a fishing environment.
The helmsman can select Stay Point® to hold a vessel's position and heading as though you were at anchor. Choose Fish Point® to hold a position in a low power mode to avoid excessive turbulence, which might spook fish. You can also choose Drift Point® to maintain a constant compass heading while drifting, which is extremely useful to minimize roll or keep your vessel pointed into the wind for kite fishing. All of this maneuverability is accomplished at the hands of the industry's first electric steering motor. Quicker response time through digital commands sent directly from the helm to the electronic steering control unit along with a super clean rigging application which eliminates the need for hydraulic pumps, oil, and plumbing at the transom and in your bilge makes you wonder why it took this long for this technology to emerge.
The benefits of the New XTO are significant. Despite size and weight the XTO's capabilities open the door for boaters to power vessels with one engine—where a pair of 225s were required in the past. Now, only two XTOs are needed instead of three 300 horsepower outboards.
By the numbers, the power to weight ratio of the 425 at 2.24 pounds per horse (when compared with the F350 at 2.18 pounds per horse) is greater than its predecessor. Concerning costs, the XTO is going to cost less than twin 225 horsepower engines, and the total maintenance cost is reduced in half.
While there is no telling what the future holds for outboard technology, it's safe to assume we'll see more high horsepower options in one form or another and increasingly higher output as the market demands it. The one thing you can be sure of is that the industry has its work cut out for it trying to one-up the new big dog on the block.
