Motor Boat & Yachting — August 2017

Compact gear sets allow for a slimmer

casing for reduced hydrodynamic drag

● The use of counter-rotating

propellers on marine engines

is nothing new. Volvo Penta

has been offering it for years

on its duo-prop sterndrive legs.

However, this is the first time it

has been successfully offered

on a production outboard engine

● The theoretical advantages

are clear. By having two smaller

propellers spinning in opposing

directions, it naturally balances

out the torque steer created by

a single big propeller

● Propeller efficiency can also

increase as the rear propeller

recovers some of the rotational

losses from the wake of the

leading propeller

● The disadvantage is that it

necessitates running a shaft

within a shaft in order to transfer

power to the second propeller.

This is considerably more

complicated and expensive

than a single solid shaft, helping

to explain why no outboard

engine manufacturer has offered

it until now

● Some of the hydrodynamic

gains of the counter-rotating

propellers are also offset by

the frictional losses in the more

complex shaft and gear design

● Suzuki’s new engine appears

to be aimed at larger, heavier

boats where acceleration, fuel

efficiency and balanced handling

are more important than outright

speed

● If it’s outright power and

performance you’re after,

Mercury’s 400hp Verado still

leads the outboard engine pack

in terms of power-to-weight ratios

30 SECOND

BRIEFING:

COUNTER-ROTATING

KEEPING A COOL HEAD PROPELLERS
One of the key innovations is a new
dual-louvre system that feeds air
into the cylinders. These specially
shaped air intakes, comprising two
rows of fin-like louvres, not only
filter out every last drop of water
and moisture before it enters the
combustion chamber but also
helps to cool the air as it passes.
Suzuki claims the average intake
air temperatures are 10° cooler than
ambient, supplying the combustion
chamber with cooler, denser air for
a more powerful combustion cycle.
In effect, this acts as a mild form of
forced air induction.
The use of two fuel injectors per
cylinder also helps to reduce knocking
by ensuring a greater density of
atomised fuel is injected into the
centre of the combustion chamber
rather than around the edges, as
is the case with the cone-shaped
spray from a single injector. This
reduces the amount of off-centre
combustion, one of the causes of
knocking. Because the same amount
of fuel can be injected faster, it also
aids cooling, delivering an additional
3% power boost.
The surface of the pistons have
been treated with a shot-peening

process that leaves microscopic

dimples on the piston face. These

help distribute the high pressures

created during combustion more

evenly for reduced wear and a

more consistent power stroke.

The connecting rods and other

moving parts have also been

strengthened to cope with the

increased loads.

TWO PROPS BETTER THAN ONE

Putting this much power into the

water is always going to be a challenge

from the point of view of both grip

and mechanical strength. A single

propeller typically uses three big

blades to transmit power into the

water, while the gears need to be

strong enough to withstand the

torque of the engine as the blades

come up to speed. This in turn

requires a larger gearcase which

generates increased drag through

the water. Suzuki’s answer is to adopt

twin contra-rotating propellers that

use six smaller blades to give better

acceleration while simultaneously

reducing the loading and size of the

gears. This has allowed it to design

a sleeker leg without the torque

steer associated with a powerful

single engine spinning a big prop

in one direction only. It claims this

is particularly noticeable on larger

boats carrying heavy loads of fuel

and people.

Last but not least, the new engine

tips the scales at a relatively modest

330kg (midway between the 346kg

Yamaha F350 and the 303kg Mercury

Verado 350), while the slender design

of its narrow-angle V6 engine makes

it easier to fit multiple installations

on transoms with limited space.

Suzuki’s usual lean burn technology

and electronic throttle control should

ensure impressive fuel economy and

easy handling for such a powerful unit,

although no comparisons with rivals

are currently available.

The new engine will be launched

at the Southampton Boat Show in

September with the first deliveries

expected in December. Pricing will

be announced at the show.

Contact

http://www.globalsuzuki.com/marine

COMPARISON OF 350HP OUTBOARDS

SUZUKI

DF350A

MERCURY

VERADO 350

YAMAHA

F350

Power 350hp 350hp 350hp

Max rpm 5,700-6,300 5,800-6,400 5,000-6,000

Capacity 4,390cc 2,600cc 5,330cc

Configuration V6 Straight 6 V8

Weight 330kg 303kg 346kg

Warranty 5yr 5yr 5yr

Price TBA £28,228 £25,797

BOAT MASTER NEWTECH