The market for high-performance outboard motors is growing. In the 300 PS and above category alone, 1,100 units were sold in the past twelve months, out of a total of 19,000 outboard motors sold. Mercury This segment covers engines ranging from 300 to 600 PS. At the entry-level mark of 300 PS, the manufacturer offers something special: five different models, all based on the same engine block.
The Mercury 300’s shared V8 engine
At the heart of all five versions lies a naturally aspirated V8 engine with a displacement of 4.6 litres. The cylinder banks are set at an angle of 64 degrees to one another. The engine features two overhead camshafts and 32 valves. The bore and stroke measure 92 by 86 millimetres. Depending on the version, the engine revs at between 5,800 and 6,200 revolutions per minute at full throttle. The engine block is made from an aluminium-silicon alloy with a low copper content. A balance shaft reduces vibrations and noise. The exhaust system runs downwards through the centre between the cylinder banks, making the engine narrower. This allows the outboard motors to be mounted side by side with a centre-to-centre distance of just 66 centimetres.
Fuel supply and control
Each cylinder is supplied with fuel via its own injector. This indirect multipoint injection (port fuel injection) sprays the petrol into the intake manifold upstream of the valves, rather than directly into the combustion chamber. The SmartCraft PCM 112 control unit processes sensor data and controls the injection, throttle valve and ignition timing of the digital ignition system. The Advanced Range Optimisation system adjusts the engine parameters during continuous operation to save fuel. The Transient Spark function briefly advances the ignition timing during acceleration. According to Mercury, this is designed to improve response time and increase torque in the lower rev range by five to seven per cent.
Mercury FourStroke 300 and Verado 300
The FourStroke 300 uses a reduction gearbox with a ratio of 1.75:1. This means that when the engine is running at 1,750 revolutions per minute, the propeller turns 1,000 times. The Verado 300 operates at a ratio of 1.85:1, which means the propeller rotates more slowly at the same engine speed. According to Stefano Arrigoni, After-Sales Manager at Mercury Italy, both models achieve comparable performance figures when fitted with suitably selected propellers. The main difference lies in the shaft: the Verado features integrated electric steering, an eight-stage engine mount system for vibration damping and an adjustable exhaust system that can alter the sound. The FourStroke does not include these features and is cheaper. It is particularly suitable for re-engining projects. The Verado offers greater comfort in terms of noise, vibration and steering response, but is not faster than the FourStroke.
Mercury Pro XS 300 for performance boats
The Mercury Pro XS 300 uses a conventional shaft but features the TorqueMaster gearbox for high-performance applications. The gear ratio is 1.75:1. The propeller shaft has a reinforced diameter of 31.75 millimetres. The engine can be mounted higher on the transom, allowing the propeller to run partially on the water’s surface. This reduces the shaft’s drag. On suitable hulls, this can result in higher speeds. The electronic tuning allows for higher revs than with the FourStroke and Verado and delivers more direct throttle response. Mercury originally developed the Pro XS for American bass boats, where acceleration and speed are crucial. In Italy, the model is selling well on sporty inflatable boats between 7.5 and 8 metres in length, which are approved for 300 PS.
Mercury Racing 300R for extreme conditions
The 300R features a reinforced shaft designed for heavy-duty use and the Sport-Master gearbox. Mercury has designed it for very high mounting positions and surface operation of the propeller. The engine allows for more extreme installations than the Pro XS, but requires a hull designed for this purpose. For propellers that run partially out of the water, at least one blade should always be submerged. This is why four- or five-blade propellers are used, such as the Ventera and Bravo I models with four blades, or the Cleaver with five blades. More blades distribute the forces more evenly and reduce the load on each individual blade. All propellers for this application are made of stainless steel.
Mercury SeaPro 300 for commercial users
The SeaPro 300 is designed for users who require high operating hours. The electronic tuning is set more conservatively, with the maximum speed around 200 revolutions lower than that of leisure models. This reduces mechanical stress and extends the service life. Customers include professional fishermen, commercial operators, public authorities and sailing teams that use support boats. The Guardia Costiera, for example, uses boats fitted with two SeaPro engines. For these users, the priority is not top speed, but reliability under intensive use.
Which Mercury 300 is suitable for which boat?
Choosing the right model depends on the boat, its weight and its intended use. For a heavy boat 10 metres long, Arrigoni recommends the Verado or FourStroke. A light, fast inflatable boat between 7 and 7.5 metres in length could benefit from the 300R. Dealers should find out about the boat and its intended use before making a recommendation. A 300R fitted to a 9-metre-long, heavy boat may be slower than a FourStroke, as the racing gearbox creates more drag when fully submerged. Some customers report being disappointed because the boat does not deliver the expected performance. In most cases, the problem lies in an unsuitable combination of engine and hull.
Operation and Control
The Verado is available exclusively with electronic controls. The FourStroke, Pro XS and 300R can also be fitted with mechanical levers. This is important for engine retrofits and certain commercial applications. In this case, actuators convert the movement of the Bowden cables into electronic signals. The engine itself is always electronically controlled. In multi-engine set-ups, the availability of different shaft lengths depends on the model. Mercury developed the Pro XS primarily for single-engine installations, but it can also be fitted in pairs. The other models are available in L, XL and XXL lengths to suit different transom geometries and multi-engine set-ups. In twin-engine set-ups, a clockwise-rotating and a anti-clockwise-rotating propeller are usually used. In three-engine set-ups, a configuration with two clockwise-rotating and one anti-clockwise-rotating propeller is common. Mercury can control up to six engines with an integrated system, whilst other manufacturers often use two separate groups of three for six-engine set-ups.
Joystick and integration at Mercury
The joystick is in high demand for the Verado, particularly on new boats. Shipyards frequently specify this feature. The FourStroke does not feature a joystick, as it lacks the necessary steering and electronics. Mercury has integrated its joystick with bow thrusters from Sleipner, Quick and Vetus. The system controls the thrust of the bow thruster proportionally and coordinates it with the outboard motors. The driver operates everything via a joystick. This also works with single-engine set-ups: an integrated bow thruster, together with the engine control system, enables the boat to move sideways. Engine data can be displayed on multifunction displays from Garmin, Raymarine and the Navico brands Simrad and Lowrance. Like Mercury, Navico is part of the American Brunswick Group. The boat owner can view engine data on the same screen they use for nautical charts.
Maintenance and weight of outboard motors
For normal recreational use, the service interval is 100 operating hours or once a year. For genuine racing use of the 300R, involving frequent full-throttle operation, Mercury recommends shorter intervals – in extreme cases, every 25 hours. Anyone who uses the engine for 200 hours per season should carry out an interim service. The weight varies depending on the shaft and systems. The Verado weighs around 15 to 20 kilograms more than versions with a conventional shaft, due to its more complex design, integrated steering and vibration damping. The lightest Mercury 300 PS weighs around 306 kilograms. This low weight at the transom affects the trim. Some hulls perform better with more weight at the stern. Mercury argues that weight can be added, but not removed. With a lightweight engine, boat builders can specifically adjust the weight distribution. Additional weight can be placed where it makes the most sense for the trim.
Colours and prices of the Mercury 300
On the European market, Mercury mainly offers these engines in black and ice white. The FourStroke, Verado and Racing are available in both colours, whilst the Pro XS is available only in black. The FourStroke 300 starts at 26,932 euros, the Pro XS 300 and the Verado 300 are both priced at 29,707 euros, the SeaPro 300 starts at 29,724 euros and the Mercury Racing 300R at 33,517 euros. These prices apply to the engine in its basic configuration, excluding controls and installation accessories.
Jan-Ole Puls
Editor Test & Technology