A saildrive is a transmission system for a boat whose inboard engine has a horizontal output shaft. The saildrive's input shaft is therefore also horizontal. That input shaft is geared so as to drive a vertical intermediate shaft extending downward through the hull. The intermediate shaft is then geared so as to drive a horizontal propeller shaft mounted on a
skeg
A skeg (or skegg or skag) is a sternward extension of the keel of boats and ships which have a rudder mounted on the centre line. The term also applies to the lowest point on an outboard motor or the outdrive of an inboard/outboard."A small fin f ...
outside the hull.
The transitions from horizontal to vertical and then back to horizontal can be seen as a Z shape, and saildrives are indeed similar to the
Z-drive transmissions used on larger vessels. The difference between a saildrive and a Z-drive is that a saildrive's propeller shaft is fixed in place, pointing aft, whereas a Z-drive's propeller shaft can be rotated to any azimuth.
Traditional sailboat transmissions consist of a simple horizontal output shaft extended rearward from the engine, through the stern via a
stuffing box
A stuffing box or gland package is an assembly which is used to house a gland seal. It is used to prevent leakage of fluid, such as water or steam, between sliding or turning parts of machine elements.
Components
A stuffing box of a sailing boat ...
. The saildrive has several advantages over the traditional sailboat transmission: smaller horizontal size, no stuffing box to maintain, and the propeller is mounted horizontally instead of at a downward angle.
Saildrive also reduces
propeller walk
Propeller walk (also known as propeller effect, wheeling effect, paddle wheel effect, asymmetric thrust, asymmetric blade effect, transverse thrust, prop walk) is the term for a propeller's tendency to rotate about a vertical axis (also known as ...
, reduces vibration, and is more efficient.
History of the saildrive
Among the earliest representatives of the technology was the OMC Zephyr Saildrive, produced from 1977 to 1984. This was a two-stroke gasoline engine to which oil needed to be added on a 50:1 mixture. It was a derated version of the 32 cubic inch engine block used at that time on the OMC Evinrude 25/30/35 hp outboards.
See also
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References
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Marine propulsion