Boat/Yacht Propulsion
Gordon Ringberg
Senior Underwriter
The following are the most common engine and
drive configurations used in “modern” boats and
yachts. Much of the following was borrowed from
Discover Boating’s “How to Buy a Boat Engine.”
Outboard
The definition of an outboard motor is a detachable engine mounted on outboard
brackets on the stern of your boat. Today's outboard motors range in horsepower from
1.0 HP to over 300 HP. Outboard motors can be used in all types of waters.
There are two general types of outboard motors; 2-Stroke and 4-Stroke motors One
noticeable difference between 2 stroke and 4 stroke engines is the weight of similar horse
powered engines. 4-strokes tend to be heavier than a 2-stroke engine of comparable
horse power. 2-Stroke engines uses a gas and oil mixture, while a 4-stroke burns straight
gasoline. A 4-stroke engine is more like a car engine including an oil filter and generally
has lower torque than 2-strokes. All outboards built for the US run on gasoline, but
Yanmar once built a diesel outboard for the Asia markets.
Common manufactures are: Evinrude, Mercury, Honda, Nissan, Tohatsu, Yamaha,
and Yanmar
Gasoline Inboard
While similar to what is under the hood of your car, gas engines used in marine applications
are modified to make them marine engines. Gasoline inboard engines range in horsepower
from 90 HP to over 1000 HP and are used in a variety of vessels from tow sport boats to
large cruisers.
In an inboard engine configuration, the engine sits amidships, with a drive running through
the bottom of the boat to a propeller, with a separate rudder used for steering. A
transmission is often used to transfer power from the engine to the propeller. Exhaust is
passed through the stern of the boat. Inboards are common for tow sports such as
waterskiing, wakeboarding as they allow the propeller to be brought forward of the back of
the boat, providing area for platforms to assist skiers in entering and exiting the boat.
Common manufactures are: Crusader, Flagship Marine, Indmar, Marine Power, Mercury
Marine, PCM , Teague Custom Marine, and Volvo Penta
Diesel Inboard
Diesel engines rely on compression to power the engine instead of spark. Compared to
a traditional gas engine, the engines are similar by design, as they have crankshafts
and cylinders and pistons; however, the fuel systems are completely different and more
complex on a diesel engine.
Diesel engines range in size and horse power. Diesel engines are widely used in other
parts of the world, while in the United States, they are typically found in boats larger
than 35 feet. The main reason that diesel engines are not used in smaller boats is
weight. The engine in general weighs more than a gas engine. However, they are used
in larger vessels because of their ability to produce torque. In general diesel engines
run at lower RPM's than a traditional gas engines.
Common manufactures are: Caterpillar, Cummins Mercruiser, Detroit Diesel,
John Deere, MAN, Volvo Penta, Westerbeke, and Yanmar
Inboard V-Drive
Inboard V-Drives are precision gear drives which allow inboard engines to be placed in the
stern (rear) of a boat for greater safety, better handling, increased space, reduced drag,
lower bow rise, shallower draft and less maintenance.
The V-Drive enables the propeller to be tucked under the hull in front of the rudder (below
left) -- instead of hanging off the stern next to the swim platform as with a stern drive or
inboard/outboard. This safety feature is important on small recreational runabouts designed
for the water sports.
Common manufactures are: Argo, Brog-Warner/Velvet Drive, Twin Disk, Walter,
and ZF Marine
I/O – Stern Drive
A Stern Drive consists of an engine and drive connected to one another through the
transom of a boat. A stern drive is sometimes called an Inboard/Outboard,
Inboard/Outdrive or I/O, reflecting its design. It is designed so that its engine is inside and
enclosed by the boat, while the propulsion system (out drive) is outside of the boat and in
the water.
The out drive can be trimmed-moved up or down- and has a propeller attached to the end
of it to propel the boat forward and reverse. This is also the main steering system for the
boat as it can turn side to side. Because the engine is in the boat, rather than being
supported by the transom as in an outboard, larger engines with greater horsepower can be
used.
Stern drive can come in a variety of different functions depending on their intended use;
drives are available with one or two propellers on a single stern drive. They can also be
used in tandem with two drives on a single boat, each with single or multiple propellers.
Common manufactures are: Ilmor, Mercruiser, and Volvo Penta
I/O – Stern Drive
Stern Drives have flexible gaskets, called “bellows,” that seal the water out where the
universal joint, shift cable and exhaust run between boat and drive. Their flexibility allows
the drive to trim and turn while under way, but they can dry out and crack due to heat,
harsh weather or age. In fact, since you can’t apply antifouling to flexible rubber like a
bellows, marine growth, especially barnacles, can create a tear.
I/O – Surface Drives
Surface drives are high performance I/O drives where only ½ the prop is in the water.
Surface drives are favored by high performance boaters for their minimal drag and high
speed capability.
Common manufactures are: Arneson, Seafury, Flexdrive and ZF Marine.
I/O - Saildrive
A saildrive is the sailor's equivalent of a stern drive. The saildrive looks like the bottom
part of an outboard motor sticking out of the bottom of the hull.
A sail drive can be installed in a smaller engine compartment, allowing for more living
space in the yacht.
Common manufactures are: Volvo Penta , Yanmar, and Mastervolt (electric)
I/O – Pod Drives
State of the art drive systems that are mounted through the bottom of the boat (vs the
stern.) to provide better performance, fuel economy and easy docking through joy stick
controls.
Common manufactures are: Volvo Penta IPS , Cummins MerCruiser Zeus
and ZF Marine.
Diesel/Electric Hybrid
With recent spikes in fuel prices, concern for the environment and the advent of new technology,
the boating industry is looking for alternatives to gasoline and diesel powered engines. One
alternative is diesel/electric hybrid systems. A hybrid generates propulsion using a diesel engine or
through a stored energy source. Boat owners who purchase hybrid systems are generally looking
for substantial reductions in diesel fuel consumption, noise, fumes, routine maintenance and
repairs.
Although various configurations are on the market, a popular hybrid system has a DC electric drive
motor that is powered by the yacht’s batteries. When the batteries run low, they can be charged
from renewable energy sources (solar panels or wind generator), from an onboard DC diesel
generator, or while connected to shore side power.
Common manufactures are: E motion, Fischer Panda, Hybrid-Marine,
Kräutler Elektromaschinen , Mastervolt, and SteyrMotors
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