Selecting a boat or a yacht, part 2: Engine and drive considerations

(Direct) shaft drive has an engine mounted typically towards the center of the boat and a direct shaft going through the hull with a propeller in the end. Shaft drive also has a rudder to steer the boat.

GOOD

• very few service parts needed and no need to service often,
• most reliable drive method and therefore used where highest reliability is needed,
• inexpensive to build and to service,
• easy to maneuver if dual engine installation,
• can provide a good protection to the propeller with a propeller guard

BAD

• because the engine is typically under the salon, the noise level in there is louder than with other engine options,
• cannot trim the propeller so the angle may not always be optimal,
• engine compartment is often more cramped due to being under the salon,
• in single engine installations close quarters maneuvering (especially backing up) is more challenging as rudder does not work when backing up

A special case of a shaft drive is a V-drive. In V-drive, the front of the engine faces aft and the shaft and gearbox form a V in order to have the propeller facing astern. The engine can therefore be mounted under the cockpit providing some advantages over direct shaft drive. V-drive is a bit more complex and also moves the weight distribution astern, which may be unwanted.

Stern drive has an outboard transmission and drive unit while the engine is located inside. The outboard unit steers the boat by turning.

GOOD

• engine can be mounted under the cockpit so the noise level in salon is lower,
• engine compartment is often easier to access than with shaft drive,
• can be trimmed to shift the planing position (angle of the propellers can be adjusted),
• excellent maneuverability both in slow speeds and in high speed

BAD

• much more complex than a shaft drive and therefore less reliable,
• more expensive to maintain (has to be serviced every second year),
• propeller is more prone to make contact as there is no propeller guard (on the other hand, stern drive can be lifted up by trimming)

Pod drives have the engine mounted either to the aft or midship. They have a pod that comes straight through the bottom of the boat. The pod turns left and right.

GOOD

• king of maneuverability,
• fuel efficient,
• engine can be mounted under the cockpit so the noise level in salon is lower,
• engine compartment is often easier to access than with shaft drive

BAD

• expensive compared to all other options,
• requires special hull design,
• more expensive to maintain than shaft drives,
• complex and therefore more prone to have issues

Outboards are attached to the transom of the boat. They have the engine and the transmission in one package. The entire unit turns to steer the boat. Most outboards are gasoline engines. Traditionally, outboards have been powering smaller boats, but nowadays there are even 11 m boats with outboards.

GOOD

• easy to service and easy to replace if needed,
• can be trimmed and tilted all the way up,
• great maneuverability,
• doesn’t take space inside the hull,
• quiet,
• inexpensive compared to inboards

BAD

• moves weight distribution to aft (and up) which must be compensated by changes in the buoyancy of the hull,
• as outboards are typically gasoline engines, the lifetime is much shorter (~1500 hours) than with diesels, and other drawbacks of gasoline engines,
• cannot have a good swim platform,
• tilting also takes space from the cockpit

Single engine

• cheaper to buy
• cheaper to maintain
• consumes less fuel (depending on the speed, 0-50% less than dual engine, based on the tests)
• 1/2 chance that something breaks
• takes less space
• less weight

Twin engine

• much better maneuverability at close quarters
• more reliable because of the redundancy (assuming there are two separate fuel tanks and separate fuel lines¹)
• more stable at sea (based on some tests)

Go to part 3:
Boat selection criteria