Yamaha V8 XTO Offshore, over the LIMIT

Yamaha V8 XTO Offshore is  motor that marks a new stage in the world of big outboards, the 425 horsepower model has recently been joined by one at  375 horsepower. Our motor expert, Emanuele Pastori, gives us some details and reveals a few secrets

by Emanuele Pastori

Very few people know that in the first half of the last century a factory in Milan made huge outboard engines that were used to power the large barges that navigated the inland canals. This was a conscious technical decision designed to overcome the problems of manoeuvring in these narrow waterways, while also leaving more useful space on board for stowage, and it was probably one of the reasons Yamaha has today created this big and powerful outboard engine.

We therefore seem to be entering the ‘Sunset Boulevard’ era for the stern units that have been the leading protagonists among medium-length craft for almost half a century. The existing outboard formula, with its intrinsic characteristics of easy fitting, undisputed space savings on board and high efficiency, combined with the low consumption rates of modern motor technology, mean their success is assured.

Yamaha V8 XTO Offshore cc

It is no coincidence that the major manufacturers are leaning towards ever higher power, as demonstrated by this fine creation from Yamaha. Its ease of use, despite weighing over 400 kilograms, and its overall dimensions, make it flexible in the ways it can be fitted onto the stern of boats: we saw one vessel with four of these engines, making a total of 1800 hp.

Yamaha V8 XTO Offshore is not a racing motor, although it is capable of shifting at over 50 knots, but rather an outstanding thruster in which preference is given to mid-range running speeds, which also mean extremely low fuel consumption, while always maintaining a good balance with the other power ranges.

The engine layout is based on a compact V-block with a 60º aperture, which reduces forces and first- and second-order moments to 0, with a perfectly square architecture since the bore diameter, 96 mm, is equal to the stroke, giving a total cylinder capacity of 5600 cc. Yamaha has opted for a solution that they had already tried in the previous 8V, but they have redesigned it and created different cylinder capacities by varying the bore and passing from 94 to 96 cc.

The Yamaha V8 XTO Offshore cylinder block is made of aluminium with the slim-design cylinder barrels interference-fitted and plasma-treated to enhance toughness and wall smoothness. The cast aluminium pistons run inside these, adequately guided and provided with three cavities for seal segmentation and floating pins connecting them to the rods and then to the engine shaft.

The shaft runs on four supports plus a fifth at the upper flywheel and is effectively counterweighted. It is worth mentioning that, due to its architecture and careful balancing, this unit runs perfectly smoothly, without vibrations. There are four cam shafts, driven by a toothed belt and moving thirty-two valves, four per cylinder, in addition to triggering the direct hydraulic fuel injectors.

This is managed by a system comprising as many as five elements – two mechanical pumps, two auxiliary pumps and a vapour separator. From here the fuel is injected directly into the combustion chamber at a pressure of 200 bar. The high compression ratio of 12.2 to 1 unit for precise micronisation of the fuel mist enhances overall performance. In true Yamaha style, the distribution timing is entrusted to a tried-and-tested electro-hydraulic system located in the lower section of the cam shafts and controlled by silent chains in a hydraulic lubrication chamber.

Exhaust fumes are released from the centre of the V, in manifolds connected into a single outlet system. Here we noticed that the release of gas under forward drive is through the propeller hub, while in reverse the fumes are conveyed into the upper anti-cavitation plate so as not to interfere with the action of the propeller and consequently enhance the rear thrust.

The entire combustion system is well anchored with four highly rigid supports. Cooling is conducted with external water driven by a two-stage pump, while the flow is regulated by thermostats housed in the two cylinder heads. There is a digitalised ignition system, while the electrical supply depends on a generator capable of producing up to 90 amp/h.

This high power production is justified by the fact that the class of vessel suited to being fitted with these engines is typically equipped with hefty electrical systems like, for example, refrigerators, electronic devices, electrical winches and other systems requiring power.

Yamaha V8 XTO Offshore lubrication is entrusted to a pressurised system with two stages based on the motor rotation rate, which has the functional task of enabling an oil change while the down shaft is in the water, simply using a suction pump. The oil in the down shaft can be changed in the same way.

Yamaha V8 XTO ò

The motor casing is large but elegant, and its ‘bodywork’ is divided into four sections that offer very good inspection and intervention options when required. The down shaft is well profiled and streamlined with water intakes on the leading side. The engine power is transmitted by a robust drive shaft and conical coupling with a ratio of 1.79, or 25 crown teeth to 14 pinion teeth.

The entire complex is fixed to a robust steering head, combined with an axial drive system similar to an electrical car power-assisted steering system, electronically controlled and allowing immediate and precise control functions. The motor tilts on a central piston in the mounting, providing a tilt angle of up to 77 degrees. The motor parameter readings for this model are very interesting and bear witness to Yamaha’s desire to demonstrate not just high manufacturing quality but also excellent design skills.

The drive torque is a hefty 542 Newton/metre at just 2000 r.p.m., generating an average effective pressure of 17.6 kilograms/cm2, figures that point towards considerable elasticity and very high thrust just when it is needed most. The average piston speed is not particularly low (17.6 metres/sec), again indicating the capacity of this enginer to exceed the average standards for modern engines without conceding anything in toughness.

Yamaha V8 XTO llThe motor’s propellers are also new and designed in relation to its high-torque performance, which is also particularly efficient when conducting reverse manoeuvres, greatly assisted in this by the integrated Hellmmaster control system managed with a simple Joystick.

When put to the test, this good-looking motor appears to fully live up to its promise, even though our first contact with it was a configuration of three Yamaha V8 XTO Offshore fitted on a 14 metre Rib. The engine noise is decidedly low, full and completely free of any mechanical acoustics. Engagement is precise, silent and stutter-free. The acceleration is certainly impressive, rising very rapidly to maximum speed, which is above 50 knots, corresponding to about 5500 revs.

As already noted above, this is not a racing motor, despite the high speeds it can achieve, and it runs best at between 2000 and 3000 r.p.m., when it achieves optimum filling and develops outstanding torque exactly when it is needed, and at the most favourable rates of fuel consumption. Cruising, for example, at around 30 knots (already a reasonably good speed), consumption remains below 50 liters/hour. We will certainly be bringing you more detailed figures after a more thorough test cycle.

Yamaha V8 XTO Offshore
Displacement 5600 cc • Bore and stroke 96 mm x 96 mm • 8 V-shaped cylinders, 60° •  Outlet meachanical power 316.9 kW (425 hp) •  Maximum rotational speed 5700/min • Gear ratio 1.79/1 (25/149) • Oil carter capacity 7.8 l •  Maximum engine torque 542 Nm • Weight 442/463 kg.

Yamaha Motor Italia
Via Tinelli 67/69
I-20050 Gerno di Lesmo (Milano)
T. +39 039 60961

(Yamaha V8 XTO Offshore, over the LIMIT – Barchemagazine.com  – May 2019)