Does size matter? 350hp vs 300hp outboard in head-to-head test

It’s a dilemma that almost every RIB and sportsboat owner faces; do you opt for the smaller engine and save yourself a pretty penny on the purchase price or treat yourself to the more powerful engine, enjoy the extra performance and hope that by not working it so hard you actually burn less fuel and increase its reliability and resale values in the long run?

While it’s almost impossible to quantify reliability and resale values, we did get a chance to test the performance and fuel consumption of two similar outboards with different power outputs during a recent back-to-back sea trial. Both were fitted to identical Brig Eagle 8s but one boat was fitted with a 300hp Suzuki outboard, while the other was fitted with the 350hp version of the same engine.

Other than a few differences in tube and trim colours the boats were to all intents and purposes identical, bar the engines and the addition of power assisted steering (£1,900) and an electronic jack plate (£1,495) to the 350hp rig. This was deemed advisable to get the best out of an engine that has been designed for use on heavy sportsfishers as well as faster, lighter RIBs. As you’ll see, it made quite a difference.

The DF350 benefits from twin counter rotating propellers putting its power into the water

Before we get to driving impressions it’s also worth recapping on the technical differences between the two engines. The most obvious is the drive system: the DF300 puts its power through a single propeller whereas the DF350 has counter rotating twin props.

Suzuki claims a number of advantages for this; by spreading the torque over two propellers the gears can be smaller meaning a slimmer housing with less drag. It also delivers superior thrust in forward and reverse gears as well as reduced torque steer and stronger acceleration.

Both engines use the same six cylinder V6 block but the extra power and torque of the DF350 come from a slight increase in capacity from 4.0 litres to 4.4 litres, a higher compression ratio of 12.0:1 (the highest of any outboard engine), a revised air intake and a new dual injection system. Together they raise the power from 300hp to 350hp, and in conjunction with the twin props, add around 40kg to the weight.

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Power play

On start up the first thing you notice is the rortier noise of the DF350. Even at idle it sounds more purposeful than the silky smooth DF300, hinting at its deeper reserves of power. The addition of power assistance to the 350hp boat’s steering made it feel lighter and more responsive than the 300hp boat’s hydraulic set up, albeit at the expense of some steering feel.

I actually like the way the helm loads up as you wind on lock and speed with the unassisted set up. Both seemed to grip well at the kind of cornering speeds you are likely to use on a leisure boat but in extremis there did seem to be slightly less slip with the twin prop set-up.

However, it was the difference in performance and fuel consumption that proved to be the biggest eye-opener. Flat out the 350hp boat recorded a two-way average of exactly 50 knots – fully 10 knots quicker than the 300hp boat.

The 350hp Suzuki pushed the Brig Eagle 8 to 50 knots – 10 knots faster than the same boat with a 300hp motor

I reckon a big chunk of that is down to the jack plate, which by raising the engine vertically rather than tilting it, kept the thrust horizontal while simultaneously reducing drag and enabling the propellers to spin faster and reach the engine’s maximum revs of 6000rpm.

The difference was pronounced, almost as if we’d found an extra gear that suddenly added an extra 5 knots to the top speed. The DF300 by contrast ‘only’ managed to reach 5800rpm for a top speed of 40 knots – quite fast enough for most purposes and not as far off the DF350 as the unfair advantage of the jack plate suggests.

Bragging rights aside, it’s the acceleration onto the plane and the ease and efficiency of cruising that matter to most owners more than outright speed. Here too there were substantial differences between the two engines.

The DF300 has a single propeller and uses a slightly smaller version of the same V6 block

At 3000rpm the 300hp engine was still labouring to get on the plane at 11.2 knots whereas the torquier 350hp was already flying along at 21.0 knots. You can feel the difference in acceleration too. The DF350 just gets up and goes as soon as you apply the throttle.

Fuel saver

But it’s the fuel consumption figures once they are both on the plane that tell an even more interesting story. At a cruising speed of 30-32 knots the 300hp engine was spinning at a relatively busy 5000rpm and burning 60.5 litres per hour while the more powerful 350hp engine was turning over at a more relaxed 4000rpm and burning just 46.5 lph.

To put it another way the more powerful engine was consuming 1.5 litres for every mile travelled while the less powerful one was using 1.9 litres to cover the same distance. Even at their respective top speeds the smaller engine was burning 2.3 lpm at 40 knots while the bigger one was hammering along at 50 knots and still using less fuel per mile (2.2 lpm).

In summary it seems you really can have your cake and eat it, provided you shell out more in the first place for the engine and the jack plate.

First published in the March 2020 edition of Motor Boat & Yachting.