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→Mechanical V-Drive
In a constant-velocity (CV) anti-vibration system such as the AquaDrive®,<ref>AquaDrive, http://www.aquadriveusa.com/advantage/advantage.htm</ref> there is a propeller shaft and a separate drive shaft, coupled with a CV joint. The propeller shaft ends in a [[ThrustBearing|thrust bearing]] mounted to the hull.
[[File:AquaDrive.jpg|thumb|250px|left|Standard mechanical drive (top) compared to AquaDrive® using intermediate thrust bearing and CV joints
– Graphic with permission © Aquadrive, http://www.aquadriveusa.com/]]
This bearing takes the end-thrust from the propeller and transmits it to the hull to move it through the water. The drive shaft has a CV joint at both ends, and is splined inside so that its length is adjustable. This allows the engine to be mounted on flexible rubber mounts.
=== Mechanical V-Drive ===
[[File:BorgWarnerVDrive.jpg|thumb|250px|left|Borg Warner V-drive
– Graphic with permission © Trans Atlantic Diesel, http://www.tadiesels.com/borg_warner-vdrive.html/]]
A mechanical V-drive is a compact reduction-gear transmission that reverses the direction of the drive shaft. It allows the engine to be placed aft, facing aft, with the output shaft facing forward. The V-drive reverses the direction of the output to connect to the propeller shaft, usually at an angle of 10-15 degrees. Typical manufacturers are: Borg Warner, Halibrand, Twin Disc Marine Transmis-sions, Yanmar, and ZF.
Diesel-electric propulsion systems are not new. They have been used in submarines, trains and buses. In a diesel-electric system, the engine drives a high output DC generator. The current from this drives one or more electric motors connected to the propeller shaft. The chief advantage of an electrical drive is increased performance throughout the speed range. This results because the engine can be run at its optimum RPM, while the speed of the boat is regulated by the electrical drive.
Much is said about the higher efficiency of mechanical drives, but electric drives are more or less as efficient as mechanical ones. The efficiency of an electric motor is measured in Watts-out/Watts-in, where Watts out are measured in hp x 746. Generally, modern electric motors (NEMA standard) have an efficiency between 87 and 97%, with larger motors being more efficient. Overall the efficiency will run around 81%, assuming the electrical generator converts 90% of the diesel engine's output into electrical energy and the traction motors convert 90% of this electrical energy back into mechanical energy on the propeller shaft.
Relevant standards are IEEE 112, JEC 37, and IEC 34-2. The main energy loss in electric drives is through energy transformed to heat.<ref>AC Motor Efficiency Guide, Rockwell Automation, http://www.reliance.com/mtr/b7087_5/b7087_5_3.htm</ref> <ref>Advanced Energy, http://www.advancedenergy.org/progressenergy/motor_efficiency.html</ref>
=== Continuously Variable Transmission ===
[https://en.wikipedia.org/wiki/Continuously_variable_transmission#Hydrostatic_CVTs Continuously Variable Transmissions ] (CVT) are mentioned here only for completeness. A CVT has a nearly infinite range of gear ratios. Like a controllable-pitch propeller, a CVT allows the engine to operate at its optimum rpm regardless of the speed. Until recently, CVTs were too expensive and unreliable for use in cars; although hydrostatic types have been used in main battle tanks. They are unlikely to be adapted for marine use.
== References ==