>>943593A slow turning diesel will not be efficient on a psv, with its highly varying load. For a tanker or similar ships where 95% of its sailing is done at nominal power on the engines, a diesel-mechanic solution is optimal. A PSV needs several smaller engines, and run as many as is needed at all time, to keep load up. A diesel at low load is an unhappy diesel. Of course, within the 500m zone of rigs most vessels run all engines, with split bustie.
The contra rotating propellers run at identical speeds, but are different sized. Big one was 3 or 3.5MW I think, the smaller one 2.4MW. All based on memory. I'm no fluid mechanics engineer, but I think a contra rotating propeller is more efficient compared to a normal propeller. Also "torque steer" is further eliminated, though this effect is eliminated almost completely I think by having main propellers always turn opposite each other. By using slow turning PM motors the need for a gearbox reduction is gone, and some loss is saved there.
Volvo Penta duoprop is also contra rotating, for daycruisers etc.
http://www.volvopenta.com/volvopenta/na/en-us/marine_leisure_engines/drives/dph_duoprop/the_benefits_of_duoprop/Pages/the_benefits_of_duoprop.aspxI'll post another interesting vessel next.
