Hi
Most efficient is a slow speed diesel engine direct coupled to a propeller.
Trouble is that you can only do that with diesel’s large enough to be run efficiently at low enough rev’s for the propellor.
Small diesels run best at much higher revs than the prop can handle.
For any small diesel you need a transmission to cut the thousand/s rpm of the diesel to the hundred/s required for the propeller.
Any transmission, hydraulic, geared, electric generator/motor, belt/chain, will lose some power.
A hydraulic system can be as efficient as some geared transmissions.
A hydraulic transmission is a common piece of kit used in many commercial applications.
It is especially useful where you want the engine in a different position from the prop shaft or you want to run two props off one engine.
Though you can get the same result, without all those oil filled pipes and junctions, with diesel/electric
There is no advantage in having an electric motor driving a hydraulic system, as in this case, as you can size an electric motor for any combination of revs or torq you need.
I suggest, using an electric transmission. I.e. Diesel/electric, then you do not need the hydraulic’s. Much simpler.
if you still want hydraulics, suggest have both the hydraulic motor and the electric motor (direct drive) on the propeller shaft with a clutch on the hydraulic motor.
An advantage is that the electric motor can be used as a shaft generator when running the diesel.
But.
Regards
Captain Kerry Thomas
kerryjthomas@gmail.com
From: electricboats@yahoogroups.com [mailto:electricboats@yahoogroups.com] On Behalf Of Dave Steere
Sent: Sunday, 19 May 2013 4:20 a.m.
To: electricboats@yahoogroups.com
Subject: Re: [Electric Boats] Hydraulic motor drive with Hybrid Electric and Diesel Motors
It's not clear why an 18 HP diesel engine driving a pump driving a hydraulic motor driving a propeller has an efficiency gain over a diesel engine driving a propeller. And then to add the complexity of a second, smaller 3 HP electric motor driving a separate hydraulic pump which then pumps in parallel with the 18 HP diesel-driven pump. Ignoring any inefficiency in a reduction gear, if we assume 98% efficiency in each component of the diesel driven motor, that's a 4% loss in efficiency in the first chain (diesel to pump to hyd motor to propellor would be .98*.98*.98 = .94). I have no idea how to calculate the parallel lash up (solar cell to battery to motor to pump to hyd motor to propellor) but it's sure to be way lower total efficiency than the other path.
While I kind of like the idea of using hydraulics -- they can be quite efficient -- it seems like you've drifted away from the KISS principle.
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