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→Design Considerations
The other reality is that the marine market for small boats is too small for many manufacturers to develop specialized versions of their drives for continuous duty marine applications. This is why some areas innovate slowly, and at great expense.
=== Twin Screws are Better ===
Hydrodynamic studies show that twin screws are clearly superior to a single screw.<ref>Naval Science 302: Navigation and Naval Operations II, Ol-Dominion University, http://www.odu.edu/webroot/orgs/ao/mo/nrotc.nsf/0/714fcf01b722201185256a0100447b19?OpenDocument</ref> <ref>
J. D. Van Manen and P. Van Ossanen, Principles of Naval Ar-chitecture, Second Revision, Volume II: Resistance, Propulsion, and Vibration, Society of Naval Architects and Marine Engineers, Jersey City, New Jersey USA, 1988, E. V. Lewis, Editor</ref> The twin-screw environment under the stern is less chaotic. This means the propeller-wake interaction is more constant and, therefore, more efficient.
Twin-screw ships are generally easier to manoeuvre. A single-screw ship suffers from stern walk, a tendency for the stern to counteract the rotation of the screw by moving sideways. Twin screws are de-signed to rotate in opposite directions, cancelling any stern walk. Also, the two shafts can be used to "twist" the ship in docking manoeuvres.
To drive twin screws, you need two main engines or one engine with hydraulic or electric drive. Twin engines are less fuel efficient.
=== One Rudder is Best ===
Contrarily, note that one rudder is better than two. Two rudders make the stern environment more chaotic again, and require a complicated steering system like on the front wheels of a car.
Unlike [[Propellers|propellers]], rudders are more likely to be a Naca shape (symmetrical). As a rule of thumb, rudder area = 0.5 * (waterline length * displacement). Articulated/deflector/high-lift-flap rudders common on work boats are appearing on yachts. They can reduce turning circles by 50%.
=== Number of Engines ===
From an emergency perspective, the debate over one engine or two will never be settled. Each person’s comfort zone is different when it comes to managing risk. From this perspective, there are four choices:
* No backup: One main engine with one propeller.
* Emergency power: One main engine with one propeller, plus a small wing engine with a small propeller.
* Redundant power:
* One main engine with dual electric drive, two propellers and battery backup.
* Two main engines with two propellers (drive systems are unim-portant).
If you’re comfortable psychologically with the proven reliability of diesel engines, you will happily go to sea with one engine and one propeller, and save some money in the bargain. You will be in the excellent company of most small fishing trawlers.
If your mind baulks at the risk this represents, then, hang the cost!, you will want two engines and two propellers.
Realistically, a small wing engine with a wimpy propeller is unlikely to have enough power in a severe-weather emergency. Two main engines is the only way to provide true redundancy. Alternatively, you might consider a single engine with an electric drive – it can be configured to use the house bank as an emergency source of power.
From a layout perspective, twin engines require a larger engine room, and may dictate an amidships layout. Twins mean twice as much cost and maintenance, and duplicate charging and fuel systems (not necessarily bad). Sometimes this results in two systems in some degree of disrepair.
==References ==
[[Category:PropulsionSystemsGeneral]]